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The Mars Surveyor Program will launch a lander vehicle to the southern polar regions of Mars in January, 1999 to evaluate Martian earth sciences and climatology. The lander vehicle will examine Martian soil samples take pictures of the Martian surface in the vicinity of the landing site and record weather data. As a “Faster, Better, Cheaper” program exposed to widely variant thermal and gravity environments, a robust yet cost-effective thermal design is mandatory. The need is amplified further by the extraordinary resource constraints on vehicle mass and power. A capillary pumped loop design has been selected because it provides a flight-proven adaptive control capability to match the range of environments encountered on the Martian surface, as well as during Earth-to-Mars cruise. During pre-launch spacecraft operation, the two phase system will be operated at low power in 1-g field.

In recent years, loop heat pipe (LHP) technology has transitioned from a developmental technology to one that is flight ready. The LHP is considered to be more robust than capillary pumped loops (CPL) because the LHP does not require any preconditioning of the system prior to application of the heat load, nor does its performance become unstable in the presence of two-phase fluid in the core of the evaporator. However, both devices have a lower limit on input power: below a certain power, the system may not start properly. The LHP becomes especially susceptible to these low power start-ups following diode operation, intentional shut-down, or very cold conditions. These limits are affected by the presence of adverse tilt, mass on the evaporator, and noncondensible gas in the working fluid.

This paper describes the flight test results of the fifth generation cryogenic capillary pumped loop (CCPL-5) which flew on the Space Shuttle STS-95 in October of 1998 as part of the CRYOTSU Flight Experiment. This flight was the first in-space demonstration of the CCPL, a lightweight heat transport and thermal switching device for future integrated cryogenic bus systems. The CCPL-5 utilized nitrogen as the working fluid and operated between 75K and 110K. Flight results indicated excellent performance of the CCPL-5 in a micro-gravity environment. The CCPL could start from a supercritical condition in all tests, and the reservoir set point temperature controlled the loop operating temperature regardless of changes in the heat load and/or the sink temperature. In addition, the loop demonstrated successful operation with heat loads ranging from 0.5W to 3W, as well as with parasitic heat loads alone.