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In the Spring of 1993, the Clinton administration called for a scaled down space station as part of a reduction in the funding level available for NASA projects. To take advantage of the products of the SSF program in containing costs, NASA considered the use Space Station Freedom (SSF) components and concepts to develop a new design. The redesigned space station concept that was developed calls for a significantly different Thermal Control (TC) than was used in the SSF design. This paper provides an overview of the redesigned space station thermal control system. The top level design requirements are summarized as they relate to the functions the TC is to provide. The functional link of TC components and space station elements into the Internal Active TC (IATC), External Active TC (EATC), Photovoltaic Active TC (PVATC), Russian Segment TC (RSTC) and Passive TC (PTC) subsystems is discussed and presented in system schematics.

This paper presents a design and operational overview of the Space Station Freedom external thermal control system. The system utilizes two-phase ammonia for its operation. The paper discusses the three-loop design architecture for the overall system and its relationship to other station equipment. It also reviews the heat acquisition, transport, and rejection attributes of each as well as the thermodynamic mechanisms associated with each process. These involve ammonia boiling at the heat exchanger and coldplate heat acquisition sites, liquid ammonia supply and two-phase return flow from these sites, and direct condensing, flow-through radiators for heat rejection. Critical sizing and performance parameters such as two-phase pressure drop (and impact on loop isothermality), maximum/minimum load variations, and transient heat load variations are discussed.