Recent advances in nuclear thermal propulsion (NTP) technology could make future manned missions to Mars significantly faster, cutting travel times in half compared to traditional chemical rockets.
NASA and the Defense Advanced Research Projects Agency (DARPA) are working together to develop this technology, which could also enable improved maneuverability on space missions. But the design of the reactors that power these nuclear rockets poses significant challenges that engineers are still working to overcome.
The potential of nuclear thermal propulsion for space travel
Chemical rockets, the current standard for space missions, rely on the combustion of chemical propellants such as hydrogen or oxygen to generate the thrust needed to propel a spacecraft. Although this system is reliable, it is slow and requires large amounts of oxygen, which adds considerable weight to the spacecraft. For a trip to Mars, this could mean travel times ranging from several months to more than a year.
In contrast, nuclear thermal propulsion uses the enormous energy produced by nuclear fission to heat a propellant, such as hydrogen, which is then released at high speed to generate thrust. This method is much more efficient than chemical propulsion and can provide up to twice the specific impulse (a measure of how effectively a rocket uses its propellant). “Nuclear propulsion releases propellant from the engine nozzle very quickly, producing high thrust,” said Dan Kotlyar, associate professor of nuclear engineering at Georgia Tech. You will be able to reach your destination faster.
This increase in efficiency is critical when planning a mission to Mars. Long flights on Mars can expose astronauts to long periods of radiation and weightlessness, both of which have negative health effects. Nuclear rockets could shorten a trip to Mars from months to just a few months, significantly reducing the amount of time astronauts are exposed to the risks of space travel.
Space rocket reactor design
Despite the potential benefits, building a nuclear reactor that can operate reliably in space and provide the necessary thrust for long-duration missions remains a major engineering challenge. Unlike chemical rockets, propulsion reactors must operate at very high temperatures, and the fuel used (uranium-235) is radioactive and must be handled with extreme care.
In nuclear thermal propulsion systems, a fission reaction heats the propellant before it is released to create thrust. During nuclear fission, neutrons are fired at uranium-235 atoms, causing them to split and release huge amounts of thermal energy. This process is well understood in nuclear power plants on Earth and must be adapted to the extreme conditions of space. The reactors used in these propulsion systems must be small, lightweight, and capable of operating at higher temperatures than ground-based reactors. “Nuclear thermal propulsion systems have about 10 times the power density of conventional light water reactors,” Kotliar said, highlighting the unique challenges faced in space applications.
One of the problems is the use of high-analytical low-enriched uranium (HALEU), which has less uranium-235 than the highly enriched uranium used in early reactor designs. Although HALEU fuel is safe from a proliferation perspective, it is less efficient and requires more fuel to be carried on board the spacecraft. This increases the overall weight of the system, but engineers are trying to solve this problem by developing special materials that can use fuel more efficiently.
History and future of space nuclear propulsion
Nuclear propulsion is not a new concept. From 1955 to 1973, a NASA, General Electric, and Argonne National Laboratory program successfully developed and ground tested approximately 20 nuclear thermal propulsion engines. However, those designs relied on highly enriched uranium fuel, which posed the risk of nuclear proliferation. Today’s efforts, such as NASA and DARPA’s DRACO (Demonstration Rocket for Agile Cislunar Operations) program, aim to develop safer and more efficient propulsion systems using HALEU fuel.
The DRACO program is scheduled to test a prototype nuclear thermal rocket in space as early as 2027, marking a significant milestone in the development of this technology. Aerospace companies such as Lockheed Martin and BWX Technologies are collaborating on the design of the nuclear reactors and fuel systems that will power these next-generation rockets.
Addressing engineering challenges
To launch a nuclear-powered rocket, several technical hurdles must be overcome. Dan Kotlyar and his research group at Georgia Tech are working on the modeling and simulation needed to optimize these systems. Models are critical to predicting how an engine will behave under a variety of conditions, including starting, stopping, and the significant temperature and pressure changes that occur during operation.
Kotlyar’s team is also developing new computational tools that require less computational power to model these complex systems. The goal is to eventually create an autonomous control system for nuclear rockets, which will be needed for long-duration missions where human intervention is not possible. “My colleagues and I hope that this research will one day help us develop models that can autonomously control rockets,” Kotlyar explained.
In conclusion, nuclear thermal propulsion holds great promise for future missions to Mars and beyond, but the technology is still in its infancy and the development of safe, reliable, and efficient nuclear reactors for space travel is still a challenge. Significant design challenges remain. With ongoing research and tests planned for the coming years, NASA and its partners are steadily moving toward a future in which nuclear rockets enable faster and more efficient exploration of the solar system.
