GPHS module assembly. The unfueled spare RTG has been in bonded storage and should deliver full power output when fueled with a new GPHS module assembly. The nominal power output of the GPHS-RTG is marginal for the 300 We/RTG at BOM requirement currently projected for the Cassini spacecraft. However, there is insufficient time to develop, demonstrate and qualify a new RTG on the Cassini program schedule. Therefore, two new GPHS-RTGs will be fabricated and fueled with enough Pu-238 to meet the Cassini power requirements. Future RTG Missions NASA plans to use the Mariner Mark II spacecraft for additional planetary exploration missions where increased RTG power levels and lower RTG weights are expected to be important. Another major NASA program which will require improved radioisotope power systems is the Mars Rover Sample Return (MRSR) mission around the turn of the century. In anticipation of such future applications, the DOE is conducting, and will continue to pursue, technology programs to advance the state-of-the-art for space RTGs. These technology programs include improved thermoelectric materials development to achieve higher figures of merit and heat to electrical conversion efficiencies; more lightweight and thinner thermal insulation packages; lightweight, modular converters using multicouples; lightweight, shorter unicouple converters; improved heat source module design and development; and improved safety technology. When these advancements have been adequately demonstrated, they can be included in future flight RTG programs. An example of an ongoing RTG technology program is the modular RTG. A sketch of a typical modular RTG is shown in Fig. 3. The objective of this program is to reduce the weight of the non-heat source components (compared to the GPHS- RTG) and to modularize the power output, at the full 28 V, to match the modularity of the heat source assembly provided by the GPHS modules. The key to the modular RTG is the thermoelectric converter element called the multicouple. The multicouple is composed of 40 small thermoelectric elements (20 n-type and 20 p-type) in a close- packed array separated by thin glass insulator/barrier layers and bonded between and electrically insulated from a heat collector and a cold-shoe mounting bolt. Eight such multicouples surround each GPHS module to produce about 16 We at 28 V. The power output of the modular RTG can be tailored by stacking up the required number of GPHS modules and their corresponding rings of multicouples with minimal redesign.
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