‘Bubble-through’ nuclear engine might be a future NASA workhorse

Ben Campbell at work in the UAH Johnson Research Center

Ben Campbell, a graduate resident assistant and master’s student in aerospace systems engineering, works on the Bubbling Liquid Experiment Navigating Driven Extreme Rotation, or BLENDER, device at UAH’s Johnson Research Center.

Michael Mercier | UAH

A cutting-edge nuclear thermal propulsion (NTP) rocket engine using what’s called centrifugal liquid fuel bubble-through could one day be a ticket for NASA to go directly into deep space.

Under an NTP research contract for the Space Nuclear Propulsion Project Office at NASA’s Marshall Space Flight Center (MSFC), The University of Alabama in Huntsville (UAH), a part of the University of Alabama System, is leading a collaboration of universities across the nation including the University of Rhode Island (URI), Drexel University, the Massachusetts Institute of Technology (MIT), Pennsylvania State University and the University of Michigan (U-M) to research the concept.

NASA has made substantial advances toward a solid fuel NTP design. The bubble-through concept under study by the university collaborators is one of three proposed hydrogen-based designs for a next generation liquid fuel NTP rocket.

Either in person or virtually, all of NASA’s NTP academic partners will gather on March 11 at a workshop hosted by UAH for NASA to discuss their progress and issues.

The bubble-through centrifugal NTP concept heats hydrogen gas propellant to super-hot temperatures, but there is no combustion. Hydrogen is literally bubbled through a rotating liquid uranium core in the engine via a porous cylinder wall, causing the gas to rapidly expand. As it exits the nozzle, the expanding hydrogen provides thrust for the spacecraft.

The design’s advantages include significantly higher performance over conventional liquid fuel rocket engines that combust hydrogen and oxygen, says Dr. Dale Thomas, the project’s principal investigator and an eminent scholar in systems engineering at UAH.

Bubble Thru Liquid Fuel Reactor diagram

A simplified diagram showing the bubble-through nuclear thermal propulsion engine concept.

Courtesy Propulsion Research Center

“In conventional liquid fuel engine combustion, the resulting propellant molecules – H2O in the case of hydrogen and oxygen – are much heavier due to those relatively heavy oxygen atoms, and they will not exit the nozzle as fast, providing more thrust but less impulse,” Dr. Thomas says.

Thrust is the force supplied by the engine, for example to lift a spacecraft away from Earth’s gravity. Impulse is the change in momentum per unit of fuel, and that matters when it comes to getting a spacecraft where it’s going in space.

“Think of your car,” Dr. Thomas says. “Think of thrust as torque and impulse as miles per gallon (mpg). Both matter, just like both torque and mpg matter in your car.”

Hotter, relatively lightweight hydrogen atoms will make the ship go farther.

“If we get the propellant hotter, it has more energy and will exit the nozzle faster, which provides more impulse,” Dr. Thomas says. “Since this is a higher performing engine, it has the potential to power spacecraft on trajectories other than the minimum energy trajectories, providing options for higher energy trajectories that will shorten the trip time to and from Mars and other destinations throughout the solar system.”

Conceptually intriguing, the bubble-through engine presents a number of technical challenges, not the least of which is developing a material for its porous cylinder wall that can withstand direct contact with the molten uranium fuel.

“We’re in the very early stages on this,” Dr. Thomas says.

“This bubble-through concept has been around since the ’60s,” he says. “The physics are 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.”

The UAH work focuses in three areas, he says.

“The first part is liquid uranium and gaseous hydrogen thermodynamic heat transfer modeling and analysis. Second, we’ll be doing modeling and analysis of geometry and trajectory of gaseous hydrogen bubbles in a liquid uranium medium, and third, we’ll perform experimentation to confirm the analytical predictions of dynamic and thermodynamic models.”

Besides Dr. Thomas, who is in charge of modeling missions, faculty involved in the research from UAH are Dr. Keith Hollingsworth, professor and department chair of mechanical and aerospace engineering, in charge of thermodynamics; Dr. Robert Frederick, professor of mechanical and aerospace engineering and director of the Propulsion Research Center, overseeing experimentation; and Dr. Jason Cassibry, associate professor of mechanical and aerospace engineering, in charge of bubble dynamics.

Mitchell Schroll observing at Johnson Research Center

Mitchell Schroll, a graduate resident assistant and doctoral student in aerospace systems engineering, watches as air bubbles rise in a water column inside the Ant Farm static test apparatus at UAH’s Johnson Research Center.

Michael Mercier / UAH

Aerospace systems graduate research assistants involved are Mitchell Schroll, a doctoral candidate; Pongkrit Darakorn na Ayuthy (a.k.a. Boom), a doctoral candidate; Ben Campbell, a master’s student; Jacob Keese, a master’s student; and Will Ziehm, a master’s student.

At MSFC, the researchers are working with Dr. Michael Houts, nuclear research manager.

Partner URI is doing senior design projects on the drive systems for the engine’s centrifugal fuel elements, including how to spin them up to operating speed, keep them at the desired rotational speed and spin them down. Drexel is developing the material properties of the cylinder wall and MIT is studying bubble dynamics. At U-M, researchers will look experimentally into the physics of the reactor itself, which is called neutronics. Penn State is researching neutronics and heating.

At Johnson Research Center, UAH’s scientists are building experimental apparatus to confirm their analytical predictions of heat transfer and bubble dynamics. Two exist so far, called the Ant Farm and the Bubbling Liquid Experiment Navigating Driven Extreme Rotation, or BLENDER. The devices use air bubbles in water to simulate the bubbling of hydrogen through the engine’s core.

The centrifugal NTP engine research fits well with other UAH research that Dr. Thomas leads for NASA to develop a spacecraft designed for use with solid fuel NTP engines.

“We are conducting mission studies, looking at what all can you do with a solid fuel NTP propulsion system other than a crewed mission to Mars,” he says. “Our work so far indicates that it will enable direct trajectories for un-crewed scientific missions to the outer planets in the solar system, and perhaps even sample returns from the Jovian moons.”

In a direct trajectory, a spacecraft flies directly to a destination. Current chemical propulsion systems must rely on proper planetary alignments to take advantage of gravity assists when flying by planets.

“Those planetary alignments only come around once every few years,” Dr. Thomas says. “With this liquid fuel NTP, you can perhaps even get to the Kuiper Belt on a direct trajectory.”

That would be quite a ride. The Kuiper Belt starts 4,400,000,000 km from the sun.