Thermal Performance of the Cosmic Background Explorer Superfluid Helium Dewar, as Built and with an Improved Support System

Abstract

Launch of the Cosmic Background Explorer (COBB) is planned for 1990. A critical element of the COBE is the 650 L superfluid helium dewar that will maintain the cryogenic instrument assembly at ~2 K for an estimated 14 months. Life testing of the dewar was completed in the autumn of 1985. The support system heat conduction is a major part of the overall heat load to the cryogen. The COBE dewar uses a tension strap system made of fiberglass/epoxy; this is the most thermally efficient system used to date in flight hardware. However, the thermal efficiency of the COBE support system could be considerably improved by using a more optimized orientation of straps and a newly-developed composite material instead of the fiberglass/epoxy. A thermally optimized tension strap support system is defined based on recent fatigue testing of alumina/epoxy straps. This material has a modulus of elasticity over twice that of fiberglass/epoxy, and therefore displays greater fatigue strength. A support system using alumina/epoxy can have both smaller strap size and greater stiffness than a fiberglass system. The thermal conductivity is somewhat higher than that of fiberglass/epoxy. The more optimized strap orientation provides considerably greater strap length and thermal resistance, and the alumina material provides about 50 percent higher resonance frequency. Thermal performance predictions are compared for the COBE dewar (1) as built, (2) with fiberglass/epoxy straps in an optimized configuration, and (3) with alumina/epoxy straps in an optimized configuration. The optimized support designs provide a predicted cryogen lifetime improvement of about 50 percent.