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. Significant parameters affecting heat rejection are also reviewed, and resulting system operational constraints and limits are identified.
The paper then tracks the buildup of the thermal control system that occurs naturally as the station is assembled on orbit. The paper discusses unique design and operational factors that occur during the early assembly periods of the station. These include the need to cross connect two of the three loops for ammonia circulation to all sites, sequentially connecting and activating additional sections of the system throughout the buildup period, and issues associated with activation of electrical power consuming equipment before cooling is provided to them.
Finally, the paper reviews the overall hardware arrangement on the station and the principal hardware assemblies (orbit replaceable units). Issues related to launch safety pertaining to ammonia-filled equipment and related design impacts for isolation and containment are reviewed.