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The International Space Station's (ISS) External Active Thermal Control System (EATCS) radiator wings are rotated to provide benign thermal environments. The Radiator Goal Angle Calculation (RGAC) software determines the radiator wing orientation relative to the space station. The Thermal Radiator Rotary Joint (TRRJ) is the hardware that rotates the radiator wing on the ISS. The radiator wing orientation is determined by two opposing criteria. First is the requirement to have a cold thermal environment to reject waste ISS heat. Second is the requirement to keep the EATCS working fluid of ammonia (NH3) from freezing. The compromise between these two opposing requirements is to maintain a constant thermal environment of Tcmd = -51°C (-60°F). The actual rotation profile is based on an edge-to-sun orientation on the sun side of the orbit and an Earth facing orientation on the shadow side of the orbit.

The International Space Station's (ISS) thermal radiators are designed to tolerate ammonia freezing conditions. The cold case thermal design environment for ISS is -92.8°C (-135°F). This environment is below the freezing point of ammonia, the External Active Thermal Control System's (EATCS) working fluid, Tfreeze = -78°C (-108°F). Ammonia contracts 10% by volume when it freezes. Liquid ammonia can fill in this 10% volume and hard pack the individual flow tubes in the radiator. A hard packed flow tube filled with frozen ammonia would have to be able to tolerate this 10% volume increase when the ammonia thaws. The ISS radiator flow tube design accommodates the volume change of thawing ammonia. The most severe condition will arise if the center of a flow tube thaws while the ends remain frozen; thus, any increase in pressure has no axial relief. The volumetric expansion of thawing ammonia will strain the flow tube by exerting a high pressure.

The International Space Station Alpha's (ISSA) thermal radiators are designed to tolerate ammonia freezing conditions. The cold case thermal design environment for ISSA is -92.8°C (-135°F). This environment is below the freezing point of ammonia, the Active Thermal Control System's (ATCS) working fluid, Tfreeze = -78°C (-108°F). Ammonia contracts 10% by volume when it freezes. Liquid ammonia can fill in this 10% volume and hard pack the individual flow tubes in the radiator. A hard packed flow tube filled with frozen ammonia would have to be able to tolerate this 10% volume increase when the ammonia thaws. The ISSA radiator flow tube design accommodates the volume change of thawing ammonia by three mechanisms. The most severe condition will arise if the center of a flow tube thaws while the ends remain frozen; thus, any increase in pressure has no axial relief. The first mechanism to accommodate the volumetric expansion is the straining of the flow tube when exposed to high pressures.