Design and Flight Qualification of a Paraffin-Actuated Heat Switch for Mars Surface Applications

The Mars Exploration Rover (MER) flight system uses mechanical, paraffin-actuated heat switches as part of its secondary battery thermal control system. This paper describes the design, flight qualification, and performance of the heat switch. Although based on previous designs by Starsys Research Corporation^1,2, the MER mission requirements have necessitated new design features and an extensive qualification program. The design utilizes the work created by the expansion of a paraffin wax by bringing into contact two aluminum surfaces, thereby forming a heat conduction path. As the paraffin freezes and contracts, compression springs separate the surfaces to remove the conduction path. The flight qualification program involved extensive thermal performance, structural, and life testing. Thermal performance testing both at the component level and in a simulated flight configuration³ has shown thermal conductance values greater than 1.2 W/K in the closed position and less than 0.018 W/K in the open position. The dynamics tests included random vibration, hard landing loads, and pyrotechnic shock. The life testing included 20,000 mechanical cycles on the seal boot containing paraffin and 350 temperature induced cycles. The switch has met all performance and qualification requirements and is flight qualified for use on the MER mission.