Illustration of a spacecraft enabled by nuclear thermal propulsion. NASA
Researchers have reported new progress in developing a liquid uranium-fueled rocket engine, a Centrifugal Nuclear Thermal Rocket (CNTR).
“The Centrifugal Nuclear Thermal Rocket (CNTR) is a Nuclear Thermal Propulsion (NTP) concept designed to heat propellant directly by the reactor fuel,” explained the researchers in a new study.
The technology, being developed by teams at the University of Alabama in Huntsville and The Ohio State University, aims to deliver nearly double the specific impulse – a key measure of rocket efficiency – compared to current advanced nuclear propulsion concepts for space travel.
Doubling efficiency of spacecraft with uranium
Nuclear thermal propulsion (NTP) has long been considered a potential successor to chemical rockets, which are now primarily seeing efforts to reduce cost rather than improve efficiency.
NASA’s DRACO Program, a solid-core NTP system, targets a specific impulse of around 900 seconds. This is about twice that of chemical rockets but half that of many ion thrusters.
The CNTR uses liquid uranium fuel instead of solid fuel in traditional NTP designs for a specific impulse of approximately 1500 seconds. This could significantly increase the “delta-v” (change in velocity) capabilities of spacecraft while maintaining similar thrust levels.
In the CNTR design, molten uranium fuel is rapidly spun in a centrifuge. Hydrogen gas is bubbled through the superheated liquid and expelled through a nozzle to produce thrust.
“The primary difference between the CNTR concept and traditional NTP systems is that rather than using traditional solid fuel elements, the CNTR uses liquid fuel with the liquid contained in rotating cylinders by centrifugal force,” added the study.
Addressing challenges
Developing the CNTR involves considerable engineering challenges. A new paper in Acta Astronautica, the fourth in a series on the engine’s development, outlines ten such issues. The research teams concentrated on four of these in their latest work.
Progress has been made in managing the engine’s nuclear reactions, or “neutronics.” Adding Erbium-167 to their models is intended to help stabilize internal temperatures. The researchers also highlighted that fission byproducts like xenon and samarium could negatively affect the reaction if not properly removed, an area requiring further simulation.
Understanding how hydrogen bubbles move through the liquid fuel is another focus. Experiments using “Ant Farm” (static) and “BLENDER II” (rotating) setups are providing data. The BLENDER II system uses X-rays to study bubble behavior in uranium-surrogate materials, though mathematically modeling these dynamics remains difficult.
Using a genetic algorithm, engine integration modeling indicates a potential specific impulse of 1,512 seconds under ideal conditions. Achieving this would require more centrifuges and higher rotation rates than the original design.
Another challenge is preventing uranium fuel from escaping through the nozzle with the hydrogen propellant. Significant uranium loss could reduce the engine’s specific impulse by up to two-thirds. The researchers propose using dielectrophoresis (DEP) to capture vaporized uranium, targeting a 99% recovery rate.
Future developments and potential
The authors state that the CNTR is not ready for a full prototype and requires more modeling and optimization. Future research will focus on uranium loss and testing the DEP solution with bench-top experiments.
“The CNTR concept offers game-changing potential for in-space propulsion in that it offers the potential for a factor of two performance increase versus the planned solid fuel NTP effort presently under development by NASA for the DRACO mission,” concluded the study.
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