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Loop Heat Pipes (LHPs) are passive two-phase heat transport devices that have been baselined for many spacecraft thermal management applications. The design life of a spacecraft can extend to 15 years or longer, thus requiring a robust thermal management system. Based on conventional aluminum/ammonia heat pipe experience, there exists a potential for the generation of noncondensible gas in LHPs over the spacecraft lifetime. In addition, some applications would have the LHP evaporator attached directly to spacecraft equipment having large thermal mass. To address the potential issues associated with LHP operation with noncondensible gas and large thermal mass attached to the evaporator, a test program was implemented to examine the effect of mass and gas on ammonia LHP performance. Many laboratory test programs for LHPs have heat delivered to the evaporator through light-weight aluminum heater blocks.

In response to the identified needs of emerging high power spacecraft applications, a multiple evaporator Hybrid Loop Heat Pipe (H-LHP) was developed and tested as part of a Dual Use Science and Technology (DUS&T) program co-sponsored by ATK and AFRL/PRP. During the course of the DUS&T program, a two-kilowatt system with three evaporators was developed and tested to identify viable system architectures and characterize system performance capabilities as a function of heat load profiles and spatial distribution of the evaporators. Following the successful development of the two-kilowatt system, a 10-kilowatt system with six evaporators was fabricated and tested. Tests were performed with the system operating in a totally passive, capillary-pumped mode, where applying a small amount of power to a sweepage evaporator provides the auxiliary flow through the primary evaporators, and as a self-regulating, capillary-controlled mechanically pumped system.