International Space Station Design-to-Freeze Radiators 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. At the maximum temperature/pressure, the flow tube volume will increase by 0.3%. The melting temperature increases at high pressure, and the volumetric expansion of the solid to liquid phase change is reduced to 6.7% at the maximum temperature/pressure. Because of the small increase in tube volume at the flow tube center line maximum temperature/pressure, the NH3 will remain in a frozen state until the ends of the flow tube have thawed allowing a small liquid film to exit the two dimensional frozen plug. The flow tube maximum temperature is -67.8°C (-90.1°F), and the corresponding pressure is 124 MPa (18.0 ksi). The flow tube design has adequate structural margin to accommodate this high pressure.
The radiator panel is designed with variable flow tube spacing to ensure all of the flow tubes within a single panel do not freeze simultaneously. The flow tubes on the edge of the panel with the longest fin length freeze first. Their flow, and corresponding heat load, are then transferred to the remaining non-frozen flow tubes.
The radiator flow tube design-to-freeze approach and panel freezing pattern have been tested. Testing of the design-to-freeze approach verified that the analysis contained in this paper is accurate. Testing of the flow tube freezing pattern has also been performed and shown to correlate with computer models of the panel.