Coupled Thermal and Electrical Analysis of Obstructed RTGs

Abstract

A Radioisotope Thermoelectric Generator (RTG) with an unsymmetrically obstructed heat rejection path can have significant axial and circumferential variations in the temperatures, currents, and voltages of its thermoelectric couple network. The present paper describes a methodology for analyzing the thermal and electrical performance of such an RTG, and the development of a computer code for implementing that methodology. The code derives coupled solutions of the RTGs thermal, thermoelectric, and electrical equations. It accounts for the Peltier effect, Ohmic heating, and the Thomson effect, and treats the electrical power produced in each couple as an effective heat sink. It satisfies the condition that all parallel couples produce the same voltage, and that all series-connected couple groups produce the same current. Finally, the paper illustrates the use of the code by applying it to the detailed analysis of the RTGs for the CRAF and Cassini missions. In each of these, there are two adjacent RTGs which are obstructed by each other and by the nearby spacecraft. The results of the analysis will be used by the spacecraft designers in selecting the location, orientation, and spacing of the two RTGs.