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

The Gravity Probe B spacecraft has forward and aft thrusters to maintain attitude control and for drag-freeflight. The thrusters are mounted in clusters of four, with eight total thrusters on the forward and eight total onthe aft. Having sixteen thrusters was a robust design, which allowed for redundancy throughout the thrustersystem. During the Initial Operation and Checkout (IOC) Phase one of the thrusters failed. This failure,although unfortunate, had little effect to the overall attitude control due to the redundancy of the system. Adiscussion of this failure is thoroughly discussed in Chapter 7, Attitude & Translation Control SubsystemAnalysis. However, as a result of this failure, four thrusters became “single-string” thrusters. Meaning that if oneof these four thrusters were to now fail, some functionality in the attitude control system would be lost. All 16thruster temperatures were monitored throughout the mission and due to their single-string nature, four of thethrusters were monitored with special care and a little more visibility. The four single-string thrusters are:Thrusters 5, 7, 10, & 12. Figure 8-12, Figure 8-13, Figure 8-14, and Figure 8-15 display the forward and aftthruster temperatures for the duration of the mission. It can easily be seen that the yellow and red limits werenever violated and the seasonal variations in the thruster temperatures are evident as well. The forwardthrusters cool as the gamma angle increases and the aft thruster cool as the gamma angle decreases.214 March 2007 Chapter 8 — Other Spacecraft Subsystems Analyses