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

Figure 12-3. Helium flow rate predicted by dewar model based on both the measuredshell temperatures and predicted temperatures.12.4.2 Helium mass gaugingThere are three independent ways of determining helium usage. Two of the three, the flow meter and the heatpulse measurement system, are instruments built into the dewar. The remaining is a mass flow rate estimategenerated by the ATC software, based on commanded control force and the nominal specific impulse of eachthruster. The two flow rate measurements must be integrated downward from the initial quantity of liquidhelium (337 kg at launch as determined by a superconducting liquid level sensor coupled with a solid model ofthe interior of the main tank), whereas the heat pulse measurement yields a direct measurement of remainingliquid. Other factors involved in these measurements will be reviewed below.12.4.3 Initial lifetime estimateThe prediction for on-orbit performance made prior to launch was 18.8 months with no contingency. Thisresult did not include impacts of planned thermal events. For purpose of a prediction near the end of IOC,vacuum shell temperatures were adjusted upward by a fixed temperature increment. This was a conservativeway to incorporate elevated dewar shell temperatures. Using this approach, the payload model predicted anannual average boil-off rate of 8.3 mg/s. Based on the heat pulse measurement result of 269 kg on 28 June 2004,we expected a remaining life of 10.5 months as of 8/23/04 (near the end of IOC). Total predicted lifetime was14.7 months from launch compared to the mission requirement of 16.5 months. According to the ProgramChief Scientist, however, this was deemed sufficient to meet mission needs.340 March 2007 Chapter 12 — Cryogenic Subsystem Analysis