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

of eclipses each day (approximately 15-16 per day). Figure 8-6 graphically shows the annual variation of thegamma angle, in conjunction with the spacecraft’s corresponding time in eclipse. Accordingly the thermal datais highly cyclical and dependent on the eclipse cycle of the vehicle as well as the seasonal variations.Figure 8-6. The annual gamma angle and eclipse cycle of GP-BDespite the thermally complicated nature of GP-B’s mission, there were no unexplained thermal transients orlimit violations. It should be noted, however, that the dewar vacuum shell temperatures deviated 25–30 degreesfrom model predictions. Chapter 12, Cryogenic Subsystem Analysis explains the reason for this deviation indetail. However, it is important to note that although the temperatures are higher than predicted, the dewar metits mean-mission-duration requirement of 16.5 months.8.2.1 System PerformanceThe Gravity Probe B Thermal Control System has performed exceptionally well from launch through thecompletion of the mission. All major TCS elements have demonstrated flawless performance and stability sincelaunch. The following series of plots are intended to graphically display the thermal trending of the spacecraft’smajor components throughout the duration of the mission.The spacecraft batteries are located on the battery pallet at the aft end of the vehicle. Each battery has anassociated heater which keeps the temperature around 0 degrees Celsius. The battery temperatures are cyclicdue to the oscillation of the heaters turning on and off. Additionally, the batteries have some seasonal variationin their temperature. As the sun moves to the front of the vehicle, the batteries become more dependent on theirheaters to maintain their 0 degree average temperature. Where as, when the sun is on the aft end of the vehicle,the heaters turn off. Figure 8-7 below, graphically displays the both batteries temperatures for the duration ofthe mission along with their associated yellow and red limits. It can easily be seen that the average temperaturestayed around the desired 0 degrees Celsius and no limits were violated.208 March 2007 Chapter 8 — Other Spacecraft Subsystems Analyses