Nuclear rockets may reach Mars in half the time, but designing the reactors to power them is not easy

Nuclear-powered rockets may one day enable faster space travel. Credit: NASA
NASA plans to send a crewed mission to Mars over the next decade, but the 140 million mile (225 million kilometer) journey there could take months or years round trip.
This relatively long flight time is a result of the use of conventional chemical rocket fuel. The agency is currently developing an alternative technology to chemically propelled rockets called nuclear thermal propulsion, which uses nuclear fission and could one day power rockets in just half the time.
Nuclear fission involves harvesting the incredible energy released when atoms are split by neutrons. This reaction is known as a nuclear fission reaction. Nuclear fission technology is well established in power generation and nuclear submarines, and its application to propelling or powering rockets could one day provide NASA with a faster, more powerful alternative to chemically powered rockets.
NASA and the Defense Advanced Research Projects Agency are jointly developing NTP technology. They plan to deploy a prototype system in space in 2027 to demonstrate its capabilities, which could be the first system of its kind built and operated by the United States.
Nuclear thermal propulsion could one day power maneuverable space platforms that protect U.S. satellites in and out of Earth’s orbit. However, this technology is still under development.
I am an associate professor of nuclear engineering at Georgia Tech, where my research group builds models and simulations to improve and optimize the design of nuclear thermal propulsion systems. My hope and passion is to help design a nuclear thermal propulsion engine to carry a manned mission to Mars.
Nuclear propulsion and chemical propulsion
Traditional chemical propulsion systems use chemical reactions involving a light propellant, such as hydrogen, and an oxidizing agent. Once mixed, these two ignite, resulting in the propellant coming out of the nozzle very quickly and propelling the rocket.
These systems do not require an ignition system and are therefore more reliable. But these rockets can be heavy because they need to carry oxygen into space. Unlike chemical propulsion systems, nuclear thermal propulsion systems rely on nuclear fission reactions to heat the propellant, which is then ejected from a nozzle to produce propulsion or thrust.
In many fission reactions, researchers send neutrons toward uranium-235, a lighter isotope of uranium. Uranium absorbs neutrons and produces uranium-236. The uranium-236 then splits into two fragments (fission products), and the reaction releases various particles.
There are currently more than 400 nuclear reactors in operation around the world that 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 absorb and transfer heat. The water directly generates steam in the reactor core or steam generator, which drives turbines and generates electricity.
Nuclear thermal propulsion systems work similarly, but use a different nuclear fuel with a higher content of uranium-235. It also operates at very high temperatures, making it extremely powerful and compact. Nuclear thermal propulsion systems have approximately 10 times more power density than conventional light water reactors.
Nuclear propulsion may be superior to chemical propulsion for several reasons.
In nuclear propulsion, propellant is injected very quickly through the engine nozzle, creating high thrust. This higher thrust allows the rocket to accelerate faster.
These systems also have high specific impulse. Specific impulse measures how efficiently propellant is used to produce thrust. Nuclear thermal propulsion systems have about twice the specific impulse of chemical rockets, so they have the potential to cut travel times in half.
History of nuclear thermal propulsion
The U.S. government has been funding the development of nuclear thermal propulsion technology for decades. From 1955 to 1973, programs at NASA, General Electric National Laboratory, and Argonne National Laboratory built and ground-tested 20 nuclear thermal propulsion engines.
However, these pre-1973 designs relied on highly enriched uranium fuel. This fuel is no longer in use due to proliferation risks or risks associated with the proliferation of nuclear materials and technology.
The Global Threat Reduction Initiative, launched by the Department of Energy and the National Nuclear Security Administration, aims to convert many research reactors that use highly enriched uranium fuel to Highly Analyzed Low Enriched Uranium (HALEU) fuel. The purpose is
High-concentration, low-enriched uranium fuel contains less material that can undergo fission reactions compared to highly enriched uranium fuel. Therefore, the rocket needs to carry more HALEU fuel, making the engine heavier. To solve this problem, researchers are investigating special materials that allow nuclear reactors to use fuel more efficiently.
NASA and DARPA’s Agile Sys Lunar Operational Demonstration Rocket (DRACO) program aims to use this high-grade, low-enriched uranium fuel in nuclear thermal propulsion engines. The program is scheduled to launch a 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 being developed by these groups must comply with specific performance and safety standards. It requires a core that can operate for the duration of the mission and perform the operations required for high-speed travel to Mars.
Ideally, the engine should be able to produce high specific impulse while also meeting the requirements of high thrust and low engine mass.
ongoing research
Before engineers can design an engine that meets all these criteria, they need to start with models and simulations. These models help researchers, such as those in my group, understand how engines handle starting and stopping. These are operations that require rapid and large changes in temperature and pressure.
Because the nuclear thermal propulsion engine is different from all existing nuclear fission power systems, engineers will need to build software tools to work with this new engine.
My group uses models to design and analyze thermal propulsion reactors. We model these complex reactor systems to see how things like temperature changes affect reactor and rocket safety. However, simulating these effects requires large amounts of expensive computing power.
We have been working on new computational tools to model how these reactors behave during startup and operation without using as much computing power.
My colleagues and I hope that this research will one day help us develop models that can autonomously control rockets.
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