The US Defense Advanced Research Projects Agency (Darpa) said on 9 April that it had awarded a $22 million contract to General Atomics to design a small nuclear reactor for space propulsion under the demonstration rocket for agile cislunar operations (Draco) project. “Cislunar” refers to space between the Earth and the moon. Draco, set up in 2020, aims to demonstrate nuclear thermal propulsion, namely, using a nuclear reactor to heat rocket fuel in order to generate thrust. In May 2020, Darpa’s Tactical Technology Office issued a “broad agency announcement” asking for proposals to test a nuclear thermal propulsion system in orbit by 2025.

Currently space systems depend on electric and chemical propulsion. However, Darpa believes other options may be needed for future exploration beyond Earth’s orbit. It said the Draco programme seeks to develop novel nuclear thermal propulsion (NTP) technology, which “can achieve high thrust-to-weights similar to chemical propulsion but with two to five times the efficiency”. In order to monitor cislunar space, a “leap-ahead in propulsion technology” will be required, Darpa noted.

The Draco programme will attempt to demonstrate a NTP system on orbit. A nuclear reactor will heat propellant to extreme temperatures before expelling the hot propellant through a nozzle to produce thrust.

NTP is a “leap ahead of conventional propulsion technology and will enable spacecraft to travel immense distances quickly,” said Christina Back, vice president of nuclear technologies and materials at General Atomics Electromagnetic Systems. She told SpaceNews that “agile spacecraft are critical to maintain space domain awareness and significantly reduce transit times in the vast cislunar region”. For human missions to Mars, “nuclear propulsion will allow for more versatility of launch windows, and enable longer stays on the planet itself”.

NASA is also looking at the possibilities of NTP. In a statement in February it said: “As NASA’s Perseverance rover homes in on the Red Planet, engineers on the ground are furthering potential propulsion technologies for the first human missions to Mars. NASA is looking at two types of nuclear propulsion systems – nuclear electric and nuclear thermal propulsion.” It added that NTP technology “provides high thrust and twice the propellant efficiency of chemical rockets”.

NASA, in coordination with the Department of Energy (DOE), is asking industry for preliminary reactor design concepts for a nuclear thermal propulsion system. “The agencies plan to fund several efforts to explore different approaches. Future follow-on contracts will generate more detailed reactor designs and build preliminary testing hardware,” NASA said.