The modular RTG incorporates several technology improvements besides the multicouple. A new type thermoelectric material with a gallium phosphide additive has demonstrated a somewhat higher figure of merit than SiGe alone. A new lighter-weight multifoil insulation package has been included and demonstrated. By using ultra-thin molybdenum foils, separated by particles of zirconia, several kilograms of insulation mass can be saved compared to the current molyfoil/fibrous insulation without suffering additional heat losses. The new insulation is thinner, about half the thickness of the current insulation, and matches the shorter multicouple thermoelement length. This permits a smaller lighter generator housing. At a comparable 300 We power output, the modular RTG is expected to weigh about 13 kg less than the current GPHS-RTG, with a specific power of over 7 We/kg. The multicouple requires a good barrier material which must operate at temperatures up to 1000°C. To date, the glass used for this has evidenced degradation after several thousand hours at operating temperature. Efforts are continuing to develop a suitable barrier material which will permit long-lived, stable performance of the multicouple. Meanwhile, an improved GPHS-RTG option is being examined which incorporates most of the weight-saving features of the modular RTG, without the power modularity feature, By using a shorter SiGe unicouple (and only half as many per RTG), the thinner multifoil insulation package and the smaller generator housing. The power-to- weight ratio of the improved GPHS-RTG is expected to be similar to that of the modular RTG. The mechanical, electrical and thermal performance of the short unicouple remains to be demonstrated, and the improved GPHS-RTG would require requalification for flight use, but this option is a viable alternative for improved RTGs for future NASA missions. The MRSR mission to Mars presents further technical challenges to the RTG designer. The Mars Rover vehicle is expected to require an average power of 500 We for a period of several years on the Martian surface. Weight constraints on the MRSR mission will be extremely important. Thermal integration considerations during all phases of the mission are unique to this application. There are several major differences in design considerations between an RTG for use on the surface of Mars compared to one for use in deep space. The converter must be hermetically sealed to exclude the Martian atmosphere, yet the helium from the decay of the fuel must be vented overboard so as not to spoil the effectiveness of the thermal insulation. The waste heat must be rejected under cruise conditions on the way to Mars, during descent to the surface and during Rover vehicle operations on the surface. The RTG must operate under extreme day-night temperature cycles, and in the high winds and sandstorms which occur on Mars. Its waste heat must keep the electronics warm under cold night conditions, but must not overheat the electronics under direct solar heating conditions. The RTG must also withstand the acceleration and shock loads during landing, deployment and Rover vehicle movement over the surface. All of these unique design requirements must be met with a minimum weight RTG, which means that a different RTG technology will have to be employed than that which was used on the 1975 Viking Mars Lander RTGs. Conceptual design studies of RTGs for the MRSR mission are already under way within the DOE program in support of NASA planning and design efforts.
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