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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.