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Loop Heat Pipes combine the advantages of both heat pipes and Capillary Pumped Loops, while overcoming the limitations of each. Loop Heat Pipes provide very high thermal transport capacities; they can transport heat over long distances, through small cross-sectional tubes and have the capillary pumping capacity to overcome high gravitational heads. Most of these features are available from capillary pumped loops (CPLs), but unlike CPLs, Loop Heat Pipes are inherently self priming and totally passive in operation. This paper describes the operating principles of Loop Heat Pipes, provides performance data from hardware tests, describes some areas of ongoing development, and discusses applications, terrestial as well as space, where Loop Heat Pipes could confer major benefits.

Thermacore, Inc. along with the Idaho National Engineering Laboratory (INEL) is developing a Thermionic Heat Pipe Module (THPM) for use with small, Thermionic Space Nuclear Power Systems. The THPM concept is a core length, cylindrical thermionic diode with a near-isothermal heat pipe emitter sleeve coupled to reactor fuel by radiation, and a heat pipe collector which also forms the waste heat radiator. The design allows for independent, non-nuclear development and testing of the THPM hardware for later integration into the nuclear reactor. A hardware development program is currently underway to demonstrate key feasibility components and to fabricate and test prototype THPMs for application to the Small Externally-Fueled Heat Pipe Thermionic Reactor (SEHPTR) concept. This paper describes the scope and status of the THPM development program at Thermacore.

Reliable, low mass rotating heat transfer joints using heat pipes have many potential on-board spacecraft applications including hinges, rotating platforms and moveable radiators. These joints require a high thermal conductance and low outgassing and fluid leakage rates to prevent spacecraft contamination. This paper describes two concepts: the first has the joint in the heat pipe envelope; the second utilizes a high thermally conductive fluid in a gap between two separate heat pipes, one located inside the other. The second concept has been pursued through the testing of a developmental heat pipe-to-heat pipe joint. Thermal performance up to 1500 watts showed a joint thermal resistance of 0.008 to 0.01 C/watt (thermal conductance of 100-125 watts/C) in the temperature range of 25-50 C. Theoretical calculations show the gap fluid leakage to be less than 1 × 10−5 grams of NaK in five years.