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

free suspension parameters to de-tune this harmonic coupling. Wehave since returned the spacecraft to drag-free operation around gyro#1, and it is performing nominally.8 OCTOBER 2004—GRAVITY PROBE B MISSIONUPDATE: Day 171At the end of Mission Week #25, GP-B is in its sixth week of sciencedata collection. The spacecraft continues to be in good health, flyingdrag-free around gyro #1. All four gyros are digitally suspended andperforming nominally. The spacecraft’s roll rate remains at 0.7742rpm (one revolution every 77.5 seconds). The dewar temperature isnominal (1.82 Kelvin), and the flow of helium, venting from the dewarthrough the micro thrusters remains within expected limits.As noted in last week’s update, we have continued to monitor theperformance of gyro #3, which had been serving as the drag-free gyrountil it suddenly transitioned to analog backup suspension mode twoweeks ago. At that time, we quickly implemented a recovery plan,switching drag-free flight operations to gyro #1 and reinstating digitalsuspension on gyro #3. It remained digitally suspended all last weekuntil last Friday, 1 October 2004, when it once again transitioned intoanalog backup mode. This past Monday, 4 October 2004, we resuspendedgyro #3. Subsequently, we changed the Gyro SuspensionSystem (GSS) bridge setting for this gyro to the high-level excitationmode used during gyro spin-up to increase the signal-to-noise ratio ofthis gyro’s position readout. The GSS team will continue to monitorthe performance of this gyro, and they are preparing to test severalhypotheses regarding the root cause of this gyro’s loss of digitalsuspension.Last week, we also reported detecting oscillations in the drag-freecontrol force, which we suspect were caused by a sympatheticresonance between the ATC drag-free control efforts and a sloshingwave on the surface of the now reduced superfluid helium in thedewar. We adjusted the ATC drag-free suspension parameters to detunethis harmonic coupling, and the oscillations disappeared for mostof last week, but last Friday, they began to re-appear. We have sincereduced the gain in the drag-free controller to its initial level, prior tothe appearance of these oscillations, and this seems to have mitigatedthese oscillations. Following is a more detailed explanation of thissuspected harmonic resonance phenomenon, provided by our GP-Bcryogenic expert, Mike Taber.GP-B recently experienced a minor instability, characterized byexcessive thruster venting and a slowly rotating force transverse to theroll direction. This force oscillation was observed by the suspensionsystem of all four gyros and had a period of approximately 220 seconds(3.7 minutes). At the time this occurred, the system was in “drag-free”mode, where the control effort needed to center gyro #1 was beingminimized by the helium thrusters. This is normal operation inscience mode, wherein the space vehicle is being flown in such a wayas to minimize the suspension forces and torques that can perturb thegyro spin direction. As a precautionary measure, in response to thisoscillation, the drag-free control system was commanded off. Theoscillation was then observed to slowly decay over a period of aboutsix hours.The initial assessment of this phenomenon was that it was due to aslosh instability in the liquid helium. The helium in the main tankexists in two phases: liquid and vapor. Since the space vehicle is slowlyrolling, centrifugal force causes the denser liquid to reside near theouter wall of the main tank with the vapor forming a bubble at thecenter. Normally, the liquid rotates as a solid body with the rest of thevehicle. However, if excited, it is dynamically possible for a surfacewave to form at the liquid-vapor interface that can propagatecircumferentially. The fact that the oscillation persisted, but thenslowly decayed after the drag-free control was turned off, supports thisscenario. The energy to build up and sustain the wave was supplied bythe drag-free system reacting to the force of the propagating wave withan appropriate phase shift. A subsequent examination of telemetrydata indicated that this phenomenon actually started when the gain ofthe drag-free controller was doubled some time ago, but the amplitudewas too small for it to be readily apparent. Ten days (and a number ofevents) actually passed before the oscillation grew sufficiently largethat it was noted in the Attitude and Translation Control (ATC)system and dewar data. The drag-free controller gain has since beenreduced back to its original value. There is no indication that any ofthese events had any adverse impact on the GP-B experiment.15 OCTOBER 2004—GRAVITY PROBE B MISSIONUPDATE: Day 178Six months into the mission, GP-B is performing remarkably well. Thespacecraft is in fine health, flying drag-free around gyro #1. All fourgyros remain digitally suspended, and they are all generating sciencedata. The spacecraft is rolling at 0.7742 rpm (77.5 seconds perrevolution), and all subsystems are performing well. The dewartemperature remains nominal at 1.82 kelvin, and the flow of helium488 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission