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Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies (2000)

Chapter:III Survey of Technologies for the Human Exploration and Development of Space

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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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Suggested Citation:"III Survey of Technologies for the Human Exploration and Development of Space." National Research Council. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. Washington, DC: The National Academies Press. doi: 10.17226/9452.
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III Survey of Technologies for the Human Exploration and Development of Space This chapter provides the essential foundation for subsequent discussion of microgravity phenomena and the determination of related research needs important for HEDS. That foundation consists of rather detailed descrip- tions and assessments of the various technologies that seem most important for HEDS systems. In the committee's view, these are the types of technologies that must operate reliably and efficiently in the various space environ- ments of interest. Two important considerations were involved in the selection of technologies for discussion. First, it was clear that this report could not usefully embark on studies of systems and mission architectures, with their specific design problems, based on premature mission assumptions. Secondly, the range of technologies identified needed to be broad enough to cover reasonable possibilities for incorporation into future systems but did not have to cover all conceivable possibilities. Therefore, this report emphasizes technologies that have a wide range of potential applications and that are expected to be significantly influenced by gravity level. In this chapter, the technologies selected for discussion are grouped according to their probable functions in the HEDS program. Since some are quite well developed already, while others exist only as concepts, the level of detail varies considerably. Systems to serve HEDS functions are identified first, followed by their components or subsystems. Especially at the subsystem level, microgravity concerns are identified and summarized in a table for each function. Tables III.G.1 to III.G.3 at the end of this chapter relate microgravity phenomena to the various subsystems and processes. However, the phenomena are not treated in detail, nor are relevant research issues described. Rather, in Chapter IV, the identified microgravity concerns are related to physical phenomena, and physical research areas are identified that may provide the knowledge base needed to design systems and components that will be reliable and effective in the microgravity environments of interest. III.A POWER GENERATION AND STORAGE Introduction Future HEDS missions for the exploration and colonization of the solar system will require enabling technolo- gies for adequate, reliable electrical power generation and storage. Advanced, high-efficiency power generation and storage will be required for deep-space missions, lunar and planetary bases, and extended human exploration. 21

22 MICROGRAVITY RESEARCH Extensive up-to-date discussions are available (NRC, 1987, 1998; Bennett et al., 1996; Brandhorst et al., 1996; Detwiler et al., 1996; Bennett, 1998) and there is no need to repeat them here. Because the electrical power technology requirements for spacecraft are similar to the requirements for the extended human occupation of the Moon or Mars (when energy demand is not constant) they are discussed together. Space propulsion is, of course, the dominant and limiting power-generation requirement for HEDS. However, due to the wide range of systems that must be considered, propulsion is discussed separately in Section III.B. Many of the means of power generation applicable to spacecraft and station power discussed in this section are also applicable to propulsion. For the purpose of the present discussion, the primary energy sources for conversion to electrical power on a spacecraft are the following: (1) solar radiation, (2) chemical and electrochemical, and (3) nuclear (radioisotope thermoelectric generators (RTGs), dynamic isotope power (DIP) sources and fission and fusion power). The choice of energy source and power-generation system and subsystem is dictated largely by the mission require- ments. These energy sources can be utilized in open or closed thermodynamic systems. A closed-cycle system is one in which a working fluid is heated, does work, and is recycled (Figure III.A.1~; an open-cycle system is one in which a working fluid is heated, does work, and is discharged, carrying waste heat with it. The electric power generated requires a power management and distribution system that includes regulators, converters, control circuits, etc. (Figure III.A.2~. Energy storage devices may also be required, since some energy sources (e.g., solar radiation) are not continuous. The power-to-mass ratio (in kWe/kg) of the power system is an important consideration for space missions. Small versus large power needs and autonomous versus manual control are additional factors. The fact that electrical power generation and onboard propulsion subsystems can account for one-half to three-fourths of the mass of the typical Earth-orbiting satellite or planetary spacecraft provides the motivation to reduce their mass, which would allow more of the spacecraft's mass to be devoted to payload. The desire to reduce costs and maintain reliable performance has led to the consideration of both some old and some new technologies for electric power generation; these technologies are reviewed and discussed in detail in Brandhorst et al. (1996), Detwiler et al. (1996), and Landis et al. (1996~. A= OR PUMA ~ r TURBINE tENERATOR | POWER HEAT SINK ~ MANAGEMENT | LOAD l FIGURE III.A. 1 Generic diagram of a simplified closed-cycle space power system. For a Brayton cycle, the heat source is a heat exchanger where heat from the source is added to the working gas, and the heat sink is also a heat exchanger (i.e., space radiator) where the working fluid is cooled. For a Rankine cycle, the heat source is a boiler where the working fluid is boiled, and the heat sink is a heat exchanger (i.e., space radiator) where the working fluid is condensed.

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE ENERGY SOURCE SOLAR NUCLEAR CHEMICAL CONVERTER SOLAR CELL ARRAY SOLAR DYNAMIC NUCLEAR ETC. POWER MANAGEMENT AND DISTRIBUTION REGULATORS CONVERTERS CONTROL CIRCUITS ETC. T 1 ENERGY STORAGE RECHARGEABLE BATTERIES REGENERATIVE FUEL CELLS FLYWHEELS CAPACITORS, ETC. FIGURE III.A.2 Schematic of a generic electric power system. Based on Bennett (1998~. 23 SPACECRAFT SYSTEMS OADS Gravity is an important consideration in active (thermal) subsystems for power generation. Many of these subsystems involve single and/or multiphase fluid and thermal management. Such important subsystems as boilers, condensers, evaporators, heat exchangers, normal and cryogenic fluid storage units, fuel cells, radiators, and heat pipes involve fluid flow and/or transport phenomena, including heat and mass transfer, phase separation, and others. Because fluid flow and transport phenomena are affected by gravity, a full understanding of the phenomena is needed for the design of the systems and for their safe and efficient operation in microgravity or reduced-gravity environments. Power Generation Systems Solar Power Systems The principal solar/electric power systems are of two types: passive (i.e., photovoltaic or photoelectric) and active (thermal). In the literature (Bennett, 1998) the former is referred to as static and the latter as dynamic. The solar cell arrays used on most spacecraft usually consist of a large number of cells that convert a fraction of the solar radiation incident on them to electricity by means of the photoelectric effect. The solar cells are connected into appropriate series/parallel circuits to produce needed power at required voltages and currents. On the recent Mars Pathfinder mission, the lander, the Sojourner, and the cruise system were all powered by gallium-arsenide solar cells. This was the first use by NASA of solar/photovoltaic power on the surface of Mars. Recent discussions of the advances in solar cell technology are available (Landis et al., 1996; Bennett et al., 1996; Bennett, 1998~. There are two ways of using solar energy: directly, by thermoelectric means, or indirectly, using solar radiation to heat a working fluid, which then drives a turbine/alternator (generator). The former method simply requires thermoelectric elements placed at the focus of a concentrator. While simple to construct, the power density is low and the system has not been used by NASA to power a spacecraft. Moreover, solar power will be of limited value for deep-space missions and may be unreliable at extraterrestrial sites, e.g., Mars. Indeed the use of photovoltaic solar cell arrays will be restricted to spacecraft that do not travel beyond the Mars orbit. This is the case primarily because the solar irradiation (insolation) decreases as a square of the distance from the Sun. The collectors/concentrators would have to be larger, and this would prohibitively increase the mass of the propulsion subsystem. Ionizing radiation, low intensity, and low- and high-temperature (for the solar probe) degradation effects are other problems that must be addressed in the use of solar cell arrays for photovoltaic electric power generation (Bennett, 1998~. Solar/thermal dynamic systems can overcome some of the problems,

24 MICROGRAVITY RESEARCH such as radiation damage to solar cells and the limited life of chemical energy storage systems, but there are issues with the technology that have not yet been solved. The need for moving components and the reliability of parts of solar dynamic systems are some of the viability issues that have not yet been addressed. The active method of using solar radiation to produce electric power is to heat a working fluid that can drive a turbine/generator in much the same way as is done in the electric power industry. To this end, a large number of solar collectors can be used to convert a fraction of the incident solar radiation to sensible and/or latent energy of the fluid. Heat is transferred from the structural collector elements to the fluid by forced convection and/or flow boiling. The energy collected can then be stored in a thermal energy storage device for use when insolation is not available. The component technologies for active power conversion for both the Brayton and Rankine cycles are fully mature. However, waste heat rejection in Brayton and Rankine thermodynamic power-generation cycles is of major concern, because space radiators represent a significant portion of the weight of the system. NASA's Glenn Research Center has performed the first full-scale demonstration of a complete space-configured 2-kW solar active system based on the Brayton cycle in a relevant space environment. However, one of the criticisms that has been leveled at solar/thermal active systems is that the conversion systems depend on moving parts that are considered to be intrinsically less reliable and shorter-lived than those in photovoltaic conversion systems. In summary, solar power systems may not be feasible for many deep-space missions, lunar and planetary bases, and extended human exploration missions or for powering high-thrust, high-efficiency propulsion systems (NRC, 1998). Chemical Power Systems Power systems based on chemical energy sources include batteries and fuel cells. Storable chemical reactants (e.g., nitrogen tetroxide, mixed amines, hydrogen, oxygen, and other chemicals) can be stored aboard spacecraft or on the surface of Mars for power generation using a mass open or closed thermodynamic system. Considerable technology relevant to this application is available from the Apollo program. The principal unknown in using chemical reactants to produce electric power is the ability of the spacecraft systems to tolerate the effects of any chemical effluents that are released. Although chemical energy sources appear attractive because they offer rapid response, as Figure III.A.3 illustrates, chemical energy sources for electric power generation are suitable only for short-duration functions and/or missions. Also, as can be seen from Figure III.A.3, a fundamental shortcoming of the chemical energy sources for power generation is that the mass of the chemical reactants becomes prohibitive for burns and/or missions of long durations (NRC, 1987; Bennett, 1998~. Stored chemical energy could be used to meet short-duration peak or emergency power demand, but for long-duration missions, solar or nuclear energy sources with energy conversion technologies based on a Brayton or Rankine cycle power-generation system would be the most suitable. Fuel cells are electrochemical devices that convert the chemical energy of a fuel directly into electrical and thermal energy (Blomen and Mugerwa, 1993~. In a typical fuel cell, gaseous fuels are fed continuously to the anode (negative electrode) compartment and an oxidant, e.g., oxygen or air, is fed continuously to the cathode (positive electrode) compartment. The electrochemical reactions take place at the electrodes to produce an electric (direct) current. The fuel cell theoretically has the capability of producing electrical energy for as long as the fuel and oxidant are fed to the electrodes. In reality, degradation or malfunction of components limits the practical operating life of fuel cells. Besides directly producing electricity and having the capacity to serve as energy storage devices, fuel cells also produce heat and water. The heat can be utilized effectively for the generation of additional electricity or for other purposes, depending on the temperature. A practical consideration for fuel cells is their compatibility with the available fuels and oxidants. For HEDS missions, at least four applications of fuel cells are possible: (1) electric power generation in a space vehicle or at an extraterrestrial site, (2) surface transport on Mars or the Moon, (3) production of oxygen (O2) from carbon dioxide (CO2) on Mars, and (4) production of potable water for life support. One of the main attractive features of fuel cell systems is the expected high fuel-to-electricity efficiency and the fact that they can also be used as storage devices. This efficiency, which runs from 40 to 60 percent based on

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 105 104 - a, 103 UJ > ~ 102 o ~ 10 C: UJ 10° 1o-l ,1 ~ ~ DYNAMIC ISOTOPE POWER SYSTEMS // // TRI C 1 HOUR 1 DAY 1 MONTH 1 YEAR 10 YEARS DURATION OF USE 25 FIGURE III.A.3 Qualitative diagram illustrating the regimes of applicability of various space power systems. Courtesy of Gary L. Bennett, Metaspace Enterprises. the lower heating value (LHV) of the fuel, is higher than that of almost all other energy conversion systems. In addition, high-temperature fuel cells produce high-grade heat, which is available for cogeneration applications. If waste heat is utilized, the theoretical efficiency can reach 80 percent (Klaiber, 1996~. Because fuel cells operate at near-constant efficiency, independent of size, small fuel cells are nearly as efficient as large ones. Thus, fuel cell power plants can be configured in a wide range of electrical power levels from watts to megawatts. Fuel cells are quiet and operate with virtually no noxious emissions, but they are sensitive to certain fuel contaminants, e.g., carbon monoxide (CO), hydrogen sulfide (H2S), ammonia (NH3), and halides, depending on the type of fuel cell. Thus, the contaminants must be minimized in the fuel gas. Fuel cells have been identified by the National Critical Technologies Panel as one of the 22 key technologies the United States must develop and implement in order to achieve economic prosperity and maintain national security (National Critical Technologies Panel, 1993~. A variety of fuel cells have been developed for terrestrial and space applications (Blomen and Mugerwa, 1993~. Fuel cells are usually classified according to the type of electrolyte used in the cell: alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), and proton-exchange membrane fuel cells (PEMFCs). The operating temperature ranges from ~80 °C for PEMFCs to ~1000 °C for SOFCs (Kroschwitz and Bickford, 1994~. The physicochemical and thermomechanical properties of materials used for the cell components (e.g., electrodes, electrolyte, bipolar separator, and current collector) determine the practical operating temperature and useful life of the cells. The properties of the electrolyte are especially important. Solid polymer and aqueous electrolytes can be used only at ~200 °C or lower because of high water-vapor pressure and/or rapid degradation at higher temperatures. The operating temperature of high-temperature fuel cells is determined by the melting point for MCFCs or the ionic-conductivity requirements for SOFCs of the electrolyte. The operating temperature dictates the type of fuel that can be utilized. Interfacial and transport (flow, heat, mass, charge) phenomena in the membranes (porous media) under reduced or microgravity conditions are also important issues in the design and safe operation of fuel cell systems.

26 Nuclear Power Systems MICROGRAVITY RESEARCH Nuclear energy sources for power generation come in three types: radioisotope, fission reactor, and fusion reactor. Since sustained fusion has not yet been demonstrated in a laboratory and no reactors are likely to be available even for terrestrial applications until well into the twenty-first century, this type of nuclear reactor is not considered. An up-to-date discussion of nuclear power technology for spacecraft applications is available (Bennett, 1998; NRC, 1987~. Suffice it to summarize that since 1961, the United States has flown 44 radioisotope thermoelectric generators (RTGs) and one nuclear fission reactor (see below) using thermoelectric conversion to provide power for 25 space systems. The Galileo mission to Jupiter, the Ulysses mission to explore the polar regions of the Sun, and the Saturn-bound Cassini mission are powered by RTGs operating at 1000 °C (Bennett et al., 1995; Bennett, 1998~. For example, the Cassini spacecraft was developed and launched in October 1997 on a mission to investigate Saturn and its rings, satellites, and magnetosphere. It is powered by three RTGs. RTGs have been used by NASA for many years, and this technology is mature and reliable. It is not sensitive to gravity; however, it is currently limited to relatively low power levels (see Figure III.A.3~. The United States has flown one space nuclear fission reactor (SNAP-1OA), which was launched in 1965 and provided 500 W of electric power. SNAP-1OA was a liquid-metal-cooled nuclear reactor with a thermoelectric conversion. A ground-test twin of the flight version of SNAP-I OA operated unattended for over a year, demon- strating the feasibility of the fission nuclear reactor. According to reports (Bennett, 1998), the former Soviet Union launched perhaps as many as 33 low-power (~1 to 2 kWe) nuclear fission reactors from 1967 to 1988 to power its radar ocean reconnaissance satellites. All of the reactors used thermoelectric elements to convert thermal energy to electricity. Fission nuclear reactors can be characterized as having a very good power-to-weight ratio. An example of a reactor designed for space use is the reactor that was being worked on for SP-100. Jointly undertaken by the Department of Energy (DOE), the Department of Defense (DOD), and NASA, the SP-100 program had the goal of developing a space nuclear reactor technology that could support a range of projected missions, including nuclear electric propulsion and planetary surface operations. Several systems were designed for a power output of 100 kWe and incorporated a high-temperature, liquid-metal-cooled reactor. One design concept used an inert-gas Brayton cycle with a turbine generator, while another was designed for use with an advanced thermoelectric converter. Most of the nuclear component development had been completed on SP-100 before the project was cancelled in 1994. Currently the United States has no useful space nuclear reactor program, even though recent studies continue to show (AIAA, 1995; NRC, 1998; Friedensen, 1998) that human exploration of the Moon and Mars will require this technology. To quote a recent National Research Council report (1998, p. 19~: "The committee is well aware that political constraints may make R&D on advanced space nuclear power systems unpopular. However, the committee could not ignore the fact that space nuclear power will be a key enabling technology for future space activities that will not be able to rely on solar power." Energy Storage Reliance on intermittent (i.e., solar) energy sources as a primary means of electrical power generation requires some method of energy storage. The storage methods may be chemical (primary and secondary rechargeable- batteries, and primary and regenerative fuel cells), electrical (capacitors), mechanical (flywheels, gravitational liquid or solid), or thermal (latent or sensible heat). The design and performance of storage systems are judged on lifetime, reliability, safety, efficiency, and specific energy. For example, the two principal systems that are being used or being considered are nickel-based and lithium-based batteries. An up-to-date review of storage systems used or under development for spacecraft applications is available (Bennett, 1998~. In summary, great progress is being made by both NASA and DOD on a range of battery technologies that promise improvements by a factor of 10 in specific energy over the old nickel-cadmium batteries. Lithium-based batteries (i.e., rechargeable lithium ion batteries) have a potential to achieve a specific storage of up to 200 We-h/kg. Fuel cells have been used by NASA to power the internal systems of Gemini and Apollo spacecraft and are currently used to power the space shuttle. The lower operating temperatures and higher efficiencies (which

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE If '''''' - ~~.~ ' 'a ~ ^ Off- H2'f'0 err...... __ Fuel Electro- Cell Iysis Cell . ~ Lo ~ t _ ~~ L]~^ ~ 27 aaan.~a.ea.~aa. Sad ·-aa.~eaaea aaa- ·.aa.~'aaaea an aa.~-~eal ·~.aa.~eaaaaae ·.aae.~-al~aaaaa ·~-~-~ea.~-a ·~-~--~.aaa. ·alaaaaa-aa.~a ·a I.. Solar l: a Panel '.a la --.-- ·aa.~ea.~aae Baa ·~ea.~ea.~.aaa. sea ae---~ea'--- ·~ala.~. Al ·~aa.~-~eala.~a ·~eeaa-—aaa.~ea ·~.aa.~-----~-e A------ - - - --- ·-------------- ·---------~-~e ·---~-~----~--- ·-------------- ·-----------—a- ·-- - - - - -e——ae— · - - - - -—~—~——~—— me—- -—~—- -—~—a. · - -------e—-~e ·-.—- -.—a..——~— ·e——~—-—~—-—a-a ·--~-a---~----e · -—~——~—- - -—aa— ·-------------- FIGURE III.A.4 Schematic diagram of the fuel cell-electrolysis cycle. SOURCE: Mayer (1992~. Reproduced with permis- sion of the American Society of Civil Engineers. translate to reduced weight) of PEMFCsimake them attractive options for planetary missions. Their superior performance and longer life have led NASA to look closely at PEMFCs. The first PEMFC used in an operational space system was the GE-built, l-kW Gemini power plant. Two 1-kW modules provided primary power for each of the seven Gemini spacecraft in the early 1960s. Each module could provide the full mission power require- ments. The performance and life of the Gemini fuel cells were limited by the polystyrene sulfonic acid membrane used at that time. In 1968 an improved Nation membrane was introduced, significantly improving performance and life. New fuel cell technologies for electric power generation continue to be developed, and there are about 200 units being used for Earth applications in about 15 countries (Hirschenhofer,1996~. As discussed previously, four types of fuel cells (classified primarily on the basis of electrolyte and ranging in operating temperature from about 100 to 1000 °C) are being developed for terrestrial applications in North America. To reduce risk due to unreliability of fuel cells, NASA has decided to use batteries on the International Space Station (ISS), but these batteries are heavy and may not be the most efficient or cost-effective means of power generation or storage on the surfaces of the Moon or Mars. An attractive alternative for power storage is the regenerative fuel cell or, even more simply, separate electrolysis and fuel cell systems (Figure III.A.4~. During the day, excess solar power may be used to electrolyze water and generate hydrogen and oxygen. At night these gases are recombined in a fuel cell to produce electricity and water. Electrolysis and fuel cell systems require a supply of oxygen and hydrogen, and platinum (wire) is used for electrochemically active surfaces. Potassium salts serve as the electrolyte. Fuel cell technology is mature and the efficiency of electrolysis is also high. One very significant advantage of the regenerative H2-O2 fuel cell is that it can have a dual function it can be used not only for energy storage but also for life support on a spacecraft (Eckart, 1996~. The electrochemical reactions involving hydrogen and oxygen are the only practical ones at the present time. The oxygen is usually derived from air, but it can be produced on Mars using solid oxide electrolysis (Sridhar and Vaniman, 1997~. Hydrogen may be obtained from several fuel sources, e.g., steam-reformed Earth fossil fuels. Other fuels such as methanol can also be used (Klaiber, 1996~. {Proton-exchange membrane fuel cells (PEMFCs) are also referred to as solid polymer fuel cells (SPFCs). By virtue of its intrinsic simplicity and high power density, the PEMFC/SPFC has a distinct advantage over other fuel cell technologies. In this discussion the name PEMFC is used.

28 MICROGRAVITY RESEARCH The intermittent nature of solar energy availability (when, for example, spacecraft or satellites enter into planetary shadow) for low-Earth-orbit or other applications presents a particular challenge for space power man- agement schemes. One alternative to photovoltaic (PV) cells with battery storage is a solar dynamic system with latent heat thermal energy storage (LHTES) via solar heat receivers. During the charging phase heat is stored in the phase-change material while it is melted; during discharge the latent heat is released as the material is solidified. Solar receivers with integral LHTES are needed for generating electric power in space when using solar energy in conjunction with a Brayton cycle (Shaltens and Mason, 1996~. A eutectic mixture of LiF-CaF2 salts, which has a melting temperature of 1413 OF (767 °C), is used as a phase-change material in this application. Electrical (capacitor, superconducting magnet), mechanical (flywheel), and thermal (latent and sensible heat) storage systems also have potential for use on specific missions but have not yet been developed for spacecraft applications (Bennett, 1998~. For example, studies have shown that a solar active system can produce a unit of power from a smaller collector area than is required for an array of solar cells. The improvement in system efficiency results from the increased conversion efficiency of a solar active power cycle compared with solar cells and from the higher specific (kWe-h/kg) energy storage capacity compared with batteries. Some Selected Subsystem Technologies There are many passive and active electric power-generation systems and subsystems. The conversion system that changes the thermal power into electric power distinguishes passive from active power-generation systems. If the conversion system does not use a working fluid it is considered to be passive, but if it employs a working fluid then it is considered to be active. For example, the alkali metal thermal-to-electric conversion (AMTEC), which utilizes high-pressure sodium vapor supplied to one side of a solid electrolyte of beta-alumina, causing sodium vapor to be removed from the other side, is considered to be an active system in spite of the fact that it does not have any rotating parts. The more important subsystems are identified in Table III.A. 1, along with the potential of reduced gravity to affect their operation. It is beyond the scope of this report to identify and discuss all of the subsystems in detail; rather, some selected ones are mentioned. Since some of the components of space propulsion systems are the same as those of power-generation and storage systems, reference is made to Section III.B of this report for a discussion of the subsystems common to both. Passive power systems can use solar (photovoltaic), nuclear-radioisotope (thermoelectric, thermoionic, and thermophotovoltaic), and chemical (fuel cells) energy sources. In the past, static electric power generation systems have enabled, or enhanced, some of the most challenging and exciting space missions, including NASA missions such as the Pioneer flights to Jupiter and Saturn, the Voyager flights to Jupiter, Saturn, Uranus, and Neptune, and the Galileo mission to orbit Jupiter (Bennett et al., 1996~. The main disadvantage of the passive power generation systems is that they are much less efficient than the active systems and therefore have a significant weight disadvantage vis-a-vis those systems. There are various designs for closed-cycle, active power-generation systems. The three most important types are the Brayton, Rankine, and Stirling cycles. It should be mentioned that solar, nuclear, and chemical energy can be used as a source of heat for all cycles. A Brayton cycle is a conventional closed-cycle system that employs a gas turbine and in which the working fluid is a gas flowing throughout the power-generating loop. A Rankine cycle is like a conventional steam cycle in which the vapor is produced in a boiler, does work in a turbine, and condenses in a condenser (radiator). A Stirling cycle is a closed-cycle reciprocating engine whose working fluid is a high- pressure gas, either helium (He) or hydrogen (Hat. For space power systems the use of both solar and nuclear energy conversion systems based on the Stirling cycle engines has been considered, and a recent discussion that cites a large number of relevant references is available (de Monte and Benvenuto, 1998~. A schematic of a closed-cycle Brayton space power generation system is shown in Figure III.A. 1. The energy source could be solar, nuclear, or chemical. The technology for the conventional Brayton cycle is well established. The main advantages of the cycle are its high efficiency and very good specific power (in kWe/kg). As already discussed, a 100 kWe power unit based on nuclear energy (SP-100 program) was designed but never tested (Bennett, 1998~. The problem of rejecting heat from the Brayton cycle space power-generation system is the main concern. The large mass and size of the radiator make it a dominant component of the overall power system. The

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 29 choice of optimum temperature level for power conversion depends on the compromise between materials limita- tions and thermal performance. Low heat rejection (radiator) temperature improves thermal performance but results in large, massive radiators. The block diagram for a closed-cycle Rankine system is the same as that illustrated for a Brayton cycle (see Figure III.A.1~. The main difference between the two is that the Rankine cycle involves liquid/vapor mixtures. Boiling takes place in the heat source (boiler, nuclear reactor core) and condensation of the vapor occurs in the radiator. A dynamic system based on the Rankine cycle, which is expected to be more efficient and lighter in weight per unit power generated, has not been designed and operated. The primary reason for NASA's lack of interest in the Rankine cycle is that it involves two-phase flow and boiling/condensation heat transfer in some of its components (i.e., boiler, condenser, separator piping, etc.) and these processes are not sufficiently well under- stood in microgravity or fractional gravity environments to allow for designing active electric power-generation systems for spacecraft. Nevertheless, the Rankine cycle for space power generation is very attractive because of its relatively high efficiency and the lower mass of the conversion system compared with the Brayton cycle (Gilland and George, 1992~. It should be noted that for use in space the boiler, condenser, piping, valves, pump, and thermal management systems need to be designed for safe, efficient, and long-life operation. However, until there is a better understanding of how multiphase systems behave in space, NASA will not be in a position to utilize them. Boiler for the Rankine Cycle As noted above, the Rankine cycle is quite efficient for electric power generation and has a higher power-to- weight ratio than a Brayton cycle. However, as noted previously, the cycle (see Figure III.A.1) has components (heat exchanger-boiler, condenser-radiator, phase separator, etc.) in which the working fluid has both liquid and vapor phases. A boiler is an essential subsystem for electric power generation using a Rankine cycle for either spacecraft or stationary power at extraterrestrial sites. However, the operation of a heat exchanger in which the working fluid is boiled (evaporated) will be greatly affected by gravity. The two-phase flow and heat transfer processes and the flow separation processes in microgravity (near zero) environments are significantly different from those on Earth or on the Moon or Mars. Predictive models for two- phase transport developed for Earth applications are often empirically based and are inadequate for a microgravity environment. Thus, designers of space power-generation systems will be challenged to develop reliable sub- systems and technologies that involve two-phase flow and transport phenomena in reduced-gravity environments. The theoretical models and computer codes need to be capable of modeling two-phase flow, boiling and conden- sation heat transfer, and flow separation and distribution phenomena for all gravity levels. This would permit simulation of microgravity in a continuously variable manner and would not only lead to an increased understand- ing of, and insight into, the fundamental multiphase phenomena but would also allow NASA engineers to design and evaluate the performance of multiphase systems for use in HEDS missions. Radiators Radiators are the only effective means of rejecting heat in space without altering the mass of the spacecraft. Of course, heat can be stored in mass that is ejected from the spacecraft, but this method is not practical for missions of long duration. A comparison of different space power systems has been made (NRC, 1990), and it was found that depending on the type of system and power capability, the radiator can account for between 35 and 60 percent of the total system mass. Radiators that take advantage of a two-phase working fluid are more efficient and are relatively lightweight, but they are subject to possible two-phase flow instabilities, freezing, structural damage caused by oscillatory forces due to periodic or condensation-induced loads, damage by meteorites, and other phenomena. Innovative radiator systems based on moving-belt, liquid-droplet, liquid-sheet, bubble membrane, heat pipes, and other concepts have been proposed (Massardo et al., 1997; Ohtani et al., 1998), but neither the fundamental physical processes of two-phase flow and heat transfer nor the proposed concepts appear to have been studied in sufficient detail to determine their practical feasibility. Space system designers continue to demand

30 MICROGRAVITY RESEARCH design-specific data because they do not understand two-phase flow and phase-change heat-transfer phenomena in microgravity. In addition, the radiators must be robust and reliable, since it is difficult to repair them in space and their impact on life support, mission success, and cost can be very large. Liquid-droplet radiators and liquid-sheet radiators are among the most promising technologies for achieving lightweight heat exchangers for space applications. In such radiator concepts, neither flow affected by surface tension and thermocapillary forces, nor radiation heat transfer from, say, a cloud of small droplets to the ambient surroundings, has been studied in long-duration microgravity environments, so neither is fully understood. Opti- mization of the liquid-droplet radiator has revealed that the minimum specific mass is estimated to be 27 percent less than the specific mass of the system with a heat-pipe radiator (Massardo et al., 1997~. A recent experimental study of the liquid-droplet radiator has been performed, and the rate at which energy is radiated by a cloud of droplets as a function of droplet velocity and frequency has been measured (Ohtani et al., 1998~. At the droplet velocities being considered, there do not appear to be any major microgravity issues associated with fluid dynam- ics for the system, but heat transfer may be affected by atomic oxygen, and in the space environment micromete- oroids and space debris are of concern. Proton-Exchange Membrane Fuel Cells Of the various existing fuel cell systems, the proton-exchange membrane fuel cell (PEMFC) is the most promising, especially for space power-generation and transportation, because of the simplicity of its design and its low-temperature operations. Today' s PEMFC membranes are solid, hydrated sheets of a sulfonated fluoropolymer similar to Teflon. The acid concentration of the membrane is fixed and cannot be diluted by product or process water. The acid concentration of a particular membrane is characterized by equivalent weight, EW (grams dry polymer/mole ion exchange sites). This number is the reciprocal of the ion-exchange capacity in moles per gram. Generally, a lower EW and thinner membranes result in higher cell performance. However, thinner membranes also result in higher parasitic cross-diffusion of reactant gas. As noted above, fuel cells have been used and tested in microgravity. PEMFCs have limited life, and the STS- 84 mission in April 1997 was terminated after only 3 days due to problems with an onboard PEMFC. The cell showed low voltage output, and there was concern on the part of the space shuttle crew and NASA ground personnel that the H2 and O2 in the cell could cause an explosion. No formal report on the cause for the low cell voltage appears to have been released by NASA or its contractors. NASA has decided to use batteries on the ISS to reduce risk from potentially unreliable fuel cells, but the batteries are relatively heavy and may not be the most cost-effective means of power generation and transportation on the surface of the Moon or Mars. Like all fuel cells, PEMFCs can serve the dual purpose of generating electric power as well as producing water for human consumption. A PEMFC typically consists of a membrane sandwiched between two gas-diffusion electrodes, which are porous composites made of electrically conductive material. The assembly is pressed between two current collectors. The electrodes are hydrophobic so that gaseous reactants can be transported through the electrodes during cell operation. The outer faces of the electrodes are exposed to the reactant gases that enter and exit the gas chamber. Heat generated during the electrochemical reaction must be removed from the system, and proper water and heat management is essential for obtaining high power density at high energy efficiency. A heat pipe, replacing a coolant pump, heat exchanger, and thermal and other controls, can remove waste heat from the system. Effective removal of the liquid water product is required to prevent flooding of the electrode, which would prevent gas from reaching the catalyst-membrane interface, where the reactions take place. The water is liquid because of the low operating temperature (<100 °C) of the PEMFC. Some PEMFC designs have used a wicking arrangement to remove the liquid water. At the cathode, or air electrode, this process is a countercurrent and competing process. The oxygen flows toward the interface and product water moves away. Two-phase (gas- liquid) countercurrent flow in microgravity is a critical phenomenon that can greatly affect the performance of a PEMFC. Flooding hampers the rate of mass transfer and results in poor cell performance, which is characterized by the cell's inability to maintain high current at a given cell voltage. Full hydration of the cell membrane is required for the fuel cell to perform well and reliably. The membrane' s

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 31 requirement for water is a function of the conditions of operation, the amount of water required for hydration, and how and where the water should be added to maintain a fully saturated membrane. Water management schemes that allow for complete membrane hydration have been and are commonly used, and for a broad range of practical current densities there are no external water requirements since enough water is produced at the cathode to adequately hydrate the membrane. There are, however, very limited data for PEMFC performance under reduced or microgravity conditions. In summary, there are a large number of gravity-related issues that are not fully understood and that could affect PEMFC performance and safety (e.g., explosion caused by the sudden reaction of H2 and O2 in the cell). These issues include, but are not limited to, the following: (1) the effect of gravity on capillary flow in a porous membrane; (2) the effect of gravity on membrane hydraulic permeability; (3) membrane dehydration due to boiling in the porous structure, which could occur if the temperature exceeds ~100 °C due to some problem with the thermal control; and (4) transport of species and mass through porous electrodes in the noncontinuum regime, including the conjugated heat transfer through the structures. NASA is considering whether a flight experiment is needed to qualify the PEMFC and to resolve the issue of two-phase flow in microgravity. According to a recent report,2 the Jet Propulsion Laboratory (JPL), the Johnson Space Center (JSC), and the Glenn Research Center (GRC) are working to resolve some of these issues, but no specifics were provided. Capillary-Driven, Two-Phase Devices Thermal management is relevant not only to power-generation systems but also to life-support fluid and thermal systems during long-duration HEDS missions. Put simply, during space travel waste heat must be rejected to space. Fluids can be circulated through the spacecraft components, collected, and then eventually transferred to the radiator, where the heat is rejected to space. Alternatively, capillary-driven, two-phase devices (heat pipes, capillary pumped loops, loop heat pipes, rotating heat pipes, etc.) may be used as key subsystems in thermal control systems of space platforms (Faghri, 1995; Andrew s et al., 1997; Vasiliev, 1998~. Such devices are characterized by capillary-driven flow of the liquid in a wick structure or in axial grooves. The pure liquid working fluid flows to the heat input section where it evaporates and carries awaY thermal eneraY as latent heat (see Figure III.A.5, for example). ~ _ . . The working fluid changes phase from vapor to liquid in the heat sink tconcrenser) section as energy Is rejected. The working fluid is then transported back to the evaporator by means of the capillary forces in a wick. Heat pipes with different designs and operating in different temperature ranges are two-phase devices that have been recognized as key elements in the thermal management and control systems of space platforms (Faghri, 1995~. A heat pipe is an evaporator-condenser system in which the liquid is returned to the evaporator by capillary action. In its simplest form, it is a hollow tube with a few layers of a porous material (e.g., wire screen) along the wall to serve as a wick, as shown in Figure III.A.5. Typical working fluids are sodium or lithium for high- temperature applications, and water, ammonia, or methanol for moderate-temperature applications. If one end of the heat pipe is heated and the other end is cooled, the liquid evaporates at the hot end and condenses at the cold end. As the liquid is depleted in the evaporator section, cavities form in the liquid surface as the liquid clings to the wick. In the condenser section, meanwhile, the wick becomes flooded. The surface tension acting on the concave liquid/vapor interface in the evaporator section causes the pressure to be higher in the vapor than in the liquid. This pressure differential causes the vapor to flow to the condenser section, where the vapor and liquid pressures are nearly equal. Heat removal from the condenser causes the vapor to condense, releasing the heat of vaporization. The condensate is then pumped back to the evaporator section by the capillary force generated at the liquid/vapor interfaces of the pores in the wick. Since heat pipes rely on surface tension to return the condensate to the evaporator section, they can operate in 2Singh, B.S., Glenn Research Center. Multiphase flow and phase change in space power systems. Presentation to the Committee on Microgravity Research on October 14, 1997, Washington, D.C.

32 MICROG^VITY RESEARCH Heat input Heat output ' ~ ~ i Liquid Vapor 1` 1 1` 1` Wick flow Container flow I / I / f ~ 1 1 ~ 7 T , ~ ~ t /~~ i! ~ ~~~ NISI CI_ ~~ t~~ r ~ _ . ~ /~f~ ~ >/ AS _f~ (,]1 ~~D~ ~ ~ —_ I,Gl, \~--~-; ~Y~-~,~-~.~,~-~ ~1~~ :!~ :.-d- ~ Ad/ \~ ~ f 1111 Evaporator_ section Adiabatic soon FIGURE III.A.5 Schematic of a simple heat pipe. Based on Faghri (1995~. Condenser_ section a microgravity environment. A number of different heat pipes have been designed and used in U.S. and Soviet- Russian space missions. Heat pipe designs for low- (including cryogenic), intermediate-, and high- (using liquid metals) temperature working fluids are well in hand. According to a recent review (Vasiliev, 1998), more than 10 former Soviet space projects used different types of heat-pipe-based thermal control systems. In addition, the results of research and development on a heat-pipe-based radiator system to cool the Stirling cycle microcryogenic machine with a heat output of 110 W and an operating temperature range of 252 to 313 K have been reported (Vasiliev, 1998~. A capillary pumped loop (CPL) and a loop heat pipe (LHP) are two-phase heat transfer devices capable of transporting a large heat load over long distances with very small temperature differences across the system. Significantly, these devices require no external pumping (rather, they use the surface tension forces developed in a fine-pore wick to circulate the working fluid), and a large number of different designs have been proposed (Ku, 1997~. Over the past two decades the CPL has been studied extensively with the aim of developing instrument thermal control for future spacecraft. Applications have included the Earth Observing System, the Mars Surveyor, the Hubble Space Telescope, and others (Ku, 1993, 1997~. A CPL available for use in a low-gravity environment could represent a new, more effective way to transport thermal energy in space. It has several advantages over a standard heat pipe when used to transport thermal energy in space applications. A CPL system, for example, can provide heat rejection over a wider range of temperatures, and it avoids the limitations of the simultaneous countercurrent flows of liquid and vapor typically encountered in heat pipes (Herold and Kolos, 1997~. Industry has developed an LHP system that uses deployable radiators to increase the heat rejection from inside the spacecraft to the space environment (Parker et al., 1999~. As noted above, capillary-driven, two-phase flow devices can operate reliably in a microgravity environment because the liquid flow is driven by surface tension. However, there are inherent limitations on the liquid flow rates. Moreover, a number of other physical phenomena and critical factors can affect the design, operation, and performance of such devices. Examples include obstruction of the liquid flow due to nucleate or film boiling in the wick as a result of overheating in the evaporator section of the heat pipe, freezing of the liquid due to operation of the device under off-design conditions, start-up (melting) from a frozen state, and critical heat flux limitations. In spite of the fact that CPLs and LHPs have been used successfully on several spacecraft, a number of phenomena that can affect the thermal performance of the devices are not fully understood. Examples include research issues such as incipient superheat, the effect of noncondensable gas generation, bubble dynamics, and two-phase behav- ior in the wick structure of the evaporator under microgravity conditions.

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE SB ~C~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Qin ~ SA ~ _________ Qin Non —- - - - - - - . DB DA ~ jet _Qout 33 FIGURE III.A.6 Schematic of the vapor-pressure pumped loop concept. SOURCE: Lund et al. (1993~. Reprinted with permission from Elsevier Science. Vapor-Pressure Pumped Loops A novel concept known as a vapor-pressure pumped loop (VPL) has been proposed (Lund et al., 1993) that uses differences in the vapor pressure between an evaporator and condenser to drive the convective heat transfer process. This device is shown schematically in Figure III.A.6. As can be seen, it is composed of multiple evaporators (two shown EA and EB) and condensers (CA and CB). These are ducted together and the flow is controlled by solenoids (SA and SB) and check valves (DA and DB), which drive an oscillatory flow vapor and condensate around the loop. This concept allows for much larger driving pressures and power densities than can be achieved with capil- lary-driven devices (e.g., heat pipes). Therefore, the VPL may prove to be an important type of multiphase system for use in a microgravity environment. Nevertheless, there will be inherent limitations in power density compared with other active phase-change power production and utilization systems (e.g., Rankine cycles). Moreover, since the VPL does not involve forced convection, there will be significant thermal limitations (i.e., critical heat flux). Alkali Metal Thermal-to-Electric Conversion With its inherently noise-free, high-efficiency operation, alkali metal thermal-to-electric conversion (AMTEC) has potential for use in space applications. Today's AMTEC cells are also compact and lightweight, and they have the potential for achieving 35 percent efficiency (Levy et al., 1997~. Because of their potential advantages, these cells are being considered by JPL as power systems for Europa Orbiter, Pluto Express, and other space missions (Schock et al., 1997~. These static converters can achieve a high fraction of Carnot efficiency at relatively low temperatures. The heart of the multitube AMTEC converter is a beta-alumina solid electrolyte (BASE) tube that is exposed to high-pressure sodium vapor on its inside and much lower pressure sodium on its outside. The tube wall, under a suitable pressure gradient, can conduct sodium ions but not neutral sodium atoms. In effect, the pressure gradient drives the electrical current through the external load resistance. The high pressure is produced by the high-temperature evaporator near the hot end of the cell, and the low pressure is produced by the low- temperature condenser at the cold end of the cell. The sodium condensate is then pumped back to the high- pressure anode by a wick-filled artery, similar to the wicked tubes in heat pipes. The evaporation and condensation

34 MICROGRAVITY RESEARCH processes are expected to depend on gravity, but AMTEC units have not yet operated under microgravity condi- tions. The flow, thermal, and electrical components for analyses of an AMTEC unit are interconnected. The design of the AMTEC BASE tubes is complicated because the flow of sodium vapor in the tubes is not in the continuum or molecular-flow regimes but in the transitional regime. The AMTEC cell requires an evaporator and a con- denser, which are connected by a wick for returning liquid sodium (condensate) to the bottom of the evaporator. Adequate temperature margin between the top of the BASE tube and the evaporator is required to prevent condensation in the tubes, which can lead to internal shorting of the multitube cell and degradation of performance. The processes of evaporation, condensation, and capillary-driven flow occurring in an AMTEC cell have not been studied under microgravity conditions, and the length of the wick for optimum cell power output has not yet been established. Liquid/vapor separation in microgravity is also of concern. Summary of the Impact of Reduced Gravity on Selected Subsystems The list of potential subsystems tor power generation and storage is very large, and it is not practical in this limited account to be comprehensive. For the sake of conciseness, the more important subsystems have been identified and are presented in Table III.A. 1. (It should be noted that AMTEC can use either a solar or a nuclear (radioisotope) energy source and is classified as an active electric power-generation system since thermal energy TABLE III.A.1 Selected Subsystems for Passive and Active Power Generation and the Potential Impact of Microgravity on Their Operation Representative Passive Active Subsystem PV TE TI TPV BR RA ST AMTEC Batteries L L L L Boiler H Capillary pumped loop H H H Compressor L L Condenser H H Converter L L L L Controls L L L L L L L L Evaporator H Heat exchanger M H M M Heat pipes H H H Phase separator H Pipes L M L Pumps M Radiators M H M Regenerative heat exchangers L H Regulators L L L L L L L L Solar array L L Solar collector M H M Storage M M M Turbine/alternator L L L Valves L M L NOTE: PV, photovoltaic; TE, thermoelectric; TI, thermoionic; TPV, thermophotovoltaic; BR, Brayton; RA, Rankine; ST, Stirling; and AMTEC, alkali metal thermal-to-electric conversion. The letters H. M, and L designate high, medium, and low (preliminary assessment) impact of reduced gravity on the operation of the subsystem. Where no letter is given, the subsystem is not applicable to the system listed.

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 35 is converted to electricity using a working fluid.) Table III.A.1 also indicates the estimated impact of reduced gravity on the operation of the subsystems as high, medium, or low (little or no impact). References American Institute of Aeronautics and Astronautics (AIAA), Aerospace Power Systems Technical Committee. 1995. Space nuclear power: Key to outer solar system exploration. AIAA Position Paper. Reston, Va.: AIAA. Andrews, J., A. Akbarzadeh, and I. Sauciuc, eds. 1997. Heat Pipe Technology: Theory, Applications and Prospects. Amsterdam: Pergamon. Bennett, G.L. 1998. Electric power technologies for spacecraft: Options and issues. AIAA Paper No. 98, p. 1022. Reston, Va.: American Institute of Aeronautics and Astronautics. Bennett, G.L., R.J. Hemler, and A. Shock. 1995. Development and use of the Galileo and Ulysses power sources. Space Technol. 15:157-174. Bennett, G.L., R.J. Hewer, and A. Schock. 1996. Space nuclear power: An overview. J. Propulsion Power 12:901-910. Blomen, L.J.M.J., and M.N. Mugerwa, eds. 1993. Fuel Cell Systems. New York: Plenum Press. Brandhorst, H.W., P.R.K. Chetty, M.J. Doherty, and G.L. Bennett. 1996. Technologies for spacecraft electric power systems. J. Propulsion Power 12:819-827. de Monte, F., and G. Benevenuto. 1998. Reflections on free-piston stirring engines, Part 1: Cyclic steady operation. J. Propulsion Power 14:499-508; also Part 2: Stable operation. J. Propulsion Power 14:509-518. Detwiler R., S. Surampudi, P. Stella, K. Clark, and P. Bankston. 1996. Designs and technologies for future planetary power systems. J. Propulsion Power 12:828-834. Eckart, P. 1996. Spacecraft Life Support and Biospherics. Torrance, Calif.: Microcosm Press, and Dordrecht, Netherlands: Kluwer Academic Publishers. Faghri, A. 1995. Heat Pipe Science and Technology. Washington, D.C.: Taylor and Francis. Friedensen, V.P. 1998. Space nuclear power: Technology, policy, and risk considerations in human missions to Mars. Acta Astronautica 42:395-409. Gilland, J., and J. George. 1992. Early track NEP system options for SKI missions. AIAA/SAR/ASME/ASEE 28th Joint Propulsion Confer- ence and Exhibit, Nashville, July 6-8. AIAA Paper No. 92-3200. New York: American Institute of Aeronautics and Astronautics. Herold, K.E., and K.R. Kolos. 1997. Bubbles aboard the shuttle. Mechanical Engineering (October):98-99. Hirschenhofer, J.H. 1996. 1996 fuel cell status. Pp. 1084- 1089 in Proceedings of the Thirty-First Intersociety Energy Conversion Engineering Conference, Vol. 2. New York: Institute of Electrical and Electronic Engineers. Klaiber, T. 1996. Fuel cells for transport: Can the promise be fulfilled? Technology requirements and demands from customers. J. Power Sources 61:61-96. Kroschwitz, J.I., and M. Bickford, eds. 1994. Fuel cells. Pp. 1098-1121 in Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., Vol. 11. New York: John Wiley & Sons. Ku, J. 1993. Overview of capillary pumped loop technology. Pp. 1-17 in Heat Pumps and Capillary Pumped Loops, HTD-Vol. 236. New York: American Society of Mechanical Engineers. Ku, J. 1997. Recent advances in capillary pumped loop technology. AIAA Paper No. 97-3870. Reston, Va.: American Institute of Aeronautics and Astronautics. Landis, G.A., S.A. Bailey, and M.F. Piszczcor, Jr. 1996. Recent advances in solar cell technology. J. Propulsion Power 12:835-841. Levy, G.C., T.K. Hunt, and R.K. Sievers. 1997. AMTEC: Current status and vision. Pp. 1152-1155 in Proceedings of the Thirty-Second Intersociety Energy Conversion Engineering Conference, Vol. 2. New York: American Institute of Chemical Engineers. Lund, K.O., K.W. Baker, and M.M. Weislogel. 1993. The vapor pressure pumped loop concept for space systems heat transport. 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Human Exploration of Space: A Review of NASA's 90-Day Study and Alternatives. Washington, D.C.: National Academy Press. NRC, ASEB. 1998. Space Technology for the New Century. Washington, D.C.: National Academy Press. Ohtani, Y., M. Fujiwara, and M. Watabe. 1998. Study of radiative heat transfer characteristics of droplet radiator. Pp. 441-442 in Proceedings of the 35th National Heat Transfer Symposium of Japan, Nagoya, Vol. II. Tokyo: Heat Transfer Society of Japan.

36 MICROGRAVITY RESEARCH Parker, M.L., B.L. Drolen, and P.S. Ayyaswamy. 1999. Loop heat pipe performance Is subcooling required? Proceedings of the 5th ASME/ JSME Joint Thermal Engineering Conference, March 15-19, 1999, San Diego, California. Paper No. AJTE 99-6285. New York: American Society of Mechanical Engineers. Schock, A., H. Norviar~, arid C. Or. 1997. Coupled thermal, electrical and fluid flow analyses of AMTEC converters, with illustrative application to OSC's cell design. Pp. 1156-1164 in Proceedings of the Thirty-Second Intersociety Energy Conversion Engineenng Conference, Vol. 2. New York: American Institute of Chemical Engineers. Shaltens, R.K., arid L.S. Mason. 1996. Early results from solar dynamic space power system testing. J. Propulsion Power 12:852-858. Sridhar, K.R., arid B.T. Vaniman. 1997. Oxygen production on Mars using solid oxide electrolysis. Solid State tonics 93:321-328. Vasiliev, L.L. 1998. State-of-the-art on heat pipe technology in the former Soviet Union. Applied Thermal Engineering 18:507-551. III.B SPACE PROPULSION Introduction Energy-conversion systems for the space propulsion required for HEDS missions have to be selected and developed. Propulsion requires an energy source and a corresponding power generation system (many of these are described in the preceding section) serving a propulsive jet that provides thrust by reaction. A wide range of propulsive capabilities will need to be provided, and therefore a large number of system possibilities must be considered, each comprising the many devices and subsystems needed to carry out essential functions within each system. Each potential system and its subsystems will need to operate reliably and efficiently at various, and perhaps variable, gravity levels. It is concern for the effects of gravity level that motivates this report and provides the focus for the descriptions and discussions that follow. In this section the anticipated capability requirements are first listed and then potential systems are identified and discussed. Judgments and preferences are, however, avoided; while there is no more portentous decision for a HEDS mission than the selection of a propulsion system, it is not the purpose of this report to urge particular choices. No "baseline" selections are recommended. Rather, the aim is to include a range of systems sufficiently wide to bring out the significance of gravity level; any emphasis of one system over another reflects only its interest for microgravity research. After the potential systems are described, major subsystems, or components, are identified by the functions they fulfill (e.g., a boiler). Then, those functions are associated with gravity-dependent phenomena, and, again as appropriate, specific subsystems are discussed in terms of requirements for microgravity research. Required Space Propulsion Capabilities Many quite different propulsive capabilities are needed for the various potential HEDS operations, including the ability to do the following: · Achieve Earth orbit; · Transfer and adjust orbit; · Transit to the Moon and the planets of the solar system and return; · Transit beyond the solar system and return; Achieve orbital capture at destination or on Earth return; Descend to surfaces of large bodies with significant gravity; Rendezvous with small bodies such as asteroids, which have very low gravity; · Send and receive sampling probes; and Perform actions involving momentum or position change, such as the deployment or aiming of antennas. Providing these propulsive capabilities will require that attention be paid to a variety of issues, including availability, security and dependability, and duration of the energy source; human safety, health, and effectiveness; and endurance, range, versatility, and reliability of the propulsion system. Commonality with other needs,

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 37 especially those of spacecraft and station electric power, will also be an important concern, as will cost and the feasibility of timely development. While judgments and choices concerning these issues are not part of this study, awareness of such general concerns will of course lead to specific concerns about microgravity effects. For example, human health may require microgravity countermeasures, and providing for operations in a range of gravity levels may seriously affect system reliability and cost. Space Propulsion Systems Various potential propulsion systems may be postulated to provide the capabilities mentioned above; some are now commonplace, while others are speculative in the extreme. They all convert or absorb energy effect momentum change, usually by means of a propulsive jet. Before describing these systems and identifying salient low-gravity issues pertaining to them, the committee first summarizes them according to energy source. in order to AS a space-propuls~on system, the chemical combustion rocket can in principle provide all propulsion capa- bilities, but for distant missions its economic performance may not compare well with that of other possibilities. The nuclear thermal rocket (NTR) may be suitable for transit beyond Earth orbit, but probably only within the solar system. Quite different is nuclear electric propulsion (NEP), which uses ion and electromagnetic thrusters: it would be of particular interest for distant transits beyond Earth orbit, especially travel beyond the solar system. Solar thermal or solar electric (also using ion and electromagnetic thrusters) is of interest for all tasks within the inner solar system. The solar sail is also of interest for transits within the solar system. A laser thermal system may be of special interest for minipropulsion applications, such as sampling probes. For very long distances, the laser sail may be useful, especially for small or moderate loads. Finally, systems deriving energy from planetary fields and atmospheres may be mentioned. The tether is of potential interest for some orbital transfer tasks, while the aeroassist technique may facilitate orbital maneuvers or capture operations. These systems are described and discussed below in varying degrees of detail, depending on their perceived significance for microgravity research. Besides the capability issues already mentioned, a number of specific technical themes appear, many of which, such as method of energy conversion and heat transfer, were thoroughly discussed in Section III.A. Other technical themes of interest include fluid handling and storage, cryogenics, mechanical machinery, structures, scale (micropropulsion), range of applicable gravity level, and complexity. Gravity level, be it microgravity or fractional gravity, will be an important consideration in all these technical topics, especially as it affects heat transfer and fluid handling. Chemical Rocket In a chemical rocket, the combustion of propellants produces hot gas at high pressure. Expansion of this gas through a nozzle converts its high thermal energy to the directed kinetic energy of a high-velocity jet exhaust; this kinetic energy provides the thrust needed for propulsion. Various types of chemical rockets may be of interest for providing propulsive capabilities for HEDS. These types differ in the way the propellants are provided for combustion; liquid, solid, or hybrid (solid/fluid) systems may each play a role in future HEDS missions. Typical liquid-propellant rockets for space propulsion include Russia's kerosene-powered engines and the NASA space shuttle's main engine, in which cryogenically stored H2 and O2 are pumped to a combustion chamber, where they are injected and mixed and where they then react to produce water vapor at high temperature and pressure (Ficure III.B. 1). The thrust that a rocket produces is proportional to the flow rate of reaction product ~ ~ O ~ ~ ~ ~ _ _ _ __ _ _ _ _ _ _ _ _ _ · · _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ ~ _ through the nozzle. The nozzle of the engine must be cooled by liquid H2 on its way to the combustor. A measure of the effectiveness of propellant flow in producing thrust is the specific impulse, which is the ratio of thrust force to the rate of consumption of stored fuel. Commonly, the rate of consumption of fuel is expressed as Earth weight per unit time and the specific impulse is given in seconds. The specific impulse of a hydrogen-oxygen rocket can be on the order of 360 s (Hill and Peterson, 1965~. A system of the sort just described is quite simple and reliable; it has already taken humans to the Moon and has propelled unmanned spacecraft to the outer edge of the solar system. Chemical combustion rockets in

38 MICROGRAVITY RESEARCH ~ )PRESSURANT TANK ~V: Fl IFI TANK arc _ _ ENGINES i\ 'Y: _d 3 \~/OXIDIZER TANK hi_ _ _ ~- ~ ~ MANUAL FILL VALVE in SOLENOID VALVE [3 LATCH VALVE [3 FILTER E3 CHECK VALVE FIGURE III.B.1 Schematic of bipropellant chemical rocket. SOURCE: Cassady (1990~. Reprinted with permission of the American Institute of Aeronautics and Astronautics. principle are capable of any mission, at any scale, that one can imagine. They can produce high thrust and can be well controlled. However, the hydrogen-oxygen rocket produces this high thrust for a rather short period of time before the fuel is consumed, which limits mission flexibility, unless fuel can be replenished from in situ resources. In fact, its economic use probably does not extend beyond Mars (AIAA, 1995~. For this report, which focuses on issues of reduced gravity, a dominant technical feature of the hydrogen- oxygen system is the cryogenic storage apparatus; liquid gases must be refrigerated and kept cold in large tanks for long periods of time before use. Fuel replenishment, or transfer between tanks in space, is not a fully developed process. Fuel must also be acquired from the tanks and transferred, by high-pressure pumps, to the combustion chamber. The operation of the refrigeration system and of the various pipes, valves, seals, and bearings involved in handling the propellants can be expected to be sensitive to gravity, yet they must all operate reliably at various gravity levels. In the rocket engine itself, including the combustor and propulsion nozzle, the fuel, propellant, and coolant are all subject to forced flow with high accelerations and therefore will not be affected by gravity level. That is, none of the engine's essential processes will depend on gravity. However, any start-up processes in space could be affected by gravity because initial velocities will be small. Liquid or solid propellants that can safely be stored in stable form can be used in chemical rockets, thus avoiding the need for cryogenic storage. Many well-developed, storable liquid propellants (such as kerosene and

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 39 ethanol) are now employed in space operations. However, these propellants are less energetic than the hydrogen- oxygen mixture and hence suffer the penalty of lower specific impulse. One concept for avoiding cryogenics while keeping the high-impulse benefit of high-energy fuel is to diffuse a light gas (such as hydrogen) into the interstices of a solid matrix, where the gas would be held and then "boiled off" as needed (Carrick and Harper, 1998~. This option is in the research stage and is therefore of uncertain applicability for HEDS. The boiling-off process would presumably be affected by microgravity. There is great interest now in very small (micro) low-cost rockets for special purposes such as sample acquisition and return (JPL,1998~. Because of their small scale, cryogenic storage would not be feasible for these devices. However, they could well be based on chemical combustion of reactants stored at high pressure, as in the pressure-fed rocket, or even in solid form. Such micropropulsion devices would often be used in low or microgravity situations but would not usually be sensitive to gravity changes, owing to their small dimensions. The solid-propellant rocket merits further discussion. As suggested above, it is free of the fluid-handling issues that become problematic in reduced gravity, and it will surely be of interest for HEDS use, especially in micropropulsion applications. In the solid-propellant rocket, the fuel-oxidizer mixture is formed into a solid matrix (known as a grain) that burns exothermally after ignition (Sarner, 1968~. Thus, while there are complex fuel-handling problems during manufacture, no such problems arise during operation. No microgravity issues appear during steady operation, although ignition in microgravity could be problematic. Nevertheless, a very severe operational limitation of a solid rocket is the lack of control over thrust after ignition; once started, it can be stopped only by burnout or destruction. Because of the complexity of the manufacturing processes involved, it seems unlikely that solid-propellant rockets would be constructed extraterrestrially during HEDS activities. Their use would probably be limited to launches from Earth (as with the solid rocket boosters attached to the space shuttle during launch) or to small- thrust applications of rockets previously transported from Earth. For HEDS applications, the so-called hybrid rocket may be more interesting than the pure solid type. In the hybrid rocket, a liquid or gaseous oxidizer is injected into a combustion chamber already loaded with solid fuel (Seifert, 1968~. Unlike the solid-propellant rocket, a hybrid rocket can be throttled to control thrust by adjusting the rate of injection of the fluid oxidizer, thus providing operational flexibility. Of course, the hybrid incurs the complications of handling the fluid oxidizer, but these complications are much less severe than those of the liquid- propellant rocket, in which both fuel and oxidizer must be managed. Thus, any reduced-gravity issues for the hybrid rocket would concern only the handling of a liquid oxidizer. The hybrid rocket is easier to construct than the solid one, because the grain contains only fuel. As a result, fuel processing is less complex and perhaps could be managed in space or in a planetary colony. For example, carbonaceous fuel could conceivably be extracted from the Martian atmosphere and then formed into solid fuel matrices for hybrid rockets using fluid oxidant derived from in situ sources to serve local propulsion needs or perhaps even enable liftoff from the Martian surface. Nuclear Thermal Rocket In the NTR system, a reactor generates heat by nuclear fission. This heat is transferred, within the reactor, to a propellant gas that has been pumped at high pressure from a cryogenic storage tank. This thermal energy is then converted to kinetic energy, and hence thrust, in an exhaust nozzle (Dearian and Whitbeck, 1990; NASA, 1991; Rosen et al., 1993~. The nozzle is cooled by the propellant on its way to the reactor (Figure III.B.2~. The NTR is likely to be a very large system if it is based on a massive, graphite-moderated nuclear reactor.3 Because of safety concerns, such a rocket would probably be assembled and launched from Earth orbit. Thus, an NTR would generally be less flexible in design and operation than a chemical rocket, but it would be useful for sit should be noted that very small NTRs have been proposed that have perhaps one-tenth the power of a conventional NTR.

40 REACTOR 7 l /~-lt I ~ ~ r ~ /~ - -—= =F—_ ,:.\ - \ 1 _~ V1 ~ ~ -by NOZZLE ~ lit \ ~ 3 REJECTOR ~ C ONTROL D RUM ~ FIGURE III.B.2 Sketch of a nuclear thermal rocket. SOURCE: NASA (1991~. MICROGRAVITY RESEARCH Lid PUM PS -—{URBINES missions of transit between Earth, the Moon, and other destinations no more distant than Mars (Bennett and Miller, 1992~. The nuclear electric propulsion option (discussed below) should prove preferable beyond Mars. Like the chemical rocket, the NTR would be a high-thrust, short-duration device. The advantage of the NTR is that its specific impulse, based on propellant consumption, is about twice that of the chemical rocket, because pure H2 can be used and there is no need for a relatively heavy oxidant. This advantage would be about 3:1 if based only on a comparison of H2O and H2 molecular weights, but it is diminished to about 2:1 by the rather lower allowable temperature to which the propellant can safely be heated in the reactor, as compared with a combustor. In addition to the nuclear reactor and its associated control system, a cryogenic system will be needed to store and maintain hydrogen in a liquid state for the length of time needed for the mission. Propellant pumps and gas turbines to drive them, with their bearings and seals, will be required. Liquid hydrogen piping and control valves will be needed, both to provide the main propellant flow and for auxiliary purposes, such as to drive pump turbines and provide nozzle cooling. Provisions must be made to ensure the proper rates of heat transfer to the hydrogen propellant in the reactor and to the liquid H2 coolant in the nozzle protection system. As in the case of the combustion rocket, the engine itself, including the reactor and probably its heat-transfer processes, will have forced flow and high velocities and should be insensitive to gravity during operation at or near design thrust. However, start-up and shutdown in space will be required for nuclear propulsion systems (Dearian and Whitbeck, 1990), and fluid flow and heat transfer processes will generally be affected by gravity level during those operations, depending on reactor design. The NTR is far beyond the conceptual stage; under the nuclear engine for rocket vehicle application (NERVA) program of 40 years ago, NASA and the Atomic Energy Commission together developed and ground-tested a full- scale NTR (Rosen et al., 1993; Bennett et al., 1994~. Though it is not a "shelf" item, there is confidence that the conventional NERVA type of NTR is a practical option, probably achievable at a rather low cost for final development. However, the future use of NTR propulsion for HEDS may depend on advances in high-power- density reactors involving, for example, rotating-bed or fast-fission approaches. Indeed, fast reactors do not require neutron moderation (with, e.g., graphite) and thus are inherently much lighter than thermal neutron fission reactors. The rotating-bed reactor might be particularly attractive for use in variable gravity. Centrifugal force would be expected to maintain fuel pellets in a stable bed through which the propellant is forced. High power density and

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 41 relative insensitivity to gravity level could thereby be achieved, although issues of pellet-bed stability and of start- up would be of concern. The gas-core nuclear rocket (GCNR) represents a variant of the NTR in which the necessary heat transfer between the fissile material and propellant would occur across a stable gas interface rather than a material wall. The gases in question might be uranium hexafluoride and hydrogen. If such a stable interface could be achieved and maintained, the material temperature limitations, mentioned above, on NTR specific impulse would be avoided, and full advantage could be taken of the low molecular weight of hydrogen. The difficulty with this propulsion system is that no feasible method has been found to achieve the necessary stable gas interface, despite vigorous research emphasizing vortex flows, which peaked about 30 years ago (Schneider and Thom, 1971~. The GCNR idea lingers, however, because of the hoped-for large potential increase ~ · ^. . 0~ specific 1mpu se. Despite the impracticality of the foregoing application, the general problem of vortex containment of dissimi- lar fluids should be revisited with microgravity applications, such as phase separation, in mind. A NASA publication (Schneider and Thom, 1971) contains interesting studies on this topic. Nuclear Electric Propulsion The energy source for NEP would be nuclear fission, with the generated heat transferred from the reactor to a suitable working fluid rather than directly to a propellant gas, as in the NTR. The working fluid is then used to generate electric power through a thermodynamic cycle, and that electric power drives a plasma or ion thruster (Barrett, 1991~. Such a system escapes the temperature limitation on specific impulse that characterizes the NTR, and specific impulse can be thousands of seconds. Electromagnetic thrusters are used very successfully today for small-scale intermittent applications (station keeping, for example). Even when used for spacecraft propulsion, the thrust itself will still be quite small, as will the reactor, which would typically be one-tenth the size of the comparable NTR reactor. When used for long-distance transit propulsion, the NEP system must therefore operate continuously for periods comparable to the duration of the mission itself, in an environment of very low effective gravity. Clearly, NEP must meet a special burden of reliability for long missions. Since the effective gravity during acceleration by NEP will be quite low (of the order of 0.1 gO), microgravity issues are likely to arise, depending on the thermodynamic cycle used. The closed Brayton cycle using helium as a working fluid would probably be the simplest cycle and the least subject to microgravity concerns.4 A heat exchanger in which heat is transferred from the reactor coolant (perhaps liquid lithium) to the gaseous helium working fluid would involve no phase change. Heated helium would pass through a turbine, which would drive the electric generator and, incidentally, the helium compressor. Then the helium would reject heat to space, probably through a secondary heat-transfer loon which might involve heat nines and chase chance of a medium No 1 1 ~ ~7 . . .. . . .. . . . . . . . . . ~ . . . . such as ammonia. Naturally the helium pump and turbine would require high-performance bearings and seals, as well as associated piping and controls. The Rankine cycle is nonetheless commonly proposed for NEP because it promises higher thermal efficiency. It has many components, however, and its advantages depend on phase change, making it inherently more sensitive to gravity. In a typical Rankine cycle NEP (Figure III.B.3) the working fluid (potassium, for example) is evaporated in a boiler. Then, following power generation in a turbine-generator, the working fluid must be condensed in a heat exchanger, with heat rejected to space via a radiator system. Then the condensate is pumped to a high pressure and fed into the boiler. Special care must be taken to be sure that there is no liquid carry-over to the turbine and vapor carry-under from the condenser. For this purpose a phase separator, probably operating on the centrifuge principle, must be used. Fluid handling equipment such as piping, valves, pumps, turbines (with 4Nieberding, J., Lewis Research Center. Propulsion, power, and cryogenic fluid systems. Presentation at Workshop on Research for Space Exploration, May 8, 1997, Lewis Research Center, Cleveland, Ohio.

42 ' ~ Nuclear Reactor ( Boiler) Feed Pump ( Liquid) Condenser ~ I ~ ~ redo I ~ I l/ , J 1, ,<. - __ hi_ ~ I ~ 1 r—- —J I l ~~ a_ Space Radiator Vapor Separation System FIGURE III.B.3 Schematic showing major elements of a nuclear electric propulsion system. MICROGRAVITY RESEARCH Vapor Turbine ~ l Electric Generator 1 ~ , . Power I I Conditioning J~ ~ter /~;opella)\ Storage, Delivery associated bearing systems), seals, and controls tends to be especially complex for the Rankine cycle, and design for high reliability will be correspondingly important. Like NTR, the NEP system would presumably be assembled and launched from Earth orbit. Start-up or shutdown in orbit poses complex issues of design and operation, especially for a Rankine cycle with a liquid-metal working fluid and a nuclear heat source. Thawing (in zero gravity) of the nuclear reactor is required and the entire system must, in a controlled way, be brought to an equilibrium appropriate to steady-state operation. Transient, intermittent, or variable operations generally must also be considered carefully, taking account of system dynam- ics and possibilities for system instabilities. NEP would not depend as much on cryogenic storage as would chemical or NTR systems; however, the propellant material, be it liquid metal or noble gas, must be stored and maintained for especially long times. A special feature of the NEP concept is the potential for dual use; electric power can be produced for purposes other than propulsion, i.e., for spacecraft or colony needs, as is more fully discussed above in Section III.A. These uses might include the powering of various microthrusters needed for spacecraft positioning or other mechanical actuation. It has also been proposed that commonality between the propulsion and power systems might only exist with respect to the reactor, which would supply heat for a thermal rocket and/or, via a suitable thermody- namic cycle, electric power. Another possible application for electric power generated by nuclear or solar means is to power an electro- magnetic accelerator, or "mass driver" (Snow and Kolm, 1992), perhaps to convey materials between the Moon and Earth, for example. Fusion propulsion ideas have been proposed and should continue to be studied with the aim of achieving specific impulses adequate for missions to the edge of or beyond the solar system. If plasma can be confined well enough to achieve "ignition," that energy could presumably be converted to a high plasma velocity in a rocket nozzle. Obviously, such ideas have no near-term relevance to HEDS, but in the far future they may be of crucial importance for space exploration. The same remarks apply to antimatter propulsion, and it should be noted that antimatter has been proposed as a trigger for fusion. 5Neiberding; see footnote 4.

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE Solar Thermal 43 Absorbed solar radiation can serve as an energy source for propulsion, although beyond Mars, the solar flux is too small to be useful. Very large collector arrays and concentrators could deliver heat directly to a propellant gas, such as hydrogen, to power a thermal rocket. Such an approach would be attractive for small thrusters. The heat collection process could require extensive fluid distribution systems, perhaps utilizing phase change, with difficult problems in structural dynamics and attitude control. Absorbed solar radiation could also heat a working fluid in a thermodynamic cycle, such as Brayton or Rankine, to generate electric power for plasma propulsion (solar electric propulsion) as well as for the spacecraft or outpost needs discussed above in Section III.A. The issues described for NEP apply here as well, as would the structural and fluid-handling problems of large collector arrays peculiar to the solar option. Solar propulsion systems would inherently have high specific impulse but low thrust and therefore would be assembled and launched from orbit. Solar Sail A propulsion system deriving momentum change directly from solar radiation pressure and the solar wind would require a very large solar "sail" (Staehle,1981~. This sail would be subject to severe problems of structural dynamics and control in a microgravity environment but would of course be free of fluid-handling problems. The solar sail might be attractive for small-scale spacecraft in the inner solar system, where solar radiation is suffi- ciently high. The probability of meteorite impact would be of particular concern for such a system. Laser Thermal An interesting possibility, especially for small rockets designed to communicate between an orbiter and a planet or asteroid surface, would be use of the highly concentrated, continuous-wave radiation of a laser to heat a propellant gas, which would then power a rocket (Mead and Myrabo, 1998; Harris, 1999~. The laser would be housed on an extraterrestrial site, using electric power assumed to be available there. Apart from the obvious tracking, control, and lost-line-of-sight problems of such a system, the propellant would pose heat-exchange and fluid-handling issues for the target vehicle. Laser Sail Studies by NASA and DOD have suggested the feasibility of "sail" propulsion powered by a fixed laser that illuminates a large collector on a spacecraft (Perry and Powell, 1998~. Radiation pressure due to reflected radiation would provide thrust useful for small payloads and long distances, taking advantage of laser beam coherence. The greatest microgravity issues for such a scheme would presumably be those of positional stability and control of the very light collecting antenna. Tether A tether can be used to exchange momentum between bodies in space (Cutter and Carroll, 1992) and thus to provide a direct mechanical propulsive effect that can be especially useful in achieving orbital changes. Also, if a conducting cable, or tether, is suspended from a spacecraft orbiting a body having a substantial magnetic field (Earth and, especially, Jupiter are examples), the tether will experience a force provided current is caused or permitted to flow along the tether. The force may be a drag on the spacecraft or it may be propulsive, depending _ _ _ i' · , · , · r i' a, ~ i' , · r. ~ ~ ~. · ~ ~ i' , , ~ ~ ~ A, ~~ ~ , ~ on the relative motion of the spacecraft and the magnetic held lines carried by the rotating body (Gallagher et al., 1998~. The presence of an ambient plasma, or ionosphere, would be important in establishing the appropriate current flow. The electrodynamic tether concept may prove useful for orbital adjustments, including de-orbit for descent. Another potential use is, of course, the generation of electric power. Obviously, there would be a gravity

44 MICROGRAVITY RESEARCH gradient along such a tether, and the dynamic behavior of such a system would be problematic. Moreover, protecting the system against the effects of a meteorite impact would seem to require redundancy in the tether design (Hoyt and Forward, 1998~. Atmospheric Drag Aeroassist A spacecraft approaching a body to orbit around it, or perhaps descend to its surface, can achieve the necessary velocity reduction by descending into the outer edges of its atmosphere, assuming the body has an atmosphere of sufficient depth and density. The resulting drag would replace the reverse thrust that would otherwise be required from the propulsion system (French, 1981~. This would save energy and, depending on the propulsion system, mission time as well. Such a system would obviously require exquisite control accuracy and dependability. Both crew and spacecraft systems would be subject to great changes of apparent gravity, from zero to many times Earth' s gravity, during typical atmospheric drag, or aeroassist, maneuvers, posing important issues of human and technical system performance. Major Subsystems, Their Purposes, and Their Sensitivities to Reduced Gravity What follows is a list of the major subsystems and important devices, together with some discussion of their uses or purposes, that would make up the various propulsion systems described above. The list is selective in that only those devices are mentioned that seem to pose issues for microgravity science, directly or indirectly. For example, thruster nozzles, although important, are not listed, because their large pressure differences and high velocities will mask any effects of microgravity. Of course, nozzle coolant passages may contain cryogenic liquids subject to flashing, and they would therefore be subject to multiphase effects of reduced gravity. Many subsystems of interest are of course common to a number of propulsion types and, indeed, to other functions such as power generation or construction and maintenance, as discussed in later sections of this report. Furthermore, some design elements, such as pipings and bearings, for which there are microgravity concerns are themselves common to different subsystems. These are mentioned later in this section, and then discussed more fully in Chapter V. Nuclear Fission Reactor A large nuclear reactor would provide heat for NTR, or a small reactor would serve an NEP system or provide for electric power generation. Coolant flow and heat transfer to a working fluid or propellant in such a reactor could be sensitive to gravity level, particularly during the transient operations of start-up or shutdown, as the coolant changes phase from a solid to a liquid. In addition, the required reactor control system would include robotic elements, bearings, seals, and actuators, whose performance may be affected by gravity level. A hypothetical gas-core nuclear fission reactor would presumably need a special control system to maintain core containment. Such a reactor, if developed, could provide heat for NTR or NEP. It would be especially sensitive to gravity level with respect to the containment process (as yet undefined) essential to its steady operation. Cryogenic Storage System A cryogenic storage system for maintaining liquefied gases6 comprises the following elements: storage tank, gas liquefaction system, refrigeration, insulation (passive cooling), liquid acquisition device, feed and fill systems, and pumps for pressure control and liquid transfer. The purpose of such a system would be to store and provide 6Neiberding; see footnote 4.

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 45 propellant and reactant gases for chemical rockets or any rocket in which a propellant gas is heated, perhaps by a nuclear reactor or by solar or laser radiation, or is accelerated electromagnetically as a plasma. Such a fluid-handling system is inherently a multiphase system because it must maintain liquid and vapor in proper equilibrium, condense vapor, and, finally, deliver liquid via pumps, valves, and pipes to the rocket cham- ber. As such, a cryogenic system is highly sensitive to gravity level in all its aspects. For example, in microgravity, the storage tank poses problems of ullage location, temperature control, and filling procedure, while the valves and pumps are subject to flashing. Cryogenic storage problems are discussed in more detail in Section III.E. Radiator System Heat rejection to space requires a system that has a large surface area, a pipe and header system for coolant, and heat pipes to convey the heat from source to radiator. Space radiators and heat pipes are discussed above in Section III.A in connection with power generation. Here, it is emphasized that space radiators are necessary in order to reject waste heat from the energy conversion processes used for electric propulsion. The need for a heat- rejection subsystem is a feature of closed power cycles such as those used to generate electric power, for whatever purpose. In contrast, chemical or thermal propulsion (nuclear or solar, for example) are open cycles, which reject heat via the propulsive stream itself and therefore have no need for a radiator system and its attendant microgravity problems. It should also be noted that the power requirement for electric propulsion of a large spacecraft implies a very large radiator area having the form of a large, light (and therefore flexible) structure. When gravity is absent, such a structure would be subject to dynamic mechanical behavior, which may be difficult to control. Solar Collector For propulsion systems depending on heat from the Sun, a large collector surface is required, probably containing a tube and header system of phase-changing fluid to absorb heat and deliver it to the spacecraft. Therefore, as in the case of the heat-rejection radiator, multiphase fluid mechanics in reduced gravity would presumably be important, as would the structural dynamics of an extended structure. Boiler for a Rankine Cycle ~ 1 ——— —— ~ —_ _~ Gas or Vapor Turbines If a Rankine cycle is adopted, a heat exchanger for evaporation and separation and collection devices for gas and liquid phases are needed. As pointed out in Section III.A, such a boiler would be an essential part of any method of electric power generation by means of the Rankine cycle, whether the energy source is chemical, nuclear, or solar, and whether the purpose is to make electric power for propulsion or for spacecraft or station power. The performance of a heat exchanger in which the working fluid is evaporated is expected to depend on gravity level. On Earth, the separation of vapor from liquid is often accomplished with the help of gravity; in microgravity, other effects, such as viscosity or acceleration, may be effective, depending on the design. The completeness of phase separation is vital for the physical integrity of the equipment and for the efficiency of the cycle; in reduced gravity, special effort must be made to achieve complete phase separation. On Earth, cyclone separators are often used, and since their workings are independent of gravity, they should be carefully considered for space applications. ~ . , Gas or vapor turbines, including control valves, seals, and shaft bearings, will be needed, as on Earth, to drive electric generators for electric-propulsion schemes and perhaps to drive the various liquid and vapor pumps for the cycle working fluids, combustion reactants, coolants, and propellants that are used in any of the propulsion systems of interest. Owing to the high fluid velocities expected in such devices, no microgravity effects in the fluid streams are to be expected, unless cavitation or flashing of vaporizable liquids occurs. Such effects fre-

46 MICROGRAVITY RESEARCH quently occur in control valves, which suggests careful attention to shaft seals. Film bearings, particularly cryogenic bearings, may be sensitive to load changes associated with variable or reduced gravity. Liquid Pumps Pumps, which also require control valves, seals, and shaft bearings, perform liquid transfer duties in all propulsion systems. In particular, high-pressure pumps are needed to pressurize working-fluid condensate in Rankine-cycle electric propulsion, and pumps are needed to circulate working fluids generally, combustion reac- tants, coolants, and propellants for all propulsion systems. The same concerns about microgravity effects apply as for turbines, except that cavitation is not to be expected at the high-pressure end of pumps. Compressor A compressor is needed to raise the pressure of the (gas) working fluid in the Brayton-cycle option for electric propulsion, raise the vapor pressure in the refrigeration cycle for cryogenics, and propel gas-phase fluids as needed in other systems. In the gas stream, unless condensation occurs, no microgravity effects should arise. Microgravity would be an issue for valves, seals, and bearings, as it would be for pumps and turbines. Condenser for a Rankine Cycle A condenser for a Rankine cycle would contain a heat exchanger for use between working fluid and heat- rejection coolant as well as devices for separating and collecting fluid phases. In any electric propulsion system employing the Rankine cycle, the working fluid passes through a vapor turbine and then must revert to the liquid phase before being pressurized and returned to the boiler. This condensation process releases heat at a rate proportional to the power generated, and that heat must be delivered to space via the radiator system mentioned above in Section III.A. Therefore, a high-capacity heat exchanger must connect the working-fluid vapor and the radiator surfaces either directly or through an intermediate coolant such as ammonia. On Earth, the condensed fluid is removed from the heat exchanger surfaces by gravity. In reduced gravity or microgravity, other mechanisms, such as surface tension in a heat pipe, centrifugal phase separators, or direct contact condensation, must be used. Significantly, these condenser designs may incur size and weight penalties because of microgravity. Moreover, the condensate pumps that return the condensate to the boiler must be carefully designed to make sure that there is sufficient net positive suction head in microgravity environments. To assure the integrity and efficiency of a Rankine cycle, it is very important that there be no liquid carry-over or carry-under. On Earth, gravity helps accomplish this task. In microgravity, the centrifuge effect (e.g., a rotary fluid-management device, or RFMD) would presumably be used. This kind of microgravity countermeasure is discussed in Chapter V. Vaporizer for Propellant In electric propulsion systems, the propellant must be provided to the nozzle system as a vapor. However, the propellant might be a noble gas stored as a liquid, cryogenically, or it might be a metal such as cesium. Vapor might be formed by adding heat or by reducing pressure (flash boiler), depending on the propellant (Cassady, 1990~. In microgravity, vaporization methods dependent on buoyancy to collect the vapor would have to be avoided; therefore, vaporization by sudden reduction of flow pressure would presumably be preferred. Switch Gear and Electric Power Conditioning For electric propulsion, the power output of the generator must be modified and controlled to provide the proper voltage and current to the thruster system. Such power conditioning generates waste heat in large quanti-

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 47 ties, which must be delivered ultimately to a space radiator. Thus, heat-rejection methods needed for the thermo- dynamic power cycle itself must also deal with large losses in the electrical system. Further, the electrical apparatus would be kept at rather low operating temperatures, which would further burden the heat-rejection design. Common Design Elements The following paragraphs discuss specific elements or components of technology that seem to be important for propulsion systems and that are also important for almost all functions described in this chapter, especially those that involve fluid flow and heat transfer. These elements are affected by microgravity but often indirectly or secondarily, in ways easily overlooked but nevertheless important. Heat exchangers between unmixed elements, fluid or solid, are needed for thermal management of spacecraft systems and as thermal connections between primary heat collection fluids, cycle working fluids, and propulsive gases. Heat exchangers in which phase change is not expected and which handle high flow rates, such as the cooling passages in a fission reactor, would not be expected to perform differently in microgravity. Any heat exchanger for which, on Earth, buoyancy might be expected to propel the fluid, due either to phase change or to thermosiphon phenomena (a two-phase riser would be an example of the latter), would be problematic in reduced gravity. An interesting multiphase heat-transfer device, the vanor-nressure numbed loon (Section III.AN. mav be helpful in microgravity. ~ ~ ,, ~ Heat exchangers, especially those with a fine flow-passage structure, are subject to loss of performance by surface fouling, and this would be especially troublesome for space systems that are hard to clean or replace. In microgravity, atmospheres in spacecraft or space colonies might contain high concentrations of suspended par- ticles or droplets that lead to fouling of heat exchanger surfaces. Thus, microgravity could indirectly but substan- tially affect heat-exchanger performance. The purity of the fluids passing through heat exchangers requires careful assessment. Piping systems, including valves, would be used for fluid handling in all propulsion and power systems, and in a wide variety of other systems, for all fluids, including liquids, gases, mixed phases, slurries, and suspensions. The various microgravity phenomena that can occur in multiphase flows in straight pipes and channels are discussed in Chapter IV. Still other phenomena will come into play at pipe bends and fittings, where transverse fluid accelerations occur. Certain mechanical problems can also be foreseen. Because flow passages used in reduced gravity would presumably be designed to be of low mass, the piping is likely to lack the strength and rigidity one would expect for terrestrial installations. Piping designed for reduced gravity would therefore be expected to be highly vulnerable structurally, especially at bends and elbows, to surge and liquid-hammer effects and to flow-induced vibrations, especially if multiple phases are present. Abrupt valving may initiate or aggravate such behavior, for example because of cavitation. These issues are discussed again in Section V.A. It has been mentioned that HEDS technology will require various machines for fluid-handling tasks, such as pumps, turbines, and motors, and these will typically need bearings. Fluid handling will certainly require valves for control, and bearings and valves both require seals to segregate liquids or gases from the surrounding spaces. Often, the operation of bearings and seals will not be directly affected by microgravity, but the magnitudes and nature of the loads for which the bearings are to be designed will be affected. This indirect influence of microgravity is discussed further in Section V.A. For many HEDS missions, robots of various types will probably be used, for long periods, in order to protect the crew from the environmental hazards of space. As mechanical devices, these robots will experience wear and decay, which are processes affected by gravity level. Microgravity will also have indirect consequences (in terms of structural dynamics) for robot design and performance, as is discussed in Section V.A. Antennas for commu- nication and solar collectors and space radiators for distant HEDS missions will be large and will require accurate control of orientation. Tanks for storage of liquids and gases will also be large. These components, to be used in low gravity, will also present structural dynamics problems, as is discussed in Chapter V.

48 MICROGRAVITY RESEARCH TABLE III.B. 1 Selected Subsystems Found in Propulsion Technologies and the Potential Impact of Microgravity on Their Operation Technology Nuclear Nuclear Nuclear Subsystem Chemical Thermal Electric (Rankine) Electric (srayton Reactor (fluidized) H H H Evaporator (liquid or metal) H H cryogenic system H H M Condenser H Liquid/vapor separator H Radiator (two-phase) H H Heat pipes M M Liquid pumps M M M vapor compressor M vapor turbines M vapor heat exchange M Solar collector Sails (solar or laser) Reactor controls M M M system controls L L M M Seals L L M L Valves L L M L Film bearings L L M L Piping L L M L Robots L L L L NOTE: The letters H. M, and L designate high, medium, and low (preliminary assessment) impact of reduced gravity on the operation of the subsystem. Where no letter is given, the subsystem is not applicable to the system listed. General Concerns Regarding Propulsion and Power in Reduced Gravity Certain general issues affecting power and propulsion component design and performance, seemingly impor- tant in microgravity, merit more thorough study in the HEDS context than they have received so far. Touched on in Section III.A on power and above in this section on propulsion, they deserve further emphasis here to round out the committee's discussion of the key topics of power and propulsion. Variable Gravity Most propulsion and power systems should be able to operate in a range of gravity or acceleration environ- ments, from coasting to full thrust, or on a station on the surface of Mars; this is especially true of the electric power-generation modes of the bimodal NTR or NEP7 systems. The exploration of asteroids, with its varied acceleration levels during flight and complex docking maneuvers, would test the ability of components to accom- modate various gravity levels. While it is true that a device designed for Earth gravity may not work well in zero gravity, the converse is also true, that a device designed for zero gravity might fail to perform properly under Earth gravity. 7Neiberding; see footnote 4.

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 49 Solar Solar Solar Thermal Electric (Rankine) Sail Laser Laser Thermal Sail Tether Aeroassist H H L L L L L L H H H H M M M H M M M M M L M M H L L L L L H H H L L L Transient Operation and Unsteady Processes Intermittent or variable operation, including start-up in space, must be considered carefully. System dynamics and system instabilities are very important issues for power generation generally; the generation system and its load must be managed together. Some issues of this kind, especially nuclear start-up, have been carefully studied (Kirpich et al., 1990), but the full range of transient issues probably has not. Unsteady processes can be very important for system performance and can be favorable or unfavorable. Harmful effects include instabilities: in rotating systems, either fluid or solid, instabilities are common (Yih, 1965), liquid-hammer loads may occur in conduits (Streeter and Wylie, 1985), and structural fatigue can occur due to cyclic loading. Beneficial effects may include enhanced mixing or diffusion by wave processes. Both ultra- sonic and very low frequency waves may be of interest. Generally, gravity level must be considered a factor in these various processes. Unsteady flow is the basis of the proposed vapor-pressure pumped loop. Multiphase Flow It appears that the power and propulsion systems needed for HEDS will require devices that depend on multiphase flow and heat transfer processes to achieve low mass, high efficiency, and low cost. However, as is discussed in Chapter IV, there are currently large uncertainties surrounding the behavior of multiphase flows in

so MICROGRAVITY RESEARCH reduced gravity. In particular, the transverse distribution of phases within flow passages will be quite different from what it is in Earth's gravity. Moreover, surface-tension-induced forces (e.g., Marangoni forces) will be much more important in space than they are on Earth. If multiphase systems and processes are to be used in space, then reliable, physically based predictive tools will need to be developed and used by NASA for the design and analysis of candidate propulsion and power systems and subsystems. Need for Artificial Gravity Amid concerns for the effects of low or variable gravity, it is important to keep in mind that technical means exist for supplying artificial gravity, and these should be explored by NASA, the purpose being not to make things more familiar, but to counter the real technical penalties suffered when body force is not available. The technical means include, but are not limited to, rotation to provide centrifugal body force. Rotation might be imposed on the scale of the spacecraft itself or on the smaller scales of components such as swirl separators of liquids and vapors. The costs and benefits of a full range of microgravity countermeasures should be studied. This topic is discussed more fully in Chapter V. Reliability The more complex a system, the more opportunities it provides for failure and accident. This is not a matter of being careful in initial design; it is a matter purely of probabilities. Designs of 30-year-old aircraft are still being corrected, after thousands of mission cycles. In contrast, HEDS systems must be designed once and for all to be reliable, without repair, for years. For HEDS missions then, designers must surely learn to be fanatical on the subject of simplicity and reliability. Systems and components should be simple, with a minimum of moving parts, shafts, bearings, valves, or anything that may fail or wear out. This issue of reliability has affected the growth of commercial nuclear power; NASA should learn from that history and, especially for HEDS, put special emphasis on design for simplicity and reliability. Techniques of probabilistic risk assessment (PRA), discussed in Section V.C, should be useful for this purpose. It should be emphasized that variability of gravity, or loading due to accelerations, also affects design and performance and, as such, can increase the possibility of failure and corresponding decreases in reliability. Nuclear System Development Nuclear fission can support electric power generation for propulsion and power and can also directly support thermoelectric, thermoionic, and advanced thermoelectric and thermoionic converters. Such systems are not affected by gravity and have the potential to provide higher power levels in the future, if development is pursued. However, NASA has stopped funding research and development on nuclear space power (Bennett et al., 1996; Bennett, 1998), and it appears that the United States will soon lose its ability to develop nuclear power and space propulsion systems. Since it appears that nuclear fission power will be essential for the success of the long-range goals of HEDS, there is a need for NASA to maintain a steady effort in this field, with attention paid both to newer reactor types and to the many advanced components needed to ensure desired performance at various gravity levels. Summary of the Effect of Reduced Gravity on Selected Subsystems Summarized in Table III.B.1 are the various subsystems and components discussed in this section and the various propulsion systems where they are found. The impact of reduced gravity on the operation of these subsystems is estimated as high, medium, or low (little or no impact). It should be remembered, however, that the impact of the gravity level on these technologies will depend greatly on design context, which cannot be predicted.

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE References 51 American Institute of Aeronautics and Astronautics (AIAA), Aerospace Power Systems Technical Committee. 1995. Space nuclear power: Key to outer solar system exploration, AIAA position paper. Reston, Va.: AIAA. Barnett, J.W. 1991. Nuclear electric propulsion technologies: Overview of the NASA/DOE/DOD nuclear electric propulsion workshop. Pp. 511-523 in Proceedings of the 8th Symposium on Space Nuclear Power Systems. College Park, Md.: American Institute of Physics. Bennett, G.L. 1998. Electric power technologies for spacecraft: Options and issues. AIAA Paper No. 98, p. 1022. Reston, Va.: American Institute of Aeronautics and Astronautics. Bennett, G.L., and T.J. Miller. 1992. NASA program planning on nuclear electric propulsion. AIAA paper 92-1557. New York: American Institute of Aeronautics and Astronautics. Bennett, G.L., H.B. Finger, T.J., Miller, W.H., Robbins, and M. Klein. 1994. Prelude to the future: A brief history of nuclear thermal propulsion in the United States. In A Critical Review of Space Nuclear Power and Propulsion, 1984-1993. M.S. El-Genk, ed. New York: Springer-Verlag. Bennett, G.L., R.J. Hemler, and A. Schock. 1996. Space nuclear power: An overview. J. Propulsion Power 12:901-910. Carrick, P., and J. Harper. 1998. High energy propellants at the Air Force Research Laboratory. Pp. 169-175 in Proceedings of the 9th Advanced Space Propulsion Workshop. Pasadena, Calif.: Jet Propulsion Laboratory. Cassady, R.J. 1990. Propulsion systems. Pp. 69-80 in Thermal-Hydraulics for Space Power, Propulsion and Thermal Management System Design. AIAA Progress in Astronautics and Aeronautics, Vol. 122. New York: American Institute of Aeronautics and Astronautics. Cutler, A.H., and J.A. Carroll. 1992. Tethers. Pp. 136-144 in Space Resources Energy, Power, and Transport. NASA SP-509, Vol. 2. Linthicum Heights, Md.: National Aeronautics and Space Administration Center for Aerospace Information. Dearien, J.A., and J.F. Whitbeck. 1990. Advanced multimegawatt space nuclear power concepts. Pp. 41-67 in Thermal-Hydraulics for Space Power, Propulsion and Thermal Management System Design. AIAA Progress in Astronautics and Aeronautics, Vol. 122. New York: American Institute of Aeronautics and Astronautics. French, J.R. 1981. An expedition to Mars employing shuttle-era systems, solar sails, and aerocapture. Pp. 245-250 in The Case for Mars (American Astronautical Society), April 29-May 2, Boulder. Science and Technology Series, Vol. 57. P.J. Boston, ed. San Diego: Univelt. Gallagher, D.L., L. Johnson, F. Bagenal, and J. Moore. 1998. An overview of electrodynamic tether performance in the Jovian system. P. 427 in Proceedings of the 9th Advanced Space Propulsion Workshop. Pasadena, Calif.: Jet Propulsion Laboratory. Harris, H.M. 1999. Light sails. Sci. Am. 280(2):90. 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52 MICROGRAVITY RESEARCH III.C LIFE SUPPORT Introduction Arguably, the central and paramount HEDS function is to provide an environment consistent with the sus- tained existence of personnel outside of Earth's atmosphere at a comfort level that will enable high performance. The systems necessary to fulfill this function include those that protect against ionizing radiation; control tempera- ture, pressure, humidity, and waste products to within prescribed limits; provide adequately balanced food, potable water, and hygienic water; and afford adequate physical activity. Because the psychological aspects of life support may be even more problematic than the physiological aspects, additional amenities consistent with creature comforts will be important. Systems considered important for life support are described qualitatively in an NRC report (NRC, 1997) and summarized in an excellent monograph (Eckart, 1996~. The former report provided a useful framework for categorizing the various technologies cited in this section, while the latter provided invalu- able design details for a wide range of technologies. Neither document identified scientific or technical issues associated with possible failure modes arising from reduced gravity. This section emphasizes these issues as it looks at selected technologies that are likely to be both important to life-support design and significantly affected by gravity levels. Key to understanding life-support systems and their subsystems is the concept of homeostasis, or maintaining constant, optimal levels of the various physical, chemical, and biological systems necessary for life support. An important goal is to achieve as nearly as practical a closed ecological system requiring the input of a minimum of mass and energy and in which as many subsystems as possible utilize recycling. In order to close cycles, different forms of matter and of energy must be interconverted and stored, preserving the necessary balance of each within a habitable module. These are precisely the processes most heavily affected by the reduced influence of density differences on gravity-density coupling and interracial phenomena (discussed in Section IV.A) in microgravity. Systems required for life support fall into five principal areas of activity (AIAA, 1990~: Regulation within suitable limits of the temperature and relative composition, purity, and pressure of the ambient gas phase of the habitat and/or the equipment for extravehicular activity; 2. Management of the quantity and quality of drinking and hygienic water and the associated recovery and processing systems; 3. Collection, processing, and recovery procedures associated with biological waste and trash; 4. Food management, including production, preparation, and storage; and 5. Crew safety management, including radiation shielding and fire detection and suppression. The first four activities consist of mass and energy conversions involving interrelated processes. For example, atmospheric parameters include the relative humidity, which is at the same time an issue for water homeostasis. Food production and human waste management are potentially closely coupled and interact with water and atmospheric management systems. The fifth area requires countermeasures to limit the biological effects of ionizing radiation and is discussed further in Section III.D. The adverse effects of microgravity on certain aspects of human physiology such as bone metabolism are covered in detail in a recent NRC report (NRC, 1998) and are not considered further here. Conceptual guidelines dealing with interconnectedness for the purpose of design and development efforts are outlined in NRC (1997) and detailed in Eckart (1996~. To understand these issues, regenerative and nonregenera- tive systems must first be distinguished. Regenerative systems explicitly recover and recycle in order to minimize the mass required for a mission and the ultimate waste that must be stored and jettisoned from it. They usually involve closed loops in which the flow of mass to and from the habitat is limited. It is useful to note briefly that wastewater recycling can reduce the relative supply mass by nearly 50 percent and that water, carbon dioxide, and oxygen recycling can reduce it by nearly 90 percent. Trace contaminant and particulate removal and temperature, pressure, and humidity control cannot be easily accomplished by regenerative systems. Other requirements, including oxygen generation and carbon dioxide reduction, are nearly always carried out by regenerative cycles.

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE TABLE III.C. 1 Gaseous Component Management Systems 53 Carbon Dioxide Removal Carbon Dioxide Reduction Oxygen Generation Molecular sieve Bosch cycle Static feed electrolysis Electrochemical depolarization concentration Sabatier cycle Water vapor electrolysis Solid amine-water desorption Carbon formation reactor Carbon dioxide electrolysis Photochemical Photocatalysis In situ resources LiOH Direct electrolysis Plants Electroreactive carriers Catalytic decomposition Cryogenic storage Plants Ultraviolet photolysis High-pressure storage Plants SOURCE: Adapted from Eckart (1996). Generally, closed-loop systems are superior wherever resupply costs are high, whereas open-loop systems are simpler to implement because they require less technological development initially and have lower power require- ments. The advantages of the former are offset somewhat by their more intensive dependence on phase changes and multiphase (fluid and solid mixtures) processing, which makes them harder to implement in microgravity. However, they will become increasingly necessary as enabling technologies as missions become more distant and of longer duration. Life support is distinct from other HEDS functions owing to the introduction of biological, as opposed to physicochemical, strategies into systems that interchange the chemical compositions of different supply and waste streams. Such biologically based systems, or bioreactors, will probably be involved in both nutrient production and waste management. Crucial to the successful development of suitable closed ecological life support systems (CELSS)8 will be the development of interfaces between bioregenerative and physicochemical systems. Biological and hybrid systems exploit the metabolism of living organisms to effect some or all of the requisite transformations, while physico- chemical systems rely exclusively on well-understood mechanical or chemical processes. In general, biological systems are bulkier, more difficult to maintain, consume less power, and respond much more slowly than do physicochemical systems. However, bioreactors have the unique potential to provide food. Moreover, many bioreactor subsystems rely heavily on capillarity and other interracial phenomena whose behaviors in microgravity cannot yet be adequately modeled (see Section IV.B). Improved modeling capabilities would decrease the difficulty and expense of bioreactor development and testing. Atmospheric Homeostasis Systems in this category regulate and monitor atmospheric composition and supply (pressure), temperature and humidity, decontamination, and cabin ventilation. They must function with little or no human intervention for long time periods. Air Revitalization The most important functions in maintaining an appropriate atmosphere are those that regulate the partial pressures of oxygen and carbon dioxide (Table III.C.1~. These two gases are interconverted by the dominant metabolic cycles of humans and plants. This reciprocity may ultimately be exploited to help regulate both gases bv balancing the respective metabolic outputs. However, physicochemical control over both gases will remain 8Note that this acronym has appeared in the literature with slightly varying designations depending on the literature source and period.

54 MICROGRAVITY RESEARCH Water in— Return to cabin ~ I ,~ ~ CO~sorbent I ~ \ \ rip \T/ / Pump CO2 reduction, waste FIGURE III.C. 1 Generic carbon dioxide (CO2) collector. Gray sections represent the desiccant beds used in the technologi- cally mature four-bed zeolite system. Sorbent beds represent either zeolite (four-bed system), carbon molecular sieves, or solid amine resin beds, which must be steam heated. Subsystems adversely affected by microgravity are shown in reverse contrast. SOURCE: Based on Eckart (19961. central in the short term and may serve to fine-tune even advanced bioregenerative systems. The physicochemical cycles used to interconvert carbon dioxide and oxygen are independent, however, and systems with a high technical readiness level (TRL) therefore deal with three separate processes: carbon dioxide removal, carbon dioxide reduction, and oxygen generation. Carbon Dioxide Removal and Concentration Carbon dioxide is a waste product produced by human respiration at the rate of ~1 kg/man-day and is potentially toxic. Treatment involves concentration followed by chemical reduction to a reduced form of carbon, such as methane, and molecular oxygen. Concentration or "removal" can be effected using a variety of regener- able and nonregenerable systems with a range of technological readiness, weights, ambient operating conditions, and power requirements. The carbon dioxide removal system with the highest TRL is a four-bed molecular sieve incorporating synthetic zeolites or metal ion aluminosilicates to collect carbon dioxide (Eckart, 19961. These collecting materials cannot tolerate excess moisture, so a preabsorbing sieve is necessary to dry the air before carbon dioxide absorption. Moreover, in the Resorption cycle the sieve is heated while it is exposed to space vacuum, so the carbon mass is lost. Thus, although the system is "regenerable," it cannot be incorporated into a closed system without the development of additional vacuum technology to recover and concentrate the carbon dioxide. Moreover, use of this system has shown that it periodically builds up residual carbon dioxide, which must be eliminated by a bakeout at 478 K. Regenerative carbon dioxide collectors compatible with closed systems, illustrated schematically in Figure III.C.1, are likely to be affected by microgravity. One near-term technology is the solid amine-water Resorption (SAWD) system, which uses chemical reaction with solid amines in the absorption phase, similar to aqueous ion exchange. Subsequent Resorption with steam requires a boiler, which is highly vulnerable to the gravity level and adds to the loading of heat rejection systems. Newer, carbon-based molecular sieves have been developed that are insensitive to water vapor and that offer more efficient trapping and can be cycled through Resorption at closer to ambient pressure and temperature. These materials can be incorporated into two-bed sieve systems that do not require the desiccant beds and are roughly twice as efficient as four-bed systems. Two-bed systems can be integrated more readily into closed-loop regenerative systems. However, no prototypes have as yet been designed. The remaining systems listed in Table III.C.2 currently have very low technological readiness but are of interest because of their potential economies of power and/or weight. Of these, ultrafiltration using osmotic membranes (membrane removal), electroactive carriers (ion-exchange dialysis), and, possibly, combinations of these could potentially provide for the most direct processing. Moreover, all closed-cycle regenerative systems require a carbon dioxide condenser/separator, which would be very sensitive to microgravity.

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE TABLE III.C.2 Physicochemical Systems for the Concentration of Carbon Dioxide 55 Nonregenerative Systems Regenerative Systems System TRLa Efficiency Products System TRLa Efficiency (%)Products LiOH 8 LiCO3, Four-bed molecular sieve 8 66 Sodasorb H2O Two-bed molecular sieve 2-3 90 Superoxides Solid amine-water desorption 6 Electrochemical depolarization 6 H2O, heat, DC power Air polarization 2 Membrane removal 2 Electroactive carriers 1 ion-exchange electrodialysis aTRL, technical readiness level, the maturity of a system ranging from level 1 (a basic principle observed and reported) to level 8 (a design qualified for spaceflight). Reduction of Carbon Dioxide The Bosch and Sabatier reactors, particularly the Sabatier reactor, are crucial for generating oxygen for various propulsion systems, and both are described further in Section III.E. Both produce water vapor, which must be condensed and separated from the vapor phase. On Earth this separation would be driven by gravity. Sub- systems involved in steam generation involve two-phase fluid management, so they would operate much differ- ently in a microgravity environment. Oxygen Generation Water Electrolysis A regenerative CELSS will almost certainly require a water electrolysis system. Electrolysis involves the delivery of electrons via an electric current to water, yielding hydrogen, and the removal of an equivalent number of electrons from the hydroxide ion, yielding molecular oxygen. The process is inherently two-phase, as the dissolved gases must be recovered and concentrated. It is thus also highly sensitive to gravity level. Oxygen Supply and Regeneration The chemical storage of oxygen is the least affected by operation in microgravity, but it is not suited to regenerative cycles. Cryogenic or supercritical oxygen storage has the significant advantages of high density and low pressure, and liquefied gases could serve as an important heat sink for integration into larger thermal control cycles. Development of these technologies is therefore crucial for HEDS. However, they are highly sensitive to gravity, the former requiring a condenser and the latter a compressor. Temperature and Humidity Control Control of ambient temperature and humidity within tolerable limits involves the physical processes of heat exchange, multiphase flow, phase changes, and phase separation, all of which are strongly affected by the gravity level, as described in Chapter IV. The system responsible for all of these processes is a condensing heat exchanger (CHX). Cooling water is used to bring the incoming air to a temperature below the dewpoint of water, inducing formation of a condensate, which must then be separated from the airstream. The phase separation generally takes place via a "slurper," which is a flat, thin volume bounded by plates, one of which is in contact with the airstream and condensate film. Holes in this plate serve to entrain the condensate into the volume between the plates (Kuhn, 1988).

56 MICROGRAVITY RESEARCH TABLE III.C.3 Physicochemical Systems for Recovering Water from Urine and Hygienic Waste Process TRLa Efficiency (%) Distillation Vapor-compression distillation Thermoelectric integrated membrane evaporator Vapor-phase catalytic ammonia removal Filtration Reverse osmosis Multifiltration Electrodialysis 70 91 3 95 aTRL, technical readiness level, the maturity of a system ranging from level 1 (a basic principle observed and reported) to level 8 (a design qualified for spaceflight). Trace Biological, Chemical, and Particulate Contaminant Control The isolated nature of a spacecraft environment introduces the critical need to detect and eliminate airborne hazards arising from outgassing of insulation and plastics, from human metabolism, and from accidental spills and degradative processes. Contaminant detection by gas chromatography, mass spectrometry, and/or Fourier trans- form infrared spectroscopy would seem to pose no obvious problems in microgravity. Water Homeostasis Recovery Water can be recovered from aqueous waste by either fractional distillation or membrane filtration (Table III.C.3) (Eckart, 1996~. Urine and waste wash water present significantly different problems for water recovery. Urine contains urea, sodium chloride, and various organic salts and acids at total concentrations of up to 5 percent. It supports bacterial growth. In contrast, the contaminants in waste wash water are considerably more diverse, ranging from dead skin, hair, and dirt to fats, soaps, detergents, and other organic compounds found in sweat. These differences in composition have led to the perception that distillation is the preferable treatment for urine, while filtration is to be preferred for other waste water sources. The water condensates dealt with by humidity management (from respiration, perspiration, and transpiration sources) are technically, although not explicitly, in the former category. Distillation requires paired phase changes and separations and is strongly affected by gravity. The four distillation schemes in Table III.C.3 incorporate different solutions to the problems posed by a microgravity environment. Vapor-compression distillation (VCD) uses a compressor to raise the saturation temperature of the process water vapor, while the actual condensation is engineered to take place in direct contact with the evapora- tor, exploiting the latent heat of condensation for the evaporation process (Eckart, 1996; Schmidt, 1989~. Vapor- phase catalytic ammonia removal (VAPCAR) combines distillation at 523 to 723 K with catalytic decomposition of contaminating volatiles to molecular nitrogen, hydrogen, and oxygen by a series of two platinum/ruthenium catalytic reactors. The high ambient temperature ensures that the water produced is not only chemically very pure but also free of microbial contamination, requiring only adjustment of the pH to render it potable. The low TRL ratings for these subsystems reflect, in large part, the technical difficulty of using multiphase systems in microgravity without further substantial research and development. VCD and VAPCAR technologies use a compressor and separator, respectively, that must operate in microgravity. The thermoelectric integrated membrane evaporator uses reduced pressure outside a hollow membrane fiber system to evaporate water from urine. The extensive pretreatment required to fix free ammonia and prevent microbial growth currently uses

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 57 oxone, a hazardous component that must be used as a liquid in microgravity, under which conditions it becomes unstable. Reverse osmosis, electrodialysis, and filtration may become alternatives to two-phase systems if they can be brought to maturity. Solid Waste Management Whereas liquid waste poses physical separation problems, recycling requires that solid waste be chemically transformed, generally by oxidation. To date, solid waste treatment has been limited to drying the various organic solids, with or without recycling of the extracted water. Three unique problems impede further development: Isolating and managing the organisms and toxins present in biological wastes; 2. Processing mixtures of solid materials whose composition varies significantly; these mixtures may be characterized as ill-defined, multiphase systems; and 3. Coupling solid waste processing with plant growth chambers and other biologically based waste treatment systems, as well as with physicochemical gas/liquid/solid mixers and separators. Both incineration and supercritical wet oxidation are being studied for recycling solid wastes (Eckart, 1996; Bilardo and Likens, 19911. Both systems show promise for future spacecraft designs. Incineration systems process mixtures of human feces, urine, and other solid biological wastes previously dried (by evaporation) to water content levels of less than 50 percent. Subsequently the feed material is combined with air or oxygen in a reactor operated near ambient pressure and heated to temperatures above 800 K to effect combustion. Incomplete combustion, produced by supplying the reactor with substoichiometric oxygen, can produce partial pyrolysis and a more stable exhaust gas stream of nearly pure water and carbon dioxide and with larger particulate sizes and fewer toxic emissions than are produced by providing excess air (or oxygen). Partial incineration of nitrogenous waste also produces ammonia, rather than nitrate and nitrite ions, and the ammonia can be converted to nitrogen and water in an afterburner. Supercritical wet oxidation utilizes the unique solvent capabilities of supercritical water above 647 K and 22.1 MPa. Above these conditions, and at significant levels of oxygen, normally insoluble organic solids can be absorbed in water, enabling rapid oxidation of undesirable organic waste products almost completely, in a single supercritical phase. Because supercritical water does not have separate liquid and vapor phases, many of the microgravity phase-separation problems are avoided. However, the high temperatures and pressures of the reactor, as well as the corrosiveness of the supercritical water itself and the toxicity of the product gases, raise safety concerns. As noted, the solid waste recycling systems have not been operated in space and are not currently justified for Earth-orbiting systems like the International Space Station. Because of the inherent variability and the low immediate value of the recycled materials that can be recovered, waste recovery systems should probably not be developed as stand-alone systems but eventually should be integrated into bioregenerative systems that produce food and oxygen from plants while recycling solid waste at some level. Accordingly, several types of biologically based solid waste recovery systems are being considered at this time. To develop those systems for operation on spacecraft, advances are required in the following areas: · Systems that chop or grind solid wastes effectively, collecting the product; · A process for blending solid wastes with water to produce a controlled liquid feed compatible with pumping; · Systems that dissolve gases in liquids; · Systems that remove dissolved gases from liquids; · Liquid water/nutrient management systems that do not flood plant chambers; and · Systems that can maintain immobilized cell colonies efficiently in microgravity.

58 MICROGRAVITY RESEARCH TABLE III.C.4 Major Subsystems Found in the Various Life-Support Systems and the Potential Impact of Microgravity on Their Operation System Solid Electrochemical Molecular Amine-Water Depolarization Subsystem Sieve Desorbtion Process Bosch Sabatier Process Adsorber (gel) L Adsorber (solid matrix) M L Boiler H Catalyst bed L L L Centrifuge Chopper/grinder Compressor L L Condenser/separator H H Electrochemical membrane L Fan/blower L L L Filter L L L L Heat exchanger L L L L L Heat pipes Heater L L L Mixer Oven Pipes and valves L L L L L Pump L L L L L Scrubber Slurper Sparger Storage tank L H H H L Thermoelectric refrigeration NOTE: The letters H. M, and L designate high, medium, and low (preliminary assessment) impact of reduced gravity on the operation of the subsystem. Where no letter is given, the subsystem is not applicable to the system listed. TIMES, thermoelectric integrated membrane evaporator; VAPCAR, vapor-phase catalytic ammonia removal. Food Production In contrast to some areas discussed above, food production from recycled wastes is expected to make a relatively minor contribution to life support in the near future (Eckart, 1996~. However, longer term advances will depend increasingly on improvements in this area. Heading the list of systems required for regenerative food production is the soil-bed reactor (SBR), in which plants are grown for their respiratory production of oxygen, conversion of carbon dioxide into carbohydrate, and conversion of carbohydrate and ammonia into protein for human consumption. Unsettled questions about how microgravity can affect plant physiology are discussed in detail in a recent report (NRC, 1998) and will not be pursued here. The remaining technical issues presented by the SBR involve the simple question of how, in the absence of drainage, to supply water and nutrients to the plants without drowning them. The interfaces between roots, solid support, and liquid nutrients change in microgravity because of the increased dependence of liquid transport, as well as the solubility and diffusion of gases in liquid and porous media, on surface tension. Adequate computational models of transport, diffusion, and wetting and their mutual interactions under microgravity are critical for the ground-based research into SBR design. In addition to the issues of fluid management, there are clearly complex biological cycles relating food

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 59 Cabin Atmosphere Supercritical Loop TIMES VAPCAR Incinerator Bioreactor Wet Oxidation L L — H H H — L L L L L _ H H H L L H H H H — L L L — L L L M L L L L L L L M L L L L L M M L L L L L L L L L L L L M M — M _ M _ H H H L H H L L — production to waste management in a closed, regenerative system. However, these are outside the scope of this study and are not discussed here. Summary of the Impact of Reduced Gravity on Selected Subsystems Microgravity has pervasive impacts on the subsystems found in each of the major systems involved in life support (Table III.C.4~: · Oxygen and carbon dioxide generation and recovery procedures use a variety of differential liquefaction processes, together with separations of liquid and gas phases. · Solid waste processing involves complex and ill-defined multiphase systems requiring the separation of dispersed solids from the continuous liquid phase. · Temperature and humidity control depends on a variety of heat exchangers, condensers, and separators. · Water recovery involves controlled distillation, which in turn requires successive phase changes and separations.

60 MICROGRAVITY RESEARCH · Food production involves special problems associated with the altered wetting and capillary behavior in microgravity, as well as with the management of fluids and multiple phases. Summarized in Table III.C.4 are the various subsystems and components discussed in this section and the various propulsion systems where they are found. For each subsystem that appears in a given system, the impact of reduced gravity on its operation is estimated as either high, medium, or low (little or none). It should be remembered that the impact of gravity level on these technologies will depend greatly on the design context. References Amencan Institute of Aeronautics and Astronautics (AIAA). 1990. Final Report to the Office of Aeronautics, Exploration arid Technology, National Aeronautics and Space Administration, on Assessment of Technologies for the Space Exploration Initiative. Washington, D.C.: American Institute of Aeronautics and Astronautics, December 31. Bilardo, V., and W. Likens, eds. 1991. In-house Life Support Technology Review Book. Document No. 90-SAS-R-003. NASA Ames Research Center, Advanced Life Support Division. Eckart, P. 1996. Spaceflight Life Support arid Biosphencs. Torrance, Calif.: Microcosm Press, and Dordrecht, Netherlands: Kluwer Academic Publishers. Kuhn, P. 1988. Condensing heat exchangers for European spacecraft ECLSS. Pp. 193-197 in Space Thermal Control and Life Support Systems. European Space Agency (ESA) SP-288. Noordwijk, Netherlands: European Space Agency. National Research Council (NRC), Aeronautics and Space Engineenng Board. 1997. Advanced Technology for Human Support in Space. Washington, D.C.: National Academy Press. NRC, Space Studies Board. 1998. A Strategy for Research in Space Biology and Medicine in the New Century. Washington, D.C.: National Academy Press. Schmidt, R.N. 1989. Water recovery by vapor compression distillation. Paper 891444 in Papers Presented at the 19th Intersociety Conference on Environmental Systems. Washington, D.C.: Society of Automotive Engineers. III.D HAZARD CONTROL Introduction This section is focused on hazards to humans, defined as detrimental phenomena that may directly impair the ability of humans to function or ultimately lead to loss of life. Traveling to distant locations in space, in particular the Moon and Mars, living there, and returning to Earth is, by the nature of the activities involved, dangerous. The relative unfamiliarity and remoteness of HEDS environments increases the risks to a point that careful attention to hazard control must become an essential and specific function. Hazards include long-term exposure of personnel to electromagnetic and particle radiation, meteorite damage to spacecraft and habitats, fire, and chemical and biological contamination of the environment. These hazards, some of which are also faced on Earth, must be eliminated or at least controlled to reduce the risk of loss of life and life support function, and the approaches and technologies used must be functional in different gravitational environments, from microgravity to the partial gravity of the Moon and Mars to the gravity of Earth. Hazards often are addressed in a piecemeal fashion that would be undesirable for HEDS missions. To address hazard control optimally, total-system concepts are needed that use the methods of probability risk assessment, explained in Chapter V. For example, strategic use of particulate and carbon dioxide detectors, electrical monitor- ing, automated fire-suppression response, and manual protocols are needed for effective overall fire-protection systems for HEDS. Nearly all such systems on Earth are strongly affected by buoyancy phenomena; indeed, they often rely on buoyancy effects, such as the rising of hot gases, for their operation. Special ventilation control systems and procedures need study for each separate HEDS environment to enable design of the best system. The large gas-density changes in fires raise many reduced-gravity habitat issues that are poorly understood today (Ross, 1996; Friedman, 1998~. The usual technologies proposed for elimination and/or control of hazards include shielding and chemical radioprotection drugs taken internally to reduce the biological effects of radiation exposure, debris shielding to guard against meteorite damage to spacecraft, and chemical and particle cleaning to scrub the environment in the

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 61 case of contamination. For fire safety, there is a need to select, insofar as is possible, materials that are nonflam- mable. Because complete elimination of flammable materials is essentially impossible (in fact, minimizing the quantity of flammable materials becomes more difficult as the length of a mission increases), it is necessary to employ systems that have the ability to detect faults. For instance, electrical systems, which might lead to smoldering or flaming combustion, might require fire detection and suppression systems, as well as systems for posture particle and gas cleanup. Likely technologies for fire safety detection, suppression, and postevent cleanup include fire signature detec- tors, which sense smoke, gaseous fire precursors or combustion products, flame radiation, etc.; suppression systems, e.g., handheld and automated extinguishers using chemical scavenging, oxygen-depletion blanketing, or cooling agents; and absorption or chemical conversion systems for particle filtration and toxic-gas removal. These systems are influenced by gravity level not only because their operation can be affected by gravity but also because the behavior of the fire and its products is influenced by gravity. Protection against hazards is addressed below in several categories: fire protection, spill and cleanup, radia- tion shielding, and protection from chemical and biological contaminants. Fire Protection Electrical System Fault Diagnostic and Response System A possible source of ignition, leading to smoldering and/or flaming combustion, is the spacecraft or habitat electrical system. This hazard is a consequence of the presence of current next to electrical insulation, which generally is made of flammable material. In view of this situation, the installation of diagnostic systems to survey the health of distribution systems (e.g., to compare current levels with set points and ranges) is a reasonable approach to protecting against a substantial ignition source. Detection would be coupled to feedback to take action should a problem be detected (e.g., isolation and the suppression of ventilation), because slight ventilation gener- ally invigorates a smoldering or flaming event, particularly in microgravity. Such diagnostic systems would not be expected to be particularly affected by gravity level unless the fault detection technique incorporates the detection of flammable offgas resulting from overheating. Smoke Detectors Smoke detectors used in space applications include ionization detectors (space shuttle) and photoelectric detectors (planned for the International Space Station). In an ionization detector, particulates interrupt an ioniza- tion current; in a photoelectric detector, the particulates alter the transmission of light through a fixed path length. Regardless of the detection scheme, the smoke must be transported to the detector, a process driven by natural convection on Earth. In the reduced-gravity environments of spacecraft and the Moon and Mars, smoke transport to smoke detectors must be provided for via forced environment gas flows, because smoke production and transport in reduced-gravity fires are not the same as in normal-gravity fires. In particular, microgravity fires in a quiescent environment produce little smoke, while gentle ventilation results in substantial smoke, much of which can agglomerate into large particles. Fire Extinguishers Handheld Halon 1301 (bromotrifluoromethane) extinguishers are in use on the space shuttle. However. production of this compound is now prohibited because of its deleterious effect on the ozone layer. Additionally, Halon presents major cleanup problems following its use. As a result, International Space Station plans call for carbon dioxide extinguishers. While these may be generally effective, a better understanding of the techniques for their application in reduced gravity needs to be developed. With such knowledge, the Martian atmosphere itself becomes an attractive extinguishant. Fire-detection systems, including smoke and other signature detectors, can include automatic suppression systems to supplement handheld extinguishers that must be actuated by human

62 MICROGRAVITY RESEARCH beings. Gravity levels significantly affect the transport of suppressants to the base of the fire, where they are most effective, so gravity-related phenomena should be studied in this connection. These phenomena involve the effects of varying buoyancy on the motion of gases, liquids, and solid clouds of particles in the vicinity of burning materials. In microgravity, net flow from fires tends to be outward in all directions, and suppressants need to be applied in a manner that will overcome the outward flow effects and penetrate to the fire. Postfire Cleanup Following a fire, atmospheric cleanup is necessary, and this is particularly important in spacecraft. Filtering of fire-generated particulates and removal of products that are toxic to either personnel or the structure is neces- sary. While it would be possible, on a lunar mission, to return the vehicle to Earth for cleanup, such a scheme on a Martian mission would be essentially impossible. In HEDS operations the material cleaned up must, therefore, also be recovered and stored or disposed of safely during the mission. Again, because formation of smoke and other combustion products is affected by gravity, a need exists to determine the makeup of the materials to be removed during posture cleanup. Currently this is not a priority in space flight planning. Instead, efforts are aimed at avoiding fire and refurbishing space vehicles after their return to Earth. Spill Cleanup Fires are not the only events that require attention to cleanup during interplanetary missions. Carefully planned housekeeping procedures will be needed to maintain a healthy environment for humans on extensive voyages. Special attention will need to be paid to automatic sealing and filtering requirements as well as to the design of handheld tools that can aid astronauts in cleaning unwanted spills. Spills of solid and liquid materials behave differently and therefore need to be considered separately. Both types of materials will behave differently in reduced gravity from the way they behave on Earth. One may expect that there will be less of a tendency for spilled materials to accumulate on one face of an enclosure. Greater dispersal on all faces should be anticipated. In addition, there may be more of a tendency for spills to accumulate and remain in the atmosphere. While this can exacerbate health hazards it can also facilitate control by automatic filtration and sealing. Surface-tension phenomena are expected to play a greater role in spill behavior and cleanup, especially under microgravity conditions, than they normally would on Earth. Microfiber technology, for example, might therefore find greater application. These areas appear not to be well studied and involve reduced-gravity issues in need of further research. Radiation Shielding Gamma and particle radiation constitute a serious but reducible threat to long-term survival in space environ- ments (Wilson et al., 1997~. There are three general types of radiation to which personnel are exposed during missions. First, Earth is surrounded by the two Van Allen radiation belts, in which Earth's magnetic field has concentrated charged particles from the solar wind and solar flares. Second, the Sun is a continuing source of solar wind (mostly a plasma of protons and electrons) and solar flares (plasma containing a richer variety of atomic weights). Solar-flare radiation fluctuates in amplitude by more than four orders of magnitude; the occurrence of these flares is unpredictable, but their probability density is correlated with the 11-year sunspot cycle. Solar flares, which pose the most danger to personnel, require sophisticated systems to monitor their activity and, possibly, to deploy protective shielding rapidly. The third type of radiation is galactic cosmic radiation (GCR). This radiation includes particles of high atomic weight, which are termed HZE (high atomic number and energy) radiation. The biological impact of HZE radiation is difficult to assess and has very large uncertainties (NRC, 1996~. In the absence of solar-flare activity, however, GCR is considered to constitute about 90 percent of the exposure to

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 63 personnel, and HZE particles represent 1 percent of this exposure, which could be responsible for an appreciable fraction of the long-term biological effects. Considerable data have been accumulated, primarily from Soviet long-term flights, on the nature and extent of radiation hazards in low Earth orbit. Generally speaking, such an experience results in a dose equivalent to 150 mSv in a 115-day Mir flight, much greater than the 2.0 mSv normally encountered annually in terrestrial environments. Data from the U.S. lunar exploration program suggest that a lunar round trip involves exposures an order of magnitude greater (50 mSv) than background. Current estimates indicate that in the absence of serious solar-flare activity, a mission to Mars equipped with the shielding used on lunar missions would involve an increase in dose equivalents of yet another order of magnitude at a minimum. Such an amount would be sufficient to induce radiation sickness in about 10 percent of exposed personnel and is within one order of magnitude of the level (10 Sv) at which no survivors would be expected from acute exposures. These simple statistics underscore the importance of developing alternative systems for protecting crew from radiation hazards. Such statistics do not assess the longer-term chronic problems associated with cellular damage. Of these, the most evident is the risk of cancer. This risk is compounded by the fact that spaceflight results in alterations of immune responses that may result in changes in resistance to neoplasia. Thus, an important area for further research concerns the interactions between microgravity, the immune system, and exposure to radiation (NRC, 2000~. The additional risks to organ systems, the central nervous system, and especially germ-line cells could be comparably severe and must be taken into account in any planning for human penetration into extraterrestrial spaces. This brief consideration of the estimated risks from the three sources of radiation clearly demonstrates that radiation poses an unacceptable risk to HEDS activities unless effective countermeasures are implemented. The countermeasures for limiting exposure to radiation fall into four categories: passive bulk shielding, electromag- netic shielding, electrostatic shielding, and chemical radioprotection. Much of the current emphasis in this area is on passive bulk shielding, but other systems, while more speculative, may deserve attention because they have the potential to offer alternative and perhaps superior protection. Combination systems, particularly those that can make use of rapid deployment when the precursor phase of solar flare activity is detected, would seem to have a high priority for further study. Passive Bulk Shielding The atmosphere on Earth provides considerable passive shielding. It has been estimated that 5 meters of Martian regolith will be required (Hepp et al., 1994) to provide radiation shielding on Mars comparable to that provided by the terrestrial atmosphere. Despite the common perception, due to their use in terrestrial applications, that materials like lead provide optimal shielding, lighter elements and their compounds are more effective passive shields for the HZE particles than are heavier elements. For instance, polyethylene sheet, with a surface density of 0.19 g/cm2, provides far better shielding efficiency per unit mass than do lead or other metals (Eckart, 1996~. Water is nearly as good. An important design consideration for both spacecraft and extraterrestrial habitats will be the integration of construction and other materials so as to optimize the use of the total mass for shielding purposes. Distributing resources such as drinking water over large surface areas can contribute materially to radiation protection. Electromagnetic Shielding Magnetic fields are very effective at deflecting energetic light-nuclei radiation but cannot deflect heavier particles with high kinetic energies. This property could make magnetic fields attractive for protecting against solar flare radiation but would limit their usefulness against galactic cosmic radiation (GCR). However, electro- magnetic shielding offers the additional possibility of dynamic control, which could be integrated with dosimetric monitoring of, for example, solar-flare activity. Dynamically controlled electromagnetic shielding subsystems might therefore be a very appropriate component in an integrated radiation protection system, because they could be used when the assault from light particles in solar flares is high. Issues that would need to be examined in a

64 MICROGRAVITY RESEARCH theoretical study of such a system include the field strength and power requirements and the effects of the field on onboard electrical systems and humans. Electrostatic Shielding Shielding provided by electrostatic fields apparently was extensively studied by the Soviet space program, but details are unavailable (Helmke, 1990~. This strategy is analogous to electromagnetic shielding; in this case, Coulomb forces are employed to deflect or retard incoming charged particles. Since GCR particles have such high kinetic energies, one can speculate that novel ways might be found to exploit this energy while at the same time diverting it from crew members. Of course, such a capability would require that the energy of the particles be captured by a shield material, such as plastic, and concentrated in particular regions of the spacecraft, which could itself result in a potentially dangerous electric field. Nevertheless, physical and chemical interconversion of various forms of radiation would seem to be one of the more interesting areas for novel research. Chemical Radioprotection Considerable effort has been devoted to the development of chemical scavengers of radiation. These com- pounds react with the by-products of ionizing radiation within the body, thereby limiting the effective dose exposure (Helmke, 1990~. Their effectiveness has been found to be significant. However, it seems likely that more remains to be done to optimize their performance. Radiochemistry and radiobiology therefore constitute fruitful areas of future research aimed at successively more sophisticated applications of chemical radioprotection. The compounds most often cited, including aminopropyl-aminoethylthiophosphoric acid (APAETF), are to be taken internally (Eckart, 1996~. Other such compounds may be developed in the future. There is an appreciable amount of information available on this subject, but there also are many unknowns. For instance, serious side effects have been seen with chronic use of some radioprotective compounds (NRC, 1996~. Much more needs to be learned if chemical radioprotection is to be applied with confidence in HEDS missions. There are immune-system concerns (one of the unknowns referred to above) in using chemicals to scavenge radiation. Otherwise, apart from the construction of shields, there are few if any reduced-gravity issues associated with radiation countermeasures. However, weight is a critical consideration when selecting a shielding material for spacecraft. Protection from Chemical and Biological Contamination The dispersal of contaminants to nonhazardous levels is facilitated on Earth by the atmosphere and oceans, which serve as large reservoirs for the dilution and removal of toxic materials. In HEDS environments, which involve essentially closed systems that rely on recirculation for life support, the long-term buildup of contaminants is potentially a much more severe problem. Chemical or biological agents produced at very low rates can, over time, concentrate to hazardous levels. Special systems are likely to be needed in HEDS habitats to deal with such threats. There are two problems associated with these two types of contamination: detection and control. Knowledge of the types of contaminants that can be of concern is needed to design suitable detection systems. Much knowledge of this kind is obtainable from existing experience in the space program and more should result from operation of the International Space Station. It will, however, also be necessary to give special consideration to each specific HEDS environment. For example, contaminants unique to the lunar or Martian habitats could become of major concern. Anticipation of these possibilities and improvement of more general all-purpose detectors could help to address these problems.

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE TABLE III.D. 1 Subsystems Found in Hazard Protection Systems and the Potential Impact of Microgravity on Their Operation 65 System Subsystem Fire Spill Meteorite Radiation Contamination Protection Cleanup Protection Shielding Protection Electrical fault detector Smoke detector Fire extinguisher Cleanup filter Manual cleanup device Meteorite shield Pressure loss detector L Bulk shield Electromagnetic shield Electrostatic shield Chemical radioprotector Contaminant detector Contaminant remover L H M L M H L M M L L L M L L L L L L H L M M L NOTE: The letters H. M, and L designate high, medium, and low (preliminary assessment) impact of reduced gravity on the operation of the subsystem. Where no letter is given, the subsystem is not applicable to the system listed. Contaminants can be controlled by use of systems for their removal, and such systems will need to be designed and installed for HEDS activities. Some such systems were discussed in Section III.C. Additional systems, unique to specific environments, should be addressed. There may or may not be reduced-gravity issues associated with such systems, depending on the methods employed (filtration, two-phase flow, etch. Gravity effects will have to be considered on a case-by-case basis. Consideration can be given also to more-general removal procedures that do not require the detection or identification of specific contaminants. Summary of the Effect of Reduced Gravity on Hazard Protection Systems Table III.D.1 lists the subsystems that make up the hazard protection systems and indicates which ones may be affected by gravity levels, as discussed in the preceding sections. The degree of commonality of subsystems among the systems is also evident. As can be seen, the expected impact of gravity level on the phenomena taking place in the subsystems is appreciable in a substantial number of cases. References Eckart, P. 1996. Spaceflight Life Support and Biospherics. Torrance, Calif.: Microcosm Press, and Dordrecht, Netherlands: Kluwer Academic Publishers. Friedman, R. 1998. Fire safety in extraterrestrial environments. Pp. 210-217 in Space 98: Proceedings of the Sixth International Conference and Exposition on Engineering, Construction, and Operations in Space, April 26-30, Albuquerque. R.G. Galloway and S. Lokaj, eds. Reston, Va.: American Society of Civil Engineers. Helmke, C. 1990. Synopsis of Soviet manned spaceflight radiation protection program. USAF Foreign Technology Bulletin, FTD-2660P-127/ 105-90. Hepp, A.F., G.A. Landis, and C.P. Kubiak. 1994. A chemical approach to carbon dioxide utilization on Mars. Pp. 799-818 in Resources of Near-Earth Space. J. Lewis, M.S. Matthews, and M.L. Guerrieri, eds. Tucson and London: University of Arizona Press. National Research Council (NRC), Space Studies Board. 1996. Radiation Hazards to Crews of Interplanetary Missions: Biological Issues and Research Strategies. Washington, D.C.: National Academy Press. NRC, Space Studies Board. 2000. Review of NASA's Biomedical Research Program. Washington, D.C.: National Academy Press, in press.

66 MICROGRAVITY RESEARCH Ross, H.D. 1996. Combustion processes and applications in reduced gravity. Pp. 527-532 in Space V: Proceedings of the Fifth International Conference on Space '96, Albuquerque, June 1-6, Vol. 1. S.W. Johnson, ed. New York: American Society of Civil Engineers. Wilson, J.W., F.A. Cucinotta, J.L. Shinn, M.H. Kim, and F.F. Badavi. 1997. Shielding strategies for human space exploration. Shielding Strategies for Human Space Exploration: A Workshop, Chapter 1. J.W. Wilson, J. Miller, and A. Konradi, eds. Washington, D.C.: National Aeronautics and Space Administration. III.E MATERIALS PRODUCTION AND STORAGE Introduction Two distinct ranges of gravity level will be encountered most frequently in space exploration. One is generally referred to as microgravity (g < 0.01 go), and the other is denoted as fractional gravity (0.01 go < g < lgo). Microgravity, for example, is experienced on board spacecraft, on the International Space Station, and on the satellites of Mars, Phobos, and Deimos (which could be a source of raw materials for a colony on Mars). In operations on the Moon and Mars, one encounters fractional gravity: 0.169 go on the Moon and 0.38 go on Mars. While these levels would cause some problems in directly applying many terrestrial processes to life sustenance on these bodies, in many cases the harsh local environments provide far greater challenges than those presented by the reduction in gravity. The lunar surface is characterized by a hard vacuum with a silty, fine-grained sand regolith in a cold ambient with long days and nights (each is about 14 Earth days). The atmosphere of Mars is composed almost entirely of carbon dioxide at a pressure of 7 mbar, and although its surface temperature may reach 25 °C at the equator in mid-summer, it is generally much colder, with large temperature swings. In addition, dust storms occur about 100 days out of almost every year, with the dust consisting of ~5 ,um particles. The processing of local resources to obtain various materials will be crucial to space exploration. This in situ resource utilization (ISRU) would include the beneficiation of regolith and the extraction of essential materials such as water, oxygen, and fuels by chemical processing. The production of hydrogen and oxygen by electrolysis of water is a critical enabling technology for long-duration life support systems and for a wide range of ISRU applications on a majority of planetary bodies found in the solar system. Water electrolysis has a critical depen- dency on gravity since the generated gaseous products require liquid-vapor separation, a process that fails at zero gravity. In addition, electrolysis can be used to produce oxygen from carbon dioxide using a solid electrolyte and to extract oxygen from lunar regolith where it is combined with metallic elements, primarily as silicates. While the gases produced (oxygen, hydrogen) can be liquefied for convenient storage, at g = 0 severe problems are encoun- tered in the storage and transfer of cryogenic fluids. Although water for the electrolysis process is potentially available from indigenous permafrost, other promis- ing chemical processes that produce water as a by-product are reviewed briefly in this section. These include extraction of carbon dioxide from the Mars ambient, which could be reacted with hydrogen to form methane and water (Sabatier process) or to form carbon dioxide and water (reverse water gas shift). Except for the critical dependence of the electrolysis step on gravity level, these processes would be little affected in gravity. Other processes that are more speculative but potentially useful (and relatively independent of gravity) are pyrolysis, radio-frequency processing, and volatilization/condensation. The last-mentioned process could be an important step in obtaining water from lunar permafrost or ice frozen under a hard vacuum, where the vapor released by heating would have to be collected by an appropriate cold trap. Because of the weight-driven costs associated with transporting material from Earth's surface into space, substantial savings can be realized by using materials that are already out of Earth' s gravity well. In some cases, the ability to exploit in situ resources for the production of consumables such as rocket propellant, breathable air, and water could be the enabling technology in terms of keeping launch mass and mission costs within realistic limits, and current NASA planning for human missions to Mars relies heavily on utilization of local resources (NASA, 1997~. Furthermore, as HEDS missions expand outward from Earth, the case for exploiting useful resources from other low-gravity bodies, such as specific asteroids (including dead comets) and planetary satel- lites, that are convenient to an extraterrestrial base can become a very logical element in the overall program

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 67 evolution, provided that the microgravity environment has been mastered. Regardless of the extraterrestrial source, the key leveraging element is the ability to substitute mass acquired from outside of any deep gravity well (relative to the specific base) for mass that otherwise would come from Earth. Fortunately, there is a symmetry between (1) the opportunity to substitute relatively high-mass, low-value in situ units, such as radiation shields, construction elements, and consumables, for Earth launch mass and (2) the need to acquire low-mass, high-value units, such as digital devices and sophisticated instruments and machines, from Earth. The substitution of power- generation equipment for return propellant mass represents the fundamental difference between a touch-and-go exploration strategy and the establishment of outposts that can eventually support unlimited human stays. The development of the technology required to exploit extraterrestrial resources in the reduced-gravity environments, where the greatest leveraging exists, will ultimately enable the expansion of human presence in space. Indeed, material-processing stations distributed throughout the solar system will enable resupply and thus extend our ability to explore the far reaches of space. The surface environments associated with resource-rich extraterrestrial bodies offer many challenges. The Moon and Mars both have reduced-gravity environments, widely varying local surface temperatures, and surface pressures ranging from a hard vacuum on the Moon to (approximately) water' s triple-point pressure at Mars. Their surfaces have relatively severe dust problems, associated on the Moon with the abrasive character of very fine lunar dust and, on Mars, with the planetwide dust storms that are observed with some regularity. The two best-characterized extraterrestrial resources at this time are the Martian atmosphere and the lunar regolith. The processing of the Martian atmosphere into oxygen (and its subsequent storage) is understood sufficiently well at this time to serve as the most probable first step toward exploiting extraterrestrial resources. However, if humans are going to be protected and sustained, it will be necessary to construct human shelters almost immediately upon arrival at Mars. As noted in a previous section, neither Mars nor the Moon has magnetic fields or atmospheres that are sufficient to protect humans from the harmful radiation that is produced intermit- tently by solar flares and storms. Because the Martian atmosphere cannot be processed readily into materials that can be used for radiation shields or shelters, there is a strong need to develop site preparation and mining capabilities. These needed capabilities will range from the ability to place soil material over spacecraft modules (to serve as radiation shields) to site preparation for the deployment of surface power generating and distribution equipment. Those operations would logically use material mined from the regolith. For lunar operations, regolith must be collected, transported and processed for the basic site preparation, radiation shelter construction, and hardware deployment steps associated with any type of human outpost con- struction. It would then be logical to develop the ability to manufacture basic construction units, such as bricks, beams, and blocks, and to consider the processing of lunar material into oxygen and metals. The possible extraction of helium-3 from lunar fines has also been the subject of a great deal of research. The reduced gravity, vacuum conditions, and temperature extremes encountered on the Moon affect all of these processes. A similar set of problems will be encountered on other planetary bodies, which have not yet been characterized as accurately as the Moon. Before such a wide range of surface operations can be carried out, a better understanding of the behavior of soil in reduced gravity environments will be needed. The geotechnical aspects of extraterrestrial soils not only control their load-bearing capacity but also affect their ability to support the forces that will be required for anchoring and/or excavating regolith. These granular materials are known to exhibit surprising levels of compac- tion on the Moon and to produce a kind of a hard crusty surface layer (called duracrust) on Mars. Furthermore, their behavior affects the geophysics of the planetary body. Successful exploration of extraterrestrial regions, a primary HEDS goal, requires the ability to produce useful mass outside of Earth's gravity well. It is not logical to bring along all of the consumables needed for propulsion and life support on round-trip missions with characteristic times measured in years. Site preparation, material processing, habitat construction, mining, and infrastructure development in a robust HEDS program must evolve rapidly away from dependence on terrestrial systems that have been transported from Earth.

68 MICROGRAVITY RESEARCH Mining Terrestrial mining equipment is characterized by a heavy rugged design, and its use is both labor- and energy- ~ntensive. This terrestrial equipment could not be directly incorporated into space activities even if the transpor- tation costs were acceptable. Such factors as reduced traction at lower gravity, the high level of abrasive dust in the ambient, and operation at low temperatures in vacuum, where common lubricants fail by evaporation, preclude the direct use of conventional mining equipment. A mining operation requires planning and integrating an entire range of procedures and techniques such as beneficiation, material handling, secondary recoveries (e.g., of ores and fuels), and storage, as well as support programs such as habitat construction and the development of transpor- tation modes. In addition, there is a need for new components and tools such as the vibratory bulldozer and penetrator studied by Szabo et al. (1994) and Nathan et al. (1992~. These show a very significant increase in efficiency, typically a greater than 50 percent decrease in the force needed to advance the bulldozer. Similar studies in static angering (Klosky, 1997) showed, however, that no decrease in torque could be obtained by similar vibration of the auger. The mining of bedrock, which adds several difficult steps to the ordinary mining process, has been discussed by DeLa'O et al. (1990~. First, the bedrock must be penetrated, and then it must be broken into pieces that can be hauled to a site for comminution or pulverization. Because of numerous factors discussed in detail by Chamber- lain et al. (1993) including heavy, bulky equipment designs unsuited to the Moon's low gravity, vacuum, and temperature fluctuations terrestrial blasting and mechanical mining techniques would not usually succeed in the lunar environment. In addition, just a few centimeters below the surface, the soil is extremely dense, even denser than can be obtained with heavy conventional compaction equipment on Earth (Carrier and Mitchell, 1990~. Open pit mining on Earth uses about 0.14 kg of conventional explosives per ton of ore (Wempfen,1973~. The transport of large quantities of such hazardous material aboard spacecraft is not practical. Moreover, it is not clear that the terrestrial experience of such blasting is applicable at reduced gravity or that the distribution of explosives to achieve a desired consequence is known. For example, the explosive could generate shock waves that would cause extensive damage to previously constructed facilities in the habitat. In addition, safe haven considerations for the operator during explosions have not been evaluated. In spite of these problems with bedrock mining, there are several advantages in pursuing this goal. For example, as the lunar base matures and the facilities become more sophisticated, underground mining into the bedrock may be desirable for some applications such as providing a comfortable, secure, pressurized habitat where one can move and work freely. Also, as discussed elsewhere, energy and processing efficiencies may be obtained by mining bedrock to recover oxygen and material for building block formation. There has been promising work on the use of electromagnetic energy from, for example, carbon dioxide lasers, microwave, or solar sources to produce thermal stresses or local melting that could effectively fracture the bedrock (Lindroth and Podnieks, 1998~. During the pulverization and comminution processes, safety considerations require monitoring and control of all yield gases and small particulates in the ambient. Even if the same terrestrial process for pulverization could be applied at reduced gravity, the process on Earth is both inefficient and energy-intensive (NRC, 1981~. It is possible that all early habitat and mining operations will be limited to processing lunar regolith. Similarly, little is known of the Martian subsoil, and early ISRU activities will involve processing the regolith. One of the potential substances to be mined is water ice, which would be a valuable and essential resource; the extraction problems for this case are discussed in the following pages. In conclusion, the reduced gravity issues relevant to mining are the design of mining equipment for excavat- ing, bulldozing, transport, etc. at reduced gravity (or low traction conditions) and ejecta management during these operations. The processing and refining of regolith involve mainly problems of material handling, transport, and chemical separations and are discussed later in this section. Volatilization/Condensation Water, oxygen, and bound carbon and hydrogen molecules are available in such abundance on Earth that it is

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 69 difficult to think of them as resources worthy of mining operations. However, hydrogen, oxygen, and carbon molecules are the primary building blocks for virtually all consumable materials needed for HEDS missions. Acquiring large quantities of those molecules outside Earth's gravitational field is a crucial step in the early deployment of HEDS systems. Water ice is known to be present on most of the planets. Hydrogen and helium have been implanted by the solar wind in the surface layers of unshielded planetary bodies such as Mercury and the Moon. The helium-3 implanted in the surface layer of the Moon is sufficiently valuable for use in fusion reactors, that its acquisition and transport to Earth could become a viable industry. Most of the other volatile molecules are available throughout the solar system, either as ices or as adsorbed molecules in the surface materials of planetary bodies. Hence, a key HEDS mining goal will be to extract and collect volatile molecules. The obvious approach is to volatilize these molecules in situ and collect them either by capturing them in a container or by condensing them on a cold surface for subsequent collection. These processes are influenced strongly by gravitation and by the particular ambient pressure and temperature environment. Because of the contrast between extracting adsorbed volatiles from small particles such as lunar fines and extracting water from extraterrestrial ice, this discussion will examine those two cases as a basis for identifying the microgravity research issues. Lunar Volatiles Adsorbed volatiles are found in the surface layer of lunar fines, with the highest concentrations on the Sun- facing side. In addition, the recent Clementine and Lunar Prospector missions have provided conclusive data showing that significant quantities of bound hydrogen (probably water ice) are deposited in the Moon's perma- nently shadowed polar craters. While more definitive data on the distribution and availability of lunar water ice is needed, it is likely that water ice will be a key resource on the Moon. However, since water extraction at a lunar site requires processing hardware that is similar to the water extraction hardware that would be required on a Galilean satellite or on many other low-gravity surfaces, that discussion is deferred to the next section. Fegley and Swindle (1993) have reported on the relative abundance of lunar solar-wind-implanted molecules and on their thermal release when heated. Helium and arsenic, with abundances of 14 ppm and 1 ppm, respectively (the abundance of helium-3 was 4.2 x 10-9), were released almost completely from lunar fine samples when heated to 1200 °C. Similar release efficiencies were obtained for chemically reactive hydrogen (50 ppm), carbon (125 ppm), and nitrogen (100 ppm) over the same thermal range. It should be noted that Apollo samples showed that the volatiles were bound primarily to particles 20,um in diameter or smaller, and the Apollo missions indicated that the soil layer that was rich in volatiles was confined nominally to the first 30 cm. The capture and release of these volatiles is a significant technical challenge. Owing to the low concentrations of volatiles (in the parts per million range), large quantities of lunar soil must be processed in order to collect useful amounts of volatile material. Even if processing takes place only during daylight hours, the energy required to heat lunar fines to 1200 °C is significant, so it would be desirable to segregate the lunar material containing significant volatiles prior to heating. Because the lunar surface pressure is on the order of 10-9 tort, it will not be practical to heat the lunar fines in an unconfined space, trying to collect the volatile molecules as they are liberated. Batch processing of segregated lunar fines in sealed containers would be desirable, particularly if energy could be recovered after the volatiles are released. Heat recovery from low-conductivity granular material in lunar gravity is not understood sufficiently to permit engineering designs. The value of helium-3 warrants special consideration if it can be separated from helium-4 (after the helium has been separated from the other volatiles). If volatile recovery systems can be operated in the permanently shaded polar crater regions, where undisturbed surface temperatures can approach 4 K, helium separation and helium-3 recovery would be greatly enhanced. However, at those temperatures, major efforts will be required to prevent locally generated thermal pollution from destroying the environment and the associated resources. The subtle influences of lunar gravity on these heat and mass transfer processes must be understood fully before any sort of processing base can be designed.

70 Extraction of Water Ice on Low-Gravity Surfaces MICROGRAVITY RESEARCH Material Handling and Transport Material handling requires the design of lightweight, temperature- and dust-insensitive equipment such as bulldozers, bucket scoops, cranes, winches, conveyer belts, and trucks that can operate at reduced gravity over a large ambient temperature excursion and in a very abrasive, fine, silty soil that could extend to depths of between 3 and 10 meters. As discussed by Chamberlain et al. (1993), the key factors in the design of material-handling equipment include simplicity, flexibility, robustness, light weight, low energy consumption, potential for automa- tion and teleoperation, tribology considerations (e.g., lubrication of bearings and seals), and ability to operate at low and wildly fluctuating temperatures, in a vacuum and/or at low pressure, and often in a very abrasive and dusty atmosphere. An additional consideration is the potential for bursts of electromagnetic radiation and particle bombardment that may be hazardous to humans and to equipment components such as electronic circuits. Excavation equipment selection has been discussed by Matsumoto et al. (1990), by Okumura et al. (1998), and in an early study by Dalton and Hohmann (1972), but without an obvious choice of equipment. Given the wide

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 71 range of environmental constraints that must be accommodated, new designs for material handling equipment, such as the vibratory bulldozer and penetrator (Szabo et al., 1994; Nathan et al., 1992) discussed previously, will have to be considered. To minimize the expenditure of energy, it is desirable to carefully select the material to be processed; processing requires the ability to collect and transfer material to a primary power node. Material handling can be facilitated by incorporating local resources into the operational planning. For example, it has been suggested that after a lunar base has been established, a railroad constructed from indigenous materials by teleoperated robots could be developed to provide surface transportation for expansion of human activities to other areas of the Moon (Schrunk et al., 1998~. Although an ambitious undertaking, a lunar railroad would allow traveling on the silty, dusty lunar regolith without penetrating into the abrasive soil, and it could provide faster transport than robots or other individual surface vehicles. If a truck-type hauling vehicle were used to transport regolith to a processing area, traction and support in the soft regolith terrain could be provided by large belts connecting the drive wheels, as in an armored military tank. The handling and transport of irregularly sized granular material at reduced gravity require further studies, because frictional contact between particle surfaces depends on the normal forces between particles, and hence on gravity. The phenomenological behavior of granular materials as it affects the construction of habitats and roads and the utilization of resources is discussed in Section IV.G. Concentration and Beneficiation of Feedstock Many methods of beneficiation of ores are used on Earth, including screening, filtering, flotation, differential sedimentation or settling, leeching, and electrostatic/magnetic separation. Of these, electrostatic and magnetic separation are judged to have potential for the beneficiation of lunar regolith. A particularly important character- istic is that they do not involve the use of scarce water resources. Electrostatic and Magnetic Beneficiation Magnetic beneficiation methods are based on differences in the magnetic susceptibility of the components of an ore. If one component has a higher magnetic susceptibility than the other components, application of a magnetic field to a stream of ore particles may be used to deflect and concentrate this component. The suscepti- bility of lunarilmenite is approximately 7.6 x 10-5 cgs mass units, which is too low to allow for efficient magnetic beneficiation of the lunar soil for ilmenite. By contrast, ferromagnetic agglutinates (e.g., particles containing iron) of the lunar soil may be separated magnetically from the soil; this is desirable prior to the electrostatic treatment of the soil for ilmenite concentration. Electrostatic beneficiation methods are based on the differential charge acquired by the components of an ore via contact charging, or by induction when grounded (on a slide or drum) and subject to an applied electric field, or by exposure to an ionizing electrode. An electric field then separates the particles during free fall, depending on their acquired charge. Electrostatic beneficiation is used to separate futile and ilmenite from zircon and monazite in plants in the United States and Australia that process heavy beach sand (Fraas, 1962; Kelly and Spottiswood, 1982~. The input to an electrostatic or a magnetic beneficiation system is a stream of particles, which first may require the comminution of the raw ore (e.g., lunar regolith). The system consists of particle handling components such as hoppers, feed belts, and accumulation bins; in an electrostatic system an electrostatic generator and deflecting electrodes are required, whereas in a magnetic system deflecting magnets are required. Reduced gravity levels will generally increase the difficulties of handling a particulate stream, including the generally diminished flow out of hoppers. A specific possible negative effect of reduced gravity levels on the beneficiation process would be a reduction in the force acting to separate the particulate stream from the charging drum or belt into the free-fall zone, where deflection and enrichment occur. Under these conditions, some additional measures to ensure separation might be used, such as a directed blower. A benefit resulting from the reduced gravity level would be an increase in the residence time and hence in the deflection of particles in a free-

72 MICROGRAVITY RESEARCH fall zone of fixed design. Otherwise, no serious issues due to the gravity level are expected, although there may be serious issues related to controlling the electric charge of particles in a hard vacuum. Atmosphere Acquisition (and Compression) Systems This discussion focuses on the problem of acquiring and compressing Martian atmospheric gases (95 percent carbon dioxide) for processing in a reactor to produce propellant and/or oxygen for life support. Carbon dioxide is a resource that is available everywhere on the Martian surface, and the processing steps are simple. Since the ambient atmospheric pressure on Mars is approximately 5 tort, compression by a factor of approximately 100 or more is required to reduce the volume of gas to be processed to a reasonable level. In principle, this could be achieved with mechanical compressors using two or three stages. Assuming that the first-stage dust filter is sufficiently rugged, this approach has the advantage of allowing dust filtration to be accomplished between stages using mechanical filters with reasonable pressure drops. The disadvantage would be the system's large energy requirement and the wear on its moving parts, especially that due to ambient dust. An alternative is a sorption pump (Rapp, 1998; Ash et al., 1978~. Such a pump consists of a bed of absorbing material such as a zeolite that is exposed to the Martian atmosphere at night, resulting in the absorption of gas from the atmosphere into the zeolite. During the day the zeolite bed is cut off from the atmosphere and heated while interfaced with the reactor. The gas absorbed during the night is expelled at the higher temperature and pressure to the reactor. The diurnal cycle of Mars is about 24 h. The data reviewed by Rapp (1998) indicate a compression ratio of 136:1 between 6 torr and 815 tort, with a temperature swing from 200 K to 450 K. The net release of gas is estimated to be 0.11 gram of carbon dioxide for 1 gram of zeolite. The main problems to be addressed in operating these systems are (1) preventing dust accumulation in the bed and other components and (2) ensuring efficient heat exchange during the heating and cooling parts of the cycle. Dust is a major and ubiquitous problem on Mars, where the surface environment exhibits very different electrical and chemical behavior than Earth (see Kolecki and Hillard, 1992, for example). Passive filtration with mechanical filters seems very unlikely to be successful for the sorption compressor under consideration because of the low pressure drops available during the collection part of the cycle. Dust removal technology is discussed in Landis et al. (1997) with regard to solar cells; a favored proposed method is electrostatic dust precipitation, which may be an option for the atmosphere collection system described here. The design of an efficient heat exchange system for cooling at night and heating in the daytime that uses vacuum jackets, thermal isolators, and thermal switches is discussed in Rapp (1998~. It should be emphasized that because the absorption of gas during the night is exothermic, active cooling is necessary to prevent the bed from heating up. Other components include heat exchangers, temperature-controlled valves, and pipes and valves. At the Martian gravity level of 0.36 go, none of the processes discussed above are expected to be greatly affected compared with operation at 1 go. Some changes may occur, however, in the heat exchange equipment that involves multiphase flow, as well as in the operation of packed zeolite beds in an adsorption pump and in the management of dust. Filtration Removal of particulate from fluids is for life support, protection of fluid-processing hardware, and material separation and processing. Filters typically incorporate a porous solid matrix of some type through which fluid flows and on which particles of a specific size range are captured. The simplest filtration mechanism is called straining and works literally like a strainer, trapping particles larger than the size of the strainer holes. Straining can only be considered in space applications where very low particle loadings (particles per unit volume) are encountered or where the particles themselves are being collected for a specific use. Otherwise, since the filter becomes clogged by trapped particles, that type of system cannot be used for unattended long-duration operation. In this sense, removal and capture of the strained particles in a space environment is a problem. Terrestrial filter systems use a variety of other mechanisms to capture and remove particles from fluid streams (Hestroni, 1982; see, specifically, Cooper, 1982~. Many of the filter mechanisms require that particles be captured

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 73 on impact with a filter fiber or filter pore. Aerodynamic or inertial capture is effected when particle mass (and shape) produce flight trajectories that cannot follow the sharp turns required of the flowing fluid during transit through the filter matrix, resulting in particle capture on filter surfaces. Diffusion or Brownian motion capture occurs in filters with very low fluid velocities relative to characteristic filter passage lengths (and pore diameters), where particle transit times are so long that the particles contact and attach to the filter material, as a result of random motion, before egress. These types of filters fail or become ineffective when too many flow passages are blocked owing to particle buildup or because the filter material loses its ability to trap particles that reach its surface. Sloughing can also occur for these systems when the flowing fluid exerts aerodynamic forces on captured particles that are sufficient to pull them away from the filter surface. These mechanisms are virtually unaffected by gravity. Electromagnetic forces can also be used in filter systems. Electrostatic fiber capture occurs when an electric field is maintained by the filter system such that the electric field lines converge on the fibers of the filter, resulting in electrophoretic capture of particles as the nonconducting, particle-laden fluid flows through the matrix. Electro- static precipitation utilizes corona- or glow-discharge electric fields, generally across discrete electrodes, to charge particles as they are convected through the field. The charged particles are captured subsequently, either by attachment to one of the electrodes or by accumulation in a controlled region of the flow. Coulomb-type electric capture results when the particles to be collected carry a charge that induces an opposite charge on filter surfaces (fibers), thereby producing an attractive force that is strong enough to pull the charged particles out of the flowing fluid and attach them to the filter. Generally, the electromagnetic forces exerted by these systems greatly exceed gravitational forces, but the removal and collection of the particles captured and collected by these systems is affected by gravity. Both the particle capture method and the filter design depend strongly on the particle size distribution and loading. Typical terrestrial particle size ranges are presented in Figure III.E. 1, where it is noted that particle sizes encountered on Earth, and hence particle sizes that could be encountered within a spacecraft environment, span eight orders of magnitude. Associated with the various particle sizes are the specific particle capture methods that apply to the various size ranges, as represented in Figure III.E.2. Particles smaller than 0.1 ,um tend to remain suspended in Earth's atmosphere. That particle size will likely behave similarly in Mars's atmosphere (and gravity). It is noted that particle sizes between 0.005 and 2 ,um can act as nucleation sites in the formation of droplets and crystals (Hudson and Squires, 1973), so that great care may be required in certain types of scientific experiments if control of nucleation is necessary. For biological systems, filtration of particles of sizes between 0.01 and 0.2 ,um is required to remove viruses, whereas bacteria are filtered at sizes slightly larger than viruses but small than 10 ,um in diameter. Filtration of particles from liquids is for the most part insensitive to gravity, and cloth and screen filters can be used to strain particles as small as 10 ,um diameter from liquids. Exceptions are sedimentation separation, which obviously is controlled by gravity, and the foam and bubble fractionation technique for removing submicron-size particles, which is also influenced strongly by gravity. Since many of the small particle filtration systems in terrestrial laboratories utilize centrifuges and ultracentrifuges (for particle sizes in the 0.01 to 100 ,um range), gravity is not a factor. Particles smaller than 0.1 ,um can be removed from liquids using diffusion processes such as reverse osmosis and dialysis or by ion exchange or electrodialysis (Freeman, 1982~. In the size range of these processes, particle masses are so small relative to surface area that terrestrial gravitational influences can be neglected all together. Dust management in extraterrestrial environments deserves special consideration. Not only are settling processes controlled by interactions between atmospheric gases and local gravitation, but the process of producing new dust is also a concern. The creation of dust by particle impacts with a surface is called saltation, and that process is controlled by gravitation. Saltation has been studied for Mars atmospheric processes because of concerns related to planetwide dust storms (Greeley et al., 1980~. Furthermore, as was observed in videos of the Apollo Moon Rovers, short-lived dust clouds are created by moving wheels, even in near-perfect vacuum condi- tions. Dust removal from lunar surfaces is a major design issue, whereas dust filtration from artificial lunar environments and filtration of the Martian atmosphere present more subtle research issues. Even though the Martian atmosphere is dusty and planetwide dust storms lasting on the order of 100 days occur nearly once every

74 0.001 0.0t 0.1 1 10 ,.: ', ~ VFASr-CELLS CAMBRIDGE ABSOLUTE FILTERS APPROACH 99.97% EFFICIENCY IN THIS RANGE , ., ~ , . ,, I I , i I I i ,, _ , .: I ' DIA - ETER OF ' I ~ , ~ JUAN HAIR I ' | j POLLEN '; ~ .~. I . I j, ,, . _ `~-SPORES I, . PAGING- PA R T-C L ES : ' ; `. . ~ . ~ . | SE7TLl~G-AT~OS.-l~PUR. I, i, F9ME5 1 1 ~ ~ i+. | ~ ~ I ! I%~Y A5H ~ ~ I I A, l Of ~ ! i ~ - + i 'ii'l.!!4, 1 i 11'i -l,', ,,,I,,, ~ ; I1J-~.-nA , t'!~!'! ~°LD,si! 8AtTER", I I, of, t. 11, jet'., ! : lI !i 1' HEAVY INDt . i', i1l,1,,'1 , _ 1~ A. l 0 ABSOLUTE FILTERS ARE OVER 99.97% EFFICIENT l ON THIS RANGE OF PARTICLES J ~ 60 80 94 100% Hi-Flo 95 & Aeropac 95 70 9095100°10 Aeropac 90 EFFECTIVE ~~ 50 El0 90 i00% Hi-Flo 85 RANGES OF POPULAR ~ 0~ CAMBRIDGE fIl_TERS ~ 50 70 9,0 105~ lo HUFlo 50 ~ 65, Aeropac 60 50 70 90 100% Hi-Cap = Th1IS REPRESENTS A 10 MICRON DIAMETER PARTICLE, THE SMALLEST / SIZE VISIBLE WITH THE HUMAN EYE. |* THIS DIMENSION REPRESENTS THE DIAMETER OF ~ HU - AN HAIR, 1 - MICRONS >| THIS REPRESENTS A 0 3 MICRON DIAMETER PARTICLE, ABSOLUTE FILTERS P'EMOYE OVER 99.97a/0 OF THIS SIZE. 1 MICRON - 1 11dIICROIIdIETER - 1 IIIILLIONTH OF A DIETER FIGURE III.E.1 Sizes and characteristics of atmospheric contaminants. SOURCE: Cambridge Filter, Syracuse, New York. Mars year, Martian dust particles are thought to be smaller than 5 microns and claylike, with particle loadings on the order of one particle per cubic centimeter. The Martian atmosphere would not be considered to be dust-laden by terrestrial standards, but because of its low atmospheric density and the possibility that dust particles are either very abrasive or contain adsorbed volatile molecules (which could poison or otherwise damage chemical process- ing equipment) dust filtration is still necessary. Away from Earth, it is not necessary to maintain environmental pressures of 1 bar for systems that do not need direct human interaction. In particular, manipulation of Mars's atmosphere can be accomplished in ambient pressures of about 5 torn Filtration of particles from uncompressed environments of this type requires designed filters with virtually no pressure drop, because even small pressure drops translate to very large increases in the volume of gas that must be moved through processing equipment. At very low operating pressures like Mars ambient, mechanical equipment must have larger dimensions to accommo- date the high volume flow rate needed to achieve the necessary mass flow rate. This in turn increases energy consumption. Although the flow behavior is not influenced strongly by gravitation, the removal of dust from low- pressure-drop filter systems is dependent on the gravitational environment. The research issues associated with filtration in a microgravity environment depend on the particular type of filter system and the range of particle sizes to be filtered. In general, the influence of gravity on particle agglomeration with respect to filter flow rates and surface interaction processes must also be understood. How

75 E L~ 1 O: C~ _ t., ~ e: CL a 8000 6000 4000 2000 1000 800 600 400 200 100 80 60 40 20 10 8 6 4 0.8 0.6 0.4 0.2 0. 0.01 o.ool _ u' cn = 4 6 8 10 20 60 100 150 200 250 325 500 000 _ L~ ~r LL c`: ~c -a I C O ~ ¢ _ -; C C'' _ ~Z_ ~` CO OCI)= ~ —1~ LL] C'—O =:—~ = (n <: ~ . ~ ~n CONTAM I NANT AND DUST COLLECTOR TYPES _ t I I ~ - r. LL (n 0.001 0.01 0.1 0 100 1000 IN 1 m': ' L~ J m,=o ~o~ =) Z 0.0006 0.005 0.134 4.2 134 4240 1.3XlC5 _ 4.2 X 106 1.3XlO8 4.2X109 1.3 x 1oll , 4.2 XlOlZ i3xlo'~ 4.2 X 1015 _ 1.3xlol' L8J C~ L~ ~ 7.0 X 10~8 1.4X 10-7 4.2 X 10-7 l.3xlo6 4.2 X 10.6 1.3 X 10-5 4~2 X 10-5 1.3X10-4 l 4.2 X 10-4 1,3X 10-3 4.2 X 10-3 1,3X 10-2 4.2X10-2 1.3 x 1o- 4.2 x 10 q: ~ L~ cr: c j= Z J ~E~ UD 9.7 6.9 4.0 3.1 0.29 0_02g 2.9 x 10- 3.1 X 10-4 3 4 X 10-5 4 4X 106 l l 7~7 X 10-7 l O o l . O O LAWS OF SETTLING (Lines of demarcation are approximate) ,o cr cr = co LLI LLI o<:, - ~: cl I =0 ~LL L`J ~ CC C/) Z C,~ O ~ Z LL L~ cn IL ~ o3 m ~n 0 LLI ~ n~co ~y | N EWTONS | LAW 1 c= | Kl ~; :0 K1 ~ 0.0045 for spheres, I ~ 0.009 for | irregular | particles STOKES LAW D'g (S.—S2) '= 1o~2 18 = 2.93 x 10-5 D2 ~ for 21"C I air | S, = 10' kgym3 | CUNNINGHAM I -STO K ES LAW | C' = C{1 + K\/D) K - 2.5 x 106 C = STOKES LAW C BROWNIAN MOVEM ENT A= ~l 97 3 t_ SYM BO LS C = Velocity of fall, m/s | D_ particle diameter, ,~` m g:: gravitational acceleration, 9.81 m/s2 = particle density, kg/ m3 I S2- gas I density, | kg/ m3 1 (S2 <' S') 1 77= viscosity, | NS/m2 1 ~ = 1.86 X 105 | for air at 21 ~C) I A—mean I free path | of gas | molecule, 1 ~ 6.5 x 15-' m | A—distance (m) | particle I moves ~n | time t, sec | R = gas constant 1 = 8.314 | J / ~ K gmol | T temperature, ~K N—6 023 x 1023 I (number of | molecules! | gmol ) FIGURE III.E.2 Sizes and characteristics of atmospheric solids. NOTE: The values for particle surface, settling rate, and number and area per cubic meter of air are based on the following: particle specific gravity = 1 (density = 1000 kg/m3~; mass concentration = 70 ,ug/m3, typical of urban concentrations; particles are smooth spheres, all of equal size; and gas is air, with a density of 1.29 kg/m3, temperature is 21 °C, pressure is 1 atm, and viscosity is 1.86 x 10-5 kglmls. SOURCE: Reprinted with permission from AAF International.

76 MICROGRAVITY RESEARCH different gravity levels influence sloughing must be understood. Development of filter cleaning systems that can be operated in microgravity for long-duration missions is an important issue. Microgravity issues associated with filter recycling include (1) threshold gravity levels required for particle removal from filter surfaces, (2) the effectiveness of scraping and shaking processes for cleaning filters in microgravity environments, and (3) how to capture and remove the dust accumulations resulting from filter cleaning operations. Furthermore, the effect of the intermittent and random acceleration vector orientations that characterize typical spacecraft operations must be understood in terms of how filter systems will be affected during long-duration missions. Fluid-Based Chemical Processing Electrochemical Processing Water Electrolysis Production of oxygen from recycled water for life support during long-duration spaceflights is a key element In enabling manned missions to Mars and beyond. Production of oxygen for rocket propellant and for life support, using water extracted from a variety of ice-containing planetary bodies, will probably be a key step enabling HEDS missions to be extended throughout the solar system. Water electrolysis can be used to produce not only oxygen but also hydrogen for use as rocket fuel and/or as a feed gas for making a variety of hydrocarbons. The wide range of extraterrestrial water sources, coupled with the need for oxygen and hydrogen molecules in space operations, makes the development of ultrareliable, autonomous water electrolysis systems a pacing technology for the HEDS enterprise. Water electrolysis is energy-intensive because electric energy is used directly to break the atomic bonds between water's hydrogen and oxygen atoms (5.3 kW-h per kg). The electric power required to sustain the desired hydrogen and oxygen production rates is high enough to cause problems with electrode corrosion and electrode life that have not yet been overcome. Reduced gravity will alter the gas-liquid interface behavior associated with the production of gaseous oxygen and hydrogen in lunar and Martian electrolysis systems, but those effects should be scalable. However, the design and operation of water electrolysis cells on board spacecraft or on very-low- gravity surfaces, such as on ice-containing asteroids or on inactive comet cores, will be determined by the microgravity environment and will require fundamental knowledge of multiphase processes in these microgravity environments to achieve necessary levels of system reliability and autonomy (see Humphries et al., 1991~. Be- cause the electrochemical production of oxygen (and hydrogen) from water in microgravity environments is of critical importance to a range of HEDS missions, a hardware demonstration program on the International Space Station for the purpose of establishing the necessary reliability levels and for developing fully autonomous systems would be fully justified. Even though water electrolysis units have been operated intermittently on Mir since 1989, producing more than 700 kg of oxygen sufficient for approximately 1,000 man-days of life support the systems have not demonstrated enough reliability to be used as Mir's primary oxygen supply (Belaventsev et al., 1991~. Further- more, because of its explosive potential, generated hydrogen must be removed completely from the system. Belaventsev et al. (1991) reported that hydrogen mixtures had relatively low ignition energies for concentrations (in Mir gravitational environments) ranging from 4.1 to 96 percent. Over the past 20 years, NASA engineers have studied static feed water electrolysis (SFWE) and solid polymer water electrolysis (SPWE) system designs; both were considered for life support use on the International Space Station, but neither system was selected. More recently, a water vapor electrolysis system has been studied that is based on operating principles similar to the SFWE but that operates directly on humid cabin air rather than by utilizing a separate water supply (Wydeven, 1988~. The SFWE system is represented schematically in Figure III.E.3 (Wood, 1992; Figure 8 in Wydeven, 1988~. The SFWE reactor uses static pressure to supply liquid water to a water feed matrix, made up of thin asbestos sheets saturated with a hydroscopic aqueous potassium hydroxide solution. The cathode and anode cell matrix elements are configured similarly. The hydroxide ions provided by the potassium hydroxide in the cell matrix act

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE Product H2 Product O2 Liquid Water ~1 1 Water Vapor e e DC Power Supply Electrodes /4H2O J 4e ~ 2H2 + 40H 40H ~O2+2H2O+4e ~ Asbestos Matrices FIGURE III.E.3 Static feed water electrolysis cell. SOURCE: Wood (19921. Courtesy of NASA. 77 catalytically on the anode and cathode reactions shown in Figure III.E.3. As electrolysis occurs, the potassium hydroxide concentration is increased across the cell elements, creating a water-vapor pressure gradient and draw- ing additional water from the feed matrix into the cell matrix. Because the SFWE electrodes operate directly on water vapor, the cell compartments thus avoid two-phase flows, transferring the two-phase management problem back to the (liquid) water feed unit. The SPWE reactor utilizes perfluorinated sulfonic acid polymer membranes, which act as electrodes when wetted. The system is shown schematically in Figure III.E.4. The SPWE half reactions at the cathode and anode differ from the SFWE reactions since protons, rather than hydroxide ions, diffuse through the membrane and act catalytically in the anode and cathode reactions shown in Figure III.E.4. Deionized water remains in contact with the cathode, generating sufficient heat to require cooling. The anode remains exposed only to the vapor phase, which results in the production of almost pure oxygen. Hydrogen must be removed from the water vapor in the cathode chamber. Both types of water electrolysis are influenced strongly by gravity level. So-called water vapor electrolysis operates directly on cabin air and serves both to dehumidify the air and to produce hydrogen and oxygen. Moist air enters the electrolysis compartment, where oxygen is produced and released at the anode; the generated hydrogen ions diffuse to the cathode, where they recombine with electrons to produce molecular hydrogen, which is vented. Use of moist air as a water source eliminates the liquid-vapor separation problems associated with the SFWE water supply. Phase separation problems abound in these systems. First, water vapor humidifies the generated hydrogen and oxygen gases. Because the electrolysis cells are operated typically at temperatures above 300 K, humidity levels can become quite high. The water vapor in the generated gases must be removed if the gases are to be stored cryogenically, in order to avoid ice formation in critical valve and flow-management elements. Second, because of its explosive potential, hydrogen removal is critical in all systems. It is very difficult to prevent hydrogen from leaking across membranes or diffusing through other containment barriers. In microgravity environments, the absence of significant buoyant forces makes it very difficult to monitor, collect, and/or remove unwanted hydrogen from gaseous volumes in the separation units. Third, dissolved gases will be present in the water that is being

78 MICROGRAVITY RESEARCH Product He t Liquid Water Product O2 DC Power Supply Electrodes _ 4H++ 4e ~ 2H2 2H2O ~ O2 ~ 4H++ 4e Solid Polymer Electrolyte FIGURE III.E.4 Solid polymer water electrolysis cell. SOURCE: Wydeven (1988~. Courtesy of NASA. electrolyzed and it will be necessary to prevent gas buildup, even if the gases are inert, in order to prevent the electrochemical cells from becoming "polarized." The separation and control of dissolved gases in liquids in microgravity environments are key elements in the development of water electrolysis systems in support of HEDS. Since vapor-phase electrolysis units rely on pressurized liquid water feeds for supply, they also require liquid-vapor phase management studies in microgravity environments in order to achieve the large production rates, over long periods of time, required for HEDS missions. It will also be necessary to develop instrumentation for autonomous operation and control. Many of the autonomous control and operation problems that must be resolved before water electrolysis can actually become a key building block technology in the overall HEDS program can be addressed through long-duration testing of autonomous water electrolysis units on the International Space Station. The significance of electrochemical production of hydrogen and oxygen is potentially so great that a long-term operational program can be justified. Gas Phase Electrochemical Extraction A variety of solid and molten electrolytes have been used as fuel cells, many of them in space. These devices, in addition to their ability to produce and store electrical energy (Sridhar and Foerstner, 1998), could also serve as important chemical processing units for the HEDS program. Fuel cells can use electrical power to extract hydrogen, oxygen, carbonate, or hydroxyl ions from mixtures of other atoms and molecules. Thus they can be modified for use as electrochemical processors, but the microgravity issues for fuel cell applications are essentially the same. Hence, it is more efficient to describe the operation of solid-electrolyte oxygen-extraction systems, which are currently scheduled to be demonstrated on the Mars 2001 mission, in terms of possible reduced gravity issues, noting that these devices can be used as fuel cells.

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE Solid Electrolyte Electrolysis: Extraction of Oxygen from Carbon Dioxide Space-based extraction of ionic molecules from gaseous extraterrestrial materials allows avoiding 79 the need for phase separations ant' the concomitant gravitational Influences on chemical separation processes. The electrolyte barriers employed in these separation systems are, however, only capable of conducting specific ions usually at rather high temperatures. The systems being considered here act like reverse fuel cells, consuming electrical energy to remove and then pump a specific ion across a solid membrane for collection. Because of their ability to extract oxygen directly from the Martian atmosphere for near-term space missions, solid electrolytes that conduct oxygen ions but offer very high electronic resistance are presently the most thoroughly studied systems for extraterrestrial resource processing. This approach was first suggested by Stancati et al. (1979) and evaluated experimentally by Richter (1981~. More recently, Ramohalli and Sridhar (1991) and Sridhar and Vaniman (1995) have reported on advanced systems similar to the system that would have been operated on the Mars 2001 mission (now cancelled). Recent reviews of solid-electrolyte separation systems for Mars applications have been reported by Hepp et al. (1994) and Rapp (1998~. Most of those systems utilize zirconia (ZrO2), stabilized with yttria (Y2O3), but a variety of other dopants have also been used. Stabilized zirconia maintains a doped cubic form of the material so that it can carry oxygen ions when subjected either to a voltage or an oxygen concentration gradient. Solid electrolytes other than zirconia have been considered for ionic oxygen conduction as well, but stabilized zirconia currently appears to be the best choice as it exhibits satisfactory ionic conduction at temperatures above 850 °C and excellent ionic conductivity at 1000 °C. Reduced gravity will influence these systems in subtle ways. Systems needs relating to using solid electro- lytes for the extraction of oxygen from carbon dioxide include the following: · Systems designed so that they can survive vibration loads, prevent leaks of critical importance, and incorporate adequate instrumentation and control. Because of the high-temperature operation and probable ther- mal cycling, mechanical behavior will be affected by gravity; · Systems designed so as to minimize mass and volume; · Systems designed so that they can tolerate thermal shock and thermal fatigue associated with temperature variations ranging from Mars ambient through steady-state cell operation. The Martian thermal environment controls the thermal shock conditions, and to some extent thermal shock is influenced by gravity; · Development of electrodes that promote oxygen production on the feedstock side of the membrane, maintain good electrical contact with the electrolyte, and provide controlled voltage and current distributions across the electrolyte with minimum Joule heat losses. Gravitational concerns must be addressed; · Development of hardware elements that minimize differential thermal expansion between components and that have seal systems able to withstand repeated thermal cycles without losing their integrity; · Development of multiple cell designs that can incorporate flow path redundancies and distributed instru- mentation and control and that assure proper operation of all of the active cell units; · Development of oxygen separation units that can withstand exposure to sulfur compounds that may be present in Martian dust; · Development of system designs that are self-cleaning and that have some ability to modify or repair themselves in situ; · Development of systems that operate at maximum conversion efficiency using the least amounts of electri- cal and thermal energy; and · Development of systems that can be used intermittently and that are capable of providing electrical power when operated in reverse (see Sridhar and Foerstner, 1998~. The principal solid electrolyte processing elements are the following: feedgas filter system, carbon dioxide concentrator and/or compressor, regenerative heat exchanger, feedgas heater, electrochemical cell array (oxygen extraction and compression), exhaust gas return (through heat exchanger), radiator, oxygen cryocooler, and oxy- gen storage system.

80 Molten Metal Electrolysis Lunar Magma Electrolysis MICROGRAVITY RESEARCH Production of oxygen and metals from lunar rock and regolith is a more ambitious form of electrochemical processing. Beck (1992) has described a lunar oxygen plant design employing a pair of electrodes immersed in a pool of molten silicate produced from molten lunar rock and regolith. Using direct current, oxygen would be produced at the anode and various metals at the cathode. Systems of this type are difficult to operate reliably on Earth, and fundamental understanding of the physical chemistry pertaining to the entire system is needed, particu- larly when the basic unit operations are altered by reduced gravity. The rate at which molten mass flows to the electrodes is a very important performance and design parameter. If mass transport is slow, say by diffusion, then electrolysis at a given cell voltage will be slow and larger cell _ _ _ ., ... . . . , . , . , in. . . .. . . . .. .. .. voltages will be needed to increase reaction rates. Buoyancy-dr~ven convection Is a very desirable alternative mechanism for mass transport, and viscosity is an important operating parameter. In addition, sedimentation and buoyancy are important for product separation. These are highly gravity-dependent. The effects of gas bubbles, the porosity of the electrodes, and simultaneous electrolyte reactions are compli- cating factors. Potential and flux variations affect the distribution of current on various electrode shapes via ionic conduction, which is controlled by the boundary conditions imposed by kinetics and mass transport. Reduced gravity is undoubtedly an important controlling variable in the design of molten metal electrochemical processes. For example, a comparison between terrestrial and lunar operation of a copper refining cell predicts that 57 percent more electrode area would be required for lunar operation (Beck, 1992~. Beck has also described concepts for systems to produce iron from basalt magma, silicon and aluminum from anorthite, and iron from iron-rich regolith. For the case of iron production from basalt magma, the metal will sink to the bottom of the melt, where it will be trapped. In the fused anorthite electrolysis, aluminum silicon alloy floats to the top, where it will be trapped. Both are continuous processes. The production of iron via regolith electrolysis is a batch process. At startup of the cycle for a given batch, the in-place regolith is melted (e.g., by a solar concentrator) before the lid of the vessel and the anode are put in place. After electrolysis starts, more regolith is added to the vessel. The top of the cathode grows by settling of molten iron, and the anode is moved up to accommodate the growth. After some time, the electrolysis is terminated, material is removed, and the cell is reloaded for the next cycle. Production of Aluminum from Orbital Debris Recovery of aluminum from orbital debris is the only known HEDS application of molten metal electrolysis that can be considered seriously at the present time: because it is possible to collect feedstock of a prescribed composition, the chemical processing requirements can be known accurately enough to pursue system designs. Processing could take place in Earth orbit with the retrieval of specific types of debris, such as nonfunctioning satellites or specific classes of spent upper stages. The hundreds of tons of aluminum orbiting Earth as spent boosters are a potentially attractive source of aluminum for solar-powered satellites and other orbital applications. In addition, the recycling of this material would reduce the hazards it poses for orbiting satellites. The recycling of orbiting aluminum would require solving problems of materials management in microgravity, including those associated with the separation and segregation of raw materials and the transport of nonuniform solid and liquid materials, including multiphase fluids containing dissolved gases. Many of the research issues associated with developing technology for recovering aluminum are similar to those encountered with technologies for welding in microgravity, which is discussed elsewhere in this chapter. Radio-Frequency Processing of Materials Radio-frequency (RF) processing of materials is a rapidly expanding segment of the semiconductor industry. By proper design of electrode geometries, processing temperatures, and pressures and by optimization of appropri-

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 81 ate RF frequencies and voltages, it is possible to process gaseous molecules and particle suspensions (in gases) selectively and efficiently for a variety of terrestrial applications. RF processing of the Martian atmosphere, discussed below, has been identified only recently as a potential means of selectively dissociating the carbon dioxide into carbon monoxide and oxygen at low processing temperatures and at Mars ambient pressures (Vuskovic et al., 1997, 1998~. The RF-based glow discharge is preferred to direct current discharges (Wu et al., 1996) because it requires far less power to sustain the plasma. RF processing of molecules is an approach that can be used selectively to avoid high-temperature processes. Furthermore, because of the electrophoretic forces that can be produced and controlled by these systems, it is possible to use RF-generated forces to separate and or manipu- late solid particles, particularly in gravitational fields. RF Processing of the Martian Atmosphere RF processing of the Martian atmosphere has many similarities to the well-developed research field associ- ated with maintaining proper concentrations of carbon dioxide in CO2 lasers (Byron and Apter, 1992), except that the CO2 dissociation in RF-excited gas mixtures needs to be maximized rather than minimized. Hence, it is not surprising that the lower pressure of the Martian atmosphere (as compared with the pressure in laser cavities) is favorable to CO2 dissociation because it reduces the potential for three-body recombination processes and also because the plasma-generated electron densities are higher, thus promoting increased interactions with CO2 molecules. Although the volume fraction of carbon dioxide in the Martian atmosphere is much higher than in CO2 laser cavities, the small concentrations of argon and nitrogen (in the Martian atmosphere) do serve as buffer gases, which sustain the discharge and provide electrons for CO2 dissociation. RF processing systems can be designed to effect almost complete dissociation of Martian atmospheric carbon dioxide to carbon monoxide and oxygen. Systems can be designed that utilize natural convection as the prime mover for transporting the Martian atmosphere through the RF field and through an oxygen extraction unit. If such systems are properly designed, Mars dust can be collected and removed during the RF processing step and the (dust filtered) exhaust gas from the oxygen extraction unit can be collected and used as a carbon monoxide fuel. The average gas temperature rise associated with this type of RF processing is only about 100 °C (or less), which is too small to create large convective flows under Martian conditions, and so the reduced-gravity environment becomes a major design consideration, affecting dust particle collection and removal and the flow of the feed gas through the processing unit. Since the exhaust stream produced in the RF processing/oxygen extraction system cannot be captured and stored by utilizing natural convection, an oxidizer/fuel production system requires blowers and/or compressors to collect and compress the processed Mars exhaust (fuel). Blowers and compressors are especially vulnerable to dust damage by abrasion and from the clogging of small passages. Hence, removal of dust from the Martian atmosphere becomes an even more important requirement, and the RF approach relies completely on detailed knowledge of the influence of reduced gravity on particle behavior. The RF plasma resides in a gas mixture that is no longer in thermodynamic equilibrium since the processing is a continuous flow process and the entering Martian atmosphere molecules are relatively cold. The flow behavior of this nonuniform mixture and the associated oxygen transport to the collector surfaces can be influ- enced by the altered buoyancy effects in Mars's gravity. Separation or Filtration of Solid Particles (Dusty from RF-Processed Gas Streams The RF plasma will interact directly with any particles that happen to be suspended in a gas stream, because the RF field will increase particle surface charges by several orders of magnitude. However, because the plasma is sustained by a very high frequency, alternating voltage field, particles are not collected on electrode surfaces, as happens with conventional electrostatic precipitators. Rather, they become trapped in specific volumes of the RF field. The electrostatic, ion drag, thermophoretic, and aerodynamic forces acting on dust particles act in different directions, with different relative magnitudes, depending on particle composition, size, shape, and density. Thus, when the RF-derived forces are combined with Mars gravity forces, the net force acting on each dust particle

82 MICROGRAVITY RESEARCH propels it to a particular spatial equilibrium location, depending on its physical characteristics and the feed gas flow rate. In fact, because of relative differences between the different forces acting on different size particles, the particles will be segregated by size and, to some extent, by their properties. In the case of an inductive RF-plasma system, the smallest particles are segregated in the vicinity of the centerline of the RF field, whereas larger particles become banned in annular zones bounding the electrode sheath. These dynamic bans will become ~ _ ~ ~ _ _ a ,, _ ~ . . . . . . . . . . . ... . . . . . . . saturated with dust particles at some point, and it will be necessary to stop the plasma long enough to permit the particle clouds to be removed either by gravity or by mechanical means. Based on current limited knowledge of Martian atmospheric dust loads, it is likely that dust removal would need to occur once each Mars sol (24.66 h). Oxygen Production Oxygen requirements for life support and for most liquid chemical rocket propellants are so large for long- duration human missions that the ability to produce oxygen mass away from Earth's surface could enable near- term HEDS missions to be commissioned for planetary objects virtually across the solar system. Furthermore, when oxygen is produced from simple molecules such as from carbon dioxide and from water, the chemical processing steps are simple. Advanced life support systems and ISRU processing systems differ primarily in the scale of the hardware (Sridhar and Miller, 1994) and in the fact that life support systems will have to operate primarily in microgravity (during space travel) rather than in the fractional gravity associated with most ISRU processing sites. Hence, a discussion of direct chemical processing emphasizing oxygen production is justified. Sabatier Reactors Recovery of oxygen from carbon dioxide and water in microgravity environments has been an active area of research since the beginning of human spaceflight. The Sabatier process (Sabatier and Senderens, 1902) for transforming carbon dioxide into oxygen and methane has been one of the most thoroughly studied approaches for developing advanced life support systems. That process reacts hydrogen with carbon dioxide: CO2+4H2 - CH4+2H2O- (III.1) The water is captured and pumped to an electrolysis unit, while the gaseous methane is collected and either vented or stored. Hydrogen and oxygen are generated by electrolysis of the water, and the generated gases are filtered dry. The desired oxygen is stored for life support, while the hydrogen is returned as feedstock for further processing with the carbon dioxide. Hamilton Standard Corporation and NASA have been evaluating systems of this type more recently (see, for example, Cusick, 1974, and Martin et al.,1983, or Sullivan et al., 1995) for advanced space station life support and for extraterrestrial resource processing. A variety of resource processing options are possible because Sabatier reactions other than reaction [11 can be activated by using different catalysts and processing temperatures. At temperatures near 1100 K, Seglin (1975) reported that it is possible to employ transition element catalysts (iron, cobalt, nickel, ruthenium, palladium, osmium, indium, and platinum) or silver to promote the following chemical reactions: CO + 3 H2 ~ CH4 + H2O and CO + H2O ~ H2 + CO2- (III.2) (III.3) In addition, reactions such as the complete reduction of carbon monoxide to solid carbon and subsequent formation of carbon dioxide, as well as the hydrogenation of solid carbon to form methane, are possible but must be avoided because of difficulties in handling solid carbon, particularly in a microgravity environment. Reactions [11 and [2], both producing methane and water as products, are the primary reactions for ISRU systems. Although

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 83 water electrolysis is considered to be an off-the-shelf technology, reliable operation of a liquid water electrolysis unit in a microgravity environment has not yet been demonstrated. Zubrin et al. (1991) proposed and demonstrated an ISRU preprototype system for Mars that utilized Sabatier reaction (1) in a tube reactor filled with catalyst to produce methane and water in a highly exothermic process. The methane product was collected and stored as cryogenic liquid rocket fuel that could be used for a Mars return mission, while the water product was deionized and electrolyzed into hydrogen and oxygen. Subsequently the hydrogen was recycled back into the Sabatier reactor and the oxygen was collected and stored as a liquid cryogen to be used either for life support or for return propellant. Zubrin et al. (1991) proposed transporting hydrogen from Earth, thus avoiding the need for an in situ water requirement at Mars, which can only be avoided otherwise by transporting methane from Earth. More recently, Mueller (1998) reported an inability to identify hydrogen storage designs that could provide quantities of hydrogen, during the period between Earth launch and landing on the surface of Mars, sufficient to enable the round trip mission. However, Rapp (1998) reports that Zubrin et al. (1991) have recommended that only half of the required oxygen (and all of the methane) be produced via Sabatier, with the remaining oxygen needs being met using either a solid electrolyte carbon dioxide processor or a reactor employing reverse water gas shift, thus reducing the hydrogen mass required from Earth. The major processing elements for a Mars-based system are a hydrogen storage and supply system or a water extraction system (from polar ice, permafrost, or other sources, including the atmosphere), an atmospheric filter, a carbon dioxide adsorption/desorption compressor or a Mars atmosphere compressor, a catalyst bed reactor and thermal management system, regenerative heat exchangers, a radiator and water collector, a methane-water sepa- rator, a methane dryer, a water collection and storage unit, a water electrolysis unit, hydrogen and oxygen dryers, methane and oxygen precooler~s), oxygen storage, methane storage, and cryogenic refrigerators). The following critical questions remain to be answered by further research: · At reduced gravity, will the gas bubbles at the electrolysis electrodes separate into collectable volumes of gas es they do atg= 1 go? · Is the maximum 7 mbar pressure difference between Mars ambient and the absorber sufficient to transport the atmospheric carbon dioxide into the sorption pump? · Will the conventional zeolite adsorption material require periodic bakeout, can it recycle indefinitely without changing its properties, and will it be subject to poisoning by constituents of the Martian atmosphere? How does the reduced gravity influence the matrix when subjected to the Mars temperature-pressure cycle? Reverse Water Gas Shift Oxygen can be produced from carbon dioxide via the reverse water gas shift (RWGS), which consists of the following reaction pair: 2 CO2 + 2 H2 ~ 2 CO + 2 H2O and 2 H2O + electricity ~ 2 H2 + O2. These reactions are the reverse of the industrial reaction that combines carbon monoxide and water to produce hydrogen. Since the reverse water gas shift reaction utilizes water electrolysis in connection with a carbon dioxide (and hydrogen) feedstock, it is very similar to the Sabatier electrolysis process. The primary differences are that they use different catalysts and operate at different temperatures. Below 400 °C, the Sabatier reactions, producing methane and water, dominate. Above 650 °C, the water gas shift reactions, producing carbon monoxide and water, dominate. Because RWGS produces carbon monoxide as a product in carbon dioxide reduction, it offers the potential to exploit a variety of other hydrogenation reactions (Fischer-Tropsch reactions) for ISRU. Therefore, the reverse water gas shift approach offers the possibility of producing such chemicals as methanol and dimethyl ether (CH3OCH3), which in turn opens up the possibility of producing other hydrocarbons and plastic (Zubrin et al., 1997~. The system consists of a catalyst bed reactor, with a copper on alumina catalyst, that combines

84 MICROGRAVITY RESEARCH hydrogen and carbon dioxide in an exothermic reaction at temperatures on the order of 400 °C. For oxygen production, the reactor exhaust is cooled and the water is condensed and removed. When terrestrial hydrogen is used, it is necessary to dry the carbon monoxide in order to minimize hydrogen loss. The collected water is transported subsequently to an electrolysis unit. Microgravity issues associated with operating reverse water gas shift systems include the condensation and separation of liquid water from a gas stream and the operation of liquid water electrolysis systems. In addition, the opportunity to utilize carbon monoxide, methane, and hydrogen in a Fischer-Tropsch synthesis gas reactor to produce more complex hydrocarbons opens up a variety of issues associated with the extraction and processing of specific chemicals for more advanced material production systems in a reduced-gravity environment, which will surely alter the unit operations. A Mars-based system using RWGS for oxygen production can incorporate a hydrogen storage and supply system or a system for extracting water (from polar ice, permafrost, or other sources, including the atmosphere), an atmospheric filter, a carbon dioxide adsorption/desorption compressor or a Mars atmosphere compressor, a cata- lyst bed reactor and thermal management system, regenerative heat exchangers, a radiator and water collector, a water condenser, a carbon monoxide dryer and exhaust, a water electrolysis unit, an oxygen dryer, radiators, an oxygen liquefaction system, an oxygen storage system, and cryogenic refrigerators). Ilmenite Reduction as a Source of Oxygen One of the more promising processes for the production of oxygen on the Moon is the use of hydrogen to reduce ilmenite (Zhao and Shadman, 1993; Gibson et al., 1990; Taylor et al., 1993), an oxide of iron and titanium (FeTiO3) present in the lunar regolith. The concentration of ilmenite in the lunar regolith varies but is thought to be the highest in the mare (basaltic) regolith, where it is believed to reach 5 to 10 percent by volume. Surface mining the 2 to 5 meter thick mare regolith is considered to be the most promising source of ilmenite. Mining and beneficiation methods are discussed by Vaniman and Heiken (1990), Sharp et al. (1990), and above in this report. Electrostatic separation techniques have been proposed by Agosto (1985~. The remaining discussion here is focused on the reduction process to obtain oxygen. The hydrogen reduction process is described by the following reaction, FeTiO3 + H2 ~ Fe +TiO2 + H2O, in which hydrogen reduces the ilmenite to yield iron, titania, and water. The water is then electrolyzed to obtain the desired oxygen, and the hydrogen is recycled. Metallic iron is a by-product; further reduction of the titania is difficult and not usually considered. The reaction is slightly endothermic, absorbing 9.7 kcal/gram mole at 900 °C, with a partial pressure equilibrium ratio PH2O/PH2 of about 0.1. The overall yield of oxygen from the ilmenite is about 10 percent by weight, which means that about 90 percent of the reduced feedstock must be removed as solids on a continuing basis. Figure III.E.5 shows a schematic of a plant developed by the Carbotek Company under contract with NASA. The process utilizes a fluidized-bed reactor and a solid-state electrolysis cell that electrolyzes water in the vapor state. A stream of hydrogen is forced through a fluidized bed of ilmenite particulate at about 900 °C to achieve a reasonable reaction rate; higher temperatures tend to sinter the particulate and reduce the porosity. In the Carbotek process, the exiting gas is passed through a solid-state electrolysis cell that decomposes the water vapor into oxygen, which is collected, and hydrogen, which is recycled through the bed. If all hydrogen reacted according to the ilmenite reduction equation and was recovered subsequently during the electrolysis step, it could be reused indefinitely without the need for additional hydrogen. In practice, however, there are small losses, principally from the unreacted hydrogen entrained in the spent feedstock. Most of the unreacted hydrogen waste could be recovered in a batch processing operation by vacuum pumping sealed dis- charge hoppers, but a need for makeup hydrogen would remain. The main components of the plant are shown in Figure III.E.5. The ilmenite, stored in a hopper, is conveyed

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE COLD LMENITE FEED | / ' 1 / "] \ REACTION BED SOLDS QUENCH I GAS PREHEAT B ~ ~ o ~ o ~ C' - SOLDS PREHEAT / GAS COOLING BED COOLED ~ ~ ~C Y CLE H. 2 At/—'\ BLOWER / SOLIDS ~ OVERFLOW | _ ELECTRA ? MAKE-UP ~ HOT _~ ~1:~.,.' 1 1 _ SPENT, REACTEDi SOLIDS O2TO LIQ'N ~ STORAGE I 1 ELECTROLYSIS POWER SOLID STATE ELECTROLYSIS CELL LOCK HOPPER / VACUUM PUMP SYSTEM TO REMOVE INTER- STiTIAL H2FROM SPENT SOLDS 85 NOTE: CYCLONES AND POSSIBLY OTHER GAS- SOLDS SEPARATORS ARE ALSO REQUIRED BUT NOT SHOWN. FIGURE III.E.5 Continuous, fluid-bed ilmenite reduction/O2 production. SOURCE: Gibson and Knudsen (1990~. Reprinted with permission from the American Society of Civil Engineers. by a belt to the fluidized bed, where it reacts with the hydrogen coming up through the bed. The product gas containing water exits through the top of the fluidized bed and enters the solid-state electrolytic cell, where the oxygen is bled off to cryogenic storage and the hydrogen is recycled through the bed. The spent ore is discharged alternately into one of two hoppers, which may be locked and pumped out to recover adsorbed hydrogen. The components most affected by reduced gravity would be the fluidized-bed reactor (Gibson et al., 1990; Ness et al., 1990) and cyclone separators, if present. Fluidized beds are proposed not only for the ilmenite reduction reaction but also are considered desirable for the removal of contaminants such as hydrogen sulfide in the gas stream. A model two-dimensional, fluidized-bed test article has been operated in a KC-135 NASA research airplane experiment at 1/6 gO (Gibson et al., 1990) and found to operate predictably. Additional tests related to design optimization would be desirable for the reactor and cyclone separators. There appear to be no serious microgravity issues associated with the solid electrolyte process, but a number of design problems with gas-phase electrolysis cells (principally zirconia) remain (Rapp, 19981. Liquid-state electrolysis, on the other hand, is well developed and extensively used on submarines in Earth gravity. It is not clear, however, how well these or similar units would operate at reduced gravity. Thus, which form of electrolysis can ultimately be used is an open question. Pyrolysis Pyrolysis of lunar regolith has been proposed as another approach for large-scale oxygen production on the moon (Senior, 19931. It is possible to produce oxygen by vaporizing metal (or semiconductor) oxides. By heating lunar regolith sufficiently, some of its metal oxides reduce to other oxides, liberating oxygen in the process. That liberation of oxygen in a reducing environment is similar to terrestrial pyrolysis processes, except that liberated oxygen rather than pyrolyzed solid is the desired product on the Moon. This approach is very energy-intensive but it avoids reliance on nonabundant molecules containing hydrogen or carbon. After the regolith has been pyrolyzed the processed material is cooled in a condensation step to remove the waste, the metals, and/or the reduced oxides.

86 MICROGRAVITY RESEARCH Subsequently, the liberated oxygen is recovered. One example of a regolith pyrolysis reaction is the reduction of silicon dioxide: SiO2 (solid) ~ SiO (metastable solid) + i/^ O^. , ~ ~ Another reaction that can be sustained is the thermal reduction of ferrous oxide, where FeO (liquid) ~ FeO (gas) FeO (gas) ~ Fe (gas) + 1/2 O2 (gas). Two variations of this process have been proposed for obtaining oxygen. One is called vapor separation, or thermal pyrolysis. The other is called selective ionization, or plasma pyrolysis. In the former, material is heated to about 2000 K. The metal species, or reduced oxides, are condensed from the hot gases, leaving gaseous oxygen. In the plasma pyrolysis process, the vapor is heated to very high temperatures (approaching 10,000 K). The plasma thus generated is passed through an electrostatic field in which the ionized metals are separated from the neutral oxygen. Little work has been done on the plasma process, and extensive study would be required to determine its viability. Vapor pyrolysis oxygen production rates depend on heat and mass transport in the liquid, evaporation and oxygen dissociation, mass transport in the gas, and condensation of metal-containing species. Thermodynamic and transport properties of the molten feed material are critical to process design. Experimental and theoretical work would be needed on both the vaporization and condensation processes. Molten material that is being reduced will have to be contained and, since the temperatures are likely to be very high, selection of construction materials will be an important problem. Avoidance of tap plugging in the removal of waste must also be considered. Because of the abundance of solar energy on the Moon and the absence of an atmosphere, it is possible to sustain very high temperatures by solar means. Hence vapor pyrolysis could be competitive with other oxygen production systems that do not rely on water as their feedstock. The unit operations associated with this process will be altered by reduced lunar gravity, as will the manipulation and transport of lunar materials. Cryogenic Storage Reduced-gravity cryogenic fluid management issues fall into two distinct areas depending on whether a fractional gravity or microgravity environment is being considered. It is anticipated that the technology used to efficiently store cryogenic fluids terrestrially should transfer with few difficulties to the gravity on the Moon and Mars, where the generally much lower ambient temperature will reduce the problem of heat flux. However, in space, where the effective gravity is small, the location of the liquid in a tank is determined by the competing effects of gravity and the liquid' s surface tension in order to minimize the sum of the gravitational and surface energies (Dodge, 1990), and the storage problems are much more complex. Reynolds and Satterlee (1966) showed that, in the absence of gravity, the liquid will form one or more interfaces of constant spherical curvature, with interface radii determined by the container geometry and size, the volume of liquid, and the contact angle at which the interfaces meet at the container wall (this angle is near zero degrees for most liquids and container materials of aerospace interest). They also show that the most stable configuration of liquid minimizes the total capillary energy, so that the liquid will collect in a single volume and the gas will form a single bubble that is attached to the walls at a definite location. The introduction of gravity with perturbing accelerations such as "jitter or brief engine firings for attitude control would negate these Predictions of stable distributions of contained liquid. ~7 ~ Depending on the details of the fluid's properties and previous force cycles, the fluid could be smoothly spread over the container walls or dispersed in the container as a collection of liquid globs or perhaps in many other configurations. The position of the liquid in a container is an important factor in the process of transferring

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 87 the liquid out of the container into another container, e.g., in the very essential process of resupplying the International Space Station with propellants and liquids from a supply ship. Since, in general, some of the liquid could be dispersed as drops or globs in random parts of the container, the gauging of liquid quantity, both contained and transferred, presents new, important problems at g = 0. Surface tension of the stored liquid is an important force at g = 0, and the associated capillary forces have been used to design many liquid acquisition devices that use fine mesh screens or similar porous materials to control fluid motion in a container (Dodge, 1990~. When wet with the stored liquid, they can withstand a pressure differential from the gas side to the liquid side. The maximum possible differential is a function of the mesh opening dimension and the surface tension of the liquid. The pressure required to force fluid through the mesh openings is just the pressure exerted in a gas bubble and is proportional to the surface tension and inversely proportional to the diameter. Thus, until the fluid exceeds this pressure, it is separated from the vapor. This design principle permits the important operation of venting vapor to reduce the system pressure while saving the stored liquid. ~ . ~ Again, taking advantage of surface tension in the wetting of the liquid to the container walls, the fluid location relative to an outlet can be controlled effectively by using appropriate vanes (Dodge, 1990), which have been shown to provide stable, nonturbulent flow during container filling at g = 0.9 Excellent single-phase fluid control can be obtained with metal bellows tanks or collapsible flexible bags with a single outlet port, where the tank shrinks as fluid is drawn, so that the tank remains completely filled with fluid (no vapor) and where flow is regulated by pressure exerted on the exterior of the tank (Dodge and Kana, 1989~. In the absence of gravity there are no convection currents in the stored liquid, so that when heat is transferred into the tank through the container wall, a significant portion of the liquid can become superheated, even at very low heat fluxes. A superheated liquid represents a metastable condition that is ultimately susceptible to explosive boiling or flashing, causing pressure spikes of varying magnitude in a closed system. The system design must take into consideration the pressure rise rate of stored fluids and be capable of containing these pressure surges. Ring baffles around the container walls can minimize sloshing since the baffles interfere with the free up-and- down motion of the slosh waves. Although at Earth gravity, go, the height of the contained fluid is minimized, at zero gravity the surface tension minimizes the area of free surface. During sloshing at go, fluid displacement is resisted, but at g = 0 surface tension resists the creation of more surface area. The specific microgravity issues are related to the design of equipment for storing, transferring, and control- ling large quantities of cryogenic liquids, or more specifically, the problems encountered in transferring liquid from supply ships to the International Space Station storage tanks. At g = 0, the fluid location in a storage tank is not a priori determined, so that movement of fluid from one container to another using conventional transfer tubes may not be effective. Since some fraction of the cryogenic fluid vaporizes during transfer, it must be ensured that no liquid escapes when a vent is opened to relieve pressure. To successfully accomplish such a transfer, it is necessary to develop methods to measure the quantity of cryogenic fluid stored in the receiver tanks and monitor this during the transfer. NASA has long recognized the problem of liquid containment and management in microgravity (Reynolds and Satterlee, 1966) and had planned a systematic, 5-year study of liquid dynamics at g = 0 in a project designated the Cryogenic On-Orbit Liquid Depot-Storage, Acquisition, and Transfer satellite, or COLD-SAT for short (Dodge and Kana, 1989), but the project was never completed. Summary of the Effect of Reduced Gravity on Selected Subsystems Summarized in Table III.E. 1 are the various subsystems and components discussed so far in this section and the various materials processing and storage systems in which they are found. For each of the subsystems in a 9Hasan, M.M. Cryogenic fluid management for ISS and HEDS missions. Presentation to the Committee on Microgravity Research, October 14, 1997, Washington, D.C.

88 MICROGRAVITY RESEARCH TABLE III.E. 1 Subsystems Found in Materials Production and Storage Systems and the Potential Impact of Reduced Gravity on Their Operation System Direct Atmospheric Chemical Electromagnetic Subsystem Acquisition Extraction Beneficiation Filtration Blower pump Compressor Condenser Desiccator Dust filters Electric current source Electrostatic generator Electrostatic/magnetic separator Fluidized-bed reactor Furnace or heater Gas-ionizing electrode Gravity collection bins Heat exchanger (counterflow) High-temperature crucible Hopper Liquid/matrix electrolytic cell Particle feed systems (conveyors) Pipes (multiphase) Radiator Refrigerated tank Rotating drum charging unit Solid electrodes Solid-state electrolytic cell Vacuum pump Valves Zeolite adsorption bed L L _ _ _ L L L _ H _ L _ H H H H H L L L L L L L L _ NOTE: The letters H. M, and L designate high, medium, and low (preliminary assessment) impact of reduced gravity on the operation of the subsystem. Where no letter is given, the subsystem is not applicable to the system listed. given system, the impact of reduced gravity on the operation of these subsystems is estimated as high, medium, or low (little or no impact). It should always be kept in mind, however, that the impact of gravity level on these technologies will depend on the design. Additional Processes of Interest The processes discussed below are considerably more speculative in nature than those covered in the preced- ing sections, so the impact of gravity on their operation is more difficult to assess. Nevertheless, their potential significance to HEDS missions warranted their inclusion in this chapter. Ejecta Capture from Asteroids and Comets The ability to establish propellant fueling depots in the vicinity of Earth but outside deep gravity wells can dramatically reduce the mass that must be launched from Earth and, accordingly, the costs associated with large- scale exploration missions throughout the solar system. Water ice is highly probable in dead, short-period comets

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 89 Molten Reverse Solid Ilmenite Electrolyte Water Sabatier Electrolyte Water Reduction Electrolysis Pyrolysis Gas Shift Process Electrolysis Electrolysis L L L — L L L H H — M M M M M M M M L L L H H H H — M M M M — L L L — M M — H H — M M M — H H H H H L L L L — M M M M L _ L L — L L L L L and in carbonaceous chondrite asteroids that are in the vicinity of Earth (Lewis et al., 1993; Lewis and Hutson, 1993~. If asteroids or short-period comets contain water ice, they can be exploited as sources of hydrogen and oxygen for propellant or for other resources, such as nickel or platinum. However, these objects are likely to have masses that are insufficient to produce significant gravitational attraction forces. To exploit asteroids and comets, it will be necessary to come into physical contact with them. Furthermore, it will be necessary to attach spacecraft elements to them and to process materials in their low gravitational fields. Strategies for processing these objects will probably require the deliberate ejection or removal of large chunks of material and their subsequent capture for resource utilization. In the case of water, it is not possible to extract water ice and separate it from other materials without subjecting it to a phase change. Gradual collection of released (sublimated) water molecules via some sort of cold trap maintained on a spacecraft collector surface is possible, but more robust and efficient high-volume collection systems can be utilized if chunks of dirty ice are broken away from the asteroid or comet and collected in a container that can be sealed and pressurized intermit- tently for batch processing. With current designs of space vehicles, it would be difficult to rendezvous and make intentional contact with

9o MICROGRAVITY RESEARCH small planetary bodies without any type of propulsive exhaust during the touchdown phase or to land on the object's surface (which is probably tumbling) with a negligible impact velocity. Because these planetary bodies may be rich in volatiles and because some have accreted under the influence of extremely low gravitational forces associated more with swarms of particles than with significant mass it is probable that any impact with their surfaces will result in the ejection of materials. Furthermore, temperatures associated with a terrestrial spacecraft are relatively high (near 300 K), and as a result the spacecraft could cause sublimation of significant quantities of water ice or any number of other volatiles that may be frozen in the object's surface at the extreme vacuum conditions of the space environment. Depending on the thermal coupling between the spacecraft and the asteroid or comet surface during encounter, it would be possible to liberate large quantities of these volatiles (owing to the extremely low surface pressures) and to simultaneously scour rocks and other debris from the surface. Hence, there are a variety of processes that will result in the ejection of material from the surfaces of asteroids and comets, besides those processes associated with the deliberate extraction and collection of raw materials. Both deliberately produced and inadvertent ejecta can pose a serious hazard for space systems because they can become projectiles. However, because fracturing processes will necessarily be involved in the removal of material from asteroids, the need to capture and collect ejecta will be a primary concern. Many problems need further study and experimental validation before a serious asteroid resource collection mission can be mounted, including how to work in the long-duration dusty environment that will probably be produced by any type of ejecta production operation. All missions to the surfaces of asteroids and comets will be greatly affected by microgravity considerations. The following specific developments, many of which will incorporate microgravity issues, are needed to support such missions: · Accurate predictions of the local debris swarms that will exist in the vicinity of targets of opportunity; · Material ejection and capture strategies that are proven experimentally, using hardware systems that can produce predictable and manageable surface material releases and that maximize raw material collection and . . . - minimize potential hazards to spacecraft systems; · Experimentally verified techniques and hardware systems that will permit spacecraft devices to rendezvous with and become attached to the surface of a tumbling asteroid or comet; · Systems that can ensure that spacecraft will not become entangled with lines, cables or nets that are used to attach hardware or communication devices to tumbling microgravity objects; · Attach/detach coupling systems that can permit hardware units to "walk" on a tumbling microgravity surface; · Rock climbing equivalent elements that can be used to secure and then release processing units in such a way that forces can be exerted on those surfaces that are sufficient to remove and/or manipulate raw materials; · Development of fluid and phase separation systems that can exploit the irregular but potentially significant radial acceleration forces that are associated with the tumbling motions of typical asteroids or comets; · Raw material batch processing systems that can extract useful resource materials and dispose of the resulting wastes in an environment with a negligible atmosphere and virtually no gravity; and · Systems that can attach to tumbling objects and then process sufficient raw material into rocket propellant to first despin the object and then propel the object toward a desired target. Mining Helium-3 The future demand for electrical power when carbon-based fuels become depleted will require the develop- ment of new generating technologies. The deuterium/helium-3 fusion reaction is recognized as an attractive, environmentally friendly source of power, but the terrestrial supply of helium-3 is very limited. There is a large quantity (109 kg) of helium-3 in the lunar regolith at a very low concentration (20-45 ppm). Very large quantities of lunar regolith would need to be processed to gather helium-3 in the quantities (hundreds of kilograms) needed for terrestrial power generation. Wittenberg et al. (1986) have analyzed and discussed this concept as a means of producing electrical energy for the entire country without causing environmental degradation. The origin of the lunar helium-3 is the solar wind volatiles (SWVs) that are emitted from the fusion reactions

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 91 in the Sun, which emits a stream of ionized elements that consist principally (96 percent) of hydrogen and a smaller amount of helium and trace amounts of other elements. The absence of a lunar atmosphere and the feebleness of the lunar magnetic field allow the SWVs to strike the lunar surface almost unimpeded and penetrate into the regolith to an estimated depth of 50 to 300 A (Warhaut et al., 1979~. Small meteor impacts constantly expose new surface of regolith to the solar wind and churn up the buned, helium-3 saturated grains so that the helium-3- concentration is nearly constant to a depth of 2.4 m (Swindle et al., 1990~. The trapped gases in the regolith are released upon heating (Pepin et al., 1970~. As mentioned above, large volumes of regolith must be processed to recover useful quantities of helium-3. For example, to continuously fuel a 1000 MW fusion power plant would require 106 kg of helium-3 per full power year (Wittenberg et al., 1991; Sviatoslovsky and Jacobs, 1988~. Two strategies have been proposed for the mining operation: (1) continuous area mining with linear travel of the gathenng-processing equipment and transportation of the mined gases in sealed containers to a central processing station (Cameron, 1992) and (2) continuous spiral mining, where the gathenng-processing equipment spirals out from a central hub to which the mined volatiles are piped for process- ing (Schmitt, 1992~. The collection of helium-3 from the outer planets has been proposed as a viable process (Lewis, 1997~. Hydrogen and helium make up significant fractions of the masses of the outer planets; Jupiter is about 95 percent hydrogen and helium, Saturn is about 90 percent, and Uranus and Neptune are about 50 percent. In the cold atmosphere of all four planets, the heavier elements have condensed and precipitated out, so that all of their atmospheres contain helium-3 at a level of about 45 ppm. Using a probe that can be placed in the atmosphere of Uranus, which is the most accessible planet, a system of pumps and cryocoolers could process the atmosphere to stepwise-extract gases based on their liquefaction temperature and behavior with temperature to obtain pure helium-3. A payload of 10 tons could be transported to Earth with rocket engine performance only very slightly improved over that available in 1965. It is estimated that the energy that could be produced from the helium-3 would be 20,000 times that expended to gather it in this operation. The helium-3 reserve on Uranus is a staggering 16 trillion tons, so that if this gathering process could be implemented, Earth's energy needs would be adequately met for many generations to come. 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Mining and excavating systems for a lunar environment. Pp. 294-304 in Engineering, Construction, and Operations in Space II: Proceedings of Space 90, Albuquerque, April 22-26, Vol. 1. S.W. Johnson and J.P. Wetzel, eds. New York: American Society of Civil Engineers. Sridhar, K.R., and S.A. Miller. 1994. Solid oxide electrolysis technology for ISRU and life support. Space Technol. 14(5):339. Sridhar, K.R., and B.T. Vaniman. 1995. Oxygen production on Mars using solid oxide electrolysis, 25th International Conference on Environ- mental Systems. SAE Paper No. 951737. Warrendale, Pa.: Society of Automotive Engineers. Sridhar, K.R., and R. Foerstner. 1998. Regenerative CO/O2 solid oxide fuel cells for Mars exploration. AIAA Paper No. 98-0650. Reston, Va.: American Institute of Aeronautics and Astronautics. Stancati, M.L., J.C. Niehoff, W.C. Wells, and R.L. Ash. 1979. Remote automated propellant production: A new potential for round trip spacecraft. AIAA Paper No. 79-0906. 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94 MICROGRAVITY RESEARCH Wittenberg, L.J., E.N. Cameron, G.L. Kulcinski, S.H. Ott, J.F. Sagittarius, G.I. Sviatoslavsky, I.N. Sviatoslavsky, and H.E. Thompson. 1991. Technical Report WCSAR-TR-AR3-9107-1. Madison: University of Wisconsin. Wood, M. 1992. Oxygen generation by static feedwater electrolysis for Space Station Freedom. Pp. 127-137 in Proceedings of the Interna- tional Conference on Life Support and Biospherics. Huntsville, Ala.: National Aeronautics and Space Administration. Wu, D., R.A. Outlaw, arid R.L. Ash. 1996. Extraction of oxygen from CO2 using glow-discharge and permeation techniques. J. Vac. Sci. Technol. A 14:408-414. Wydeven, T. 1988. A survey of some regenerative physico-chem~cal life support technology. NASA Technical Memorandum 101004. Moffett Field, Calif.: National Aeronautics and Space Administration. Zhao, Y., and F. Shadman. 1993. Production of oxygen from lunar ilmenite. P. 149 in Resources of Near-Earth Space. J. Lewis, M.S. Matthews, arid M.L. Guerrien, eds. Tucson and London: University of Arizona Press. Zubnn, R.M., D.A. Baker, and O. Gwynne. 1991. Mars direct: A simple, robust, and cost effective architecture for the space exploration initiative. AIAA Paper 91-0326. New York: American Institute of Aeronautics arid Astronautics. Zubnn, R.M., B. Frankie, and T. Kito. 1997. Mars in-situ resource utilization based on the reverse water gas shift. AIAA Paper No. 97-2767. Reston, Va.: American Institute of Aeronautics arid Astronautics. III.F CONSTRUCTION AND MAINTENANCE Introduction It is an underlying assumption in this discussion that the need for power, even if it is large, will be met locally. Potential power sources and generation systems are discussed in previous sections and are not covered again here. Much thought has been given to the incorporation of novel manufacturing processes that would uniquely benefit from the extraterrestrial microgravity environment and the hard vacuum (10-9 to 10-~2 torr) in the lunar environ- ment and in space. However, to date, no examples have been found of products that could be advantageously manufactured in space for commercial use on Earth (NRC, 1992~. It is clear that the HEDS program must plan to use local resources for life support since it will not always be possible to transport the needed facilities or replacement parts from a terrestrial base. While it is acceptable to provide all the needed supplies for the nearby International Space Station, this approach would become expensive on a lunar base and for missions to Mars or beyond, and in addition, the one-way transit time of hundreds of days would make such resupply impossible. It is clear that unexpected component failures can provide challenges to the success of a mission. It is also obvious that a spare parts kit, no matter how extensive, cannot anticipate every possible emergency. Direct manufacturing, an important new technology that builds metal or ceramic piece parts by computer-controlled, step-by-step deposition rather than by the machining of bulk feedstock, may have some dependence on gravity. An alternative would be to use a universal, compact machine shop that could process parts ranging from a wristwatch gear to an antenna mount and that would have no direct dependence on gravity. The problems that might be encountered at fractional gravity in lunar and Martian inhabitation are discussed here in relation to site preparation and habitat construction. In considering materials handling and transport technologies, it becomes evident that the direct use of unmodified terrestrial equipment would involve many shortfalls in performance at fractional gravity. For example, as discussed above with respect to mining operations, the reduced traction in transport vehicles and the reduced friction available to secure tethers into the regolith make it desirable to develop new structural designs and innovative processes. Granular materials such as the lunar and Martian regolith exhibit cohesion and are arrangements of rigid particles in frictional contact. Because gravity contributes to the normal stress of interaction between particle surfaces and frictional forces are typically propor- tional to the normal (hence gravitational) forces, the behavior of granular material is strongly dependent on gravity. Dust management as gravity nears zero is of concern since, in the absence of gravitational settling, one needs a positive filtration technology capable of dealing with a large range of particulate sizes and densities, both during space travel and to cope with tenacious lunar fines and Martian dust. Construction will be facilitated by the ability to use local materials to fabricate concrete (tin and Bhattacharja, 1998), which will be useful for many applications such as securing tethers at reduced gravity and general construc- tion. There will also be a need to refine metals from local ores and perform manufacturing operations to provide needed articles and to generate replacement parts. There will also be a need to provide comfortable facilities, such

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 95 as abodes and habitats protected from radiation, where the occupants can work and dwell in reasonable comfort, safely isolated from the harsh environment. Site Preparation The ability to establish outposts on extraterrestrial surfaces represents a serious challenge for future HEDS missions. Initial site preparation for all but touch-and-go surface exploration missions will require that the landing site be prepared to support research stations, spacecraft landing sites, habitats shielded from hazardous radiation, surface transportation systems, power generating stations, and an energy distribution infrastructure. Nearly all contemplated site development scenarios assume that significant portions of these preparation activities will be accomplished robotically, before humans arrive. Based on present knowledge, fairly detailed mission designs can be developed for lunar and, to a lesser extent, Mars missions, but it is also logical to consider the technologies that might permit site development on other extraterrestrial bodies such as asteroids and the Galilean satellites of Jupiter. Furthermore, even though it seems natural to imagine conventional earth-moving and excavation equip- ment operating robotically on these surfaces, gravitational and other environmental differences ensure that that will not be the case. Even the simple task of covering a structure with regolith involves a number of unresolved engineering issues. Because of our more detailed knowledge of the lunar surface, it is instructive to frame the reduced gravity research issues in that context. Lunar soil characteristics at depths greater than 0.7 m are not well known (Klosky et al., 1998; Carrier et al., 1991~. While a few core samples were taken down to depths of 3 m using rotary drilling devices, the samples were undoubtedly altered during the collection process. However, it is likely that site preparation and resource extrac- tion activities will require excavation operations to depths on the order of 5 m, which is, incidentally, the estimated thickness of Martian regolith needed for passive shielding from radiation (Hepp et al., 1994~. This will require the ability to perform bedrock mining as well as to handle and process regolith. Both soil composition and soil density variation with depth will change from one site to another, but Carrier et al. (1991) found that the lunar soil appears to be compacted more than terrestrial soil just beneath the surface. In addition, if site preparation studies are shifted to locations near the permanently shadowed polar craters, where the Lunar Prospector has indicated significant quantities of water ice are present, consideration must be given to the manipulation of soil containing water ice in an environment that is considered to be a hard vacuum on Earth. An appropriate area of study would be the development of instruments and gravity-dependent soil models that could address soil strength, stiffness, and density on the Moon, Mars, and other extraterrestrial bodies. Creation and control of dust during excavation and soil placement operations will be a major concern (Colwell et al., 1998~. Since much of the site preparation activity (and subsequent mining activity) will involve operating small, low-power, robotic machines, their ability to perform automated tasks reliably in a dusty environment, unattended for long periods of time, is critical. Because of the high value of hydrogen and oxygen molecules for propellant and life support outside Earth's gravitational field, attention should be directed toward developing water extraction sites on a range of low-gravity surfaces whose compositions range from dirty ice to permafrost. However, the presence of water ice can pro- foundly influence the types of soil manipulation operations that can be accomplished. Under some conditions, the water ice can fill pore spaces in the soil, decreasing the friction angle of the soil and thus decreasing the energy required for excavation. Conversely, permafrost (which is probable at some locations at Mars) is extremely difficult to excavate or even penetrate, and subsequent to any site preparation operations on permafrost surfaces, design considerations associated with maintaining the permafrost in a solid state must be addressed. Because of the low pressures that exist even on Mars the excavation process will probably generate sufficient heat to liberate water. Aside from the problems of processing water ice in a vacuum, Perkins and Madson (1996a) described a number of basic geotechnical issues in some detail. There is a need for smaller-scale soil excavation tools capable of moving cubic meters of material per day (as opposed to their terrestrial counterparts, which move hundreds of cubic meters of material per day). These tools must be very low in mass and consume only small amounts of power during initial site preparation operations. These requirements are very different from those for their terrestrial counterparts, where mass and power con- sumption are variables that can be optimized to improve reliability and efficiency. Unfortunately, the reduced

96 MICROGRAVITY RESEARCH gravitational acceleration at all contemplated HEDS sites translates to reduced frictional forces, which ultimately control the magnitudes of forces that can be exerted in excavation operations. Some work has been done on possible systems (Szabo et al., 1994; Boles et al., 1997), but to date research in this area has been sporadic and has failed to properly address the scaling factors that relate to low-gravity operations. Owing to the nonlinearities in soil behavior and consistency of stress fields (Ko, 1988), it is probably more desirable to construct 1/6 scale models of lunar equipment and operate them in Earth's gravity than to attempt to conduct tests in reduced gravity on a KC 135 flight. Future excavation research should focus on properly scaling the direct and indirect gravita- tional parameters, which control both the characteristics of the in situ material being excavated and the forces required to effect excavation. The following important issues relating to preparation and native material handling remain to be addressed: · There is a clear need for better soil mechanics information from greater depths on both the Moon and Mars before a long-term human presence can be established. As part of this effort, information could also be gathered on the availability of various resources for mining at the chosen site. Augered rather than push-in technologies show promise for introducing probes to significant depths with small machines (Klosky et al., 1998~. · The energy and force needed to excavate, haul, and place extraterrestrial soils are a central design param- eter needed to define the equipment required for site preparation. Research into more efficient methods of excavation using lightweight, low-power equipment is called for. This research would need to cronerlv account for scaling factors related to operations and forces in low-gravity environments. · A method of locomotion and tractive efficiency for equipment remain important questions. Terrestrial soil-handling equipment operators have long found that continuous-loop (tracked) vehicles, such as bulldozers and backhoes, have significant advantages over wheeled vehicles in long-term operation. Further, the isolation of the drive mechanism from the surface has proven to greatly decrease maintenance. Some of these issues were discussed recently by Costes and Sture (1998~. · The behavior of ice-laden lunar and Martian soils will need to be addressed. Before this can occur, we must obtain practical estimates of the concentration of this ice and determine whether it is continuous in the pore spaces of the soils. Then, the effect of this ice on excavation forces, foundation elements, and slope stability will need to be evaluated. · Procedures for the installation of foundation elements and reliable estimation of their load-bearing capacity will be required very early in the deployment of extraterrestrial bases (Perkins and Madsen, 1996b). Helical plate anchor/foundations have been suggested as a reusable and efficient alternative for this application (Klosky, 1997; Klosky et al., 1995, 1996, 1998~. Further evaluation of the load-bearing capacity and installation energy require- ments for this and other foundation types is also appropriate. · A method for detecting and removing or otherwise dealing with large rocks in excavation areas needs to be discovered. Terrestrial methods, typically involving blasting or otherwise applying massive energy impulses, will not be practical. This is a very significant problem and has not been adequately addressed as yet. Construction For cost reasons, equipment that is transported to space will have to be as light as is feasible to perform a given task and, therefore, the design must provide stability and stiffness. On Earth, gravity plays an important role directly by providing a restoring force following mechanical vibration or movement and indirectly in that since structures are made massive enough to withstand compressive gravity loads, they are automatically stable and stiff. For HEDS structures, structural stability and stiffness are issues to be addressed in the earliest stages of design. Preliminary architectural studies will uncover the specific research issues of structural dynamics that NASA should investigate. Structures must be suitably anchored to provide the desired relationship with the surface. For anchoring on Earth, gravitational body force and friction are important factors in most designs; in very low gravity, as, for

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 97 example, on an asteroid, these factors are reduced or absent, but still the anchoring must be able to reliably attach structures to fixed positions. Wind forces must be withstood in an environment such as that of Mars, where ubiquitous dust devils result in rather strong short-duration wind loads, even though the local atmospheric pres- sures are low. Even when wind forces are small, they can be important when acting on structures that are large for operational reasons but that, for cost reasons, are of low strength and stiffness, as discussed above. Large area and volume of habitat, while difficult to provide, will be desirable because equipment and crew will need to be isolated from the environment to a degree unusual on Earth, and the crew will need room for their operational work and to maintain mental and physical health. Dust intrusion must be prevented, particularly since in reduced gravity, dust will not settle as rapidly as it does on Earth. Inside atmosphere must be maintained across a wall or membrane separating it from extreme conditions on the outside where verY low pressure and verY high or verY low (perhaps variable) temperatures would exist. Diffu- x. . . ~ . . . . . . . . . . . . . . . scrotal loss of Inside gases must be prevented, appropriate thermal Insulation must be provided, and pressure loading of the wall must be withstood. All these issues will be troublesome for the large, low-mass structures that might be contemplated for HEDS colonies. Ideally, the habitat should provide for comfortable living in a shirtsleeve atmosphere. The construction methods for such structures will utilize equipment and processes that are described in other sections of this Chapter for materials handling, transport, bedrock mining, welding, and concrete use for anchoring . . . and Joining. In-depth research is needed on the design requirements and possibilities for all potential station locations, taking into account the relevant environmental factors, including low gravity, and with the health, safety, and happiness of crew being given paramount importance. This research should pay particular attention to structural analysis, dynamic as well as static, and to specific construction methods. Concreting Weight considerations are a strong argument against the transportation of structural building materials from Earth to bodies such as the Moon or Mars. The use of in situ building materials is therefore highly desirable, and the production of concrete is often cited (tin and Battacharja, 1998) as a particularly attractive approach for a number of reasons, including the enormous experience base that engineers could bring to it. The various aspects of concrete production and use are described in detail in a textbook by Mindess and Young (1981) and are briefly summarized here. Concrete is made up of an aggregate embedded in a cement, or bonding agent. Aggregate can contain a variety of materials but is usually composed of rock of some type. There are also various types of cement, but a type known as Portland cement is used commonly in construction. For this reason, the following discussion refers to Portland cement for the purposes of illustration. (Note, however, that there is a family of Portland cements differing in composition.) Production of Portland Cement The central process in the production of Portland cement is the heating of a mixture of sources of calcium carbonate and silica in a kiln at 1400 to 1600 °C. In this temperature range, calcium silicates are formed. The raw materials are processed before burning to obtain a feed that is thoroughly pulverized and homogeneous, which ensures a product that is uniform in composition. Failure to do this would result in cement with irregular properties and performance. Similarly, the control of the burning process is critical. This heat treatment, taking place in a rotary kiln, is called clinkering. It consists of a number of steps, as shown in Figure III.F.1. The inclination of the kiln, together with its rotation of 60 to 200 rph, causes the feed to move slowly along the length of the kiln. Material may remain in the kiln between 20 min and 2.5 h depending on the type of kiln and the specific production method. The material emerging from the kiln is called clinker. It is conveyed to ball mills, where it is finely ground with a small amount of gypsum and then stored until needed.

98 Exhaust gases \~; Free water Clay decomposes , Raw feed in Gas temp. 450~C ~~ y en= ~ 8000 C IN to f Bed temp. 50°C MICROGRAVITY RESEARCH Formation of _~_ ~ C;= 600° C Limestone initial Compounds decomposes t Formation I initial formation of melt Of C^S / Formation of / C3S 7 - - 1 ~ lY 20n~'ng By_ 20n'- 1 200° C 1 350° C 1 550° C -- t—- --t- - - 1000°C 1350°C 1450°C Clinker out Cooling grate FIGURE III.F.1 Schematic outline of conditions and reactions in a typical cement rotary kiln (dry process). SOURCE: Mindess and Young (1981~. Aggregates Aggregates make up 70 to 80 percent of the volume of concrete and therefore have a profound influence on its properties. They are granular materials mostly coming from rock, crushed stone, gravel, and sand, although other materials such as slag can be used. For extraterrestrial use, aggregate will no doubt be an in situ resource. Since aggregate greatly affects the properties of concrete, the potential in situ sources of aggregate should be character- ized and their effect on concrete properties understood. Batching, Mixing, and Placement ~ . Homogeneity is absolutely necessary to ensure uniform and adequate performance of the concrete. There are well-defined tolerances on the amounts of cement, water, aggregates, and admixtures, and thorough and uniform . . . . mixing Is a requirement. Hydration of Cement and Curing The various compounds that make up cement undergo chemical reactions when the cement is mixed with water. These reactions, referred to as hydration, are responsible for the hardening of the concrete. The rate of hydration is therefore directly related to the rate of hardening of the concrete and the rate at which its properties, such as compressive strength, develop. Furthermore, the hydration reactions are exothermic, so that the concrete increases in temperature as it hardens. The temperature increase will be a function of the hydration reaction rate of each of the compounds, the amount of each compound in the concrete, and the rate of heat loss to the surroundings. Concrete must be properly cured to develop its optimum properties. The most critical parameter is the amount of water present. An adequate supply of moisture must be present to achieve as much hydration as possible. In principle, there is enough water in concrete to ensure complete hydration if the water to cement weight ratio is at least 0.42. However, water is lost by evaporation or absorption by aggregates, framework, and subgrade. Once enough moisture is lost to reduce the internal relative humidity to about 80 percent, hydration will stop. Strength development will also stop, and the concrete will not achieve its potential.

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 99 Another critical parameter is the temperature of curing. Curing at low temperatures gives an initially low rate of strength development but can ultimately result in a higher compressive strength. Hydration will occur at temperatures down to 263 K, and the heat of hydration, along with adequate insulation, should protect the concrete from freezing in the early stages. At ambient temperatures where insulation is inadequate, external heating is required. In extraterrestrial environments such as the Moon, where surface temperatures vary from 80 to 390 K, and Mars, where surface temperatures vary from 130 to 300 K, curing processes are obviously a concern. Summary of Gravity Impacts The steps described above for the manufacture of concrete are varied and are fairly challenging for environ- ments such as the Moon and Mars. They include the production of cement, followed by the production of concrete and its handling and placement. Cement production involves quarrying, grinding and blending, clinkering, and ball milling. Concrete production involves batching of aggregate and cement, mixing, transportation, placement, and curing. The production process for cement is greatly affected by gravity level throughout. First, it is most important to have a feed that is thoroughly pulverized and homogeneous. Since grinding and blending operations on Earth are basically driven by gravity, in reduced gravity they will need considerable modification. Likewise, the terrestrial clinkering process is gravity-driven. It depends on a number of fundamental reaction steps, which in turn depend on the rate of fall through the various zones of the rotary kiln. Furthermore, the reaction rates are controlled by the rates of heat and mass transport, which can themselves be gravity-dependent. All terrestrial batching and mixing processes are also dependent on gravity in that they all make use of free fall. Hence, the dynamics (or kinetics) of these processes in extraterrestrial environments are critical if concreting in those environments is to be viable. Likewise, all placement methods on Earth are reliant on gravity by virtue of its control of sedimentation and buoyancy. After placement, the concrete is worked to eliminate voids and entrapped air as well as to consolidate it into corners and around any reinforcing steel. Again, the dynamics of these processes and the resulting concrete integrity and properties are likely to differ at extraterrestrial gravity levels. Hydration and curing are not directly affected by the gravity level, but they are certainly affected by the local environment. Specifically, the temperature increase during hydration has a direct effect on the final properties of the concrete. This increase is dependent on the rate of heat loss to the surroundings, which is, in turn, dependent on ambient conditions. Likewise, as already mentioned, the temperature of curing is a critical parameter; again, ambient conditions are important and countermeasures for temperature extremes are necessary. Direct Manufacturing An important new technology, direct manufacturing, is being actively developed at a number of government, university, and industry laboratories. There are many variants of direct manufacturing, and they go by a number of names. All systems have in common that they involve a three-dimensional rendering and the production of a complex physical form by the continuous, layer-by-layer buildup of metals, ceramics, or polymers. The sub- systems are, basically, a powder delivery subsystem, a mechanical subsystem to drive the building of the part, a laser, and a control subsystem. The powder delivery subsystem consists of a compressed gas supply, a powder feeder, and a cyclone mixer. The powder is fed through a nozzle into a Bettered glove box, where the objects are made. A multiaxis mechanical subsystem is used to manipulate the object under the laser beam. A microcomputer control subsystem drives the mechanical stage and the laser. These computer-controlled layer deposition techniques allow the direct production of high-value replacement parts without use of conventional casting, forging, and machining. The ability to produce new shapes at will lends itself to rapid, flexible, customized production and offers considerable potential for the extraterrestrial production of spare parts. While many variations of the technology are being studied, some typical processes are briefly described. In selective laser sintering (SLS) (Bergen, 1998), a layer of metal powder is deposited on a surface and a laser beam

100 MICROGRAVITY RESEARCH directed by the computer numerical control program fuses and consolidates individual powder particles in selected regions. Only the particle surfaces are fused, so that complex geometry control is maintained. The interior of each shell is fused in a second heating to achieve full density, with the sintered shell acting as a mold or forming die. Promising results have been achieved with titanium, Inconel 625, and mild steel/nickel alloys. In another example (W.H. Hofmeister, private communication, 1998; Keicher, 1999), powders between 50 and 100,um are entrained in a gas flow. The powders are delivered coaxially with a laser beam to a molten pool on the workpiece. The laser and powder feed traverse the workpiece to build parts in layers. Beam diameters from 0.25 to 0.5 mm have been used with layer depths from 0.2 to 1.00 mm. Linear traverse speeds from 15 to 45 mm/ s have been demonstrated. The metal volumes deposited have been on the order of cubic centimeters per minute. The cooling rates in this process are as high as 105 °C/s, so that highly refined microstructures are produced, comparable to those made by other rapid solidification processes. A number of materials, including stainless steels, tool steels, nickel-based superalloys, and titanium alloys, have been successfully processed. Multiaxis laser control has been used to form complex geometries with this process. Complex ceramic parts have been successfully produced using deposits of ceramic-loaded polymers. For example (Danforth et al., 1998), the molten polymer is extruded out of a 250 to 635,um diameter nozzle, directed by a computer program, to a platform where the polymer freezes. Another process (Brady and Halloran, 1998) to achieve ceramic structures uses successive layers of ceramic-loaded polymers that are ultraviolet-curable so that layer patterns can be defined by stereolithography. In both cases, the polymer is removed and the ceramic is densified in subsequent healings. Though these processes are in their infancy, the potential advantages to the NASA program are obvious. In the area of fabricating a prototype part or parts in limited numbers, successful implementation could drastically reduce the cost and lead time for procurement. In remote locations such as the Moon or Mars, direct fabrication from computer numerical control programs could be used to produce items on location, reducing reliance on spare parts inventories. One of the original applications of this technology was in aircraft turbine repair (W.H. Hofmeister, private communication, 1998), demonstrating that the equipment is also capable of laser welding and repair of critical structural items. Further technology development and actual implementation of the technology require that considerable re- search be done in the area of microstructural control. The research is necessary to learn how to control the process to allow tailoring the microstructure of each part manufactured, thus ensuring that the resulting properties are appropriate for the desired application. The weld pools involved in the buildup of layers contain very large thermal gradients. As a result, both surface-tension-driven flows and gravity-driven flows can be large, leading to significant effects on the microstructure, which must be understood. Also, the powder feed is delivered by forced convection, and the powder particles not captured by incorporation into the process must be recovered for reuse. Current recovery methods depend on gravitational settling, so alternative methods will be necessary in reduced gravity. Fabrication of Components and Structural Elements from Raw or Processed Materials The success of a mission to unexplored destinations can depend on the ingenuity with which local resources are utilized to meet unexpected challenges. Such a challenge could come from the unanticipated failure of a mechanical component, which would require the fabrication of a replacement part to repair the problem. As discussed above, it would not be practical to carry a complete spare parts inventory, nor would an extensive collection of spares necessarily fulfill every emergency need. A different approach has been described whereby a universal, compact machine shop with a very broad capability that might even extend to repairing itself would be included in the spacecraft or at the base (Stryker, 1987~. This machine "shop" would be able to generate replacement parts as small as a wristwatch gear or as heavy as an antenna mount. Such a machine was developed by a mechanical engineer in the 1950s for personal use and was first described in 1974 by Urwick. Its operation is based on the common geometries of three machines: the lathe, the horizontal milling machine, and the

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 101 horizontal boring machine. The machine is commercially available and has been used by the Royal Navy and various research institutes as a general-purpose machine tool. It is completely modular and can be disassembled and reassembled. The design allows for working on a wide range of part sizes. Tiny parts can be manufactured by bringing the movable machine components close together. The largest parts may require partial disassembly of the machine, and their handling can be assisted by use of robotic positioning. In kit form, the machine weighs 300 kg and occupies a volume of 1 m3. Operation requires minimal on-site skill since the machine' s activities can be controlled by an onboard computer or remotely by an operator elsewhere on the space station or on Earth. The latter approach offers the advantage that the machining procedure can be measured, evaluated, and planned by an expert stationed at an identical machine. The feedstock would initially be aluminum or a suitable alloy from unneeded external vehicle tanks. In situ mining and refining activities could also generate iron-nickel-titanium alloys and other suitable feedstock metals and ceramics. Such a universal machine could provide the wide range of capabilities needed to sustain a space station. It would be able to mill, shape, saw, and grind metal parts and even to generate special-purpose vises, collets, chucks, faceplates, and clamps that were not available in the kit of spare replacement parts. The mechanical and thermal stability of the machining operations present no unusual problems, but operation in a hard vacuum would require appropriate lubrication of sliding and mating surfaces to prevent cold welding. The machine's operation is not sensitive to gravity, but the generated filings, cuttings, etc., must be collected at reduced gravity so that a clean environment can be maintained. At zero gravity, the manufacturing should occur in an isolated atmosphere to prevent contamination of the adjacent spaces. In addition to machining, it is useful to note here some other common metal-working processes that may eventually be needed on HEDS missions, including extrusion, rolling, drawing, forging, bending, and pressing. None of these processes are expected to be gravity-dependent and so are not discussed further. Casting in Reduced Gravity Casting is a process in which a molten material is allowed to freeze or solidify, usually in a mold, to produce a solid object of the desired shape. The liquid is introduced into the mold by pouring or by pressure, as in die casting (Scully,1988~. Containerless solidification is also possible if a uniform crystal structure is desired, free of contamination from a container, but the resulting object will probably require machining as this process allows only a limited range of shapes (Strong et al., 1987; Hofmeister et al., 1987; Naumann and Elleman, 1986~. Molds or dyes of complicated shapes would usually have to be made in situ, as it would be impossible to anticipate or transport all the ones that might be needed. In some cases, this could be done using sand molds and lost wax or similar techniques. The performance of the product is directly determined by the microstructure resulting from the casting process. The large body of knowledge on casting metals in terrestrial gravity is treated extensively in a handbook by ASM (1988~. There is also a substantial body of work demonstrating that the microstructure of castings in microgravity differs from that in terrestrial gravity (Curreri and Stefanescu, 1988~. Finally, there is considerable research in progress (e.g., Glicksman et al., 1987, 1995a-c; Abbaschian, 1996; Bassler et al., 1995) exploring the fundamentals of solidification in a microgravity environment. At present, however, there is insufficient under- standing of these fundamentals to allow predictions of the detailed effect of gravity level on the microstructure of a casting. Microgravity experiments continue to yield surprises. For example, solidification of eutectic alloys in microgravity (Larson and Pirich, 1982) shows a closer spacing of finer rods than in Earth gravity. In general, the solidification process and the resulting microstructure are affected by gravity levels. The effect is ultimately due to differences in the strength of density-induced convection in the liquid phase. These differences affect the distribution of temperature, solute, and suspended particles or bubbles, which in turn affect the solidified microstructure. Moreover, casting operations may perform differently in reduced gravity. For example, many 1OAnthony Croucher Ltd., Alton, Hampshire, England.

102 MICROGRAVITY RESEARCH casting operations depend on the gravity feed of liquid by way of risers as part of the design of the mold. At reduced gravity, such feeds would be less effective. Conduction/convection furnaces may also have different operating characteristics in microgravity, as treated in a study by Lenski and Filler (1987~. Sintering Sintering is an important manufacturing process for making near-net-shape parts from powder in the solid state. It is discussed, along with the reduction of a material to powder, in Metals Handbook (ASM, 1984~. The powder is poured or injected into a mold or dye, with or without the aid of a binder. Filling the dye is extremely important, particularly for parts with complicated geometry, and it relies on gravity feed unless injection molding is used. If injection molding is used, the powder may be introduced as a slurry or paste, in which case the part is usually subject to light machining after sintering. The powder compact is then heated to a temperature below the melting point of the solid, with or without pressure, to produce the consolidated part. The process may take place entirely in the solid state of the material or may be facilitated by the presence of a liquid phase in the solid particle interstices; in the latter case, it is called liquid-phase sintering (LPS). Sintering offers advantages over casting that include its capability to (1) use high-melting-point materials, (2) produce porous materials as used in self-lubricating bearings, and (3) use mixed powders, whose separate liquids are immiscible, to produce materials that cannot be formed by casting. In all three cases, however, molds or dyes are required. As with casting, molds or dyes of complicated shapes would usually have to be made in situ, as it would be impossible to anticipate and carry all the ones that might be needed. During the sintering process, densification of the aggregate of solid particles takes place by the formation of connecting necks between particles and the concomitant reduction of pore volume during heating below the melting point of the solid. The driving force is surface energy reduction and if the aggregate has been compacted or is under pressure plastic and elastic energy reduction. In the case of solid-phase sintering, material transport occurs by diffusion on the surface of the solid particles, volume diffusion occurs in the interior of the solid particles (including high diffusivity paths), and vapor transport occurs in the pores; in LPS, there are additional processes of flow of the interparticle liquid phase, diffusive transport in the liquid phase, and local melting/ freezing and dissolution/precipitation at the liquid/solid interface. Particle reorientation and plastic deformation or viscous flow of the solid phase may also play a role. Each of these processes is dependent on the temperature and the particle size; for example, surface diffusion dominates at smaller scales and lower temperatures and volume diffusion dominates under the opposite conditions. Sintering is not only an important technique for making precision parts but it is also considered to be potentially important for fabricating building material brick from lunar regolith (Allen et al., 1992, 1994; Pletka, 1993~. Solid-state sintering is slow and leads to very uneven heating owing to the low thermal conductivity of the regolith. Pletka (1993) has described an LPS process in which the liquid phase may derive from the glassy silicates in the regolith itself or from reactions that occur in the material when heated and which thus requires no additive. The advantage of sintering over casting is that lower temperatures are sufficient. The disadvantage is that the material must be comminuted and/or sieved to small particle sizes (typically ~100,um) for the sintering rates to be reasonable. The diffusion transport processes that occur during solid-state sintering are not affected to any significant degree by the level of gravity. The spatial distribution of particles, however, is affected by it. In Earth's gravity, particles settle, forming a skeleton characterized by an average coordination number (Yang and German, l991~. In microgravity, an aggregate of independent particles would not form a compact unless pressure is applied, with the effective coordination number depending on the pressure. Similarly, the distribution of particles in LPS is affected by gravity level. If the volume fraction of particles is low for LPS under microgravity, the particles tend to agglomerate toward the center, surrounded by liquid (Kohara, 1994; German, 1995), rather than settling toward the bottom as they do in Earth's gravity. This agglomeration has been interpreted as being driven by the reduction of surface and interface energy that can occur

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 103 when particles coalesce to form grain boundaries at their junctions (German, 1995~; the process is analogous to the coalescence of liquid drops brought into contact. On a microstructural level, LPS involves solidification and is therefore affected by gravity level by virtue of density-induced convection and sedimentation in the freezing liquid, as was discussed in the section on casting. For example, it is found that materials formed by LPS in microgravity may be more porous (German et al., 1995) than those formed in Earth's gravity, presumably because the bubbles formed by outgassing are not eliminated by buoyancy migration; the migration of liquid-filled pores is also affected by gravity level (Heaney et al., 1995~. The coarsening of particle sizes that occurs during sintering has been studied in microgravity, mostly to test theoretical models that do not include gravitational effects. Again, there are some surprises related to the behavior of pores in LPS (German et al., 1995~. The sintering operation requires a mold or dye with equipment (e.g., injection equipment) to fill it, a furnace capable of operating at sintering temperatures (typically above 1000 °C), and an atmosphere regulating system. As pointed out, molds would usually have to be made in situ. This poses no problem for simple shapes like bricks but is a serious limitation for complicated shapes unless they can be made by a lost wax or similar technique. Composite Materials On Earth, increasing use is being made of composite materials. These materials are combinations of a matrix and a dispersion. Broadly, they are classified as one of three basic types, depending on their matrix: polymer matrix composites (PMCs), metal matrix composites (MMCs), and ceramic matrix composites (CMCs). The utility of these materials stems from the synergism achieved by combining different materials into a single entity (Eckold, 1994; Schaffer et al., 1995; Callister, 1997~. It is not anticipated that exploration plans over the next few decades would include the extraterrestrial manufacture of matrix materials and particulate/fiber materials because of its complexity and the drain on re- sources that such activities would entail, including the demand for manufacturing capability. Instead, if compos- ites were needed, judiciously chosen materials could be part of the cargo. These raw materials could then be used for repair, maintenance, or replacement. The main manufacturing methodologies for polymer matrix composites are hand lay-up, filament winding, and pultrusion. These do not appear to involve gravity effects in a major way, although in hand lay-up, spray processes typically rely on free fall to distribute fiber in the mold for making the final product. Therefore, development of techniques for use in reduced gravity would be needed. For example, confinement of sprayed material such as resin, catalyst, and particulate would be necessary. Metal matrix composites can be cast to shape using an intermediate feedstock but, alternatively, these com- posites can be forged to shape. Other options are production by hot processing or casting to shape using pressur- ized feeding of liquid metal into a mold cavity containing fiber preform. Of these alternatives, casting to shape could be affected considerably by reduced gravity through its effects on flow, convection, buoyancy, and sedimen- tation. The process would be a likely candidate for experimental work in reduced gravity and microgravity. Products from ceramic matrix composites can be fabricated by pressing, hot or cold, and by sintering of prepregs as composite feedstock. In these cases, gravity level is not a factor. Joining Methods in Space Joining structural members in space is important for both construction and repair. Methods usually consid- ered are mechanical joining, adhesive bonding, and welding or soldering (including brazing). However, mechani- cal joining requires special design (e.g., provisions for O rings) to assure pressured seals, and the high polymers used for adhesive bonding are subject to degradation in space owing to outgassing and radiation damage. We therefore focus here on welding, which may be used for repair (e.g., to patch holes caused by micrometeors) as well as for construction. Welding entails the fusion of the base metals at the junction. It may be done either with or without a welding

104 MICROGRAVITY RESEARCH rod; in the latter case, it is called autogenous welding. In brazing and soldering, only the solder and not the base metal is melted; above 450 °C the process is called brazing, below 450 °C, soldering. The history of welding in space is described in AWS (1991~. It starts with Russian experiments in 1969 on Soyuz 6, followed by a number of subsequent Russian experiments and tests. The first American trials occurred in 1973 on Skylab with the welding of three metals (stainless steel, an aluminum alloy, and high purity tantalum); the results were examined at Battelle and NASA. In 1984, Russian cosmonauts spent 3 hours welding outside Salyut 7 using a handheld electron beam gun designed by the E.O. Paton Electric Welding Institute in Kiev. In 1986, two cosmonauts constructed a large truss in EVA off Salyut 7. This showed that quality welds could be done with little prior training and that the VHT (versatile handgun tool) performed well in space; however, there were dangers to the welder from emitted X rays. Numerous underwater welding experiments have been carried out at Marshall Space Flight Center in a neutral buoyancy tank. Welding experiments scheduled for the October 1997 shuttle flight STS-87 were postponed and have still not taken place. The EVA welding environment is characterized by microgravity (10-6 go) with jitter, hard vacuum (modified by outgassing from the vehicle), meteoroids and debris, sunlight and ionizing radiation, atomic oxygen, and large thermal gradients (near the Sun/shade boundary). Technical conditions or limitations for welding are limited power sources (a few kilovolts for a few minutes) and the paucity of nondestructive testing methods for space. For some welding methods the associated health hazards make robotic welding very desirable. Gravity is not a dominant factor in the welding process itself, even in Earth's gravity, as indicated by the fact that welding is routinely done upside down. Under microgravity conditions, the weld pool dynamics are com- pletely dominated by capillary and electromagnetic forces. Thus, even though gravity-induced convection and sedimentation are absent, Marangoni-induced convection (due to the dependence of surface tension on tempera- ture and composition) may be strong; added to this are electromagnetic stirring forces due to the welding current. The shape of the weld pool that moves in concert with the welding rod is determined by the interplay among these forces in a way that is not entirely understood. The shape in turn affects the weld quality. A cusp shape at the trailing edge produces a seam that is generally detrimental to the material properties since impurities tend to segregate there. 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106 MICROGRAVITY RESEARCH Lin, T.D., and S. Bhattacharja. 1998. Lunar and Martian resources utilization: Cement. Pp. 592-600 in Space 98: The Sixth International Conference and Exposition on Engineenng, Construction, and Operations in Space. R.G. Galloway and S. Lokaj, eds. Reston, Va.: Amencan Society of Civil Engineers. Mindess, S., and J.F. Young. 1981. Concrete. Englewood Cliffs, N.J.: Prentice-Hall. Nance, M., and J.E. Jones. 1993. Welding in space and low-gravity environments. P. 1020 in Metals Handbook, Vol. 6, Welding, Brazing and Soldenng. Metals Park, Ohio: ASM International. National Research Council (NRC), Space Studies Board. 1992. Toward a Microgravity Research Strategy. Washington, D.C.: National Academy Press. Naumann, R.J., and D.D. Elleman. 1986. Containerless processing technology. P. 294 in Matenal Science in Space. B. Feuerbacher, H. Hamacher, and R.J. Naumann, eds. New York: Spnnger-Verlag. Perkins, S.W., and C.R. Madson. 1996a. Mechanical and load-settlement characteristics of two lunar soil simulants. J. Aerospace Eng. 9(1): 1- Perkins, S.W., and C.R. Madsen. 1996b. Scale effects of shallow foundations on lunar regolith. Pp. 963-972 in Engineenng, Construction, and Operations in Space V: Proceedings of the Fifth International Conference on Space 96, Vol. 2. S.W. Johnson, ed. New York: Amencan Society of Civil Engineers. Ple~a, B.J. 1993. Processing of lunar basalt matenals. P. 325 in Resources of Near-Earth Space. J.S. Lewis, M.S. Matthews, and M.L. Guernen, eds. Tucson and London: University of Arizona Press. Schaffer, J.P., A. Saxena, S.D. Antolovich, T.H. Sanders, Jr., and S.B. Warner. 1995. The Science and Design of Engineering Materials. Chicago: Richard D. hwin. Scully, L.J.D. 1988. Die casting. P. 286 in Metals Handbook, 9th Ed., Vol. 15. Metals Park, Ohio: ASM International. Shong, D.S., J.A. Graves, Y. Ujiie, and J.H. Perepezko. 1987. Containerless processing of undercooled melts. P. 17 in Matenals Processing in the Reduced Gravity Environment of Space: Proceedings of the 1986 Fall Matenals Research Society (MRS) Meeting. R.H. Doremus and P.C. Nordine, eds. Warrendale, Pa.: Matenals Research Society. Stryker, J.M. 1987. A job shop for space manufactunng. Pp. 158-163 in Proceedings of the Eighth Princeton/AIAA/SSI Conference. B. Faughnan and G. Maryniak, eds. Washington, D.C.: American Institute of Aeronautics and Astronautics. Szabo, B., F. Barnes, H.-Y. Ko. 1994. Effectiveness of vibrating bulldozer and plow blades on draft force reduction. Proceedings of the Winter Meeting of the Amencan Society of Agncultural Engineers. No. 941535. St. Joseph, Mich.: American Society of Agncultural Engineers. Yang, S.-C., and R.M. German. 1991. Gravitational limit of particle volume fraction in liquid-phase sintenng. Met. Mater. Trans. A 22:786. III.G MATRICES OF SUBSYSTEMS, PROCESSES, AND PHENOMENA Tables III.G.1 through III.G.3 summarize information from the preceding sections regarding which common subsystems and processes are likely to be affected by changes in gravity level. The tables identify the specific phenomena that are most likely to play an important role in the operation of a given subsystem when the gravity level is altered. These phenomena are discussed in greater detail in Chapter IV.

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 107 TABLE III.G. 1 Phenomena Associated with the Common Subsystems Likely to Be Affected by Gravity Level Phenomenon Subsystem/ Variant Storage tanks Gas Liquid Cryogenic Pumps Condensate Liquid line Microdevices Compressors Rotary Adsorption Piping Gas-phase Liquid-phase Two-phase . Radiators Solid-state Gas-phase Two-phase Heat pipes Capillary pumped loop Simple Fans and blowers Evaporators Boilers Vaporizers Liquifiers Condensers Distillations units

108 TABLE III.G.1 Continued Subsystem/ Variant Filters/separators Gas/solid Gas/liquid Liquid/liquid Liquid/solid Vortex separators 1 _ Rotating drum separators Spargers Valves and actuators MICROGRAVITY RESEARCH Phenomenon ~ ~ i genii 6~ Heaters - Catalyst beds Seals Heat exchangers Gaslgas Gas/liquid Gas/solid Flu id ized-bed Fire extinguishers Smoke detectors

SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE TABLE III.G.2 Phenomena Associated with the Matenals Handling Equipment Likely to Be Affected by Gravity Level Equipment - Screens Hoppers Excavators Conveyers Drillers Bulldozer Anchor Trucks Cranes Bucket scoop Winch Rotating drum or slide charging unit Electrostatic generator Gravity collection bins 109 Phenomenon ., ~ : , ~ . ' ' ' 3 ' ! ~

0 MICROGRAVITY RESEARCH TABLE III.G.3 Phenomena Associated with Various Material Processes Likely to Be Affected by Gravity Level Phenomenon Process Crushing/grinding Settling Sieving Transporta Sintering (LPS) Casting Welding - aIncludes such bulk material transport processes as ore transport and slurry flow in pipes. i: t .

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The frontier represented by the near solar system confronts humanity with intriguing challenges and opportunities. With the inception of the Human Exploration and Development of Space (HEDS) enterprise in 1995, NASA has acknowledged the opportunities and has accepted the very significant challenges.

Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies was commissioned by NASA to assist it in coordinating the scientific information relevant to anticipating, identifying, and solving the technical problems that must be addressed throughout the HEDS program over the coming decades. This report assesses scientific and related technological issues facing NASA's Human Exploration and Development of Space endeavor, looking specifically at mission enabling and enhancing technologies which, for development, require an improved understanding of fluid and material behavior in a reduced gravity environment.

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