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The success of CAPL 2 flight experiment has stirred many interests in using capillary pumped loop (CPL) devices for spacecraft thermal control. With only one evaporator in the loop, CAPL 2 was considered a point design for the Earth Observing System (EOS-AM). To realize the full benefits of CPLs, a reliable system with multiple evaporators must be developed and successfully demonstrated in space. The Capillary Pumped Loop (CAPL 3) Flight Experiment was designed to flight demonstrate a multiple evaporator CPL in a space environment. New hardware and concepts were developed for CAPL 3 to enable reliable start-up, constant conductance operation, and heat load sharing. A rigorous ground test program was developed and extensive characterization tests were conducted. All performance requirements were met, and the loop demonstrated very reliable operation.

This paper presents the analytical results of a thermal hydraulic study to determine an “optimum” two-phase heat pipe/heat exchanger radiator panel configuration for the Space Station Freedom. The study was based on using conventional axially grooved heat pipes in combination with integral two-phase heat exchangers. Various design parameters were traded to arrive at an optimized panel design that satisfied the thermal requirements. For two-phase flow across a radiator array consisting of eight panels with fourteen heat pipes per panel, small diameter lines acting as flow restrictions are needed at the exit of each heat exchanger to balance the flow across each panel and the radiator array. The paper also presents the test results with a representative subscale heat pipe/heat exchanger radiator panel. In general, the heat pipes exhibit transport capabilities that exceed the design requirements. Balanced flow across each heat exchanger was also demonstrated.