‘Bubble’ nuclear engine could be NASA’s future workhorse

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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. Credit: Michel Mercier | UAH

A state-of-the-art nuclear thermal propulsion (NTP) rocket engine using so-called centrifugal liquid fuel bubble could one day be a ticket for NASA to go straight 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 at Huntsville (UAH), which is part of the University of Alabama, leads a collaboration of universities across the country. including the University of Rhode Island (URI), Drexel University, Massachusetts Institute of Technology (MIT), Pennsylvania State University, and the University of Michigan (UM) to research the concept.

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

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

The bubble centrifugal NTP concept heats the hydrogen gas propellant to very 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 expand rapidly. Upon exiting the nozzle, the expanding hydrogen provides thrust to the spacecraft.

Benefits of the design include significantly better performance than conventional liquid-fuel rocket engines that burn hydrogen and oxygen, says Dr Dale Thomas, the project’s principal investigator and a leading systems engineering researcher at the UAH.

“In the conventional combustion of liquid fuel engines, the resulting propellant molecules – H2O in the case of hydrogen and oxygen – are much heavier due to those relatively heavy oxygen atoms, and they won’t come out of the nozzle as quickly, providing more thrust but less momentum” , says Dr. Thomas.

Thrust is the force provided by the engine, for example to pull a spacecraft away from Earth’s gravity. Momentum is the change in momentum per unit of fuel, and it’s important when it comes to getting a spacecraft where it’s going in space.

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

A simplified diagram showing the concept of a nuclear thermal bubble propulsion engine. Credit: Propulsion Research Center

Hotter, relatively lighter hydrogen atoms will allow the ship to travel farther.

“If we heat up the propellant, it has more energy and will come out of the nozzle faster, which provides more momentum,” says Dr Thomas. “Because it is a higher performance engine, it has the potential to power spacecraft on trajectories other than minimum energy trajectories, providing options for higher energy trajectories that will shorten the time of journey to and from Mars and other destinations throughout the solar system.”

Conceptually intriguing, the bubble 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 molten uranium fuel.

“We are in the very early stages of this project,” says Dr. Thomas.

“This bubble-through concept has been around since the ’60s,” he says. “The physics are well understood, but engineering challenges have prevented this concept from being taken off the drawing board in the past. We are trying to see if today’s technologies will allow us to develop a prototype NTP engine at viable liquid fuel.

UAH’s work focuses on three areas, he said.

“The first part is the modeling and thermodynamic analysis of the heat transfer of liquid uranium and gaseous hydrogen. Second, we will model and analyze the geometry and trajectory of hydrogen gas bubbles in a liquid uranium medium, and third, we will perform experiments to confirm the analytical predictions of dynamical and thermodynamic models. .

In addition to Dr. Thomas, who is in charge of the modeling missions, the professors involved in UAH research are Dr. Keith Hollingsworth, professor and director of the department 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, supervising the experiment; and Dr. Jason Cassibry, associate professor of mechanical and aerospace engineering, in charge of bubble dynamics.

The Aerospace Systems Graduate Research Assistants involved are Mitchell Schroll, PhD candidate; Pongkrit Darakorn na Ayuthy (aka Boom), PhD student; Ben Campbell, Master’s student; Jacob Keese, master’s student; and Will Ziehm, master’s student.

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

At the MSFC, researchers work with Dr. Michael Houts, Director of Nuclear Research.

The URI partner performs high-level design projects on centrifugal engine fuel element drive systems, including how to spin them up to operating speed, maintain them at the desired rotational speed, and to slow them down. Drexel is developing the material properties of the cylinder wall and MIT is studying the dynamics of bubbles. At UM, researchers will experimentally look at the physics of the reactor itself, called neutronics. Penn State is researching neutronics and heating.

At the Johnson Research Center, UAH scientists are building experimental apparatus to confirm their analytical predictions of heat transfer and bubble dynamics. Two exist to date, called Ant Farm and 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 core.

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

“We’re doing mission studies, looking at anything you can 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 allow direct trajectories for uncrewed science missions to the outer planets of the solar system, and possibly 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 gravitational assists when flying near planets.

“These planetary alignments only happen once every few years,” says Dr. Thomas. “With this liquid NTP fuel, you may even be able to get to the Kuiper Belt on a direct trajectory.”

It would be quite a trick. The Kuiper Belt begins 4,400,000,000 km from the sun.


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