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

Flight Anomalies.) Furthermore, a decision to reduce noise in the experimental data signal by increasing thespacecraft’s roll rate from 0.52 rpm to 0.75 rpm (and ultimately, 0.774 rpm) made the ATC’s star fieldidentification more difficult.2.4.1.5 Full Speed Gyro Spin-up IssuesThe team, using a very thorough process, successfully completed the full-speed spin-up of the four sciencegyroscopes. Gyro #4, which showed the greatest spin-up helium leakage rate during low-speed testing was spunup to full speed first. The full-speed spin-up process, which lasted most of the day on 13 July 2004, wentsmoothly and resulted in a final spin rate of 105.8 Hz (6,348 rpm) for gyro #4. This rate was quite acceptable forcollecting relativity data. Three days later, gyro #2 was spun up to full speed, also without incident. It reached afinal spin rate of 87 Hz (5,220 rpm). Helium leakage from its spin-up caused gyro #4 to spin-down from 105.8Hz to 91 Hz.While both of these spin rates were acceptable for the science phase, the team decided to postpone the full-speedspin-up of gyros #3 and #1 for a little over a week in order to run both ground-based and on-orbit tests, in anattempt to both increase the spin rates of the final two gyro rotors and also reduce the helium leakage during thespin-up process. During the last week in July, gyros #3 and #1 were spun up to full speed. Each of these gyrospin-ups caused approximately a 15% decrease in spin rate of the other gyros, resulting in the final spin ratesshown in Table 2-1 below.Table 2-1. Final gyro spin ratesGyro # Final Spin Rate (Hz) Final Spin Rate (rpm)1 80 48002 62.3 37383 82.7 49624 65.5 39302.4.1.6 Gyro Spin Axis AlignmentThe final gyro spin rates were smaller than originally anticipated, but they were more than adequate for meetingour science requirements. However, these slower spin rates affected the process of spin axis alignment towardsthe end of the IOC period. Before beginning to collect spin axis precession data from the gyros, their spin axesmust initially be aligned with the guide star. This alignment is accomplished by the Gyro Suspension System(GSS), in conjunction with the SQUID magnetometer readouts, by applying small torques to the gyro rotors to“nudge” their spin axis positions into alignment with the direction of the guide star, as indicated by thetelescope readout.The spin axis alignment algorithm was designed with the assumption that the gyros would be spinning in therange of 120-170 Hz (7,200-10,200 rpm). At these very high spin rates, the gyro develops a significantcentrifugal bulge, and the GSS uses this bulge as a “handle” for applying the torque to nudge the rotor’s spinaxis. However, at slower spin rates, this centrifugal bulge is smaller than the natural out-of-round shape of therotor, so the SQUIDs and GSS must apply the spin axis alignment torques to the natural, polhode modulated,shape of the rotor, rather than the more predicable centrifugal bulge of fast spinning rotor. The torquesgenerated under these circumstances are very small, and they require ground processing of rotor informationand manual adjustment of the phasing and timing of the torque commands. Thus, it takes more time to alignthe spin axes of the gyros when they are spinning slowly. In fact, the smaller the level of imperfections in therotors, the longer it takes the GSS to complete the alignment. As an example, gyro #4 is apparently so perfectlyspherical that it took over two weeks longer than the other three gyros to align its spin axis.46 March 2007 Chapter 2 — Overview of the GP-B Experiment & Mission