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

Description of the GP-B experience:Two of GP-B’s gas thrusters were taken off line after failing; the identified root cause for both was foreign objectcontamination in thruster body which prevents the thruster from turning off properly. This observation has alsobeen made by numerous other projects with a variety of gas thruster designs.Lesson:• Pay particular attention to contamination control and cleanliness of all systems which interface with thecold gas thruster system. Use ultra-high-vacuum and optics procedures to maintain cleanliness. Designin as much filtering as possible to limit migration of potentially damaging particles.16.1.2.6 Attitude Reference Platform StabilityIssue Summary: The alignment sensitivity of the space vehicle's Attitude Reference Platform (ARP) was foundto be significantly more sensitive to the vehicle's thermal environment than originally calculated.Description of the GP-B experience:The space vehicle was unable to maintain the required wide-band and roll-frequency pointing requirements dueto a out of spec thermal sensitivity of the ARP to the space vehicle thermal environment. This effect significantlycomplicated the GP-B data analysis by requiring careful modelling of the effect during both guide star valid andinvalid periods.Lesson:• For such a mission critical piece of hardware, do not rely solely on analysis-only methods of verification,but place the vehicle in a thermal environment (in thermal vacuum testing, for example) in excess ofwhat is expected in orbit and verify the stability of the platform by direct testing. Formally require thevehicle subcontractor to perform this verification by test, in addition to analysis.16.1.2.7 Formal anomaly resolution processIssue Summary: A formal anomaly resolution process proved to be very effective.Description of the GP-B experience:GP-B developed, simulated, and practiced an Anomaly Resolution process during pre-launch preparations. Thesystem developed (described elsewhere in this report) was very effective, and a number of lessons were learnedand re-affirmed during the mission:Lessons:1. A formalized anomaly resolution process is very helpful to focus and guide investigations. Each anomalywas evaluated with a checklist to ensure all key issues were addressed. Recording of anomaly data andclosure information was enforced with this process. It provided a framework in which to develop faulttrees and form root cause investigations and assists in the rapid assimilation of new team members viaclear processes and standards.2. Leadership of the anomaly review board by senior, mission-cognizant technical staff critical for effectiveanomaly resolution. Senior technical staff, especially at the managerial level, have a deep understandingof the operation of the space vehicle and can effectively direct investigations in “high yield” directions.They have the broadest knowledge of the skills of the team members and are most effective in forminganomaly teams. ARB members that are senior managers from core subcontractors and technical450 March 2007 Chapter 16 — Lessons Learned and Best Practices