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

On Friday, 2 July 2004, the spin rates of gyros #1 and #3 was increased to 3 Hz (180 rpm), by streaming purehelium gas through their spin-up channels for 90 seconds each, and a few days later, gyros #2 and #4 werelikewise spun-up to 3 Hz (180 rpm). Note that the exact spin rate of the gyros cannot be controlled. Rather, wecontrol the length of time that ultra-pure helium gas flows through the spin-up channel for each gyro, and thenthe SQUID readouts tell us the resulting spin rates—which may differ slightly from one gyro to another.During the week of July 4 th , helium gas was flowed over gyro #4 for another 90 seconds, increasing its spin rateto approximately 6 Hz (360 rpm), and a number of tests were subsequently performed to ensure that gyro #4and its suspension system were functioning properly in preparation for full-speed spin-up. Then, on 13 July2004, Gravity Probe B achieved a major milestone with the successful spin-up of gyro #4 to a science-readyspeed of 105.8 Hz (6,348 rpm). Second to the launch, the full-speed spin-up of the gyros was the next most longawaitedevent in the history of GP-B. Members of the team were very attentive at their stations in the MissionOperations Center (MOC) here at Stanford for 3 hours and broke into applause when the final announcementcame over the MOC intercom that the gyro #4 full spin-up had been completed successfully.On July 16 th , gyro #2 underwent the same spin-up procedure, reaching a final spin rate of 87 Hz (5,220 rpm).Spinning up the gyros to science-ready speed is a complex and dynamic operation that exercises the fullcapabilities of the Gyro Suspension System (GSS) and requires a high level of concentration and coordinationon the part of the GP-B Team. Following is an overview of the full-speed spin-up process.First, commands are sent to the GSS to move the gyro rotor (sphere) very close to the spin-up channel (about1/100th of the edge of a sheet of paper) in one half of the gyro’s housing. Ultra-pure (99.999%) helium gas isstreamed from the Gas Management Assembly (GMA), mounted in a bay on the spacecraft frame, throughtubing that enters the “top hat” (the thermal interface at the top of the Probe) and travels down to the gyrohousings in the Science Instrument Assembly (SIA) at the lower end of the Probe. As the helium gas descendsinto the Probe, which is at a temperature of approximately 2.7 Kelvin, the gas cools down from 273 Kelvin toaround 12 Kelvin.Before entering the spin-up channel in one of the gyro housings, the gas is passed through a combinationfilter/heater. The filter, which is made of sintered titanium, removes any particles that may have been carried bythe helium on its journey into the Probe. The heater enables the helium to be warmed slightly, which increasesits adhesion to the ultra-smooth surface of the gyro rotor. The filtered and warmed helium then passes throughthe spin-up channel in one half of the gyro housing, and most of the gas evacuates into space through anexhaust system. However, some of the helium gas leaks into the housings of the other gyros, causing their spinrates to decrease up to 20% over a full spin-up period of 2-3 hours.For this reason, the order in which the gyros are spun up is very important. Earlier in the IOC phase, the 3 Hz(180 rpm) spin-up provided information on the helium leakage rate of each gyro. Gyro #4, which had thehighest leakage rate, was spun-up to full speed first, so that helium leaked from its spin-up would not affectother gyros that were already at science mission speed. The remaining gyros are then spun-up in decreasingorder of their helium leakage rates—gyro #2, gyro #1, and finally, gyro #3.Each full-speed spin-up takes most of a day. In the morning, helium gas is flowed over the gyro for 90 seconds,and tests are run to ensure that the helium leakage rate for that gyro corresponds to previous measurements. Ifeverything checks out, the full-speed spin-up, in which helium gas is flowed over the rotors for 2-3 hours,commences early in the afternoon. The GP-B team controls the spin-up process by sending commands from theMission Operations Center (MOC) here at Stanford to the spacecraft in real-time. For example, they sendcommands to open or close the GMA valves to flow helium through the gyro’s spin-up channel. They alsocontrol the amount of heat applied to the gas before it enters the gyro spin-up channel, and they controlopening and closing of exhaust valves. Real-time telemetry provides immediate feedback on the progress of thespin-up so that various parameters can be adjusted as necessary.54 March 2007 Chapter 2 — Overview of the GP-B Experiment & Mission