GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

Lessons:1. M/OD design and analysis must be considered and evaluated early in the program/project life cycle froma complete system level approach, integrated with other systems such as structures, radiation shieldingand thermal protection materials, and not on component by component comparison.2. M/OD critical item lists may be different from failure effects mode critical items.3. Several items were omitted from the “critical items” list, since they were functionally redundant, orimpossible to shield at the point M/OD analysis was done in the program design (i.e., the dewar). ForM/OD impacts, if items are located adjacent to each other, they may not always be consideredredundant. For example, critical damage to the first item can generate debris, which could easily damagethe second. Thorough assessments should be performed for every item on the spacecraft, consideringlocation, redundancy, inherent shielding in the design, for the entire spectrum of environmentalparameters (particle density, velocity, direction, size, shape, etc.). This kind of assessment must begin atthe beginning of a program, not at the end, to assure items such as the dewar are, in fact, not high-riskitems.4. Some items were omitted from the list, such as the aft thruster plumbing. These lines are protected onlyby the MLI blanket surrounding them, and may cause mission failure if punctured and pressure is lost.5. For a better understanding of the quantitative risk of GP-B, and similar spacecraft, hypervelocity impactdata of flight-like items is required.16.1.2 Lessons Learned from On-orbit OperationsThe following subsections summarized various lessons that the GP-B team learned from operations of thespacecraft on orbit.16.1.2.1 Validation of pre-launch vehicle testingIssue Summary: Ensure that testing is backed-up by thorough data analysis & review.Description of the GP-B experience:One of two transponders (4 th generation TDRSS transponder, GFE from NASA/General Dynamics)experienced low link margins immediately following launch, and this resulted in degraded performance on theGP-B forward omni antenna. This particular transponder was retrofited to GP-B’s frequency prior to launch; itreplaced a transponder with a low power output anomaly found during system testing.The root cause was determined to be a low-level, out-of band oscillation in the second LNA stage that results ininterference inside the transponder. The unit level test data suggests that the cause of the problem was inducedduring the retrofit but was not observed during retrofit testing. Thermal cycle testing was performed, but athermal vacuum test was omitted with the rationale that a thermal cycle test in atmosphere would provide largerthermal stresses on the vehicle, and thus provide a more thorough test. Current data indicates that crystaloscillator performance and center frequency is sensitive to the pressure environment—this was not evident tothe team at the time of the rework Vehicle level testing did not find the problem because RF signal levels wereunrealistically high, thus masking the link margin problem. At the vehicle level, the program review of thetransponder test data was not able to uncover the problem, though there was evidence of the issue upon further,detailed review.Gravity Probe B — Post Flight Analysis • Final Report March 2007 447