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

7.3.3.1 DFS – PerformanceThe GP-B drag-free control system has for most of the mission performed nominally. Drag-free control reliablyactivates and deactivates on command and has operated essentially without fail. Drag-free control has beenshown to reduce control forces by a factor of 10 or more. The rms control efforts applied to the proof massgyroscope are on the order of 10 nanonewtons, and the cross track control force at the roll rate frequency, whichcan generate gyro torques, is roughly 1 nanonewton. Figure 7-13 shows the control effort time history beforeand during a six hour drag-free run on August 15, 2004, with gyroscope 3 acting as the drag-free sensor.Gyroscope 3 and 1 were used as the drag-free sensor gyroscopes throughout the Science Mission phase. Thecontrol efforts are clearly reduced once drag-free control is activated.Figure 7-13. Drag-Free performanceBecause the performance of the back up drag-free control system has typically been superior to that of the primedrag-free control system at low gyro spin rates, the back up drag-free configuration was selected for use duringthe Science Mission phase.Since the science mission has started, the spacecraft was configured in backup drag-free on gyro 3 for most ofthe time. There is one stretch of time where the spacecraft used gyro 1 instead of gyro 3, due to an unexpecteddrop out of drag-free on gyro 3. There have also been several instances where the vehicle dropped out of dragfreeentirely to enter a safe mode, but the cause has been something external most of the time. Furthermore,most of these periods have lasted for only a matter of hours, short enough that negligible effects on the scienceresults are expected.7.3.4 Pressure Controller/Mass FlowThe helium mass flow rate from the dewar is set with the primary function of maintaining the dewar at aconstant temperature and pressure. That is, the flow rate is set to match the seasonal heat input to the dewar.The ATC uses mass flow as required to maintain attitude and a drag-free orbit. Short periods may exceed theaverage heat input do not appreciably change the tank temperature. Any excess mass flow output by the dewar,but not required for ATC control, is null dumped by adding equal flow to thruster pairs, thrusters aligned inopposite directions. Should the ATC require more flow than output by the dewar, the ATC may demand a briefadditional amount of gas flow beyond the dewar setpoint. This is termed “negative excess mass flow”.In Figure 7-14, mass flow data is shown starting on 2005/239. At this time, the commanded dewar flow rate is7.0 mg/s (although the dewar temperature controller can adjust this amount as needed by 10- 20%). In thefigure, the flow rate data spanning from 0-2 days, represents two days of testing drag-free on science gyro #3, ina 2-axis control mode (the Z-axis is not under drag-free control). During this time, the average ATC mass flowis approximately 3.0 mg/s, with the remaining 4.0 mg/s null dumped to space. The flow rate data spanning from186 March 2007 Chapter 7 — Attitude & Translation Control Subsystem Analysis