A New Type of Propulsion Could Revolutionize Space Travel

Here’s what you’ll learn when you read this story:

As NASA and DARPA work toward launching their first nuclear thermal rocket (NTR) in 2027, scientists and engineers are already working on innovative concepts to make the next era of human spaceflight more powerful and more efficient.
One of these designs, known as a Centrifugal Nuclear Thermal Rocket (CNTR), uses a centrifuge to turn uranium fuel into a molten liquid that heats hydrogen and produces thrust.
A new paper analyzes the benefits of this engine—which is almost twice as efficient as regular NTR engines—and the many challenges that are keeping this engine from its planet-hopping mission.

Humanity owes a debt of gratitude to the chemical rockets that have launched satellites, astronauts, and entire space stations into space. But if we have any hope of reliably settling on other planets, those engines will likely need to be swapped out with nuclear ones.

The U.S. is very aware of the nuclear future of spaceflight—it’s why NASA and the Defense Advanced Research Projects Agency (DARPA) are launching a nuclear thermal rocket (NTR) powered by solid uranium in 2027 called the Demonstration Rocket for Agile Cislunar Operations, or DRACO. But that spacecraft’s design isn’t the only horse in the spaceflight race. For one, there’s an entire other category of nuclear propulsion known as NEP, which converts nuclear heat into electricity to power ion thrusters. But even within the NTR family, there isn’t absolute certainty about what these rockets might look like in the near future.

DRACO is the standard-bearer for this technology, but a new paper published in the journal Acta Astronautica details the potential engineering challenges facing a rival NTR concept known as a centrifugal nuclear thermal rocket, or CNTR. True to its name, this engine uses a centrifuge to keep uranium molten (i.e. liquid), and then bubbles through hydrogen to eventually produce thrust.

One of its key advantages over other NTR designs is its efficiency. According to Universe Today, DRACO uses a solid high-assay low-enriched uranium (HALEU) to generate lots of heat that turns hydrogen from a liquid to gas (and thus propulsion), providing a specific impulse of roughly 900 seconds. Without getting too much into it (after all, it is rocket science), specific impulse describes how efficiently an engine converts fuel to thrust.

CNTR, on the other hand, can pull off a specific impulse of 1,500 seconds—a dramatic improvement.

“The bubble-through reactor design features a reactor fuel which is rotated at high speed so as to maintain a layer of liquid fuel around the hydrogen-permeable inner cylindrical surface,” the authors wrote. “As the hydrogen propellant is bubbled through this liquid fuel, it is heated to the temperature of the liquid fuel, exiting the engine through the nozzle to produce a thrust.”

Of course, there’s a reason that DRACO isn’t adopting this carnival ride approach to its own nuclear propulsion engine—it’s easier said than done. In the paper, scientists from the University of Alabama in Huntsville and Ohio State University talk through just a few of the engineering challenges that lie ahead. One problem is that certain unwanted elements (like xenon and samarium) would need to be continuously removed from the system, and the researchers note that modeling the interaction of the bubbled hydrogen is immensely difficult.

But as Universe Today also notes, the big problem is guaranteeing that only hydrogen escapes the propulsion nozzle, and not uranium fuel. If uranium does escape, the nuclear reaction slows down, and so does the engine’s efficiency.

Although scientists have worked on designs for CNTR engines for years, these centrifugal machines still remain in the modeling phase, and are likely years away from prototyping. By then, nuclear propulsion engines will likely be whizzing around the Solar System. But just as the V-2 rocket was far from the apex of chemical propulsion, DRACO and its NTR brethren will be far from the last word in the next chapter of human spaceflight.

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