DARPA plans to demonstrate a nuclear thermal propulsion (NTP) system that can be assembled on orbit according to the Pentagon advanced research agency’s fiscal 2020 budget request.

The agency is asking for $10 million in 2020 to begin a new program, Reactor On A Rocket (ROAR), to develop a high-assay low-enriched uranium (HALEU) propulsion system.

“The program will initially develop the use of additive manufacturing approaches to print NTP fuel elements,” the DARPA budget document says.

“In addition, the program will investigate on-orbit assembly techniques (AM) to safely assemble the individual core element subassemblies into a full demonstration system configuration, and will perform a technology demonstration,”

In a nuclear thermal rocket, propellant, usually liquid hydrogen is heated to a very high temperature in a nuclear reactor then expanded through a rocket nozzle to produce thrust.

Propulsive efficiency, or specific impulse, can be twice that of a chemical rocket.

HALEU is being developed as a fuel source for next-generation U.S. nuclear reactors. Where U.S. naval reactors use highly enriched uranium with a concentration, or assay, of the fissionable isotope uranium-235 (U-235) greater than 90%, commercial reactors use low-enriched uranium with 3-5% U-235.

HALEU has a U-235 assay of more than 5% but less than 20%. Because the U-235 is more concentrated than in current commercial systems, reactors can be smaller. They do not need to be refueled as often, and they can achieve higher “burnup” rates, requiring less fuel and producing less waste.

In fiscal 2020, DARPA plans to demonstrate AM techniques using surrogate materials, followed by proof-of-principle additive manufacturing of natural uranium reactor components. The agency also plans to begin development of a modular nuclear propulsion system, incorporating AM fuel into a low-enriched uranium reactor and additively manufactured engine.

NASA looked at NTP in the 1960s as a way to enable faster manned missions to Mars, and tested reactors and rockets over several years under the NERVA (Nuclear Engine for Rocket Vehicle Application) program. Budget cuts halted the work in 1972, but NASA is again looking at NTP using low-enriched fuel.