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The external active thermal control system (ATCS) of International Space Station (ISS) in the assembly-complete configuration has 24 radiator panels on each side, designed to reject the Station's waste heat to deep space. The radiators use liquid ammonia as the working fluid. The effective sink temperature of the radiators may vary from -112° C to 116° C (-170° F to 240° F) depending on several factors including the orbit parameters and attitude and orientation of the Station. At sink temperatures below the ammonia freezing point of -77.7° C (-107.9° F) and low heat load conditions, the radiator flow tubes may freeze. Therefore, the radiators are designed to withstand freezing and thawing. The purpose of this study is to develop a detailed SINDA/FLUINT model of the ATCS radiators and use it to predict the freeze/thaw cycles of the radiator flow tubes at extremely cold environments, and examine its impact on the performance of the ATCS loop.

The thermal radiators are a major subsystem of the Space Station Freedom (SSF) Active Thermal Control System (ATCS). They dissipate to deep space the excess heat transported from the modules and truss mounted equipment. Condensation of the ATCS twophase working fluid occurs directly in small diameter tubes which are bonded to a thin aluminum face sheet in the flow-though radiator panels. The Permanently Manned Capability (PMC) configuration of the Space Station will have a total of 48 radiator panels grouped in 3 replaceable units of 8 panels on each side of the Space Station. Accurate prediction of radiator performance on orbit is important to keep the ATCS from getting too hot (exceeding its capacity) or getting too cold (freezing). For this reason, detailed models of the radiator system are being developed using the SINDA/FLUINT thermal and fluid systems analyzer.