Advanced AMTEC radioisotope power systems for deep space applications

Abstract

The alkali metal thermal to electric converter (AMTEC) used in the Advanced Radioisotope Power System (ARPS) program has been developed around a specific system configuration that was derived from the radioisotope thermoelectric generator (RTG) used for the last 20 years. This configuration, and the restrictions it places on the converter technology, limits the overall generator performance. An advanced AMTEC system design, that is not based on RTG heritage, using AMTEC chimney cells, is better suited for the AMTEC technology and provides substantially better power density and efficiency. It also provides system scale-ability from 10 to 100 Watts. While technology development continues to proceed with the ARPS cell design in order to validate the technology, efforts are also underway to develop the advanced technology to meet the more aggressive mass, efficiency, life and electromagnetic performance requirements of future missions. INTRODUCTION The alkali metal thermal to electric converter (AMTEC) has been considered to be a potential candidate for small radioisotope space power systems since the early 1980's. Recent converter technology advances and system design work by Hendricks et al. and Schock et al.' have shown these systems to be very attractive relative to the state of the art radioisotope thermoelectric generators (RTG's), currently used in all spacecraft radioisotope power system. The AMTEC technology not only reduces the system mass relative to RTG technology, it also requires significantly less radioactive material than the RTG's for any given power requirement. From mid-1997 to mid-1999, the Advanced Radioisotope Power System (ARPS) program, was focused on developing the first AMTEC power system for the Europa and Pluto missions. This power system, which has been described in the literature, is the first generation of AMTEC space systems. It is derived from the RTG system technology and has several features in common with the RTG systems. • The cells are mounted to the inside of a cylindrical housing. This The general purpose heat source (GPHS) is stacked, and the stack is held from each end in much the same way as the RTG. The cells use the same mounting feature used in the RTG unicouple. The cell must support the weight of the multilayer insulation package, system is 14% efficient and 21 kg for approximately 100-120 W, end of mission power. The use of RTG system reduced the system level design effort, however it also increased the AMTEC cell development risk by requiring that the technology be configured for the system as opposed to configuring the system to meet the technology needs. The combination of developing the technology under the constraint of the RTG heritage requirements and a number of changes in the spacecraft power requirements made AMTEC technology development very difficult. During the course of the two years of development, there were two major changes in system power level and one change in configuration. This resulted in 3 different cell revisions that were actually designed and tested. The final E-revision cell is shown in Figure 1. Figure 1. ARPS AMTEC Cell, Revision E. American Institute of Aeronautics and Astronautics (c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. With the challenges of developing the materials and fabrication processes for the refractory metal cell (previous designs were stainless steel), these design changes made it very difficult to mature the fabrication process. In mid-1999 DOE and NASA made the decision that AMTEC would not be ready for the first planned ARPS mission in 2003 or 2004. Cell testing had not yet reached the point of demonstrating reliable performance and was not yet ready for life testing at that time. Cell development has continued since mid-1999 and substantial progress has been made, with cells now capable of producing power at or near predicted levels and, in most cases, to operating reliably at nominal and elevated temperatures. These cells are still designed for the RTG heritage system, and while they are not the design that would provide the highest efficiency or power, they are a baseline to fully develop and prove the technology for mission use. Life testing could begin early next fiscal year. ADVANCED SYSTEM DESIGN An AMTEC system design that is significantly lower mass than the current ARPS design has been described in the literature. This design is not constrained by RTG heritage. It places the cells at only two ends of the heat source stack. A new method is used to support only two GPHS. It eliminates the cell mounting features used in the ARPS cell. This system is over 20% efficient and 8.9 kg for approximately 100 W, end of mission. Closure Plate Cell Interconnects