Analytical Thermal Model Validation for Cassini Radioisotope Thermolelectric Generator

The Saturn-bound Cassini spammft is designed to rely, without preccdcn~ on the waste heat from its three radioisotope thcrrnocl~”c generators (RTGs) to warm the propulsion module subsystem, and the RTG end dome tcmpcmture is a key determining factor of the amount of waste heat delivered. A previously vatidatcd SINDA thermal model of the RTG was the sole guide to understanding its complex thcrmat behavior, but displayed large discrcpmcics against some initist thermal development test data. A careful rcvatidation effort M to significant modifications and adjustments of the model, which result in a doubling of the radiative heat transfer from the heat source support assemblies to the end domes and bring up the cad dome md flange tcmpcraturc predictions to within 2W of the pertinent test data. The increased inboard end dome tcmpcmturo has a considerable imoact on thcrrnat control of the soawcdt central body. Ile &tidstion Process offbrs an cxfkplo of physicstiy-&vcm anatyticat model calibration with test ~ from not onty an elcctricat simulator but also a nuclear-tiled flight uni~ and has established the end dome tcmpcmtums of a flight RTG where no in-flight or ground-tat data existed before. INTRODUCTION The Cassini spacecraft has ken developed for a mission to investigate Saturn and its rings, satellites and magnctosphcrc. The spacwraft will be launched in October 1997 and powered by three Radioisotope Thcrmoclcctric Generators (RTG, see Figure 1), l’hc utilimtion of the RTG waste heat as a major heat source for thcrmst control of the Propulsion Module Subsystem (FMS) is a new concept that has never been applied bcfors, neither for Gatilco nor for Ulysses (missions to explore Jupiter and the sun, rcspcctivcly). Thcrmat development test was conducted using an electrically heated RTG simulator to demonstrate that the RTCls can provide a signiflcmt part of the heat necessary to warm the PMS (Mircles and Stul@ 1994), and it was found that the RTG end dome tcmpcmturc is cnticat in dctcrminiig the amount of heat entering the PMS cavity (a large MU blanket drapes over the propellant tanks forming the cavity, not shown in Figure 1 for clarity). However, analysis indicated that there was a large discrepancy between the flight RTG therms! analytical model pmdktions and the test results based on the RTG simutator, cspcciatty with regard to the end dome tcmpnturcs (the initial modcthcst deviation on the cnd dome tcmpanturc was as large as 62W). This raised questions concerning the adequacy of the simulators as well as the analytical model. FIG. 1 THE CASSINI SPACECRAFT