Technical Paper
International Space Station Design-to-Freeze Radiators
1997-07-01
972345
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.