Multi-Mission Radioisotope Thermoelectric Generator (MMRTG): Proven Power for Next Generation Radioisotope Power Systems

Abstract

The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) was developed to serve as a power source for a variety of space missions, from planetary surface to deep space interplanetary missions. Its special capability to handle the harsh surface environment of Mars makes it unique among recent RTGs, which could not survive on the surface of Mars. The special design characteristic of an isolated and hermetically sealed thermoelectric converter compartment enables long term performance in a multitude of environments. The heat source compartment is separated from the thermoelectric converter compartment by means of a metallic liner. The heat source generated helium gas is slowly vented to the exterior through a gasket which is designed to maintain a nominal 1 atmosphere pressure inside the heat source compartment. The heart of the MMRTG, its thermoelectric couple, is based on the proven design heritage of the Pioneer and Viking RTGs. The Pioneer RTGs provided power for more than 30 years, long beyond their design life of only three years. The Viking RTGs for the Mars landers had a design life of only 90 days but they were operating when the landers were shut down after four years and they continued to operate for at least another 14 years after that. All crucial design aspects developed over the decades that are key to the rugged, reliable and long life of the Pioneer/Viking family of RTGs have been carried into the MMRTG design. Notable among these is the thermoelectric module cold end hardware thermal management system. This applies a constant spring force on each individual thermoelectric leg while providing a minimal temperature drop from the thermoelectric cold junction to the external radiator, which maximizes thermoelectric and system efficiency. This paper will expand on these design features and present test results which demonstrate the MMRTG’s capability to reliably provide power for all types of deep space and planetary surface missions.