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

12.2 Temperature / pressure control of the Main TankThe primary function of the dewar is to maintain a tank of superfluid helium at approximately 1.8 K so that thescience instrument can be maintained at its operating temperature of ~2.5 K. Thermal stability can bemaintained by allowing the heat leaking into the main tank to evaporate the liquid helium at an appropriaterate. This evaporation occurs at the external surface of a sintered porous plug, which acts as a phase separatorthat retains the superfluid liquid helium in the tank. (The porous plug was invented and developed by GP-B andhas been utilized by other flight programs requiring superfluid helium, e.g., IRAS, COBE.) The vapor thusproduced is then vented through a chain of four heat exchangers that warm the gas to the external boundarytemperature. The heat used to warm the gas is intercepted heat that otherwise would be conducted or radiatedto the liquid helium. By this means, the rate of evaporation of the liquid is significantly less than what it wouldotherwise be. After traversing the heat exchangers, the gas enters the thruster manifold and then is supplied tothe sixteen thrusters.The ATC subsystem controls the gas flow from each thruster according to the needs not only for attitude andtranslation control but also to maintain the appropriate pressure in the main tank. In general, the boil-off rate ofthe dewar exceeds the amount of gas needed for thrusting, and, in that circumstance, the excess gas is “nulldumped” such that no net torque or force is produced. When there is positive excess gas flow, the ATC pressurecontrol algorithm controls the total gas flow in such a way as to maintain a constant pressure in the thrustermanifold. If, on the other hand, the demand for thrusting exceeds the amount dictated by the pressure controlloop (negative excess mass flow), ATC allows the needed flow regardless of the pressure control loop.If the negative excess gas flow is such that on average the flow exceeds that provided by the main tank undernormal heat load, the pressure will decrease in the main tank causing the temperature also to drop. This isaccompanied by excessive cooling of dewar heat exchangers due to the high flow rate. If this condition were topersist, it could be compensated for by applying electrical heat to the main tank. It was not necessary to do this,however. In fact, except for special events and circumstances, ATC did maintain the main tank within a 5 mKtemperature band, well within the 30 mK requirement.12.3 Events and conditions related to Dewar performanceThe liquid helium consumption rate is not only determined by long-term conditions, but also by various eventsand conditions. Most of these events occur during the IOC (Initialization and On-Orbit Checkout) because theyare either part of planned IOC activity or because most unanticipated events occur early in the mission as partof the shakedown process. In fact, no significant additional thermal events occurred during the remainder of themission.12.3.1 LaunchLaunch occurred with the main tank 95.5% full (337 kg) and at a temperature of approximately 1.8 K. Thesemeet the requirements of launching at least 95% full and at a temperature less than or equal to 1.9 K. The guardtank was maintained at 4.2 K with normal boiling point liquid helium in order to reduce the heat rate to themain tank and avoid any need for servicing the main tank on the launch tower. During ascent the guard tankvented through a flow restrictor and a check valve that fully opened in zero-g. (The flow restriction wasrequested by Boeing in order to ensure that the cold gas would not damage the fairing on ascent.) This blowdownand depletion of the guard tank caused a substantial sub-cooling of the guard tank, heat exchangers, andvapor-cooled shields. This temporarily maintained a heat rate into the main tank that was below the steady-statevalue, which, if anything, is beneficial to the performance of the system. (The guard tank normally runs in the 25– 30 K range when cooled only by vent gas from the main tank.) The flow restrictor prolonged the completedepletion of the guard tank, but this had no material impact on the performance of the cryogenic subsystem.336 March 2007 Chapter 12 — Cryogenic Subsystem Analysis