NASA plans to send astronauts to Mars in the coming decade. This presents many challenges, not the least of which is the distance involved and the resulting health risks. To this end, they are investigating and investing in many technologies, ranging from life support and radiation protection to nuclear power and propulsion elements. A particularly promising technology is Nuclear-Thermal Propulsion (NTP), which has the potential to reduce transit times to Mars significantly. Instead of the usual one-way transit period of six to nine months, a working NTP system could reduce the travel time to between 100 and 45 days!
In January of this year, NASA and the Defense Advanced Research Projects Agency (DARPA) announced that they were launching an interagency agreement to develop a nuclear-thermal propulsion (NTP) system – known as the Demonstration Rocket for Agile Cislunar Operations (DRACO). Just yesterday, DARPA announced that it had finalized an agreement with Lockheed Martin to design and build a prototype NTR system – the Experimental NTR Vehicle (X-NTRV) – that will be sent to space for testing by 2027.
The announcement was followed by a live media teleconference held by NASA and DARPA at 01:00 PM EDT (10:00 AM PDT) on Wednesday, July 26th. The teleconference was live-streamed on NASA’s website and featured an expert panel discussing the latest developments in the DRAGO program and fielding questions from various media outlets. The panel consisted of four members, including:
- Dr. Anthony Calomino, space nuclear technologies portfolio manager, NASA
- Dr. Tabitha Dodson, DRACO program manager, DARPA
- Kirk Shireman, vice president, Lockheed Martin Lunar Exploration Campaigns
- Joe Miller, president, BWXT Advanced Technologies
Nuclear Propulsion
Nuclear propulsion concepts generally fall into two categories: nuclear electric and nuclear thermal propulsion (NEP/NTP). NEP concepts consist of a nuclear reactor generating power for a Hall-Effect thruster, which uses electromagnetic fields to ionize and accelerate inert gas (like xenon) to generate thrust. Conversely, NTP involves a nuclear reactor heating liquid hydrogen or deuterium, then channeling the rapidly-expanding gas through nozzles to generate thrust. While no NEP concepts have been realized to date, experiments involving NTP go back to the Space Age.
In NASA’s case, these efforts resulted in the creation of the Nuclear Engine for Rocket Vehicle Application (NERVA), which was successfully tested between 1961 and 1969. With the closing of the Apollo Era, efforts to further develop the technology and integrate it into a spacecraft were shelved. This reflected the changing budget environment and new priorities, which included the Space Shuttle program and the development of space stations. In 2017, with crewed missions to Mars on the horizon, NASA reignited its NTP program.
In addition to improved efficiency, nuclear thermal propulsion systems have a virtually unlimited energy density, significantly reducing the propellant needed. This eliminates the need for orbital refueling or fabricating propellant on Mars for the return journey. Since the majority of a chemical rocket’s mass consists of its propellant and large propellant tanks, it also means that future spacecraft will be less massive and cumbersome. Moreover, NTP engines also enable abort scenarios on journeys to Mars that are impossible using chemical propulsion systems.
The DRACO project represents the fruition of these efforts, focusing on more efficiently and quickly transporting payloads and crews through cislunar space and, eventually, to Mars. As Dr. Tabitha Dodson, the DRACO program manager for DARPA, indicated in an agency press release:
“The DRACO program aims to give the nation leap-ahead propulsion capability. An NTR achieves high thrust similar to in-space chemical propulsion but is two-to-three-times more efficient. With a successful demonstration, we could significantly advance humanity’s means for going faster and farther in space and pave the way for the future deployment for all fission-based nuclear space technologies.”
Kirk Shireman, vice president of Lunar Exploration Campaigns at Lockheed Martin Space, said in a separate Lockheed Martin press release:
“These more powerful and efficient nuclear thermal propulsion systems can provide faster transit times between destinations. Reducing transit time is vital for human missions to Mars to limit a crew’s exposure to radiation. This is a prime technology that can be used to transport humans and materials to the Moon. A safe, reusable nuclear tug spacecraft would revolutionize cislunar operations. With more speed, agility and maneuverability, nuclear thermal propulsion also has many national security applications for cislunar space.”
Old Idea, New Fuel
The DRACO program leverages advances achieved through the NERVA program but relies on a new fuel known as High-Assay Low-Enriched Uranium (HALEU). Compared to conventional uranium fuel, which relies on 5% enrichment with Uranium-235 – the main fissile isotope that produces energy during fission reactions – HALEU fuel is enriched between 5% and 20%. This allows for smaller reactors, more power per unit of volume, and the optimization of reactors for longer core life, increased efficiency, and better fuel utilization.
The use of HALEU fuel was made possible through the National Security Presidential Memorandum 20 (NSPM-20) issued in August 2019, which updated U.S. policy regarding nuclear power and propulsion in space. The U.S. Department of Energy (DoE) will provide the HALEU metal, which will be converted into fuel by BWX Technologies (BWXT), a leading supplier of nuclear components and fuel to the U.S. government and one of Lockheed Martin’s partners for the DRACO program. This Virginia-based company will also manufacture the nuclear reactor that will power the X-NTRV.
“The award of this contract further demonstrates BWXT’s ability to design, manufacture and deploy nuclear reactors and fuel on a scale that is unmatched elsewhere in the world today,” Joe Miller, president of BWXT Advanced Technologies LLC, said in a company press release. “This partnership with Lockheed Martin working for DARPA adds another important dimension to BWXT’s already-impressive line-up of nuclear reactor designs for commercial and defense applications.”
DARPA also indicated it would engineer the system so the reactor remains “cold” (switched off) until it reaches its designated orbit. As Dr. Dobson explained during the teleconference:
“The Draco program upfront sets very clear rules and guidelines which hopefully can help to take a variety of programmatic and technical questions off the table. For instance, we are never turning the reactor on, on the ground. Therefore, transportation and watch site operations are all greatly simplified because a reactor that has never been turned on is cold and benign. You can put your hand right inside the core and touch the fuel like you would any other heavy metal.”
The Test
The development of the DRACO thruster will be conducted primarily at BWXT’s Mt. Athos Road complex near Lynchburg, Virginia. Once the reactor is complete and integrated into the X-NTRV, the U.S. Space Force will launch the vehicle to High Earth Orbit (HEO). As Dr. Dobson indicated, this will likely involve a commercial launch vehicle from NASA’s Kennedy Space Center in Cape Canaveral, Florida, depending on how far into space they decide to send it.
“We’re looking at launching out of the Cape, and it will require a heavy lift vehicle,” she said. “Although that would be nice to have if, for some reason, we needed to [launch] it to a much higher orbit. But at this point, we’re looking at just an NSSL [National Security Space Launch] with a standard fairing – not a heavy lift. So Falcon 9 or Vulcan Centaur.”
According to Dr. Calamino, the vehicle will carry an estimated 100 kg (220 lbs) of HALEU fuel in its core and about 2000 kg (4400 lbs) of hydrogen propellant. The panel also addressed some key concerns regarding the safety of the upcoming test and the issue of radiation hazards. In particular, there was the question of what might happen if there is a mishap during launch or an accident in orbit, resulting in debris falling to Earth. As Dr. Calamino explained, the NTP reactor will be much safer than a fission system that relies on uranium-235 or plutonium:
“Fission systems are not radioisotope systems; they’re very very much different. Radioisotope systems are radioactive from the time that they are prepared to go into an electric generator all the way up to when they’re incorporated into the payload and sitting on the launch pad. Plutonium is just basically a radioactive material. Uranium-239, without it having been fissioned and actually being surrounded by fission products, is basically a metal.
“It is safe to work around. It is safe to be around. It doesn’t need the protection measures that need to be a place for plutonium. If you have a mishap during launch or on the launchpad itself, there is the debris that would potentially be generated by that, isn’t any worse than the debris that would be produced by the turbo machinery that might also be dispersed in such an accident or placed anywhere else. It is not a radioactive material at that point in time.”
Another safety measure NASA and BWXT are considering is inserting “poison wires” and systems in the reactor to prevent it from going critical. This refers to neutron-absorbing materials often used in reactors to lower the high reactivity of their fuel load if the reactions become unstable. The combination of these elements, said Dr. Calamino and Miller, will prevent the contamination of Earth’s atmosphere and any biosphere the vehicle encounters should something go wrong during the test.
When NASA inaugurated its “Journey to Mars” program in 2010, it envisioned a Deep Space Transport (DST) that would ferry crews between the proposed Lunar Gateway and a similar station orbiting Mars using a 500-kilowatt (kWe) Solar Electric Propulsion (SEP) system. Unfortunately, there have since been delays with the development of the Space Launch System (SLS) and other mission elements and complications imposed by parallel programs (the Artemis Program). Because of this, there are legitimate fears that NASA will not be ready to conduct crewed missions to Mars by 2033, as originally hoped.
The realization of a nuclear thermal propulsion system is seen as one of two possible solutions to these problems – the other being an “orbit-only” mission architecture. In addition to enabling rapid transits to Mars and back, it would also allow for greater flexibility. Instead of launching every 26 months when Earth and Mars are closest to each other (a “Mars Opposition“), it could be possible to send missions during a “Mars Conjunction.” The reduced transit would also reduce the amount of radiation crews would be exposed to and the time spent in microgravity (the two greatest health hazards of a crewed Mars mission).
Beyond Mars, nuclear-thermal, nuclear-electric, and systems that combine both (bimodal propulsion) will enable missions to the Main Asteroid Belt, Venus, and other deep-space destinations, effectively expanding the reach of human-rated exploration. It will also enable shorter transits for robotic explorers destined for the outer Solar System, drastically increasing the rate of scientific returns. There has been some doubt that the technology will be ready in time. But if the X-NTRV test is successful and completed on schedule, NASA may have an NTP system ready for the 2033 launch window.
Further Reading: DARPA, Lockheed Martin
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