U.S. Edges Closer To Nuclear Thermal Rocket Vision

Artist's concept of DRACO
Credit: DARPA

LOS ANGELES­—As NASA continues feasibility studies into nuclear propulsion for deep space missions, DARPA has awarded an initial contract to help pave the way toward possible orbital tests of a nuclear-powered rocket for U.S. military use in and around cislunar space.

Nuclear power for space propulsion has been studied intermittently since the 1950s but in recent years has attracted renewed interest for both potential use in military space applications closer to Earth as well as for exploration missions into deep space. With the capability of generating high thrust and 100% more specific impulse than the best chemical rockets, nuclear thermal propulsion (NTP) systems offer shorter trip times, lower mission mass, reduced heavy-lift launch requirements and extended mission-abort capability.

For military space operations in cislunar space—a region extending from Earth to beyond the Moon’s orbit—NTP potentially provides rapid transit velocity, extended mission capability and greater operational flexibility.

In support of this latter military role, DARPA has awarded a $14 million task order to Gryphon Technologies, a Washington, D.C.-based engineering company, to aid development and demonstration of an NTP system. The work, which will be undertaken as part of DARPA’s Demonstration Rocket for Agile Cislunar Operations (DRACO) program, will specifically cover development of a High-Assay Low Enriched Uranium (HALEU)-based fission reactor propulsion system.

The engine will be based around a HALEU reactor through which liquid hydrogen (LH2) will be pumped. Passing through the core, the LH2 will be heated up and expanded to a gas which will be expelled at high velocity through the nozzle to generate thrust. Unlike most current reactors—which typically operate on low-enriched uranium, usually containing up to 5% uranium-235—the DRACO’s HALEU fuel will be enriched to between 5% and 20% uranium-235. In comparison, most reactors used by the U.S. Navy use weapons grade (greater than 90%) highly enriched uranium fuels (HEU).

Artist’s impression of an NTP-enabled spacecraft. Credit: NASA / Marshall Space Flight Center

The DRACO program will be split into two parallel tracks with the first covering the design of the NTP reactor, while the second will focus on development of an operational system concept as well as a demonstration system focusing on the propulsion subsystem.

NASA’s nuclear propulsion studies include NTP options as well as nuclear electric propulsion (NEP) systems, which convert heat from a reactor to generate electricity for thrust. Although recent funding has up until recently supported studies of a potential NTP flight demonstrator originally targeted at a launch in 2024, a NASA review completed late in 2019 directed work be refocused on a longer-term flight demonstrator later in the decade with greater applicability to an operational system.

NASA meanwhile remains undecided on whether to also put further effort into NEP concepts, recent studies of which have shown positive results. To help it decide, the agency has commissioned a National Academies study committee to study the relative merits of NEP and NTP. The committee study also includes considerations such as HALEU-based systems versus HEU which, because of the potential weapons grade material involved, are considered politically sensitive for proposed roles in space exploration.

Guy Norris

Guy is a Senior Editor for Aviation Week, covering technology and propulsion. He is based in Colorado Springs.