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

and if these two were both to malfunction, it is still possible to operatethe thruster system by partially opening the valves in the isolatedthrusters.Once the revised thruster-control software is up and running, thehelium bubble is wrapped, and the spacecraft’s mass has beentrimmed, we will lock onto the guide star, IM Pegasi, in a drag-freeorbit, in preparation for the science mission.2 JULY 2004—MISSION UPDATE: DAY 73At a little over ten weeks into the mission, the spacecraft is in excellenthealth, with all subsystems performing well. The spacecraft’s orbitremains stable, ready for the transition into the Science Phase. All fourgyros are digitally suspended and have completed calibration testing atapproximately 0.3 Hz (18 rpm) spin rates. Two problematic microthrusters, which were preventing the spacecraft from sustaining adrag-free orbit, have been isolated, and the thruster-control softwarewas modified to optimize Attitude and Translation Control system(ATC) functionality without them. Over the past two weeks, thespacecraft’s roll rate was increased from 0.3 rpm to 0.9 rpm as part ofthe process of uniformly distributing and balancing the mass of thespacecraft.During this past week, we completed the mass trim procedure at 0.9rpm, using movable weights on long screw shafts to alter thespacecraft’s center of mass from front to back and from side to side.The mass trim operation is necessary to precisely align the spacecraft’sroll axis so that it passes through the centers of the gyros and thetelescope’s line of sight. The mass trim procedure also balances themass of the spacecraft so that it rolls smoothly around this axis.Also, this past week, we decreased the spacecraft’s roll rateincrementally from 0.9 rpm back to 0.5 rpm. During the first rolldowndecrement to 0.7 rpm, we discovered that the distribution of theliquid helium in the dewar is less predictable during roll-down than itis during roll-up. When the spacecraft’s roll rate is slowed too quickly,the liquid helium begins to slosh around. The resulting displacementof the center of mass from the sloshing helium affects the microthrusters, resulting in a significant increase in the time required tocomplete the roll-down.Last weekend, in preparation for optimizing ATC performance with14 instead of 16 micro thrusters, we uploaded revised drag-freethruster-control software to the on-board computer. After completingthe mass trim maneuver and stabilizing the spacecraft at the 0.5 rpmroll rate, we re-booted the on-board computer with the revisedsoftware. The re-boot went very smoothly, and a checkout of the newsoftware confirms that the two problematic thrusters are isolated andreceiving no helium, while the remaining 14 thrusters are respondingto commands as expected. With the new software up and running, wehave begun successfully testing both primary and backup drag-freemodes around gyro #1.Meanwhile, gyro #1 and gyro #3 are currently in the process of beingspun up to 3 Hz (180 rpm), and likewise, gyros #2 and #4 will be spunup to 3 Hz next week. Over this weekend, the team will also re-lock thescience telescope on the guide star, IM Pegasi, with the spacecraftrolling at 0.5 rpm and balanced along the telescope’s axis of sight.Before we begin calibration testing of the gyros at a spin rate of 3 Hznext week, our goal is to have the spacecraft rolling smoothly at 0.5rpm, locked onto the guide star, and in a drag-free orbit around one ofthe gyros. Photo: A picture of a gyro rotor before coating, beingmeasured for roundness many years ago.9 JULY 2004—MISSION UPDATE: DAY 80After 80 days in orbit, the spacecraft remains in excellent health, andall subsystems are continuing to perform well. All four gyros aredigitally suspended and are currently spinning at approximately 3 Hz(180 rpm). Two weeks ago, the on-board computer was re-booted witha new version of the drag-free thruster-control software to workaround two problematic micro thrusters that were isolated and takenout of service shortly after launch. The new software optimizes theperformance of the Attitude and Translation Control system (ATC)using 14, instead of 16 micro thrusters, and it has been performing asexpected since the re-boot. The spacecraft is flying drag-free aroundgyro #1, at a roll rate of 0.52 rpm, with the science telescope lockedonto the guide star, IM Pegasi. We are now entering the home stretchof the Initialization and Orbit Checkout (IOC) phase of the mission.On Saturday, July 3rd, we re-locked the science telescope on the guidestar, IM Pegasi, with the spacecraft rolling at 0.5 rpm. The spacecraftremained locked on the guide star throughout the 4th of July weekend.This past week, we continued testing and optimizing ATCperformance in both primary and backup drag-free modes. Based onthis testing, we have selected back-up drag-free as the nominal modefor the science phase of the mission. In back-up drag-free mode, theGyro Suspension System (GSS) applies very light forces on the gyro tokeep it suspended and centered in its housing. The ATC uses feedbackfrom the GSS to “steer” the spacecraft so that the GSS forces arenullified or canceled, thereby keeping the gyro centered. Applyingforces with the GSS to suspend the drag-free gyro adds a very smallamount of noise to the gyro signal, but this noise is negligible, and it isoutweighed by the added stability that has been demonstrated in ourtests over the past few weeks. This increased stability is due to the factthat the back-up mode uses both the micro thrusters and the GSS tocounteract drag on the spacecraft, whereas the primary mode reliessolely on the micro thrusters to create a drag-free orbit.Yesterday, the roll rate of the spacecraft was slightly increased from0.50 to 0.52 rpm, in preparation for the transition into the sciencephase of the mission. This slightly higher roll rate was chosen based onATC and SQUID readout performance, as well as the requirement thatthe science phase roll rate should not be a harmonic of either orbit orcalibration frequencies. We are in the process of evaluating whether ornot further mass trim operations will be needed for the science phaseof the mission.On Friday, July 2nd, we successfully spun-up gyros #1 and #3 to 3 Hz(180 rpm), by streaming ultra-pure helium gas through their spin-upchannels for 90 seconds each. This past Tuesday and Wednesday, July6th and 7th, we followed suit with gyros # 2 and #4. Note that wecannot control the exact spin rate of the gyro rotors (spheres). Rather,we control the length of time that ultra-pure helium gas flows throughthe spin-up channel for each gyro, and then the SQUID readouts tellus the resulting spin rates—which may differ slightly from one gyro toanother. Pending a final review meeting this afternoon, tomorrowmorning, we are planning to stream helium gas over the gyro #4 rotorfor another 90 seconds, thereby increasing its spin rate toapproximately 6 Hz (360 rpm). Then, after performing a number oftests to ensure that gyro #4 and its suspension system are functioningproperly, tomorrow afternoon, we plan to spin up this gyro to fullspeed by streaming ultra-pure helium over its rotor for approximately90 minutes. We anticipate that the final spin rate of the gyro #4 rotorwill be between 120 Hz (7,200 rpm) and 170 Hz (10,200 rpm). We willthen monitor the performance of gyro #4 for several days beforespinning up the remaining gyros to full speed.Gravity Probe B — Post Flight Analysis • Final Report March 2007 479