Assessment of Options for Extending the Life of the Hubble Space Telescope Final Report (2005) / Chapter Skim
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4 HST Observatory Assessment and Lifetime Projection
4 HST Observatory Assessment and Lifetime Projection
Pages 40-55
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From page 40... ...
The three key subsystems in this category are the fine-guidance sensor (FGS) units, the rate sensor unit (RSU; commonly referred to as "gyro" or "gyros")
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From page 41... ...
The model for observatory lifetime computes the failure rate of the two categories separately to derive the projected lifetime of the system as a whole. Previous shuttle servicing missions to HST have demonstrated that essentially all failures on HST are repairable.
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From page 42... ...
These ancillary upgrades, which are more difficult to implement and also add mission risk, have rightly been eliminated by NASA from the robotic mission plans. FINDING: Previous human servicing missions to HST have successfully carried out unforeseen repairs as well as executing both planned and proactive equipment and science upgrades.
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From page 43... ...
This ability to "reset the avionics failure clock" has been demonstrated on past space shuttle servicing missions but is not likely for a robotic mission due to the complexity and risk considerations discussed above. The SPATEL model used by NASA to project the reliability of HST's avionics system was originally developed by Marshall Space Flight Center and Lockheed Missiles and Space Company.
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From page 44... ...
TIME (YEARS) FIGURE 4.1 Hubble Space Telescope avionics system reliability over time.
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From page 45... ...
This means that it is unlikely that the avionics system's reliability can be reset through robotic servicing, a result with two important mission implications. First, a robotic mission with an implementation schedule of 4 to 6 years (5.4 years is the projected development time derived in Chapter 5 based on an independent assessment by the Aerospace Corporation)
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From page 46... ...
, series of personal communications, August 2004; A.H. Zimmerman, Life Projection for the Hubble Space Telescope Nickle-Hydrogen Batteries, Aerospace Report No.
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From page 47... ...
The 160 Ah on the upper red segment represents the minimum battery capacity required to support science operations. Once the 160 Ah threshold is reached (based on a battery voltage level representing a specific discharge level)
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From page 48... ...
Failure of HST's optics is most likely to occur in the May 2011 time frame when the battery capacity reaches the 40 Ah threshold. FINDING: Battery lifetime trends are consistent with supporting HST science operations through April 2008 and maintaining the telescope's optical system in a highly protected Level-1 safe-hold state until July 2009.
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From page 49... ...
FINDING: Replacement of HST gyros by the space shuttle is a straightforward operation that has been accomplished successfully on past servicing missions. Replacement by a robotic mission is more complex, entailing the attachment of multiple RSU and ECU elements plus interface electronics onto the WFC3 instrument.
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From page 50... ...
Its replacement is important if FGS redundancy is to be retained to support post-servicing HST science operations. Replacement of FGS-2R is straightforward on a space shuttle mission but is considered to be high risk for a robotic mission.
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From page 51... ...
Replacement of RWA units has been performed successfully on two previous shuttle missions in response to an unexpected anomaly and is also possible, if required, on SM-4. Replacement of an RWA is not part of the planned robotic mission and may not be possible on such a mission due to the RWA mounting locations on the telescope.
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From page 52... ...
The risk to the science instrument electronics is judged to be low due to their recent replacement. Assuming that 3 to 5 years of post-servicing science operations following a robotic mission will extend into and beyond 2014, avionics failures could eventually occur due to radiation effects.
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From page 53... ...
Associated with these bars are the orange- and yellow-star mission risk values at the bottom of Figure 4.3 and the horizontal dashed 50 percent risk line. The orange-star values represent the assessed risk for the planned NASA robotic mission, and the yellow-star values represent the assessed risk for the shuttle servicing case, with the vertical dashed lines representing the corresponding dates for the robotic and shuttle mission 50 percent values.
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From page 54... ...
units, and the gyroscopes, based on current patterns of usage. The yellow and orange segments represent the potential servicing windows for a shuttle mission and a robotic mission based on analyses in subsequent chapters and the resulting time line for a 3- to 5-year post-servicing period of science operations.
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From page 55... ...
Therefore, the total expected operational time in the science mode is projected to be at least 6.3 years for a shuttle servicing mission executed in July 2006 and 7.3 years for a shuttle mission executed in July 2007. -- The projected robotic servicing mission starts with 3 years of operations prior to gyro failure followed by a 29-month suspension of science operations, at which time the projected telescope avionics system risk value will be above 50 percent.
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