Thermal Testing of the Mercury Transfer Module
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This paper describes the thermal vacuum testing of the Mercury Transfer Module (MTM) in ESA's BepiColombo mission to the planet Mercury. The tests, carried out in February and March of 2013, faced many unique challenges, both because of the demanding thermal design of the MTM and because of the challenges of reproducing the flight environment on ground. The MTM is the work-horse of the BepiColombo mission. During a 6 year cruise, it uses solar electric propulsion, gravity-assist manoeuvres and a chemical propulsion system to bring a composite stack of three spacecraft to Mercury. A three dimensional heat pipe network spanning 3 large radiators helps distribute in excess of 2kW of waste heat from the solar electric propulsion system and nearly a further kilowatt of environmental fluxes. Not only does proximity to the sun challenge the thermal designer (the MTM flies between 1.16 and 0.298AU), but a wide range of solar aspect angles must be also accommodated, along with large temperature excursions driven by the many on/off phases of the 13kW solar electric propulsion system. Remarkably, some quite cold conditions can also occur during the cruise phase and some special thermal measures are required on the launch pad. These aspects all require verification by test, as far as is practicably possible. The constraints of a solar simulator beam which cannot fully illuminate the MTM, a finite beam collimation angle and the complex heat pipe network made the design and conduct of the test uniquely challenging. Nevertheless, the testing and thermal design proved very successful.