Uniform fin sizes versus uniform fin root temperatures for unsymmetrically obstructed solar probe RTGs

The Solar Probe will approach the sun within four solar radii or 0.02 AU. Because of that proximity, the spacecraft must be protected by a thermal shield. The protected umbra is a cone of 4 m diameter and 7.5 m height, and all temperature-sensitive flight components must fit within the cone. Therefore, the radioisotope thermal generators which power the Solar Probe cannot be separated from each other and from other payload components by deploying them on long booms, as was done on previous missions. Instead, the RTGs must be located near and thermally isolated from the spacecraft's payload. As a result, only about half of each RTGs circumference has a direct view of space. The other half is cooled indirectly, by means of a semi-cylindrical reflector. For a standard RTG design with uniform radiator fins, the unsymmetrical reflectors result in significant circumferential variations in the thermocouples' cold-junction temperatures, voltages, and currents. This can be avoided by varying the dimensions of the radiator fins, so as to produce a uniform set of fin root temperatures. The paper compares the performance of such variable-fin RTGs with that of uniform-fin RTGs. It derives the fin dimensions required for circumferential isothermicity, identifies a design that maximizes the RTG's specific power, and proves the practicality of that design option