Design and Control of Space Station Freedom U.S. Laboratory Active Thermal Control System

The Space Station Freedom (SSF) Active Thermal Control System (ATCS) “collects, transports, and rejects waste heat from the pressurized elements.” The US Laboratory (USL) ATCS is independent of other SSF elements, and supports all subsystem and payload cooling requirements within the US Laboratory including redundant cooling of life and station critical loads. The thermal transport capability of the USL internal ATCS is sized to accommodate 28.6 kW of waste heat which includes electrical power (25kW), DDCU electrical conversion losses (2.2 kW), DDCU fixed losses (0.2 kW), and crew metabolic loads (1.2 kW). Active control of the coolant flowrate is required to manage the heat rejection capability of the Thermal Control System (TCS). The ATCS accomplishes these functions through pumped, single phase water thermal transport loops which collect waste heat via coldplates and heat exchangers and transport that heat to heat exchangers external to the module. Two separate but cross-connectable loops provide either low temperature (.56°C - 5.6°C) or moderate temperature (12.8°C - 18.3°C) cooling fluid at each TCS served location (rack) within the module. Equipment racks are plumbed into the ATCS loops in a parallel, reverse-return manner. This arrangement increases flexibility of location of equipment within the module while allowing proper flow balancing for simultaneously varying heat loads in multiple locations. ATCS fluid is circulated via two centrifugal pump packages (one for each loop) either of which is capable of providing sufficient performance to support required subsystem and payload liquid cooling needs in separate loop or cross-connected operations. The pressure differential between the supply header and the parallel return header is automatically controlled at a constant value so that each rack maintains the same inlet to outlet pressure drop. The rate of flow through each rack is actively controlled based on either temperature or flow rate feedback. These provisions allow active management of the thermal loads each rack can impose upon the two thermal transport loops and enable efficient scheduling of ATCS cooling capability. The requirements applicable to the ATCS are satisfied by implementation of specific design features of the transport loops and major components. The performance of the ATCS is demonstrated through considerations of operational scenarios and relevant development test plans and results. A description of the system and component level controls methodology is also included, as well as the approach to redundancy management issues.