NASA and the Defense Advanced Research Projects Agency (DARPA) will demonstrate a nuclear-powered rocket engine that will aid long-duration deep space missions and might even power crewed missions to Mars. Under DARPA's Demonstration Rocket for Agile Cislunar Operations (DRACO) program, the agencies plan on launching a spacecraft powered by a Nuclear Thermal Reactor (NTR) engine in Earth orbit by 2027.

“With the help of this new technology, astronauts could journey to and from deep space faster than ever — a major capability to prepare for crewed missions to Mars,” said NASA Administrator Bill Nelson.

Once in orbit, the teams will conduct several experiments with DRACO’s reactor at various power levels. These data will be thoroughly examined by engineers on Earth, before conducting a full-powered test.

“These tests will inform the approach for the future operation of NTR engines in space,” said Dr. Tabitha Dodson, DARPA program manager for DRACO.

The development will be led by NASA’s Space Technology Mission Directorate (STMD) while DARPA acts as contracting authority for the experimental spacecraft and nuclear reactor. The reactor will be powered by high-assay-low-enriched uranium (HALEU) to reduce risk and logistical hurdles. It will also be engineered to ignite only in orbit — as an added safety precaution.

“DARPA and NASA have a long history of fruitful collaboration in advancing technologies for our respective goals, from the Saturn V rocket that took humans to the Moon to robotic servicing and refueling of satellites,” said Dr. Stefanie Tompkins, director, DARPA.

In 2021, DARPA awarded General Atomics, Blue Origin and Lockheed Martin with the first phase contracts for the DRACO program. Awarded $22 million, General Atomics is responsible for the preliminary design of the Nuclear engine’s reactor and propulsion subsystem. Under a $2.5 and $2.9 million contract respectively, Blue Origin and Lockheed Martin will work independently to develop other operational and demonstration systems.

Nuclear engines are a technological leap in modern propulsion systems, requiring a total redesign of how a rocket engine operates. Rather than generate heat due to the combustion of the propellant and the oxidizer, the nuclear engines — as the name suggests — are equipped with a fission-based reactor core. The extreme heat from the reactor rapidly expands liquid hydrogen through a conventional rocket nozzle to generate thrust.

This external heat source results in an unprecedented increase in performance, resulting in an exceptionally high exhaust velocity and ISP, which can theoretically double or triple payload capacity and allow for faster transit times, a key requirement for deep space crewed missions.

This isn’t NASA’s first nuclear rodeo. The development and testing of nuclear engines predate the space race, with early designs meant to replace the conventional second stage for Intercontinental Ballistic Missiles. After the Sputnik crisis, these plans were redirected and named Project Rover. They were thereafter managed by the Atomic Energy Commission (AEC) and NASA’s Space Nuclear Propulsion Office, a program created in Washington DC to manage exploration activities involving nuclear engines. Due to the classified nature of the program, the engine’s construction and tests were carried out at the Los Alamos National Laboratory and the Area 25 Test Facility in Nevada.

During Project Rover, over 20 engines were tested, with over 17 hours of engine run time.

After 1963, the NERVA (Nuclear Engines for Rocket Vehicle Application) program was established to further develop technologies specifically for the space program. Unlike modern nuclear engines, NERVA used highly-enriched uranium for their reactors. The program was considered highly successful and had strong political support from various senators. Picking up the work from Project Rover, NASA developed their first nuclear engine named NERVA NRX (Nuclear Rocket Experimental), to demonstrate ignition and engine restart without an external power source, provide teams with more data during a variety of conditions, and evaluate the endurance of the nuclear reactor during multiple restarts.

NERVA’s first test fire took place in September 1964 with NERVA A2, running flawlessly at full power. During the test, the engine showed a significant leap from any solid or liquid-powered engine, with an ISP clocked at 811 seconds. ISP or specific impulse is a measurement of the efficiency of any rocket engine. It is calculated in seconds — amounting to the time a rocket engine can generate thrust. A higher ISP indicates a higher burn time (in seconds) with the same amount of fuel, hence, a higher efficiency.

A year later, NERVA A3 was successfully tested to verify a full power run and restart. Later on, as the engine matured, the scope of test campaigns grew to include long-duration full-power burns and multiple restarts. Learnings from these experimental test programs led to the development of NERVA XE, the first nuclear engine designed as a complete flight system. After a comprehensive testing regime, the rocket engine was eventually deemed suitable for spaceflight operations by NASA and ready for missions to Mars.

During a series of tests between 1968-69, XE ran at full power for 1680 seconds and was restarted 24 times.

The agency's plans were not limited to Mars but included deep space probes to outer planets in the Solar system. Nuclear tugs were envisioned to take payloads from low earth orbit to higher orbits, to resupply space stations in Earth orbit, and to support a permanent lunar base. A nuclear-powered upper stage for the Saturn V was also planned, which could’ve launched over 150 tons to LEO.

But the rising costs of the Vietnam War and a dwindling NASA budget made it harder to fund the NERVA program. President Johnson was adamant to keep the US nuclear propulsion program alive, funding NERVA specifically twice. But as President Nixon came to power in ’69, cost-cutting went into effect, and he canceled the program by 1973 to fund the Space Shuttle.

After 17 years of research, development, and comprehensive testing, and spending over $1.4 billion, NERVA never left the test stand.

Half a century later, we’re reigniting this transformational technology, embedded with 21st-century safety features and a better understanding of nuclear science.

With DARPA providing fixed-price contracts for the DRACO program, the commercial companies involved are much more motivated to successfully complete their contracts and find innovative solutions, all while under the supervision of experienced government entities.

NTRs are not needed for an initial Mars mission but they’ll significantly accelerate the advent of a permanent and sustainable human settlement on the red planet. And a highly-efficient engine would greatly reduce transit times for scientific payloads venturing out to gas giants, the outer solar system, and beyond.

Nuclear engines reduce the exposure to radiation by drastically reducing the travel time. A conventional mission to Mars during a Hohmann-transfer window is around 9 months, however, nuclear propulsion can cut transit times by at least 50%. This results in cost savings, fewer consumables on board, and more mass for experiments, gear, and Elon's baggage.