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The first space flight cryogenic flexible diode heat pipes (CFDHPs) were developed and verified under micro-gravity conditions on the Space Shuttle during STS-94 (July 1997) and the previous, minimum mission STS-83 (March 1997). The heat pipe working fluids were oxygen (with an operating range of 60 to 145 Kelvin) and methane (95 - 175 Kelvin). The heat pipes were verified as part of the Cryogenic Flexible Diode Heat Pipe (CRYOFD) flight experiment. CRYOFD was the third and fourth flights of the Hitchhiker based Cryogenic Test Bed (CTB). CRYOFD was managed by the Air Force Research Laboratory (AFRL) Phillips Research Site with NASA's Goddard Space Flight Center (GSFC) co-sponsoring the experiment, the Air Force's Space Test Program (STP) and GSFC's Hitchhiker (HH) group provided the Shuttle integration and support. Jackson and Tull (J&T) and Swales Aerospace, Inc. (SAI) executed the program as a Phase II SBIR under the AFRL.

This paper summarizes flight test results which were obtained in the Cryogenic Two Phase Flight Experiment (CRYOTP). This was a HitchHiker canister experiment that was flown aboard the space shuttle Columbia in March of 1994. Two flight articles were tested independently. The first was a nitrogen heat pipe with five parallel fibrous copper cable wicks. It did not startup in either of two cooldown cycles. Post flight inspection of the heat pipe showed that it had its original fluid charge. The failure to startup is attributed to the large fluid inventory and the conduction gradient that existed due to nominal parasitic heat leaks along the titanium heat pipe tube. Complete success was obtained with the Brilliant Eyes Thermal Storage Unit (BETSU) which contains 2-methylpentane phase change material for temperature control at 120K. More than 200 hours of on orbit tests consisting of several cooldown cycles and 55 freeze/thaw cycles was obtained with BETSU.