Design and Analysis of RTGs for CRAF and Cassini Missions

Abstract

The design and analysis of Radioisotope Thermoelectric Generators integrated with JPL's CRAF and Cassini spacecraft are described. The principal purposed of the CRAF mission are the study of asteroids and comets, and the principal purposes of the Cassini mission are the study of asteroids, Saturn, and its moons (particularly Titan). Both missions will employ the Mariner/Mark-2 spacecraft, and each will be powered by two GPHS-RTGs. JPL's spacecraft designers wish to locate the two RTGs in close proximity to each other, resulting in mutual and unsymmetrical obstruction of their heat rejection paths. To support JPL's design studies, the U.S. Department of Energy asked Fairchild to determine the effect of the RTGs' proximity on their power output. This required the development of novel analysis methods and computer codes, described in this report, for the coupled thermal and electrical analysis of obstructed RTGs with axial and circumferential temperature, voltage, and current variations. The code was validated against measured data of unobstructed RTG tests, and was used for the detailed analysis of the obstructed CRAF/Cassini RTGs. Also described is a new method for predicting the combined effect of fuel decay and thermoelectric degradation on the output of obstructed RTGs, which accounts for the effect of diminishing temperatures on degradation rates. The computed results indicate that for the 24-degree separation angle of JPL's baseline design, the mutually obstructed standard GPHS/RTGs show adequate power margins for the CRAF mission, but slightly negative margins for the Cassini mission.