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

The helium in the dewar has now surpassed its estimated lifetime bymore than three weeks, and thus, our dewar team has been analyzingthe calculations and underlying assumptions on which their heliumlifetime predictions were originally made. (See the Mission News storybelow for more information about this.)Meanwhile, we have continued working our way through ourprioritized list of calibration tests. Last weekend, we completed theprocess of de-fluxing the SQUIDS—that is, removing electromagneticflux from the SQUIDs by heating them up a few kelvins, and thenallowing them to cool back down to their normal cryogenic operatingtemperature of 1.8 kelvin.On Monday, we attempted to switch from backup to primary dragfreeoperation on gyro #1. For various reasons, including a misconfigurationof the ATC system prior to the mode switch, the attemptfailed, and we reverted to backup drag-free mode on gyro #1.On Tuesday, we visited a virtual star 0.1 degrees east of IM Pegasi, inthe opposite direction from HR Pegasi. We remained locked in theposition for 48 hours, while the telescope team performed some darkcurrent calibration tests, and then we returned and re-locked thetelescope onto IM Pegasi on Thursday. We then attempted again toswitch from backup to primary drag-free mode on gyro #1, and againthe attempt failed. We are currently investigating the possible causesof this failure, and we plan to try this switch again nextweek...assuming that we still have helium in the dewar.GP-B MISSION NEWS—REVISING THE HELIUM LIFETIMEPREDICTIONSOur dewar team made its initial helium lifetime predictions by makingcalculations based on the results of several heat pulse meter tests thatwere performed at various points throughout the mission and someassumptions derived from the scientific literature and experience ofother spacecraft that used a helium-based cryogenic system.and the helium gas inside the dewar are in a state of thermalequilibrium. That is, they are both are maintaining the sametemperature due to close thermal contact between the liquid and gas.An alternative assumption is that there is poor thermal contactbetween the helium liquid and gas in the dewar, due to the low thermalconductivity of the vapor. If this is the case, the temperatures of theliquid and gas are not necessarily in equilibrium over the time scale ofthe measurement. That is, over short time scales, a temperatureincrease in the liquid helium does not necessarily result in acorresponding temperature increase in the helium vapor bubble.(Equilibration does ultimately occur, but over a longer time scale.)Taken to their extremes, these two assumptions result in two limitingcases for the helium depletion:1) Strong thermal contact between the two helium phases (the thinlayer of liquid helium coating the dewar wall and the bubble of heliumvapor in the center of the dewar are in thermal equilibrium over theshort time frame of the measurement—the initial assumption)2) Weak thermal contact between the two helium phases (a change intemperature in the liquid phase has little effect on the temperature ofthe gas bubble over the time scale of the measurement; rather, it takesmany hours for the helium vapor bubble to reach equilibrium with theliquid helium).The dewar team's initial predictions for the helium longevity werebased on the first assumption, which yielded the 5 September heliumdepletion date. However, if the second assumption is used, thelongevity of the helium increases by 5-6 weeks. In other words, theupper bound on the helium longevity, based on the secondassumption, places the helium depletion date around mid October. Itis most likely that the actual conditions inside the dewar liesomewhere between these two bounding assumptions. And, judgingfrom the dewar's performance thus far, it appears that the thermalcontact between the helium phases is weaker than was originallyassumed.As we have noted these past few weeks, as long as we still have heliumin the dewar, we will continue working our way through ourprioritized list of calibration tests. When the helium actually does runout, we will post a notice on our Web site and send out a message tothe subscribers of our GP-B Update email list. NASA will also issue anews release, and we will then post the content of that release on ourWeb site and send it to our email subscribers.30 SEPTEMBER 2005—GRAVITY PROBE B MISSIONUPDATEThe dewar team's initial calculations suggested that the helium in thedewar should have been depleted sometime around Labor Day (5September 2005). Because the helium has now lasted three weekslonger than the initial predictions, the dewar team has been reexaminingtheir calculations and the underlying assumptions used tomake them. The team has checked and re-checked their calculations,and it appears that no errors were made. This has led them to reevaluatethe assumptions underlying these calculations.Inside the dewar, there is some amount of superfluid liquid heliumand some amount of helium vapor (gas). The initial longevitycalculations were based on the assumption, as suggested by theliterature and performance of other spacecraft, that the helium liquidMission Elapsed Time: 528 days (75 weeks/ 17.3 months)—IOC Phase: 129 days (4.2 months)—Science Phase: 352 days (11.6 months)—Final Calibration Phase: 43 days (1.3 months)—Science II Phase: 4 daysCurrent Orbit #: 7,792 as of 4:00PM PSTSpacecraft General Health: GoodRoll Rate: Normal at 0.4898 rpm (2.04 minutes per revolution)Gyro Suspension System (GSS): All 4 gyros digitally suspendedDewar Temperature: 5.632 kelvin and risingGlobal Positioning System (GPS) lock: Greater than 95.0%Attitude & Translation Control (ATC): N/AY-axis error: N/ACommand & Data Handling (CDH): B-side (backup) computer incontrolMulti-bit errors (MBE): 0Single-bit errors (SBE): 7 (daily average)Gyro #1 Drag-free Status: OFF518 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission