Microgravity Research and the Space Station Furnace Facility Core
This chapter is divided into two sections. The first section is an overview of the materials-science microgravity research program implemented by the National Aeronautics and Space Administration (NASA) and a discussion of the relevance of microgravity research to the material science and engineering community. The second section contains a description of the specifications and capabilities of the current Space Station Furnace Facility (SSFF) Core concept.
NASA's Microgravity Materials-Science Research Program
The general goal of the programs within NASA's Microgravity Research Division (MRD) is to conduct basic and applied research under microgravity conditions (10-6g) that will further our understanding of fundamental physical, chemical, and biological processes. Specifically, the five main areas of research within the MRD are biotechnology, combustion science, fluid physics, fundamental physics, and materials science.
The microgravity environment of space provides a unique opportunity to further our understanding of various materials phenomena involving the molten, fluidic, and gaseous states by reducing or eliminating buoyancy-driven convection effects. The space environment also permits containerless processing, thus eliminating impurities and stresses introduced by contact with the container walls. Although under special circumstances density-matching can be used to eliminate convection in the Earth's gravity field, the selection of the component materials is limited. Furthermore, density-matching occurs
at only one temperature. Sample levitation for containerless processing can sometimes be accomplished in a terrestrial environment, but it requires large induced currents or acoustic pressure gradients.
The anticipated results of microgravity materials-science research range from establishing baselines for fundamental materials processes to generating results with more direct commercial significance. NASA's objectives for the microgravity materials-science program include:
- advancing our knowledge base for all classes of materials
- designing and facilitating the execution of microgravity experiments that will help achieve this goal
- determining road maps for future microgravity studies
- contributing to NASA's Human Exploration and Development of Space enterprise
- contributing to the national economy by developing enabling technologies valuable to the U.S. private sector
To accomplish these goals, the materials-science program has tried to expand both its scientific scope and the research community's involvement in microgravity research. Based on their requirements for experimental facilities, most of the current materials-science microgravity experiments can be divided into four general categories. The first category involves melt growth experiments, such as those used for processing multicomponent alloys from the liquid. The experiments in this category frequently require high temperatures and closed containers or crucibles to prevent elemental losses. The second group includes aqueous or solution growth experiments for materials like zeolite or triglycene sulfate. These experiments usually require moderate to low temperatures. Hydrothermal processing of inorganic compounds and sol-gel processing also fit in this category. The third category of experiments involves vapor or gaseous environments, such as those used for growing mercury iodide or plasma processing. Unlike the first three categories that use containers for the parent materials and products, the fourth category involves processes and experiments that require containerless processing environments. Examples of these experiments include the formation of metallic and nonmetallic glasses during levitation melting and solidification, the float-zone growth of crystals, and the measurement of thermophysical properties like diffusion coefficients and surface tension.
Although the SSFF Core on the International Space Station (ISS) is envisioned as the primary dedicated facility for materials-science
microgravity research, a number of other facilities are currently available for short-time, low-gravity experiments (e.g., drop towers and parabolic flights of aircraft and sounding rockets). These reduced gravity facilities are used primarily for preflight developmental work. The ISS will also be able to accommodate experiments using the EXPRESS racks, microgravity science glove-boxes, and other facilities that are currently used on the Space Shuttle missions.
The research areas originally identified by the MRD included the following materials classes: electronic and photonic materials, glasses and ceramics, metals and alloys, and polymers and nonlinear optical materials. The MRD subsequently formed an 11-member Materials Science Discipline Working Group (DWG) to review the science priorities, implementation plan, and long-term strategy of the materials-science program; develop advocacy and outreach programs for the MRD to promote microgravity research; and help the MRD compile information and assessments of the microgravity program for external review bodies. The primary mechanism for obtaining broader input into the MRD materials-science program and for informing the community of the current program content and future research opportunities has been through biannual Microgravity Materials Science conferences. Conferences organized by the DWG and hosted by the Marshall Space Flight Center in 1994 and 1996 (NASA 1996) were attended by approximately 300 to 350 scientists. Additional input was provided by two National Research Council reports: Towards a Microgravity Research Strategy (NRC, 1992) and Microgravity Research Opportunities for the 1990s (NRC, 1995).1 Based on the input from these conferences and reports, the DWG identified fundamental physical and chemical phenomena research areas that it believed would benefit from long-duration microgravity conditions. Promising subjects for investigation identified by the DWG included:
- nucleation and metastable states
- prediction and control of microstructure, pattern formation, and morphological stability
- phase separation and interfacial phenomena
- transport phenomena
NASA also supports some in-house research through the University Space Research Association, but these activities follow a separate funding process and were not reviewed by the current committee.
- crystal growth, defect generation, and control
- extraterrestrial processes and technology development (e.g., welding in a vacuum and exploiting extraterrestrial materials for fuel, etc.)
The DWG also recommended the support of such ground-based activities as process modeling and materials characterization that support microgravity research projects.
Space Station Furnace Facility Core Capability
The SSFF was conceived to provide a set of common services that would support a wide range of high-temperature microgravity materials-science experiments on the ISS. NASA defines the purpose of the SSFF as follows:
The purpose of the Space Station Furnace Facility project is to provide a modular facility for materials research in the microgravity environment of the International Space Station. The SSFF will be designed for crystal growth and solidification research in the fields of electronic and photonic materials, metals and alloys, and glasses and ceramics. The SSFF will allow for experimental determination of the role of gravitational forces in solidification, crystallization, and thermophysical property measurement. The facility will provide a capability for basic scientific research, and will evaluate the commercial viability of low-gravity processing of selected technologically important materials.
This report focuses on the Core of the SSFF, which will provide the mechanical, power, and control infrastructure to support an array of experiment modules (EMs). The EMs will contain the actual hardware (e.g., furnaces, samples, thermocouples) in which the experiments will be conducted and, whereas the SSFF Core is being developed and constructed by NASA, the EMs will be separately developed by Principal Investigators in conjunction with independent equipment manufacturers.
The reasons for providing a common infrastructure via the SSFF Core are to (1) reduce experiment implementation times by providing major generic subsystems that have long lead-times; (2) reduce the
up-mass and down-mass required for materials-science investigations; (3) provide flexibility in responding to evolving priorities of the materials-science research community; (4) reduce costs by eliminating the development, fabrication, and verification of redundant hardware and software systems; (5) reduce costs by providing common ground-support equipment, laboratory hardware, and operations support; and (6) facilitate the integration of new experiments. The committee did not have the expertise to assess the perceived cost/benefit advantages of a SSFF Core. The committee believes, however, that hardware integration with the SSFF Core could reduce instrument-development time by providing standardized interfaces—both in space and on the ground—to which researchers and equipment designers could efficiently and accurately respond. Over time, these standard interfaces would also permit the accumulation of experience that could be passed on to new users and provide them with the lessons learned by prior investigators.
The SSFF Core concept was initially devised during the late 1980s and early 1990s, based on recommendations from the DWG; the SSFF Intercenter Science Advisory Panel, which consisted of two representatives from the NASA Langley Research Center, one from the NASA Marshall Space Flight Center, and one from the NASA Jet Propulsion Laboratory; five public workshops hosted by the Marshall Space Flight Center; and a study by Teledyne Brown, a hardware fabricator for NASA with headquarters in Huntsville, Alabama. The SSFF Science Working Group (SWG) was formed in 1995 to provide advice directly to the SSFF project scientists during the development and early operation of the Core and to guide its functional and operational design. The SWG consists of 22 members from government, academia, and industry. Members are appointed for two years. The present membership is heavily weighted toward metals and semiconductor specialists, most of whom have active microgravity research projects with NASA. Two of the members are also on the DWG. The SWG has met twice: once in March 1995 to review the SSFF Core concept prior to its Critical Design Review and once in March 1997 to review the project status and assess potential new science requirements.
To support a broad range of high-temperature materials-science experiments, the SSFF Core must be able to support a wide variety of ''simple'' and "intelligent" EMs. Simple EMs do not contain computer or control circuitry and are totally dependent on the Core for all of their control resources (e.g., command, control, signal conditioning), as well as for basic infrastructure requirements (e.g., power, cooling,
gas, communications, vacuum/waste-gas systems). Intelligent EMs have their own computer and control capabilities and only rely on the Core for their infrastructure needs. The technical specifications for the Core are provided in Appendix A.
The current SSFF Core concept consists of three racks (Figures 1-1 and 1-2). The central, or core, rack will contain the support equipment, and each of the side racks, or instrument racks (IRs), will contain connection equipment to accommodate two standard-sized or one larger EM. In the current concept, two EMs are supposed to be able to run simultaneously, one in each IR. The operation of the EMs would have to be carefully orchestrated to ensure that their combined infrastructure-requirements remain within the limits currently specified for the Core facility (e.g., amount and quality of power, gases, vacuums). Thus, although some functionality can be provided within individual EMs, the SSFF Core imposes overall constraints.
The SSFF Core will operate for 120 days per year. Time limits for the Core were established based on the resource needs (e.g., power, microgravity, crew time) of the other scientific research facilities on the ISS. Each research facility on the ISS can function for only 20 to
30 percent of the year. This time will usually be broken down into 30-day segments. These segments will also be interrupted by periodic events that will compromise the microgravity environment (e.g., docking by space shuttles, exercising by astronauts, reorienting the ISS). One IR will be replaced every two years; thus, each IR will remain on the ISS for a total of four years. EMs can be changed within an IR if the wiring configurations are similar and crew time is available. Samples can also be changed within an EM on the ISS, depending on the complexity of the process and the availability of crew time.
Meaningful research in the SSFF Core will be impossible if a microgravity environment cannot be maintained. When the ISS was initially conceived, microgravity conditions were supposed to be maintained on a station-wide basis. In the current ISS concept, however, the IRs must maintain their own microgravity conditions.
In order to maintain microgravity conditions, the IRs must be isolated from the transient, oscillatory accelerations (termed "g-jitter") of the ISS via the active rack isolation system (ARIS), which is currently being developed by Boeing. ARIS consists of mechanical, electrical, and electronic assemblies that will be installed in the IRs
and connected to the ISS via stabilizing rods. When ARIS detects a vibratory acceleration, it will actuate devices on the IR to compensate by either pushing or pulling on the connector rods in the opposite direction of the acceleration. ARIS is designed to compensate for vibratory accelerations between 0.01 and 300 Hz. The performance of ARIS will be compromised if the vibratory accelerations are outside the specified range (e.g., if a rack is accidentally jarred) or if IR operation exceeds maximum allowed payload disturbance levels. ARIS will require approximately 2.5 cm of rattle space between racks and approximately 8 cm of floating space at the top of the racks. To minimize the levels of vibration to be controlled by ARIS and ensure that g-jitter levels remain within IR specifications, NASA has also stipulated that all umbilicals (cables and hoses) for the IRs have minimal stiffness. The umbilicals are still being developed, and a recent space shuttle flight test of ARIS was not successful.
Finding. Meaningful research in the SSFF Core will be impossible if a microgravity environment cannot be maintained. The success of the SSFF Core therefore depends on the perfection of ARIS or the development of an alternative system for controlling g-jitter.