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

Solution: That kind of window material meets all of these requirements? The answer is sapphire. Sapphire isextremely strong, it has very high thermal conductivity, it is an electrical insulator, and it is opticallytransparent. Sapphire has a crystalline structure, that could cause a phenomenon called birefringence or doublerefractions of a single light beam. However, due to the sapphire's strength, Window #4 could be made relativelythin, and birefringence was not a problem. In addition to an anti-reflective coating, which was also applied tothe three fused quartz windows, Window #4 was treated with an electromagnetic interference (EMI) coating toattenuate the ambient low-frequency and microwave electromagnetic radiation leaking into the Probe throughthe wide open optical aperture, both in space and on the ground. Because EMI coatings use metal, which isopaque, they reduce the optical transmission of the window. Thus, there was a trade-off between theeffectiveness of the EMI shield and optical transmission requirement. Finally, Window #4 also included athermal emissivity or “low e” coating that reduced the amount of heat it transmits into the Probe.Perhaps the greatest challenge with Window #4 was its hermetic vacuum seal, which took many designiterations to arrive at a working solution. At the time Window #4 was constructed, rubber O-rings weretypically used to create vacuum seals, but they would not have been able to withstand temperatures down to 200K, which was a requirement for GP-B. Another option was to braze (very high temperature soldering) a metallicseal onto the outer edge of the sapphire window. However, brazing was deemed too risky, given thermalexpansion issues and the very high cost of sapphire. Ultimately, the Stanford-Lockheed Martin engineeringcollaboration came up with a unique seal that had never before been tried: Window #4 was surrounded by anIndium-coated “C” seal, lined with a thin layer of gold against the sapphire, and held in place by a ring ofspringy nickel-chromium-iron alloy called Incline that is more typically used in formula race car exhaustsystems.Result: Window #4, along with its unique hermetic vacuum seal, was an engineering tour de force—a synergyof design constraints, ingenuity, and engineering prowess. This window and its hermetic vacuum seal havefunctioned flawlessly throughout months of pre-launch testing, and two years in orbit.3.2.3.12 Precision Pointing TechnologyFigure 3-22. GP-B—The first spacecraft with 6 degrees of freedom in position and attitude controlChallenge: Develop a spacecraft attitude and position control system that could meet the precision pointingand drag-free flight requirements of the GP-B experiment, including all of the following constraints:Constraint 1: Provide precision attitude control for a minimum of 16 months on-orbitConstraint 2: Maintain a continuous spacecraft roll at a selected rate, in a range of 0.1- 1.0 rpm.Gravity Probe B — Post Flight Analysis • Final Report March 2007 91