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

For this reason, the order in which the gyros are spun up is veryimportant. Earlier in the IOC phase, the 3 Hz (180 rpm) spin-upprovided information on the helium leakage rate of each gyro. Gyro#4, which had the highest leakage rate, was spun-up to full speed first,so that helium leaked from its spin-up would not affect other gyrosthat were already at science mission speed. The remaining gyros arethen spun-up in decreasing order 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, heliumgas is flowed over the gyro for 90 seconds, and tests are run to ensurethat the helium leakage rate for that gyro corresponds to previousmeasurements. If everything checks out, the full-speed spin-up, inwhich helium gas is flowed over the rotors for 2-3 hours, commencesearly in the afternoon. The GP-B team controls the spin-up process bysending commands from the Mission Operations Center (MOC) hereat Stanford to the spacecraft in real-time. For example, they sendcommands to open or close the GMA valves to flow helium throughthe gyro’s spin-up channel. They also control the amount of heatapplied to the gas before it enters the gyro spin-up channel, and theycontrol opening and closing of exhaust valves. Real-time telemetryprovides immediate feedback on the progress of the spin-up so thatvarious parameters can be adjusted as necessary.The successful spin-up of gyro #4 to full speed enabled us to spin-upgyro #2. Gyro #2 topped out at 87 Hz (5,220 rpm), which is onlyslightly above the minimum spin rate of 80 Hz (4,800 rpm) requiredfor the science experiment. Also, the helium gas leakage from the Gyro2 spin-up slowed gyro #4 down to 91Hz (5,460 rpm). We are takingsome time to evaluate the data from the first two spin-up proceduresand to perform some ground-based tests before spinning up gyros #1and #3 to full speed.Meanwhile, we are fine-tuning the drag-free software used by theAttitude and Translation Control system (ATC) to optimize itsperformance at the current and final spacecraft roll rate of 0.52 rpm.For part of each orbit, the spacecraft passes behind the Earth, causingthe telescope to lose visual contact with the guide star. During this“eclipse” period, two standard external rate gyroscopes, plus the startrackers on either side of the spacecraft, enable the ATC to keep thespacecraft/telescope pointed towards the guide star. When thespacecraft emerges over the North Pole and the guide star becomesvisible again, the telescope must be re-locked onto it. This re-lockinghas been taking up to 15 minutes, and the fine-tuning will speed upthis process considerably.23 JULY 2004—GRAVITY PROBE B MISSIONUPDATE: Day 94One day #94 of the mission, Gravity Probe B is poised to enter thehome stretch of the Initialization and Orbit Checkout (IOC) phase ofthe mission. The spacecraft is in excellent health, and all subsystemsare continuing to perform well. All four gyros are digitally suspended,with gyros #2 and #4 spinning at science mission speed—greater than80Hz (4,800 rpm). Gyros #1 and #3 are spinning at less than 1.5 Hz (90rpm) and are ready for full-speed spin-up next week. Fine-tuning ofthe Attitude and Translation Control system (ATC) is still in progress,and the ATC is performing well. The spacecraft’s roll rate is 0.52 rpm,and the science telescope is locked onto the guide star, IM Pegasi.Last Friday, the full-speed spin-up of gyro#2 went smoothly, with afinal spin rate of 87 Hz (5,220 rpm). Helium gas leakage from the spinupof gyro #2 caused gyro #4 to slow down from 105.8 Hz (6,348 rpm)to 91 Hz (5,460 rpm). We had hoped that gyro #2 would achieve a spinrate above 100 Hz (6,000 rpm), with less leakage effect on gyro #4.Thus, rather than spinning up gyros #1 and #3 as originally planned,we spent the past week doing analysis and running tests—both on thespacecraft and here at Stanford—in order to ensure that the upcomingspin-up of gyros #1 and #3 will result in higher speeds, with lessleakage effect on the remaining gyros.The spin rate of the gyros during the Science Phase of the missionaffects the signal-to-noise ratio in the SQUID readouts of theexperimental data. The noise level is quite small, but constant. Thehigher the gyro spin rate, the larger the London moment (magneticfield created by a spinning superconductor), and thus, the greater thesignal-to-noise ratio. In ground testing prior to launch, we determinedthat a spin rate of 80 Hz (4,800 rpm) or greater for each gyro wouldprovide a good signal-to-noise ratio for the science mission. However,the threshold of 80 Hz (4,800) rpm is not a hard and fast limit, so if thefinal spin rate of one or more gyros falls slightly below this value, thiswill not appreciably compromise the science data.One way to potentially increase the spin-up rate of the remaining twogyros, while reducing the amount of helium gas leakage during spinupis to use the Gyro Suspension System to position the gyro rotorscloser to the spin-up channel in the gyro housing. Tests and analysisperformed this past week indicate that we can move the rotors of gyros#1 and #3 up to 30% closer to the spin-up channels than gyros #2 and#4, and still have a safe margin of clearance from the suspensionelectrodes and the gyro housings. We have also determined thatopening a second exhaust valve during spin-up may help to reduce thepressure in the Probe caused by helium leakage, thereby reducing thespin-down effects on the remaining gyros. Both of these changes willbe implemented in the spin-up of gyros #1 and #3 next week.Also, this past week, we continued fine-tuning the drag-free softwareused by the Attitude and Translation Control system (ATC) tooptimize its performance at the current and final spacecraft roll rate of0.52 rpm. Tests from parameter changes we made to the ATC systemindicate that we have reduced the time it takes to re-lock onto theguide star from as much as 15 minutes to less than 2 minutes.31 JULY 2004—GRAVITY PROBE B MISSIONUPDATE: Day 102Gravity Probe B successfully accomplished a major milestone thisweek as the final two gyroscopes were spun up to high spin speed. Allfour gyros are now spinning at high spin speed. The spacecraft is inexcellent health, and all subsystems are continuing to perform well.With this major accomplishment behind us the Program is enteringthe home stretch of the Initialization and Orbit Checkout (IOC) phaseof the mission.This week's high speed spin-up operations were performed on gyro #3(Tuesday) and gyro #1 (Friday). There were no anomalies during thespin up. The SQUID readout FFT tracked the spin up in real-time. Asexpected, the other gyros spun down approximately 15% with eachhigh speed spin-up operation due to pressurization of the Probecaused by gas leakage during the spin-up. Final spin speeds of thegyros are now: gyro #1 (80.0 Hz - 4800 rpm), gyro #2 (62.3 Hz - 3738rpm), gyro #3 (82.7 Hz - 4962 rpm) and gyro #4 (65.5 Hz - 3930 rpm).At these spin speeds, GP-B’s relativity measurement is expected to besignificantly better than specification. Therefore, no further gyro highspeed spin-up operations are planned.Fine-tuning of the Attitude and Translation Control system (ATC) isnearly complete, and the ATC is performing well. This past week, wecontinued fine-tuning the drag-free software used by the ATC tooptimize its performance at the current spacecraft roll rate of 0.52rpm. Tests from parameter changes we made to the ATC systemGravity Probe B — Post Flight Analysis • Final Report March 2007 481