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

control the space vehicle without these two thrusters.) As a result, there was excessive loss of liquid helium.Some of this loss is recoverable (sub-cooling of the heat exchangers and vapor-cooled shields temporarilyreduced the heat rate; flow rate can consequently be reduced to allow temperatures to recover to normal values),but there is also some permanent loss.12.3.4 Heating events in the ProbeThere were three heating events in the Probe. These events added additional heat load to the main tankprimarily through the station 200 interface, but also to some extent through residual gas conduction in thedewar well. These events were 1) low temperature bakeout rehearsal, 2) trapped flux reduction, and 3) lowtemperature bakeout (post gyro spinup). The first and third events were relatively mild with maximumtemperatures in the 6 –7 K range and durations of less than a day. The trapped flux reduction process producedtemperatures up to 13 K and lasted for 36 hours. This event warmed the main tank over 1.9 K as the maximumvent rate (limited by choking of the porous plug) was not high enough to maintain heat balance. Functionally,however, all the heating events met their requirements and accomplished their purposes.12.3.5 Drag-Free operationAfter gyro spinup and at the commencement of drag-free operation, it was found that there was sufficientnegative excess flow rate (i.e., demand by ATC for flow in excess of that called for by the pressure control loop)to cause the main tank to cool. Compared to the stuck-thruster incidents, however, this incident was ratherminor with the negative excess occurring only during a fraction of each orbit. If this matter could not beresolved, however, it had the potential for shortening mission lifetime. It was subsequently discovered that if thepressure control loop were given additional control authority (a total range of 3 mg/s instead of 0.6 mg/s) thesubsequent reduction in null dumping was sufficient to make up for the periods of negative excess flow andstabilize the tank temperature in the long run. The main tank has enough heat capacity that it does not reactnoticeably to orbital period flow variations.12.3.6 Effect of Dewar shell temperatureThe parasitic heat rate into the main tank (i.e., the heat rate in excess of that imposed by the science instrument)is ultimately a function of the dewar vacuum shell temperature. The exposed portion of the dewar vacuum shell(the cone and cylinder) is passively cooled by use of a FOSR (flexible optical solar reflector) film. This filmreflects visible solar radiation while being an effective radiator to space in the infrared wavelengths. In general,shell temperatures followed seasonal trends as expected (colder when the sun was aft and illuminating thespacecraft, and warmer as the sun moved forward). On some occasions, however, the dewar shell temperaturewould run warmer than predicted (see discussion on lifetime estimation below), and this would have thetendency to elevate the heat rate into the main tank.12.3.7 Thermo-acoustic oscillations (TAOs)A TAO is a pressure oscillation that can occur in any gas-filled line that traverses a large thermal gradient, forexample, the vent line of a cryogenic system. It is driven by the thermal gradient, and can sometimes cause adramatic increase in heat rate to the cryogen. TAO activity has been previously observed in the main tank ventline in some non-flight configurations, and the question naturally arises as to whether it has affected heat rateon orbit. The answer is no for two reasons: The entire GP-B vent system was analyzed by Dr. Sidney Yuan, arecognized authority on TAO analysis, who determined that it was unconditionally stable against TAO.Secondly, the short-term peak-to-peak variation in the unfiltered thruster manifold pressure data during flightwas approximately 0.1 torr. (Some of this is undoubtedly electronic noise since the analog circuitry before the338 March 2007 Chapter 12 — Cryogenic Subsystem Analysis