Nuclear rockets could get to Mars in half the time, but designing the reactors that would power them isn’t easy

NASA plans to send crewed missions to Mars over the next decade, but the 225 million mile journey to the Red Planet could take several months, or even years, round trip.

This relatively long transit time is due to the use of traditional chemical rocket fuel. An alternative technology to chemically powered rockets that the agency is currently developing is called nuclear thermal propulsion, which uses nuclear fission and could one day power a rocket that makes the trip in half the time.

Nuclear fission involves harvesting the incredible amount of energy released when an atom is split by a neutron. This reaction is known as the fission reaction. Fission technology is well established in power generation and nuclear-powered submarines, and its application to drive or power a rocket could one day offer NASA a faster, more powerful alternative to chemical-powered rockets .

NASA and the Defense Advanced Research Projects Agency are jointly developing NTP technology. They plan to deploy and demonstrate the capabilities of a prototype system in space in 2027, which could make it one of the first of its kind to be built and operated by the United States.

Nuclear thermal propulsion could also one day power maneuverable space platforms that would protect U.S. satellites in Earth’s orbit and beyond. But the technology is still developing.

I am an associate professor of nuclear engineering at the Georgia Institute of Technology, whose research group builds models and simulations to improve and optimize the design of nuclear thermal propulsion systems. My hope and passion is to contribute to the design of the nuclear thermal propulsion engine that will enable a crewed mission to Mars.

Nuclear or chemical propulsion

Conventional chemical propulsion systems use a chemical reaction involving a light propellant, such as hydrogen, and an oxidizer. When mixed, these two elements ignite, causing the propellant to exit the nozzle very quickly to power the rocket.

These systems do not require any type of ignition system, so they are reliable. But these rockets must carry oxygen with them into space, which can weigh them down. Unlike chemical propulsion systems, nuclear thermal propulsion systems rely on nuclear fission reactions to heat the propellant which is then expelled from the nozzle to create the motive force or thrust.

In many fission reactions, researchers send a neutron toward a lighter isotope of uranium, uranium 235. The uranium absorbs the neutron, creating uranium 236. The uranium 236 then splits into two fragments, fission products, and the reaction emits assorted particles.

More than 400 nuclear reactors in operation around the world currently use nuclear fission technology. The majority of these nuclear reactors in operation are light water reactors. These fission reactors use water to slow down neutrons and to absorb and transfer heat. Water can create steam directly in the core or in a steam generator, which drives a turbine to produce electricity.

Nuclear thermal propulsion systems work in the same way, but they use a different nuclear fuel that contains more uranium 235. They also operate at a much higher temperature, making them extremely powerful and compact. Nuclear thermal propulsion systems have a power density approximately 10 times that of a traditional light water reactor.

Nuclear propulsion could have a head start over chemical propulsion for several reasons.

Nuclear propulsion would expel propellant from the engine nozzle very quickly, generating high thrust. This high thrust allows the rocket to accelerate more quickly.

These systems also have a high specific impulse. Specific impulse measures how effectively the propellant is used to generate thrust. Nuclear thermal propulsion systems have about twice the specific impulse of chemical rockets, meaning they could reduce travel time by a factor of 2.

History of nuclear thermal propulsion

For decades, the U.S. government has funded the development of nuclear thermal propulsion technology. Between 1955 and 1973, NASA, General Electric, and Argonne National Laboratories programs produced and ground tested 20 nuclear thermal propulsion engines.

But these pre-1973 designs relied on highly enriched uranium fuel. This fuel is no longer used because of its proliferation risks or the dangers linked to the diffusion of nuclear materials and technologies.

The Global Threat Reduction Initiative, launched by the Department of Energy and the National Nuclear Security Administration, aims to convert many research reactors using highly enriched uranium fuel to uranium fuel. high-grade low-enriched uranium, or HALEU.

Highly titrated, low-enriched uranium fuel contains less material capable of undergoing a fission reaction than highly enriched uranium fuel. So, rockets must contain more HALEU fuel, which makes the engine heavier. To solve this problem, researchers are studying special materials that would allow fuel to be used more efficiently in these reactors.

NASA and DARPA’s Demonstration Rocket for Agile Cislunar Operations (DRACO) program intend to use this high-grade low-enriched uranium fuel in its nuclear thermal propulsion engine. The program plans to launch its rocket in 2027.

As part of the DRACO program, aerospace company Lockheed Martin partnered with BWX Technologies to develop the reactor and fuel design.

The nuclear thermal propulsion engines developed by these groups will have to meet specific performance and safety standards. They will need to have a core capable of operating for the entire duration of the mission and performing the maneuvers necessary for a rapid trip to Mars.

Ideally, the engine should be capable of producing high specific impulses, while meeting the high thrust and low mass requirements of the engine.

Research in progress

Before engineers can design an engine that meets all of these standards, they must start with models and simulations. These models help researchers, like those in my group, understand how the motor would handle starting and stopping. These are operations that require rapid and massive changes in temperature and pressure.

The nuclear thermal propulsion engine will be different from all existing fission electric systems, so engineers will need to create software tools compatible with this new engine.

My group designs and analyzes nuclear thermal propulsion reactors using models. We model these complex reactor systems to see how things like temperature changes can affect the reactor and the safety of the rocket. But simulating these effects can require very expensive computing power.

We are working to develop new computational tools that model the behavior of these reactors during startup and operation without using as much computing power.

My colleagues and I hope that this research can one day contribute to the development of models capable of controlling the rocket autonomously.

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