Estimating the Next Gen RTG Mod 1 Performance Based on Analysis of the New Horizons Mission

Abstract

The Next Generation Radioisotope Thermoelectric Generator (Next Gen RTG) is a project aimed at restarting production of the high performing General Purpose Heat Source (GPHS) RTG. Unfortunately, the performance of most GPHS-RTG missions is not expected to be representative of the Next Gen RTG. Heat sources used by the U.S. today are the slightly larger Step-2 GPHS module. This will reduce the amount of $^{238}\mathbf{PuO_{2}}$ fuel that can fit within the RTG down to $\boldsymbol{\sim 3900\mathrm{W}_{\text{th}} (\text{from}\sim 4400 \mathrm{W}_{\text{th}})}$. This reduced thermal inventory will lower RTG temperatures, which will reduce performance. New Horizons is a current GPHS-RTG mission that was loaded with 3948 $\mathbf{W}_{\mathbf{th}}$ of $^{238}\mathbf{PuO_{2}}$, suggesting that New Horizons performance may be similar to the future Next Gen RTG. The rate law analysis method has been successfully used to convert RTG telemetry into equations that describe behavior. A rate law analysis of New Horizons was used to determine the degree of thermoelectric degradation and thermal inventory losses. This analysis showed that the thermoelectric degradation mechanism on New Horizons changed after 8.0 years, and that this change in mechanism is likely tied to the thermal inventory of the fuel at that time. These equations were then used to produce a preliminary estimate the performance of the Next Gen RTG. Based on this estimate, Next Gen RTG will produce $\boldsymbol{184 \mathrm{W}_{e}}$ at the end-of-design-life (EODL) if the unit is not stored before launch, and $\boldsymbol{177 \mathrm{W}_{\mathrm{e}}}$ if the unit is stored for 3 years before launch. Performance curves were also produced. It is important to realize that this preliminary estimate is based on a lot of assumptions, including: how the Step-2 module will affect performance, mission specific parameters, and potential changes to the Next Gen RTG as the design evolves. A discussion of these assumptions is provided. It may be necessary to update this estimate as additional Next Gen RTG design information becomes public.