Development of Radioisotope Stirling Generators for High Efficiency Space Power Generation

Radioisotope power systems have been powering space missions for decades, historically using radioisotope thermoelectric generators to convert heat to usable electricity. Teams at NASA’s Glenn Research Center have been studying and testing free-piston Stirling convertors, which provide significant efficiency gains for space power generation when compared to state-of-the-art thermoelectric power conversion. Stirling convertors have been validated as stand-alone units with extensive environmental and extended lifetime testing. Recently, several efforts have integrated multiple Stirling convertors in generator configurations representing the interfaces and geometry that would be present in a flight system. Three separate radioisotope Stirling generator testbeds have been designed or conceived: the Plutonium-238 Stirling Generator Testbed, an internal NASA effort; the Americium-241 Stirling Generator Testbed, an international collaboration with a team from the University of Leicester; and the Heat Source Agnostic Stirling Generator Testbed, a conceptual study. The Plutonium-238 Stirling Generator Testbed (originally named the Dynamic Radioisotope Power System Testbed) began the initial design process in 2019, with more resources for the effort added in 2021 for design, analysis, and drawing creation. First testing occurred in 2023, with follow-on testing occurring in 2024 and 2025. Initially, the Americium-241 Stirling Generator Testbed was part of a conceptual design in 2022 which showed promise and led to an international collaboration effort to build hardware and test the design in a laboratory environment. The first integration and testing of the Americium-241 Testbed occurred in 2025 at NASA Glenn Research Center, was successful, and validated thermal models of the system. Further efforts are ongoing to implement improvements to the design and test a second version in 2026. The Heat Source Agnostic Stirling Generator Testbed effort began as an idea to adapt to the changing environment surrounding fuel production and planned usage. Debates have been ongoing about the best mission options for the available radioactive material, and due to the current dynamic environment, a Stirling generator that can accommodate various heat sources, while still utilizing significantly less fuel to produce similar power output, is attractive. Recently, the Heat Source Agnostic Stirling Generator Testbed idea received funding to develop a project plan for a conceptual design study leading to hardware build, integration, and an environmental testing campaign, contingent on approval of the project plan. Design and analysis efforts for the Heat Source Agnostic Stirling Generator Testbed will show the feasibility and limitations of a Stirling generator design that can accommodate a variety of heat sources.