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

accomplish this, the ARPs are mounted on posts that, in turn, are mounted on a pair of very stiff graphite-epoxyrings that provide the warm termination points for the forward PODS. The graphite rings are mounted justinside the dewar vacuum shell at the top of the vacuum shell cylinder by means of nine titanium flexures (SeeFigure 7-20 below). Nominally, the flexures provide radial compliance to provide isolation from thermallyinduced expansion and contraction of the dewar vacuum shell, but are stiff in other directions. The graphiteepoxyrings, for their part, have a high modulus and are designed to have a coefficient of thermal expansion thatis near zero relative to that of aluminum. The aft end of the vacuum shell cylinder is bolted in turn to the dewarsupport ring, which provides the attach points for the spacecraft truss on the outside and termination points forthe aft PODS on the inside. The ARP posts pass through openings in the vacuum shell cylinder and are sealed tothe vacuum shell by flexible bellows.The foregoing arrangement is intended to meet the requirement that the drift of the boresight of the star sensorin the roll direction relative to the science instrument be less than 10 arcseconds per year and not be correlatedwith any periodic behavior of the satellite. Evidence from the ATC system (section 8.1.1) indicates, however,that significant relative motion does occur at roll, orbit, and annual rates. This motion is correlated with thetemperature of the dewar vacuum shell in a complex way: the short-term correlation (orbital period) has theopposite sign of the long-term correlation (seasonal time scale). Although other mechanisms may exist, itappears that the most likely explanation is that thermally-induced expansion and contraction of the dewarvacuum shell cylinder is being coupled to the graphite rings via the flexures and causing them to distort. Thedetails of the mechanism and, in particular, the mechanism underlying the time-scale-dependent nature of thecorrelation are currently not well understood.Figure 7-19. Side view of partially assembled dewar showing primary structural elementsexcept for forward PODS.194 March 2007 Chapter 7 — Attitude & Translation Control Subsystem Analysis