Design, Analyses, and Fabrication Procedure of Amtec Cell, Test Assembly, and Radioisotope Power System for Outer-Planet Missions

Abstract

This paper describes the results of OSC's department of energy (DOE)-sponsored alkali metal thermal-to-electric conversion (AMTEC) generator studies. The paper was prepared in response to an invitation from the International Astronautic Federation for the presentation of a 1-h keynote lecture on our AMTEC studies at the Space Nuclear Power and Propulsion Systems session of the 48th International Astronautical Congress. The paper covers a broad range of topics, which explains its unusual length. After presenting the background of those studies and the operational principles of the AMTEC converters, it describes a novel methodology for the coupled solution of the interdependent thermal, electrical, and fluid flow differential and integral equations governing their performance, and the application of that procedure to OSC's recommended cell design. As explained in the paper, that design was the result of parametric analyses of 35 cell design variations to determine the individual effects of 27 design parameters and various operating parameters on the cell's power output, efficiency, and critical temperatures. The analysis revealed a method of substantially improving the cell's performance by enhancing the internal reflectivity of the cell wall, and identified a specific cell wall composition and coating to achieve that goal. The paper presents a set of OSC-recommended cell design parameters for the ultimate long-service flight generators, as well as a set of parameters for an interim cell design for planned short-term ground tests. The primary difference between them is that the ultimate design employs refractory metal components to enhance compatibility with high-temperature sodium vapor, while the interim design is based on stainless steel, nickel, and Haynes-25, which are expected to be adequate for initial short-term tests, but not for long-duration missions at high operating temperatures. The paper also presents a detailed fabrication sequence for the recommended interim cell design, which is being implemented by AMPS, Inc., for a DOE-sponsored technology development program. It then describes the extension of that analytical procedure to a variety of OSC-designed radioisotope-heated generators employing the recommended cell design, with particular attention to the thermal insulation between the outside of the 16 cells and the inside of the generator housing. The studies found that the performance of the generator is optimized by employing a hybrid insulation system, in which the space between the cells is filled with fibrous Min-K insulation, and the generator walls are lined with tapered (i.e., graded-length) multifoil insulation. The paper then examines the performance of the OSC generator designs for various fuel loadings, output voltages, and mission phases, and assesses their ability to meet the stipulated temperature constraints and power demands of the Europa Orbiter and Pluto Express missions under consideration by NASA. Finally, it presents an OSC-recommended design and fabrication procedure of an electrically heated four-cell test assembly with hybrid insulation to simulate the prototypic 16-cell generator. That test assembly will be analyzed by OSC, built by AMPS, and tested by the Air Force Phillips Laboratory (AFPL), to check the validity of the analytical predictions. The reason for basing the recommended test assembly on the interim rather than the ultimate refractory metal cell design is to avoid the delays required to develop the new technology for the latter.