What is DARPA’s Endgame Here?

Nuclear thermal rockets typically have around twice the Isp (basically fuel economy) of chemical rockets, on the order of 800—1000, which is a lot less than than electric propulsion, which is typically ten times that of chemical rockets, but the available thrust produced is far greater, 100-1000 kilo-newtons for nuclear thermal vs. 10-500 milli-newtons for the various electric rockets.

For longer missions, Mars and the further, it’s clear that electric propulsion is better and faster, so the only applications that I see are some sort of manned moon base, or an as yet undisclosed military mission:

DARPA plans to demonstrate a nuclear thermal propulsion (NTP) system that can be assembled on orbit to expand U.S. operating presence in cislunar space, according to the Pentagon advanced research agency’s fiscal 2020 budget request.

The agency is seeking $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,” DARPA’s 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,” the document says.

In a nuclear thermal rocket, propellant such as liquid hydrogen is heated to 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.

Given the advances in electric propulsion, I do not see where nuclear thermal will have an advantage, except possibly for supplying a moon base or some as yet undisclosed military program.

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