Mars Rover RTG Study

Abstract

The paper describes the design and analysis of Radioisotope Thermoelectric Generators (RTGs) for powering the Mars Rover vehicle, which is a critical element of the unmanned Mars Rover and Sample Return mission (MRSR). The RTG design study was conducted by Fairchild Space Company for the U.S. Department of Energy, in support of the Jet Propulsion Laboratory's MRSR project.The paper briefly describes a reference mission scenario, an illustrative Rover design and activity pattern on Mars, and its power system requirements and environmental constraints, including the RTG cooling requirements during transit to Mars. It identifies the key RTG design problem, i.e. venting the helium generated by the fuel's alpha decay without intrusion of the Martian atmosphere into the RTG, and proposes a design approach for solving that problem. Using that approach, it describes and analyzes a variety of RTG designs, at! based on the proven and safety-qualified General Purpose Heat Source module.The first RTG option described is a very conservative baseline design, employing standard thermoelectric unicouples whose reliability and performance stability has been extensively demonstrated on previous space missions. The heat source of the 250-watt RTG consists of a stack of 18 separate modules that is supported at its ends but not along its length. The paper describes and analyzes the structure that holds the stack together during Earth launch and Mars operations, but allows it to come apart in case of an inadvertent reentry.It then summarizes the baseline RTG's mass breakdown, and presents a detailed description of its thermal, thermoelectric, and electrical analyses. It examines the effect of different operating conditions (beginning versus end of mission, water-cooled versus radiation-cooled, summer day versus winter night) on the RTG's performance. Finally, the paper compares the RTGs' specific powers for different power levels (250W versus 125W), different thermoelectric element designs (standard versus short unicouples, versus multicouples) and different thermoelectric figures of merit (0.00058K"The results presented show the RTG performance achievable with current technology, and the performance improvements that would be achievable with various technology developments. It provides a basis for selecting the optimum strategy for meeting the Mars Rover design goals with minimal programmatic risk and cost.

Uniform Fin Sizes versus Uniform Fin Root Temperatures for Unsymmetrically Obstructed Solar Probe RTGs

“…For unobstructed RTGs, such a simplified analysis is a useful initial design tool, since it permits closed-form solutions for the optimum area ratio Ap/Ap of the thermoelectric n-and plegs and for the optimum output voltage. For these optimized parameters, it yields simple expressions for the maximum material efficiency of the thermoelectric couples, and for the required RTG design parameters [5,6].…”

Design and Structural Analysis of Mars Rover RTG

“…A much fuller description of the work covered by both papers is contained in a separate Fairchild report [2], available from the author.…”

“…A more detailed description of the baseline RTG's design, mass breakdown, and of its structural, thermal, thermoelectric, and electrical analyses for a variety of environmental conditions is presented in a separate Fairchild report [2] available from the author. Figure 8.…”

“…The stress analyses to carry out these tasks are described in detail in Ihe full Fairchild report [2], and summarized below.…”

“…As shown in the full report [2], for the simplified case of a partitioned hollow box beam having a cross-section bounded by an outer square of side a and inner square of side b, with a length L, mass per unit length m', and subjected to a transverse g-load G, the preload P required to hold the partitioned beam together is given by The actual heat source stack is more complicated. The heat source modules that comprise the stack have curved cavities and non-uniform side walls.…”