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

SQUIDs to detect their rotation speed to an accuracy of 10 millionthsof a Hertz. Because the measurement is made between the gyro rotorsand their housings, and because the gyro housings are connected tothe spacecraft itself, the roll rate of the spacecraft was also measuredusing the gyros. In fact, two of the gyros spin in the same direction asthe spacecraft roll, and two spin against the roll. Thus, as the roll rateof the spacecraft was increased, two of the gyros appeared to spinfaster, and two appeared to slow down in accordance with theincreased roll rate. Photo: The gyro housing and SQUID lines underinspection during repairs of the Science Instrument Assembly, circaAugust 2000.As chronicled in previous weekly updates, two difficulties—now bothovercome—have made the task of locking onto the guide star takelonger than anticipated, and as a result, the IOC phase of the missionhas been extended from 60 to 90 days. First, the side-facing startrackers on the spacecraft required an extended search period toproperly identify the known field of stars. Feedback from these startrackers is used to orient the spacecraft in the vicinity of the guide star,similar to using a spotting telescope to position a high-poweredtelescope on a particular part of the sky. The second is frommalfunction of two of the spacecraft’s 16 ultra-sensitive microthrusters. Redundancy built into the system enables the spacecraft tofly without the two problematic thrusters, but to optimizeperformance with 14 instead of 16 thrusters, it was necessary to revisethe control software. This software change has now beenimplemented, and after a Flight Readiness Review on June 25, 2004, itwill be uploaded to the spacecraft. At a later stage, we will explorepartial re-activation of the two problematic thrusters.25 JUNE 2004—MISSION UPDATE: DAY 66On Day# 66 of the mission, the spacecraft continues to be in goodhealth, with all subsystems performing well. The spacecraft’s orbitremains stable, ready for transition into the science phase of themission. All four gyros remain digitally suspended, and finalcalibration tests at very slow gyro spin rates of up to 1 Hz (60 rpm)have been completed. The spacecraft’s roll rate has been incrementallyincreased from 0.3 to 0.9 rpm, as part of the process of uniformlydistributing and balancing the mass of the spacecraft. Following aFlight Readiness Review currently in progress, we will begin uploadingrevised drag-free thruster-control software to the Attitude andTranslation Control system (ATC) on-board the spacecraft over theweekend.This past week, we completed the last of a series of very low spin ratecalibration tests on the gyros and the gyro suspension system (GSS).As described in previous highlights, these tests involve briefly applyingvoltages asymmetrically to the suspension electrodes on a given gyro,causing that gyro rotor (sphere) to move off center by a few millionthsof a meter and then re-centering it. We also tested the GSS bymeasuring the level of electrostatic charge on each gyro rotor afterapplying 20% higher than normal suspension voltages to thesuspension electrodes, with the gyro rotors in spin-up position. Noneof the rotors showed a significant charge build-up after this procedure,which indicates that the GSS is functioning as anticipated.Additionally this past week, we continued with two proceduresdesigned to bring the entire spacecraft into balance, rolling smoothlyabout its main axis while the telescope focuses on the guide star. In thefirst procedure, called “bubble wrap,” the spacecraft’s roll rate wasincreased in incremental steps, from 0.3 rpm to 0.9 rpm. The increasedroll rate begins to rotate the liquid helium, effectively pushing itoutwards as it tries to move in a straight line with its inertia. Thedewar walls hold it in with a centripetal force. This wraps the heliumuniformly around the outer shell. Distributing the liquid heliumuniformly along the spacecraft’s roll axis helps to ensure that thescience telescope can remain locked on the guide star while thespacecraft is rolling.Some people might think this is centrifugal force. One might ask, whatis the difference between centripetal and centrifugal force, anyway?The words centripetal and centrifugal are in fact antonyms defined asfollows:centrifugal: tending to move away from a center.centripetal: tending to move toward a center.In the second procedure, called “mass trim,” weights mounted on longscrew shafts are attached in strategic locations around the spacecraftframe. Small motors, under control of the spacecraft’s ATC, can turnthese screw shafts in either direction, causing the weights to moveback and forth by a specified amount. Thus, based on feedback fromthe GSS, the spacecraft’s center of mass can be precisely centered, bothforward to back, and side to side, around the designated “drag-freegyro.”Finally, revisions to the spacecraft’s drag-free thruster-controlsoftware that will optimize the spacecraft’s performance, using 14instead of 16 micro thrusters, passed a design review last week. AFlight Readiness Review is currently in progress to approve theuploading of this updated software to the spacecraft this weekend.Next week, the ATC will be re-started using the new software. The 16micro thrusters are arranged 4 clusters, with each cluster having twopairs of thrusters. Redundancy built into the thruster system enablesthe spacecraft to meet the attitude control requirements for the sciencemission with only 12 functioning micro thrusters. In the event of athruster problem, the original thruster-control software was designedto isolate both the malfunctioning thruster and its paired partner. Therevised software now enables individual micro thrusters to be isolatedin any thruster pair, without affecting the paired partner. Of the 14micro thrusters currently functioning, only two are critically located,478 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission