Actively Controlled Loop Heat Pipes as a Human Spacecraft External Active Thermal Control System

Recent United States human spacecraft (Space Shuttle, International Space Station, and Orion) have used two loop active thermal control systems. In these systems, a non-toxic water-based fluid flows through an internal thermal control loop to remove cabin heat loads. The internal loop connects with an external loop at an interface heat exchanger. The external thermal loop used a low freezing point fluid that flows through radiators and rejects the spacecraft waste heat to space. A radiator bypass valve controls the external fluid return temperature which maintains the internal fluid supply temperature within its specified limits.

It has been proposed to use a system of loop heat pipes (LHPs) as an external thermal control system. Such a system would have no moving parts and could be designed to be insensitive to puncture by micrometeoroids and orbital debris. The internal loop would flow through heat exchangers attached to the LHP evaporators. The LHP condensers would be attached to the radiator. By controlling the LHP compensation chamber temperatures, the return temperature of the internal loop fluid would be controlled within the desired setpoint limits.

The present work is an analytical study of an internal loop/LHP system. Series and parallel flow arrangements to the LHPs are explored and various possible control methodologies are investigated. A detailed analysis is performed on the configuration with the simplest control scheme. The operation of the control scheme is delineated, the required hardware complexity is explored, and the LHP system is compared to the external loop system.