Empirical Performance Analysis of MMRTG Power Production and Decay

MMRTG is the only flight qualified radioisotope power system available for use by NASA. Recently, telemetry from the first flight unit (F1) was analyzed to obtain a performance prediction at 17 years of life. Despite this analysis, it is difficult to have high confidence in a life prediction from a single system. In this paper, measurements from the MMRTG engineering unit (EU) will be analyzed to improve confidence in the F1 power predictions. The EU has experienced multi-year life testing under two different conditions: simulated thermal vacuum and diurnal cycle testing similar to F1 conditions on Mars. Analysis of the EU thermal data indicates that the unit is experiencing a steady increase in hot junction temperature, which is most likely due to a concurrent decrease in thermal conductivity. Power analysis indicates that temporary degradation mechanisms play a major role in the degradation rate of MMRTG. These mechanisms produce different rates at different temperatures, but once they reach completion, the total degradation will be the same. In other words, units operated at cooler temperatures will have higher power early in life, but all units should produce similar power late in life. EU power data was adjusted to account for the differences between EU and F1 operational conditions. These results provide a prediction of power out to the end-of-design life (EODL) at 17 years after fueling. Results from the simulated thermal vacuum data predict a monthly average EODL power of 75.2±0.4. We, while the diurnal cycle data predicts 76.0±0.4. We These results are very similar to the 75.2 We prediction previously provided for F1. The EU was also placed into conditions equal to F1 10.0 years after fueling. In this F1 simulation test, the EU produced an average 90.1 We, which is similar to the 89.7 We monthly average produced by F1. These results suggest that the EU is a very good indicator for the degradation behavior of F1. In addition, the fact that two different MMRTG units produce similar performance and degradation results significantly increases confidence in the F1 predictions, suggesting that F1 could be a leading indicator of performance for other MMRTG flight units. While the EU appears to be a good predictor of flight unit performance, it is important to note that the EU has experienced significantly more physical and thermal stresses compared to a flight unit. This makes a statistical comparison of EU and F1 difficult. In addition, it is possible that the EU may produce anomalous behavior when the unit experiences conditions outside the nominal flight envelope.