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The potential for a catastrophic hazard occurring on the International Space Station United States on-orbit segment exists due to using a single barrier heat exchanger as the interface between external active thermal control systems and the internal active thermal control systems of the habitable elements and the incompatible maximum design pressures of the respective systems. Pressurized ammonium hydroxide may engulf crewmembers if proper controls are not designed into the integrated system. Engineering review and analyses were made to determine the failure mechanisms and recommend controls to mitigate the hazard and lower the risk to a program acceptable level.

This paper describes the integrated active thermal control systems analysis of proposed operational scenarios for Space Station Freedom. The analysis uses detailed mathematical models of Freedom's water, air, and ammonia cooling loops. Also included, is an electrical connectivity model to provide for proper equipment response due to any failure, either electrical or thermal. The operational scenarios analyzed for this paper are developed by the National Aeronautics and Space Administration in order to provide various disciplines with realistic operational conditions and station construction conditions. The station construction process generates many unique conditions and configurations. During this time, many of the normal system backups are temporarily disabled or reduced in effectiveness.