Ohio State engineers have developed a nuclear thermal propulsion system that could dramatically reduce travel times to Mars and the outer planets. The centrifugal nuclear thermal rocket (CNTR) uses liquid uranium instead of traditional solid fuel elements, potentially doubling rocket efficiency compared to existing nuclear propulsion concepts.
The CNTR design addresses a fundamental limitation of chemical rockets, which have powered spacecraft since the dawn of the space age but require enormous amounts of fuel and deliver relatively modest thrust. The New Horizons mission to Pluto, for example, took nine years to reach its destination using conventional propulsion.
“You could have a safe one-way trip to Mars in six months, for example, as opposed to doing the same mission in a year.”
That quote comes from Spencer Christian, a PhD student at Ohio State who leads prototype construction of the CNTR under professor John Horack. The technology could enable round-trip human missions to Mars in as little as 420 days, compared to current mission profiles that would take several years.
Liquid Fuel Advantages
Traditional nuclear thermal rockets, tested extensively in the 1960s Rover/NERVA program, achieved specific impulse values around 900 seconds – roughly double that of the best chemical engines. The CNTR concept aims for even higher performance by using liquid uranium fuel contained in rapidly rotating cylinders.
The rotating cylinders spin at 7,500 RPM, using centrifugal force to maintain the liquid uranium against the cylinder walls while hydrogen propellant flows through the molten fuel. This direct heating approach allows the system to reach much higher temperatures than solid fuel designs, potentially achieving specific impulse values of 1,800 seconds.
Associate professor Dean Wang, a senior member of the project, explains the broader implications:
“The longer you are in space, the more susceptible you are to all types of health risks. So if we can make that any shorter, it’d be very beneficial.”
Beyond Mars missions, the technology could enable entirely new types of missions to the outer solar system. Current chemical propulsion makes trips to Saturn, Uranus, and Neptune extremely challenging due to the massive fuel requirements and decades-long travel times.
Engineering Challenges Remain
The research team, supported by NASA funding, acknowledges significant hurdles before the concept becomes reality. Their recent study in Acta Astronautica outlines ten major engineering challenges that must be overcome.
The most pressing issue involves uranium vaporization. When the liquid uranium reaches the extreme operating temperatures needed for peak performance, some of it evaporates into the hydrogen propellant stream. This uranium vapor, being much heavier than hydrogen, dramatically reduces the rocket’s efficiency – potentially dropping performance below that of conventional chemical engines.
The team is exploring several mitigation strategies, including advanced fuel compositions and a technique called dielectrophoresis to electromagnetically separate uranium vapor from the propellant stream. Early analysis suggests this vapor recovery system would require less than 100 watts of power per fuel element.
Other challenges include developing materials that can withstand the extreme conditions, ensuring reliable startup and shutdown procedures, and managing the complex neutronics of a liquid fuel reactor. The rotating cylinders must maintain structural integrity while spinning at thousands of RPM in a high-radiation environment.
The researchers have established a reference design featuring 37 rotating fuel elements in a reactor core roughly three feet tall. Computer simulations indicate the system could operate for about 10 hours total runtime, sufficient for multiple interplanetary missions with 48-hour cooling periods between burns.
Safety considerations drive many design decisions. The liquid fuel naturally provides negative temperature feedback – if the reactor gets too hot, the uranium expands and the nuclear reaction automatically slows down. The team has also incorporated neutron-absorbing materials to ensure stable operation across all conditions.
Current projections suggest the CNTR concept could reach design readiness within five years, though significant testing and development work lies ahead. The next major milestone involves laboratory demonstration of a single fuel element under realistic conditions.
Wang emphasizes the long-term importance of the research: “We need to keep space nuclear propulsion as a consistent priority in the future, so that technology can have time to mature. It’s a huge benefit that we can’t afford to miss out on.”
The timing aligns with renewed interest in nuclear propulsion from both NASA and the Defense Advanced Research Projects Agency (DARPA), which plan to demonstrate nuclear thermal propulsion technology by 2027 through their joint DRACO program.
If successful, CNTR technology could transform human space exploration by making distant destinations accessible within reasonable timeframes. The ability to reach Mars in months rather than years, or conduct robotic missions to the ice giants of the outer solar system, would open entirely new chapters in our understanding of the cosmos.
Acta Astronautica: 10.1016/j.actaastro.2025.05.007
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