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

The successful spin-up of gyro #4 to full speed enabled us to spin-up gyro #2. Gyro #2 topped out at 87 Hz(5,220 rpm). Also, the helium gas leakage from the Gyro 2 spin-up slowed gyro #4 down to 91Hz (5,460 rpm).Based on these results, the team postponed the full-speed spin-up of gyros #3 and #1 for over a week, in order torun both ground-based and on-orbit tests, in an attempt to both increase the spin rates of the final two gyrorotors and also reduce the helium leakage during the spin-up process. From the results of these tests, the teamdetermined that the gyro rotors could be positioned up to 30% closer to their spin-up channels during the spinupprocess and still maintain a safe margin of clearance from the suspension electrodes and housings.Using this finding, during the last week in July, gyros #3 and #1 were spun up to full speed. Each of these gyrospin-ups caused a 15% decrease in spin rate of the remaining gyros, resulting in the final spin rates ranging fromapproximately 60 – 80 Hz (3,600 – 4,800 rpm). Table 2-3 below shows the history of the spin rates for all fourgyros over the course of the entire spin-up process.Table 2-3. Spin-up history of the GP-B gyrosGP-B's Gyro Spin-up Results (Hz)Date 13-Jul 16-Jul 28-Jul 30-JulGyro # Gyro 4 Spinup Gyro 2 Spinup Gyro 3 Spinup Gyro 1 SpinupCumulative AvgSpin-down1 1.6 1.2 0.3 80.02 1.6 87.0 75.7 62.33 1.6 1.4 100.4 82.74 105.6 90.5 79.5 65.5Spin-up Impact on other Gyros1 na na na na2 na na -13.0% -17.7%3 na na na -17.7%4 na -14.3% -12.2% -17.6%Average Spindownna -14.3% -12.6% -17.7% -15.4%Although all four gyros ended up spinning at less than half the spin rates initially anticipated, their spin rateswere more than adequate for collecting relativity data. Furthermore, several other accomplishments achieved bythe GP-B subsystems during IOC—especially extremely low SQUID noise and higher than planned spacecraftroll rate—effectively reduced the error factor in the GP-B science experiment, thereby partially compensatingfor the reduced spin rates of the gyros.2.4.3.7 Low Temperature BakeoutFollowing gyro spin-up was the low temperature bakeout (LTB) process. The gyro spin-up process leavesresidual helium gas in and around each of the science gyros. If not removed, this helium gas could reduce theaccuracy of the experimental data collected during the Science Phase of the mission by providing a higher thandesired gas pressure in the Probe, and thus causing the gyroscopes to spin down faster than requirements. Thelow temperature bakeout process involved applying a very mild heat cycle to the Science Instrument Assembly,briefly raising its temperature from 1.8 kelvin to approximately 7 kelvin. The net effect of this procedure was toremove nearly all of the residual helium. A cryogenic pump, comprised of sintered titanium, collected theremaining helium from the gyro housings. After low temperature bakeout, the SIA temperature was restored to1.8 kelvin.Gravity Probe B — Post Flight Analysis • Final Report March 2007 55