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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 Corporation1,2, 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.

A paraffin-actuated heat switch has been developed for thermal control of the batteries used on the 2003 Mars Exploration Rovers. The heat switch is used to reject heat from the rover battery to a radiator. This paper describes the development test program designed, in part, to measure the thermal conductance of the heat switch in an 8 Torr CO2 environment over the expected operating temperature range of the battery. The switch has a closed conductance of about 0.6 W/°C and an open conductance of 0.019 W/°C. The test program also included measuring the battery temperature profile over a hot case and a cold case Mars diurnal cycle. The test results confirm that the battery will remain well within the upper and lower allowable flight temperatures in both cases.

The Mars Science Laboratory (MSL1) mission to land a large rover on Mars is being planned for Launch in 2009. As currently conceived, the rover would use a Multi-mission Radioisotope Thermoelectric Generator (MMRTG) to generate about 110 W of electrical power for use in the rover and the science payload. Usage of an MMRTG allows for a large amount of nearly constant electrical power to be generated day and night for all seasons (year around) and latitudes. This offers a large advantage over solar arrays. The MMRTG by its nature dissipates about 2000 W of waste heat. The basic architecture of the thermal system utilizes this waste heat on the surface of Mars to maintain the rover's temperatures within their limits under all conditions. In addition, during cruise, this waste heat needs to be dissipated safely to protect sensitive components in the spacecraft and the rover.

Passively activated thermal control valves were developed for use in a mechanically pumped single-phase fluid liquid loop (MPFL) of the Mars Science Laboratory (MSL) rover. A key approach to the thermal control of the rover with the fluid loop is to control the flow through the rover's heat generating or heat rejecting components. This is achieved by either splitting or mixing the fluid stream coming from different branches of the system at different temperatures; actively or passively controlled flow valves are typically used for such purposes. To meet the thermal control requirements of the Mars Science Laboratory (MSL) rover, a splitting and a mixing thermal control valves with gradual control capabilities using a linear thermal actuator and a spool was developed at Jet Propulsion Laboratory (JPL). The key feature of these control valves is the balancing of the flow through the various branches of the fluid loop in order to balance the heat loads of the whole thermal system.

NASA's Exploration Mission program is planning for a return to the Moon in 2020. The Exploration Systems Mission Directorate (ESMD)'s Lunar Architecture Team (LAT) is currently refining their lunar habitat architectures. The Advanced Thermal Control Project at the Johnson Space Center, as part of the Exploration Technology Development Program (ETDP) is developing technologies in support of the future lunar missions. In support of this project, a trade study was conducted at the Jet Propulsion Laboratory on the mechanically pumped two-phase and single-phase thermal loops for lunar habitats located at the South Pole for the LAT II architecture. This paper discusses the various trades and the results for a representative architecture which shares a common external loop for the single and two-phase system cases.