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

A/D converter is broadband. Also, operation of the thruster system contributed to pressure fluctuations.) Ourexperience has indicated that TAO activity at the 0.1 torr level or less is completely inconsequential compared tothe normal parasitic heat rate. Thus, there is no evidence that TAO activity played a role in flight performance.12.4 Lifetime projectionMost missions that use expendable cryogens are typically observational in nature. Lifetime prediction is usefulfor planning and prioritization. In the case of GP-B, however, lifetime prediction is a bit more significant innature. This is because the mission plan calls for a post-science calibration phase to apply known perturbationsand measure their effects. This allows the quantification of various systematic errors. Many of these tests aredone at the end of mission because they involve perturbing fully spun-up gyros, effectively ending science andentailing some degree of risk. As a consequence, it is important to monitor dewar performance and periodicallymake updated predictions of the time of cryogen depletion.12.4.1 Thermal modelsBecause the dewar shell temperatures vary somewhat throughout the year, the heat rate to the main tank alsovaries. In order to predict future heat rates, it is useful to have a thermal model of the space vehicle that predictsthe dewar shell temperatures and a thermal model of the payload that estimates heat rate into the main tank as afunction of the dewar shell temperatures. The payload model correlates well with both ground measurementsand with on-orbit measurements when measured dewar shell temperatures are utilized. There is somediscrepancy, however, with the ability of the space vehicle model to predict dewar shell temperatures on orbit.This model predicts shell temperatures on the dewar forward cylinder that are as much as 10 K lower whenmeasured in the warm season (sun forward). Likewise, the model under-predicts the dewar top platetemperature by as much as 10 K during the cold season (sun aft). Discrepancies at other locations and seasonsare not as severe. The net effect of these discrepancies on payload performance, however, is not major. Theaverage flow rate predicted on the basis of measured shell temperatures is 7.68 mg/s, whereas the valuepredicted for calculated shell temperatures is 7.84 mg/s, a 2.1% discrepancy (Figure 12-3). It should be noted,however, that cryogen lifetime easily exceeds one year. That being the case, once the launch anniversary isreached, it is possible to look at the previous year’s data to aid in prediction. The perturbations occurring duringthe IOC phase tend to limit the utility of this approach, however.Gravity Probe B — Post Flight Analysis • Final Report March 2007 339