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The International Space Station (ISS) Thermal Control System (TCS) can be categorized into three major subsystems: Passive Thermal Control System (PTCS), External Active Thermal Control System (EATCS), and Internal Active Thermal Control System (IATCS). Each of these segments of TCS is highly automated and as such is very dependent on the on-board software for monitoring and control of the various functions. For the PTCS, software is used to monitor temperature sensor data and command heaters on and off. The active thermal systems contain a large number and a large variety of equipment that is highly dependent on software control. This paper explores the important role that software plays in the operation of the ISS TCS and the need for the hardware and software development to be well integrated. A brief overview is provided of the design and architecture of the three major thermal control subsystems and the related thermal control software.

The Active Thermal Control System (ATCS) of the Space Station U. S. Laboratory (USL) module circulates coolant fluid to remove heat from subsystem and experiment equipment located in racks and endcones. The system is designed to provide active flow control and parallel service to each of the various racks. This arrangement provides several advantages but also raises concerns over the ability to control and balance the flow to each heat load as required. Since the USL will have fluctuating heat loads due to the operation and cycling of user experiments, the ATCS must respond to the cooling needs of the individual racks while maintaining adequate service to other experiments and to the Station subsystems. This paper addresses the capability of the ATCS to meet all cooling needs of the Space Station USL by implementing the parallel heat load with active flow control design.

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.