Nuclear Propulsion Could Help Get Humans to Mars Faster
NASA and DARPA are to demonstrate a nuclear thermal rocket engine in space, with a view to using the technology to send crewed missions to Mars. The Demonstration Rocket for Agile Cislunar Operations (DRACO) program will develop and flight test in space nuclear thermal propulsion technology that could be used to transport astronauts to Mars and beyond. The research agencies plan to send the DRACO test vehicle into space and run the nuclear engine in 2027.
NASA’s Space Technology Mission Directorate (STMD) will lead technical development of the nuclear thermal engine, which will be integrated with an experimental spacecraft developed by the Defense Advanced Research Projects Agency (DARPA). A nuclear thermal rocket engine uses a fission reactor to generate extremely high temperatures. The engine transfers the heat produced by the reactor to a liquid propellant, which is expanded and exhausted through a nozzle to propel the spacecraft.
Engineers believe that nuclear thermal rockets can be three or more times more efficient than conventional chemical propulsion. Using a nuclear thermal rocket will enable faster transit times, reducing risk for astronauts. Reducing transit time is a key component for human missions to Mars, as longer trips require more supplies and more robust systems. Other benefits to space travel include increased science payload capacity and higher power for instrumentation and communication, said NASA.
DARPA is acting as the contracting authority for the development of the entire stage and the engine, which includes the reactor. DARPA will also lead the overall program including rocket systems integration and procurement, approvals, scheduling, and security, cover safety and liability, and ensure overall assembly and integration of the engine with the spacecraft. Over the course of the development, NASA and DARPA will collaborate on assembly of the engine.
Nuclear thermal rocket engine testing was last conducted by the USA more than 50 years ago under NASA’s Nuclear Engine for Rocket Vehicle Application and Rover projects. NASA and the Soviet space program spent decades researching nuclear propulsion during the Space Race. A few years ago, NASA reignited its nuclear program for the purpose of developing bimodal nuclear propulsion – a two-part system consisting of an NTP and NEP element – that could enable transits to Mars in 100 days.
As part of the NASA Innovative Advanced Concepts (NIAC) program for 2023, NASA selected a nuclear concept for Phase I development. This new class of bimodal nuclear propulsion system uses a “wave rotor topping cycle” and could reduce transit times to Mars to just 45 days. The proposal, titled “Bimodal NTP/NEP with a Wave Rotor Topping Cycle,” was put forward by Prof. Ryan Gosse, the Hypersonics Program Area Lead at the University of Florida and a member of the Florida Applied Research in Engineering (FLARE) team.
Gosse’s proposal is one of 14 selected by the NAIC this year for Phase I development, which includes a US$12,500 grant to assist in maturing the technology and methods involved. Other proposals included innovative sensors, instruments, manufacturing techniques, power systems, and more.
For Nuclear-Thermal Propulsion (NTP), the cycle consists of a nuclear reactor heating liquid hydrogen (LH2) propellant, turning it into ionized hydrogen gas (plasma) that is then channeled through nozzles to generate thrust. Several attempts have been made to build a test this propulsion system, including Project Rover, a collaborative effort between the US Air Force and the Atomic Energy Commission (AEC) that launched in 1955.
In 1959, NASA took over from the USAF, and the program entered a new phase dedicated to spaceflight applications. This eventually led to the Nuclear Engine for Rocket Vehicle Application (NERVA), a solid-core nuclear reactor that was successfully tested. With the closing of the Apollo Era in 1973, the program’s funding was drastically reduced, leading to its cancellation before any flight tests could be conducted. Meanwhile, the Soviets developed their own NTP concept (RD-0410) between 1965 and 1980 and conducted a single ground test before the program’s cancellation.
Nuclear-Electric Propulsion (NEP), on the other hand, relies on a nuclear reactor to provide electricity to a Hall-Effect thruster (ion engine), which generates an electromagnetic field that ionizes and accelerates an inert gas (like xenon) to create thrust. Attempts to develop this technology include NASA’s Nuclear Systems Initiative (NSI) Project Prometheus (2003 to 2005). Both systems have considerable advantages over conventional chemical propulsion, including a higher specific impulse (Isp) rating, fuel efficiency, and virtually unlimited energy density. While NEP concepts are distinguished for providing more than 10,000 seconds of Isp, meaning they can maintain thrust for close to three hours, the thrust level is quite low compared to conventional rockets and NTP. The need for an electric power source, says Gosse, also raises the issue of heat rejection in space – where thermal energy conversion is 30-40 percent under ideal circumstances.
And while NTP NERVA designs are the preferred method for crewed missions to Mars and beyond, this method also has issues providing adequate initial and final mass fractions for high delta-v missions. This is why proposals that include both propulsion methods (bimodal) are favored, as they would combine the advantages of both. Gosse’s proposal calls for a bimodal design based on a solid core NERVA reactor that would provide a specific impulse (Isp) of 900 seconds, twice the current performance of chemical rockets.
Gosse proposed cycle also includes a pressure wave supercharger – or Wave Rotor (WR) – a technology used in internal combustion engines that harnesses the pressure waves produced by reactions to compress intake air. When paired with an NTP engine, the WR would use pressure created by the reactor’s heating of the LH2 fuel to compress the reaction mass further. As Gosse promises, this will deliver thrust levels comparable to that of a NERVA-class NTP concept but with an Isp of 1400-2000 seconds. When paired with a NEP cycle, said Gosse, thrust levels are enhanced even further: “Coupled with an NEP cycle, the duty cycle Isp can further be increased (1,800-4,000 seconds) with minimal addition of dry mass. This bimodal design enables the fast transit for manned missions (45 days to Mars) and revolutionizes the deep space exploration of our Solar System.”
Based on conventional propulsion technology, a crewed mission to Mars could last up to three years. These missions would launch every 26 months when Earth and Mars are at their closest (aka. a Mars opposition) and would spend a minimum of six to nine months in transit. A transit of 45 days (six and a half weeks) would reduce the overall mission time to months instead of years. This would significantly reduce the major risks associated with missions to Mars, including radiation exposure, the time spent in microgravity, and related health concerns. In addition to propulsion, there are proposals for new reactor designs that would provide a steady power supply for long-duration surface missions where solar and wind power are not always available. Examples include NASA’s Kilopower Reactor Using Sterling Technology (KRUSTY) and the hybrid fission/fusion reactor selected for Phase I development by NASA’s NAIC 2023 selection.
NASA, the Department of Energy (DOE) and industry are also developing advanced space nuclear technologies for multiple initiatives to harness power for space exploration. Through NASA’s Fission Surface Power project, DOE awarded three commercial design efforts to develop nuclear power plant concepts that could be used on the surface of the Moon and, later, Mars.