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The Heat Rejection Subsystem (HRS) Radiators Orbital Replaceable Unit (ORU) for the International Space Station has undergone thermal vacuum qualification testing at NASA Plum Brook Station, Space Power Facility in Ohio. The testing was conducted from December 1996 through January 1997 and October 4 through 18, 1997. This testing included confirmation of the heater control assembly (HCA) and heater performance that was initially tested during December 1996 through January 1997. Deployment system functional operations were tested for both hot and cold conditions using both the Integrated Motor Control Assembly (IMCA) and the Extravehicular Activity (EVA) drive.

The International Space Station (ISS) Active Thermal Control System uses a single phase liquid ammonia system to collect and reject waste heat from the various space station systems. The expected cold environments in which the Heat Rejection Subsystem (HRS) radiators of the heat rejection system are to operate fall as low as -102.8 °C (-153 °F). Because the ammonia working fluid freezes at -77.7 °C (-108 °F) and since the environment temperatures are to remain below this level for 30 minutes per orbit, design approaches have been identified, implemented, and tested to ensure that the ISS Active Thermal Control System radiators will perform under these environments. There are several items of concern in a freeze-tolerant design. The flow tubes imbedded in the panel, from which heat is rejected, must be designed to tolerate potentially high pressure during a thaw. The supply and return manifold tubing must be designed to prevent ammonia from freezing within them.

The Space Station Freedom program planned to use a two-phase (vapor-liquid) ammonia system to collect and reject waste heat from the various space station systems. The predicted thermal environments in which the radiators of the heat rejection system were to operate fell as low as -117.8 °C (-180 °F). Because the ammonia working fluid freezes at -77.7 °C (-108 °F) and since the environment temperatures were to remain below this level for thirty minutes per orbit, design approaches were identified and implemented to tolerate these conditions. There are several items of concern in such a design. The flow tubes imbedded in the panel from which heat is rejected must be designed to tolerate potentially high pressure during a thaw. The supply and return manifold tubing must be designed to prevent ammonia from freezing within them.