Installing Scientific Instruments into a Cold LHe Dewar — The Gravity Probe B Approach

Gravity Probe B is an orbital test of Einstein’s general theory of relativity using gyroscopes. The precession of the gyroscopes will measure both the geodetic effect (6.6 arc sec/yr) through the curved space-time surrounding the Earth and the motional effect (0.042 arc sec/yr) due to the rotating Earth dragging space-time around with it. To achieve the extraordinary accuracies needed to measure these small precessions, it is necessary to have the gyroscopes operating in the following environments: a vacuum of < 10−10 torr; an acceleration level of < 10−10 g’s; a magnetic field of < 10−7 gauss; and a temperature near 2 K. This paper discusses designs that allow scientific instruments to be installed into a dewar at 4.2 K. Methods for structurally supporting the instruments, transferring heat across joints at low temperature in vacuum and excluding air during the insertion process are discussed. The structural support method is designed for Shuttle launch loads. The separable heat transfer joints are needed for cooling the instruments through the four cool vapor cooled shields and structural support in the dewar. Both structural and thermal resistance test data for the joints are presented. A design of a helium purged air lock is discussed that allows the insertion or removal of the instruments into or out of the dewar. The design permits multiple insertions and removals without refurbishment.