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. The panel design must guarantee that a sufficient number of flow tubes remain open during freezing conditions in order to comply with specified hydraulic parameters. In addition, the concepts adopted to address these issues must not significantly impair the hot environment performance.A fully functional radiator panel was built, instrumented and tested in a thermal vacuum chamber to demonstrate these concepts. This paper will report on the test and its results.An integral requirement for an effective test is realistic environmental transients. Heating element testing was performed to determine the optimum method to produce the severe transients experienced when the spacecraft travels in and out of the Earth's shadow. A bare Nichrome wire heating element was chosen for its fast time response characteristics.Operational modes are partially defined by the flow loops within the panel. Two independent loops are present with the potential for equal or uneven flows between loops, or one unused loop. The environments also play a role in operation, with a range of environment temperature from -117.8 °C (-180 °F) to -23.3 °C (-10 °F).Test instrumentation includes differential and absolute pressure transducers, thermocouples, and an infrared camera. The camera monitored panel performance, which included the two-phase length of ammonia within each tube. Thermocouples were installed on the test article internally and externally. They provide reference temperatures for the camera as well as differential temperature for heat flow calculations.The test results confirmed the panel thermal design by demonstrating high load heat rejection in a moderate environment and moderate load heat rejection in the cold orbits. Freezing and thawing of the ammonia in the individual tubes during the test was verified as well as the hydraulic performance during worst case freezing environments.