With three commercial teams under contract to produce reactor designs for nuclear thermal propulsion rockets that would use solid high-assay low-enriched uranium fuel to heat hydrogen propellant, NASA’s investment in nuclear thermal propulsion (NTP) has increased in recent years. But just as there is more than one way to fuel a terrestrial reactor, other fuels are under consideration for future NTP rocket engines.
Researchers at the University of Alabama at Huntsville (UAH) are leading a group of university collaborators to investigate a “bubble-through” centrifugal NTP concept that could one day propel deep space missions. According to an article titled “‘Bubble-through’ nuclear engine might be a future NASA workhorse,” published by UAH on March 8, the concept is one of three proposed hydrogen-based designs for a next-generation liquid fuel NTP rocket.
The partners: Under a contract from the Space Nuclear Propulsion Project Office at NASA’s Marshall Space Flight Center, UAH is leading a collaboration of universities, including the University of Rhode Island, Drexel University, the Massachusetts Institute of Technology, Pennsylvania State University, and the University of Michigan, to research the concept.
Dale Thomas, a UAH professor and deputy director of the university’s Propulsion Research Center, is the project’s principal investigator. Other UAH faculty involved in the project include Keith Hollingsworth, Robert Frederick, and Jason Cassibry. They are working with Michael Houts, nuclear research manager at NASA’s Marshall Space Flight Center.
The technology: In the bubble-through centrifugal NTP concept, a porous cylinder wall would allow super-heated hydrogen gas to bubble through a rotating liquid uranium core, causing the gas to rapidly expand. As it exits the nozzle, the expanding hydrogen would provide thrust for the spacecraft.Like more advanced NTP designs using solid fuel, the design would offer higher performance than conventional liquid-fueled combustion rocket engines that burn hydrogen and oxygen. Combustion engines release water molecules that due to their greater mass provide less impulse to a rocket than super-heated hydrogen atoms. Specific impulse is the change in momentum per unit of fuel, and NASA has targeted a specific impulse of 900 seconds from NTP designs, which is double the specific impulse that conventional rockets can provide.
Greater propellant efficiency could enable trips through space of shorter duration that use direct trajectories instead of carefully timed planetary fly-bys to provide gravity assists. Longer-term nuclear rocket engine concepts that could meet or exceed NTP deep space mission requirements include nuclear electric propulsion and fusion propulsion.
Engineering challenges: “This bubble-through concept has been around since the ’60s,” Thomas states in the UAH article. “The physics [is] well understood, but the engineering challenges have precluded getting this concept off the drawing board in the past. We’re attempting to see whether today’s technologies will let us develop a viable liquid fuel NTP engine prototype.”
UAH’s work focuses on three areas: liquid uranium and gaseous hydrogen thermodynamic heat transfer modeling and analysis; modeling and analysis of the geometry and trajectory of gaseous hydrogen bubbles in a liquid uranium medium; and experimentation to confirm the analytical predictions of dynamic and thermodynamic models.
According to UAH, researchers at collaborating institutions are working on other aspects of the design. The University of Rhode Island is working on drive systems for the engine’s centrifugal fuel elements; Drexel University is developing the material properties of the cylinder wall; MIT is studying bubble dynamics; the University of Michigan is studying neutronics; and Penn State is researching neutronics and heating.