Nuclear Thermal Propulsion (NTP) has stood as a promising potential alternative propulsion technology for decades. Chemical rockets have begun to reach their theoretical maximum efficiency, and their developers have switched their focus to making them cheaper rather than more efficient. NTP should answer that by offering high thrust and specific impulse. NASA's DRACO Program, the standard-bearer for NTP systems, provides a specific impulse of around 900 seconds, about double a traditional chemical rocket, but half that of most ion thrusters. To increase that number even further, researchers at the University of Alabama at Huntsville and The Ohio State University have been working on a novel configuration of NTP called the Centrifugal Nuclear Thermal Rocket (CNTR) that promises almost to double the specific impulse of traditional NTP systems while maintaining similar thrust levels. However, the system has some engineering challenges to overcome, and a new paper coming out in Acta Astronautica describes some incremental progress on making this improved engine a reality.
The biggest difference between the CNTR and traditional NTP projects is the fact that CNTR uses liquid Uranium for fuel, rather than the solid sort. To keep its fuel liquid, a CNTR system must rotate it quickly in a centrifuge. Once the uranium is molten, the CNTR bubbles hydrogen through it and expels it out of a nozzle for a thrust reaction. With this system, the researchers estimate they could achieve a specific impulse of around 1500 seconds, almost double what a traditional NTP engine would have, while only having slightly less thrust.
That seems like a significant improvement, as using these rockets could dramatically increase the "delta-v" achievable. But, as with all significant engineering challenges, there is a catch. Ten of them, in fact. The paper mentions ten engineering challenges that are holding back the development of the system, ranging from developing a coating that can handle the liquid uranium and all the different types of propellants at high temperatures to dealing with transient vibrations in the system. Throughout their work on the paper, which is the fourth in a series describing the development of this engine, the authors decided to focus on four specific challenges, including one that could have the most significant impact on the engine's specific impulse - uranium vaporization.
Fraser discusses the DRACO project, which hopes to test a nuclear rocket by 2027.
First, they tackled the neutronics of the system, or how the different elements created by the nuclear reaction impact the operation of the engine. In this iteration, the researchers added Erbium-167 to their model to manage the temperature inside the system, making it more stable. However, they also noted that the nuclear fission process could be "poisoned" by the xenon and samarium created as part of it if they are not effectively removed from the system. They will continue to develop models and simulations to work out the best way to eliminate those elements without exacerbating the possibility of losing more uranium.
Bubbles are the second challenge - the hydrogen bubbles that are needed to superheat the fuel and then eject it out are hard to model. The researchers turned to two experimental setups, "Ant Farm" (static) and "BLENDER II" (rotating), to try to understand how the hydrogen bubbles move through the liquid fuel. BLENDER II includes an X-ray to look at the bubble dynamics in the prototype systems they developed that use Galinstan, a gallium alloy, to act as a uranium equivalent, and nitrogen to act as the hydrogen would. Mapping out the bubbles mathematically remains a challenge, though.
One area where modeling was better understood was engine integration. The researchers used a genetic algorithm to maximize the engine's output given the different parameters, resulting in a specific impulse at ideal conditions of 1512 seconds. However, it would require additional centrifuges and higher rotation rates than the original design.
This video discusses the design details of the CNTR, compared to other NTP concepts.
Credit - Eager Space YouTube Channel
But all of those challenges pale in comparison to the big one - how to stop uranium fuel from leaking into the nozzle. If too much uranium (rather than hydrogen) is lost to thrust, it could slow the nuclear reaction down, cutting the potential specific impulse of the engine by two-thirds. The authors estimate that they can use a technique called dielectrophoresis (DEP) to isolate vaporized uranium molecules and redirect them back into the centrifuges, but even with a 99% recovery rate, there could be massive impacts on the viability of the engine.
As with all good research projects, more research is needed. The researchers acknowledge in the paper that the CNTR isn't ready for a complete prototype yet and will require several more rounds of modeling and optimization before it is ready. Uranium loss will get special attention with the next round of research, with a bench-top test planned to test the DEP solution. As the researchers continue to work on it, and as the need for high-specific-impulse / high-thrust propulsion systems continues to grow, the CNTR will hopefully be able to maintain funding until it finally comes to fruition.
Learn More:
D Thomas et al - Addressing challenges to engineering feasibility of the centrifugal nuclear thermal rocket
UT - Are Nuclear Propulsion Systems the Future of Space Exploration?
UT - New Nuclear Rocket Design to Send Missions to Mars in Just 45 Days
UT - NASA Invests in New Nuclear Rocket Concept for the Future of Space Exploration and Astrophysics
