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

Weekly Highlights of the GP-B Final Calibration PhaseWeek Activities & EventsWeek ending 9/16 The virtual “helium level gauge” in the dewar was clearly in the “red zone,” but when thehelium would actually run out was now anyone’s guess. So, planned calibration testscontinued.Over the weekend, after running more slewing tests, we locked back onto IM Pegasi andswitched drag-free control from gyro #1 to gyro #3 in order to perform calibration tests ongyro #1.During the gyro #1 calibration test, the pressure in the dewar dropped, causing thrusterinstability. To mitigate this situation, we switched the thrusters to “open-loop,” and theyremained in this state for the duration of the calibration tests.On Tuesday, we slewed the telescope/spacecraft to the very bright star, Alpha Pegasi (Markabor HD 218045), located 3.25 degrees South of IM Pegasi. We remained locked on Alpha Pegasifor 12 hours, before returning to IM Pegasi and switching drag-free control back to gyro #1.On Wednesday, we sent commands to the spacecraft to decrease its roll rate from 0.7742 rpmto 0.4898 rpm and changed the frequency of a SQUID calibration signal that was causinginterference at this slower roll rate. The spacecraft was then able to perform nominally at thenew roll rate.On Friday, we initiated procedures to remove electromagnetic flux from the SQUIDs.Week ending 9/23 Because the helium in the dewar had now exceeded its expected lifetime by more than 3weeks, the dewar team re-calculated the helium lifetime tests based on a new assumption—that there was weak, rather than strong thermal contact between the liquid and gas layers ofhelium in the dewar. This new assumption put an upper bound on the helium lifetime thatwas 5-6 weeks longer than the initial assumptions. Thus, it now appeared that the heliumcould last up to 3 weeks longer.Over the weekend, we completed the process of removing electromagnetic flux from theSQUIDs.On Tuesday, we visited a virtual star 0.1 degrees east of IM Pegasi and remained locked in thatposition for 48 hours. While the telescope was locked in this position, we performed somedark current calibration tests on the telescope.During this week, we twice attempted to switch from backup to primary drag-free mode.Both attempts failed, and in each case, we returned to backup drag-free control, which hadbeen used throughout the science phase of the mission. Analysis indicated that these failedswitch-overs were due to configuration issues.Week ending 9/30 This turned out to be the last week of the mission—the day the dewar “died.”On Monday, we once again attempted to switch from backup to primary drag-free control,and once again, the attempt failed—but for a different reason than the two attempts madethe previous week. Preliminary analysis indicated that this failed switch-over was similar toones that occurred during IOC and was probably due to “un-modeled” forces between thegyro rotor and its housing.On Tuesday and Wednesday, the team returned the spacecraft to its nominal science mode,pointing at IM Pegasi, and collected gyro data, later to be compared with similar datacollected during the science phase of the mission.On Thursday, 29 September 2005, at 1:55 pm PDT, the last of the liquid helium in the dewartransitioned to the gas phase, and the Science Instrument Assembly began to warm up.The team correctly assessed the status of the dewar, based on a set of pre-approvedindicators, and they initiated a planned helium depletion procedure.On Friday morning, we held our last daily “all-hands” meeting, took a group photo of theteam, and the GP-B flight mission was thus concluded.64 March 2007 Chapter 2 — Overview of the GP-B Experiment & Mission