NASA and the Department of Energy went to industry on February 12, 2021, asking for preliminary reactor designs for a nuclear thermal propulsion system. The request came as the Perseverance rover closed in on Mars, and it pointed at a harder goal: getting people there.
Think of it like this. In a nuclear thermal engine, a reactor heats a liquid propellant until it turns to gas. The gas expands through a nozzle and pushes the spacecraft. NASA says the approach delivers high thrust and twice the propellant efficiency of chemical rockets.
The agencies planned to fund several efforts, each exploring a different approach. Follow-on contracts would bring more detailed reactor designs and preliminary test hardware.
Two nuclear options
NASA was weighing a second type as well, nuclear electric propulsion. Here a reactor makes electricity, which charges a gas such as xenon or krypton and pushes the ions out through a thruster. The thrust is low, but the system uses propellant so efficiently that it can build speed for long stretches and fly a Mars mission on a fraction of what a high-thrust system burns.
“We’re exploring both nuclear electric and nuclear thermal propulsion options for crewed Mars missions,” said Anthony Calomino, NASA’s nuclear technology portfolio lead in the Space Technology Mission Directorate. “Each technology has its unique advantages and challenges that need to be carefully considered when determining the final preference.”
It was too early, NASA said, to say which system would carry the first astronauts.
Why bother
Only robotic explorers have made the trip so far, and none had to come home. For a crew, waiting for the best planetary alignment for the return could mean loitering at Mars for more than a year and stretching the round trip past three years. NASA wants to bring the time the crew spends traveling between the planets down to roughly two years, as far as practical.
Nuclear propulsion could help by taking a low-energy path on one leg and using the engine’s extra performance for the higher-energy leg. That would allow shorter stays on the surface.
Jim Reuter, associate administrator of the directorate, put the agency’s priorities in order. “While NASA’s immediate priority is returning humans to the Moon with the Artemis program, we are also investing in ‘tall pole’ technologies that could enable crewed missions to Mars,” he said.
The reactor problem
Heat is the hard part. Most of the engine runs at modest temperatures, but anything touching the reactor fuel has to survive more than 4,600 degrees Fahrenheit. “The reactor underpinning a nuclear thermal propulsion system is a significant technical challenge due to the very high operating temperatures needed to meet the propulsion performance goals,” Calomino explained.
NASA and DOE have been developing fuels that use low-enriched uranium and testing how they hold up under the thermal and radiation conditions an engine would see. Samples have gone into research reactors at Idaho National Laboratory’s TREAT facility and the Massachusetts Institute of Technology Nuclear Reactor Laboratory. Non-nuclear tests in simulated reactors have run at Marshall facilities.
NASA’s Marshall Space Flight Center in Huntsville, Alabama, leads the space nuclear propulsion project. It works with a DOE team drawn from Idaho, Los Alamos and Oak Ridge national laboratories. Nuclear thermal propulsion has been on the agency’s radar for more than 60 years.
