Experimental verification of a 60 K thermal storage unit

To reduce the cooling power requirements and weight of space-based IR sensor systems with time varying focal plane loads, cryogenic thermal storage units (TSUs) with embedded phase change materials (PCMs) represent a viable solution. This paper describes the design and verification of a dual-volume 60 K TSU, which uses nitrogen trifluoride (NF₃) as the working fluid. The dual-volume design divides the TSU into a cryogenic temperature heat exchanger volume and an ambient temperature storage volume. This approach increases TSU energy storage capacity and thermal stability while reducing total weight and volume. The TSU heat exchanger has a beryllium shell to ensure CTE compatibility with beryllium focal planes. The heat exchanger core is constructed of a 40 ppi, 6101 aluminum foam, compressed to 35% relative density. Developmental testing has shown that the aluminum foam core, which is vacuum furnace brazed to the beryllium end-caps, tends to suppress NF₃, single-phase supercooling. Also, by virtue of its slight capillary wicking capability, the foam core eliminates concerns over heat exchanger filling in 0-g. The TSU will be tested on-orbit in a Hitchhiker GAS Canister flight experiment (designated as CRYOTSU) on the STS is late-1998. In this flight experiment, the TSU storage tank will be made of thin-walled stainless steel and the system will be pressurized to less than 100 psi to minimize safety concerns