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

A second mass trim operation had been scheduled after the spacecraft was placed in drag-free mode for thescience phase of the mission, but this was not necessary. In fact, no further mass trim refinements were everrequired.2.4.3.4 Space Vehicle Roll Up / Liquid Helium Bubble WrapThe space vehicle separated from the launch vehicle rolling at 0.1 rpm. During IOC the roll rate wasincrementally increased to 0.9 rpm for two reasons:1. To get better insight into adjusting the Mass Trim Mechanisms (MTM)2. To mass balance the space vehicle by wrapping the liquid helium bubble uniformly around the inside ofthe dewar wall.In addition to the mass trim operation performed mid-June, the so-called “bubble wrap” procedure wasperformed at the same time. During the bubble wrap procedure, the spacecraft’s roll rate was increased inincremental steps, from 0.3 rpm to 0.9 rpm. The increased roll rate begins to rotate the liquid helium, effectivelypushing it outwards by centrifugal force. This process is intended to wrap the liquid helium uniformly aroundthe inner wall of the dewar shell. Distributing the liquid helium uniformly along the spacecraft’s roll axis helpsto ensure that the science telescope can remain locked on the guide star while the spacecraft is rolling.Following the bubble wrap and mass trim procedures, the spacecraft’s roll rate was decreased incrementallyfrom 0.9 rpm back to 0.5 rpm. During the first roll-down decrement to 0.7 rpm, we discovered that thedistribution of the liquid helium in the dewar is less predictable during roll-down than it is during roll-up.When the spacecraft’s roll rate is slowed too quickly, the liquid helium begins to slosh around. The resultingdisplacement of the center of mass from the sloshing helium affects the micro thrusters, resulting in a significantincrease in the time required to complete the roll-down.2.4.3.5 Drag-Free OperationsDrag-free operations were first practiced during the third week of IOC. The term, “drag-free,” means that theentire spacecraft literally flies in its orbit around one of the gyros; this compensates for friction or drag in orbit.Signals from the Gyro Suspension System (GSS) are used by the ATC system to control the position of thevehicle, via the output of the micro thrusters, to fly the spacecraft around the selected gyro.The GP-B spacecraft can fly using either a primary or a back-up drag-free mode. In primary drag-free mode, theGSS suspension control efforts are turned off on one of the gyros, so that no forces are applied to it. The ATCuses feedback from the position of the designated drag-free gyro in its housing to “steer” the spacecraft, keepingthe gyro centered. Back-up drag-free mode is similar, but in this case the GSS applies very light forces on thegyro to keep it suspended and centered in its housing. The ATC uses the GSS to “steer” the spacecraft so that therequired GSS forces are made very low.The initial Drag-Free Control (DFC) checkout used the back-up drag-free mode and lasted 20 minutes, asplanned. Then, a two-hour DFC session was tested, during which the spacecraft roll rate was increased and thenreturned to its initial rate, maintaining drag-free status throughout the test. Then, three weeks later in mid June2004, after turning off the two malfunctioning micro thrusters, primary drag-free mode was successfully tested.Both drag-free modes continued to be tested periodically throughout July and August for varying lengths oftime. When the gyros were spinning slowly, back-up drag-free mode yielded more stability.Though primary drag-free control would be preferred due to the absence of forces acting upon a gyro andresulting in a data source free of any outside disturbance, analysis of on-orbit data confirmed that eitherprimary or back-up drag-free mode would meet the science needs of the mission. Towards the end of IOC,exhaustive testing demonstrated that primary drag-free required more helium mass flow than desired, whileback-up drag-free was both robust and reliable, and thus it was chosen to be used during the science phase.52 March 2007 Chapter 2 — Overview of the GP-B Experiment & Mission