Small Stirling Technology Exploration Power (Small STEP)

The objective of the Small STEP project is to demonstrate the applicability of a Stirling Radioisotope Power System (RPS) generator for Mars applications. Development of Stirling-based RPS systems has identified numerous high-performance designs that will require a single 200-250 Wth isotope heat source and could serve as networked orbiting or landed nodes. They could provide a robust, long-lived source of meaningful power for operations and science payloads on the surface of Mars. Meaningful power could mean continuous measurement without invasive duty cycling that would prevent the system from providing continuous power. This approach uses demonstrated designs to mature a system concept in a relevant context for small RPS powered nodes. An electrically-heated Stirling RPS generator breadboard demonstration will increase the TRL of the concept and the subsystems in a system context. The approach is cost effective because the effort will utilize existing power conversion hardware, generator designs, and test infrastructure to increase efficiencies.

An electrically-heated small Stirling RPS generator, including the key Stirling convertor and controller technologies, will be designed and operated in the Stirling Research Laboratory (SRL) at NASA Glenn Research Center. SRL has the infrastructure, test capabilities, equipment, and subject matter expertise in dynamic power conversion. This project will build upon the knowledge-base obtained through the design, build, and test of other Stirling generators and their components at NASA GRC. Stirling power convertors are a mature option for high efficiency power conversion achieving conversion efficiencies up to 40%. NASA has invested in Stirling technology maturation over the past two decades, including maturation of a 35 W Stirling convertor called the EE-35. These convertors were designed by Sunpower, Inc. and tested at GRC to successfully demonstrate full power output and robustness by means of a launch vibration test. Another investment is in Stirling controller development. Some primary functions of the controller are AC-DC power conversion and regulation of the power provided to the spacecraft. Most recently, NASA GRC designed an electronic controller and through this project, the controller is being advanced to operate a Stirling generator with EE-35 convertors for Mars applications, as well as use on other Stirling projects. The small Stirling RPS generator concept is packaged to hold the isotope heat source and two EE-35 Stirling convertors. This approach is agnostic to a particular type of isotope source. Performance estimates suggest this concept, as shown in the figure, could reach system efficiencies as high as 23% while providing 59 watts of usable continuous dc power output to user loads, with an estimated specific power of 5.5 W/kg.

There are several mission concepts for Mars exploration that could utilize a small Stirling RPS generator. These include, but not limited to, networked nodes using long-lived small spacecraft for global meteorological monitoring and high-resolution imaging, RPS rovers to enable deployment of multiple science stations or communication repeaters in key locations on the surface of Mars. These small nuclear powered stations will offer long-lived operation in harsh, dusty, and dark environments or areas of low solar insolation where solar panels either cannot operate or provide optimal performance. RPS convert heat from isotope sources, like plutonium-238 or alternatives like americium-241, to usable electricity for spacecraft. Thermoelectric generators offer around 6% efficiency while dynamic generators offer 3-4 times higher efficiency. For that reason, high efficiency power conversion technologies are critical to maximizing the utility of isotope fuel for space applications.

This is early/mid-stage (TRL 3) — the most realistic path in is NASA SBIR/STTR, which funds small businesses and research institutions to develop technology aligned with NASA's needs (equity-free, phased funding). Check whether a current SBIR/STTR solicitation topic overlaps with this project's technology area, or contact the project directly (above) to ask.

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