Understanding Degradation and Heat Losses in the MMRTG and an Improved Analysis of F1 on Curiosity

Radioisotope thermoelectric generators slowly lose power over time. Power losses are attributed to two effects, heat losses and degradation in the thermoelectric converter. Understanding how heat losses and thermoelectric degradation impact performance will improve our ability to understand, analyze, and predict the behavior of an RTG. Life testing the MMRTG Engineering Unit and Qualification Unit included conditions that allow heat losses to be directly calculated for these generators. The Engineering Unit was measured at a heat loss rate of 0.979% power loss per year, while the Qualification Unit showed 1.01% per year. The similarity observed in both units improves confidence that an average value (0.99% power loss per year) will be a good representation of heat losses in other MMRTGs. This includes F1 on Curiosity, F2 on Perseverance, and the future Dragonfly mission. This represents the first report of an empirically derived heat loss value for MMRTG. Results from life testing the MMRTG Engineering Unit also provide clues regarding thermoelectric degradation. MMRTG appears to experience degradation of the electrical resistance and thermal conductivity across the interfaces in the thermoelectric couple. Degradation of the thermal conductivity appears to follow a linear trend for almost 5 years before a change in the degradation is observed. Electrical degradation, on the other hand, appears to follow a consistent trend over the entire 10 years of life testing. Using the empirically derived heat loss value, an updated analysis of MMRTG F1 has been performed. This analysis shows the individual contributions from heat losses and thermoelectric degradation. This updated analysis also shows that F1 appears to be experiencing a change in degradation mechanism, and that this new mechanism appears to have caused degradation in the MMRTG to slow down.