Nuclear Thermal Propulsion:
Old Technology, Renewed Purpose

Victoria Woodburn   | January 22, 2021

Victoria Woodburn   | January 22, 2021

Artist rendering of a potential mission outward bound NASA Nuclear Thermal Propulsion (NTP) enabled spacecraft. Credit / NASA

Nuclear thermal propulsion, originally thought to be the key to getting humans onto the lunar surface in the 1960s, has recently risen in relevancy once again as humanity attempts to take the next giant leap into our solar system – towards Mars and beyond. In a prior AAS Future in Space hangout, the panelists and host, Tony Darnell, explored what exactly nuclear thermal propulsion is, what its advantages are and how safe the technology is for space usage. This time, his guests included Chief Nuclear Thermal Propulsion engineer for BWX Technologies Dr. Jonathan Witter, Michael Houts of Marshall Space Flight Center, and the Neil Armstrong Chair in Aerospace Policy at the Ohio State University John Horack.

Interest in nuclear thermal propulsion first began in 1961 when NASA and the former Atomic Energy Commission collaborated to establish the Nuclear Engine for Rocket Vehicle Application (NERVA). Legendary rocket scientist Wernher von Braun saw great potential in the technology and proposed that its usage be applied to deep space missions in the near-future. The project was eventually discontinued due to other, less-costly options and the unapparent need for the technology at the time. Presently, aerospace engineers are once again acknowledging the potential of using nuclear thermal propulsion as a tool to advance humans into space. This re-sparked interest has led to modern developments and pushes from within the aerospace community to further develop, test and ultimately integrate these technologies into future NASA missions through their Game Changing Development Program, which is under NASA’s Space Technology Mission Directorate.

“I’m a real advocate for nuclear thermal propulsion because you need to build out that Infrastructure for higher power levels that solar or battery systems just can’t provide,” said Dr. Witter of BWX Technologies. “We want to help NASA ensure that nuclear thermal propulsion is a viable option and to bring it into the next level of technology maturation.”

Dr. Jonathan Witter, Michael Houts and John Horack discussing Nuclear Thermal Propulsion (NTP) with Tony Darnel on his Future In Space NTP hangout episode. Credit / AAS

Currently, NASA uses chemical propellant engines to fuel its rockets, which is effective, but lacks in efficiency when compared to nuclear thermal propulsion rocket engines. The largest appeal to nuclear thermal propulsion comes from its ability to cut down travel times by up to 25 percent, and that it allows for travel significantly further than a rocket using chemical propellants. This is possible through the small nuclear reactor that would generate heat from low-enriched uranium fuel. The envisioned rocket engine that NASA and BWX Technologies are developing would only require a volume of uranium that is roughly the physical size of a marble. Not only would the actual journey-time to deep space locations such as Mars be reduced, but the increased energy output would also allow for a wider launch window, including times when the Earth and Mars are not in the most optimal positions. Additionally, crewed missions on spacecraft with nuclear thermal propulsion engines would actually have the ability to abort their missions and return home before reaching their destination, which is an option that those missions would not have with chemically propelled engines due to their quick propellant usage. Rather than refueling at their destination before turning home, crews would be able to make that decision mid-transit.

Nuclear thermal propulsion engine diagram with arrows that show the flow path of the hydrogen propellant. Blue arrows represent the coldest relative gas temperatures and red arrows represent the hottest. Credit / BWX Technologies

“Once you would send humans to Mars under a chemical rocket, you would have to wait,” said Horack. “You have to wait until Mars and the Earth are back again in the same spot. So as Tom Petty said, ‘the waiting is the hardest part.’ This will allow you to come and go as you please, so to speak, as opposed to having to wait for the celestial mechanics to line up.”

For many people, the word nuclear conjures fears of dangerous technologies and weaponry; however, the integration of nuclear thermal propulsion into spacecraft could actually lead to increased safety for the individuals involved. Providing the option to return home mid-transit would increase safety levels for the crew onboard if an issue were to arise, which is an important factor to take into consideration when navigating flights into deep space that last months to years in duration. Uranium is also safer than other alternatives, such as plutonium, due to the lower levels of radiation that it emits. Additionally, when launching, the reactor is actually powered off and is not turned on until the rocket reaches space. This reduces the chances of calamity during the actual launch of the rocket.

“I think it’s important to say we’re very safe,” said Horack. “We aim to be very safe anytime we fly something in space and a low-enriched uranium nuclear thermal propulsion system is among the safest nuclear things you could decide you want to put on a rocket.”

Artistic rendering of a Mars transit habitat and nuclear propulsion system that could one day take astronauts to the Red Planet. Credit / NASA

To learn more about the advances and applications of nuclear thermal propulsion, readers can listen to this AAS Future in Space hangout here.

Victoria Woodburn is an undergraduate student studying Journalism and Integrated Marketing Communications at Northwestern University Medill School of Journalism.