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

12.3.2 Thermal transient eventsAbout a week into the mission, large thermal transient events (temperature spikes) were observed at probe anddewar station 200 and elsewhere in the Probe (Figure 12-2). These events were logged as Observation 029, andwere noted to have a period of about 24 minutes and an amplitude of 20 - 40 mK at probe station 200. After fivedays, the amplitude had increased to 150 mK with a period of 40 minutes as measured at probe station 200, andan adverse impact on SQUID readout performance was noted. Although no direct evidence was available toconfirm it, it was felt that the only feasible explanation for this phenomenon was periodic desorption andadsorption of residual helium gas in the dewar well. (A small long-term leak through cold valve RAV5 allowedhelium to leak into the well during guard tank fill operations prior to launch. A special well pumping line hadbeen installed to allow the well to be evacuated after fill operations. Normally pumping operations would extendfor about a day, but due to prelaunch schedule limitations, this was curtailed to a few hours after the last filloperation prior to launch.) Because the well was slowly venting to space through an opened pyrovalve, it was feltthat this problem would eventually resolve itself. Indeed, the period between spikes continued to lengthen andthe phenomenon was self-extinguishing by the time the flux reduction operation (see below) was initiated. Theheating that occurred during the flux flush operation undoubtedly caused additional gas desorption andremoval from the well, and the phenomenon was never observed again. It should be noted that although thisphenomenon had a significant adverse impact on SQUID readout noise performance, it did not have asignificant impact on dewar thermal performance.Figure 12-2. Temperature spiking at probe station 200 (primary cryogenic probe/dewar interface).12.3.3 Stuck thrustersOn two separate occasions, after launch and near the end of the flux-reduction operation, one thruster(numbers 6 and 8, respectively) failed in a high-flow configuration. In each case, the problem was corrected byclosing the appropriate thruster isolation valve (TIV). (There is sufficient redundancy in the thruster system toGravity Probe B — Post Flight Analysis • Final Report March 2007 337