Regan Brown
NASA has been testing new heat source fuels for deep space exploration. The space agency has been working with the University of Leicester to test the capabilities of a Stirling generator testbed powered by two electrically heated americium-241 heat source simulators. The test proved the viability of an americium-fueled Stirling RPS, which could be used for future missions to Mars and beyond. NASA has also been testing an innovative technique for super-cold fuel storage, called cryogenic fluid management, which stores, transfers, and measures super-cold fluids for the surface of the Moon, Mars, and future long-duration spaceflight missions.
| Characteristics | Values |
|---|---|
| Type of fuel used by NASA in tests | Radioisotope power systems (RPS) |
| RPS heat source fuel of choice | Plutonium-238 (plutonium oxide) |
| Alternative RPS heat source fuel | Americium-241 |
| Type of test | Two-stage cooling tests |
| Location of tests | Test Stand 300 at NASA's Marshall Space Flight Center in Huntsville, Alabama |
| Date of tests | June to September 2025 |
| Fuel for the Mars 2020 rover | Fabrication of fuel pellets using plutonium dioxide |
| Fuel for the Artemis I mission | 2.75 million gallons of super-cooled liquid oxygen and hydrogen |
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What You'll Learn
Testing a new type of RPS heat source fuel
NASA is testing a new type of RPS heat source fuel for future long-duration journeys to extreme environments. Radioisotope power systems (RPS) are a viable option for these missions and have been used for over 60 years. These nuclear batteries provide long-term electrical power for spacecraft and science instruments using heat produced by the natural radioactive decay of radioisotopes.
Historically, the radioisotope plutonium-238 (plutonium oxide) has been NASA’s RPS heat source fuel of choice, but there has been interest in using americium-241 as an alternative fuel source. In January 2025, NASA's Glenn Research Center in Cleveland and the University of Leicester in the United Kingdom partnered to test the capabilities of a Stirling generator testbed powered by two electrically heated americium-241 heat source simulators. The University of Leicester provided the heat source simulators, which were the exact size and shape of their real americium-241 heat source, and the Stirling Research Lab at Glenn provided the test station, Stirling convertor hardware, and support equipment.
The test successfully proved the viability of an americium-fueled Stirling RPS, and performance and efficiency targets were met. The Stirling convertor is a heat engine that converts thermal energy into electrical energy, with pistons that float freely within the engine instead of a crankshaft to extract power. This design allows it to operate for decades continuously without wear, as it does not have piston rings or rotating bearings that will eventually deteriorate.
The next version of the testbed will be lower mass, higher fidelity, and will undergo further environmental testing. This new RPS heat source fuel has the potential to enable future NASA missions to deliver more payloads, achieve faster trip times, and provide power for crew stations on the surface of the Moon or Mars.
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Cryogenic fluid management for super-cold fuel storage
Cryogenic fluid management is a critical aspect of NASA's mission to explore deep space, including future crewed journeys to the Moon and Mars. In the vacuum of space, where temperatures can reach -455°Fahrenheit, preserving ultra-cold fluid propellants or fuel is challenging due to the risk of overheating from onboard systems, solar radiation, and spacecraft exhaust.
The Cryogenic Fluid Management Portfolio Project, led by acting manager Kathy Henkel, is based at NASA Marshall and the Glenn Research Center. The project aims to develop and manage cryogenic fluid research and technology, focusing on reducing propellant loss for long-duration missions. This is crucial for the success of ambitious missions, such as crewed journeys to Mars, which require storing large amounts of cryogenic propellant for extended periods.
The new technique introduced by the project is called "tube on tank" cooling. This method integrates two cryocoolers, utilizing chilled helium at approximately -424°Fahrenheit, circulating through tubes attached to the outer wall of the propellant tank. The tank is wrapped in a multi-layer insulation blanket, including a thin aluminum heat shield for added protection. This two-stage cooling process effectively prevents propellant loss and enables the long-term storage of propellants, ensuring their stability during transit and on planetary surfaces.
Cryogenic fluids, such as liquid hydrogen and liquid oxygen, are commonly used as propellants for space exploration. Their low boiling points make them susceptible to boil-off, emphasizing the importance of cryogenic fluid management in maintaining the stability and functionality of these super-cold fluids. This innovative technique is a significant step forward in NASA's pursuit of sustainable and extended exploration of the Moon, Mars, and other destinations in deep space.
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Stirling RPS generator testbed
For over 60 years, NASA has relied on radioisotope power systems (RPS) to power spacecraft operating far from the Sun. Traditionally, plutonium-238 has been the radioisotope of choice. However, researchers are now turning their attention to americium-241, a fuel that has been under active development in Europe for the past two decades.
In 2025, NASA's Glenn Research Center in Cleveland partnered with the University of Leicester to test a Stirling generator testbed powered by americium-241 heat source simulators. The University of Leicester provided the heat source simulators and generator housing, while the Stirling Research Lab at NASA Glenn provided the test station, Stirling hardware, and support equipment.
The Stirling generator testbed uses electrically heated devices that mimic the thermal output of americium decay, allowing engineers to safely evaluate performance and reliability without handling radioactive materials. The testbed successfully proved the viability of an americium-fueled Stirling RPS, meeting performance and efficiency targets.
The Stirling RPS generator testbed is a significant development because it can continue producing power even when one converter fails. This is a key feature for missions that cannot afford to lose power in deep space. The testbed's design highlights the robustness and reliability of an Americium-Radioisotope Stirling Generator for potential future spaceflight missions, including long-duration missions that could operate for many decades.
The Glenn team is now working on the next version of the Stirling RPS generator testbed, aiming to make it lighter, more efficient, and able to withstand the environmental extremes of launch and space travel. If successful, americium-fueled Stirling generators could power science instruments, landers, or small surface habitats in environments with weak or unreliable sunlight, such as the permanently shadowed craters of the Moon or the icy moons of Jupiter and Saturn.
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Radioisotope power systems
The heat used in RPS is produced by the natural radioactive decay of plutonium-238, which has several unique features that make it the material of choice for electrical power in spacecraft. Plutonium-238 has a long half-life, releasing energy at a relatively constant rate for a reasonable amount of time. However, due to a shortage of plutonium-238, a new kind of RTG assisted by subcritical reactions has been proposed. In this system, the alpha decay from the radioisotope is also used in alpha-neutron reactions with an element such as beryllium, producing a long-lived neutron source.
One method to generate electricity from radioisotope heat sources is the free-piston Stirling convertor, a heat engine that converts thermal energy into electrical energy. The Stirling convertor uses pistons that float freely within the engine, allowing it to operate continuously for decades without wearing out.
NASA is currently testing a new type of RPS heat source fuel that could become an additional option for future long-duration journeys to extreme environments. The test involves an americium-241 heat source simulator, which uses embedded electric heaters to create an equivalent amount of heat to simulate the decay of americium fuel. The test proved the viability of an americium-fueled Stirling RPS, and the team is now pursuing a lower mass, higher fidelity version that will undergo further environmental testing.
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Liquid hydrogen and liquid oxygen propellants
The NASA Glenn Research Center's Rocket Laboratory played a crucial role in the development of high-energy liquid propellants, including liquid hydrogen, in the 1940s and 1950s. Over time, the laboratory's equipment was upgraded, and it tested a range of thrusters, combustors, and alternative fuels.
In the early 1950s, the Rocket Lab focused on hydrogen with oxygen or fluorine as the oxidizer, while continuing studies with hydrocarbon fuels. By the late 1950s and early 1960s, NASA had settled on the liquid hydrogen and liquid oxygen combination for its upper-stage rockets. This decision was heavily influenced by the propellant testing conducted at the Rocket Lab in the 1950s. The liquid hydrogen-liquid oxygen propellant combination became a key element of NASA's Apollo Program, powering the upper stages of the Saturn and Atlas-Centaur launch vehicles.
The selection of this propellant combination shifted the focus of NASA's rocket work from propellant evaluation to enhancing combustion, refining pumping systems, and improving the storage of cryogenic fluids. The Rocket Lab continued to be active in testing new types of propulsion systems, including "green" propellants and low-cost injectors.
In the mid-1960s, as spacecraft became larger, researchers at NASA explored the use of liquid-fueled thrusters to enhance thruster power without increasing their size. They discovered that nitrogen tetroxide-hydrazine-type thrusters performed efficiently at simulated altitudes. Additionally, NASA engineers investigated water-electrolysis to power reaction control thrusters, which generate hydrogen and oxygen as propellants or for fuel cells.
While liquid hydrogen and liquid oxygen propellants have been a significant focus of NASA's testing and have proven successful for many missions, NASA is also exploring alternative fuel sources for deep space exploration. NASA has been testing a new type of radioisotope power system (RPS) heat source fuel, specifically investigating the use of americium-241 as an alternative to their traditional choice of plutonium oxide. The successful tests conducted in collaboration with the University of Leicester Space Nuclear Power team indicate the potential for americium-fueled Stirling RPS as a viable option for future long-duration journeys to extreme environments.
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Frequently asked questions
For the successful Artemis I mission, NASA used 2.75 million gallons of super-cooled liquid oxygen and hydrogen, as well as various hypergolics.
The type of fuel used by NASA varies, but for the Artemis I mission, the fuel used was liquid oxygen and hydrogen, as well as hypergolics such as hydrazine, mono-methyl hydrazine, and dinitrogen tetroxide.
NASA is currently testing a new type of RPS heat source fuel that could be used for future long-duration journeys to extreme environments. The new fuel is americium-241, which has been a source of interest for the past two decades in Europe.
