Demonstration Rocket for Agile Cislunar Operations

In May 1946, the U.S. Air Force launched the Nuclear Energy for Propulsion of Aircraft (NEPA) project to explore the potential of nuclear energy for powering aircraft. This initiative led to a collaborative effort of the Air Force and the U.S. Atomic Energy Commission (AEC) known as the Aircraft Nuclear Propulsion (ANP) program, aimed at developing nuclear propulsion systems for aerospace vehicles. In 1955, the Air Force partnered with AEC to develop reactors for nuclear rockets under Project Rover. In mid-1958, NASA replaced the Air Force Due to funding issues, NERVA ended in 1973 without a flight test. ==New program==

In 2020, the National Academies of Sciences, Engineering, and Medicine, at the request of NASA, convened an ad hoc Space Nuclear Propulsion Technologies Committee to identify primary technical and programmatic challenges and risks for the development of space nuclear propulsion technologies for use in future exploration of the Solar System. With regard to nuclear thermal propulsion (NTP) systems, the committee identified the following technological challenges. • A high operating power density and temperature of the reactor are necessary to heat the propellant to approximately 2700 K at the reactor exit for the duration of each burn. • The need for long-term storage and management of cryogenic, liquid hydrogen (LH2) propellant. • Short reactor startup times (as little as 60 s from zero to full power) relative to other space or terrestrial power reactors. • Dealing with the long startup and shutdown transients of an NTP system relative to chemical engines. This drives design of the engine turbopumps and thermal management of the reactor subsystem. The committee also emphasized the lack of adequate ground-based test facilities, noting that "There are currently no facilities in the United States that could conduct a full-power ground test of a full-scale NTP reactor comparable to the Rover/NERVA experiments." In January 2023, NASA and DARPA announced their collaboration on DRACO, dividing the $499 million program between them for Phases 2 and 3. On July 26, 2023, DARPA and NASA announced the awarding of a contract to Lockheed Martin and BWX Advanced Technologies (BWXT) for DRACO Phases 2 and 3 to design, build and demonstrate the experimental NTR for the 2027 launch. BWXT is slated to design and build the reactor, manufacture the fuel and deliver the complete subsystem for integration into the DRACO vehicle. ==Design==

The main design features of DRACO include the following: • The nuclear thermal propulsion (NTP) engine will consist of a fission reactor that transfers heat to a liquid propellant, in this case, liquid hydrogen. That heat will convert the hydrogen into a gas that expands through a nozzle to provide thrust. • The nuclear fuel will consist of enriched uranium, that is, 238U (the most commonly-occurring isotope) together with roughly 20% of 235U, the fissile isotope. This level of enrichment is somewhat higher than the 3-5% common in light water power reactors on Earth, but lower than the roughly 90% enrichment characteristic of weapons-grade material. The choice of 20% enrichment was made in order to alleviate programmatic and regulatory overhead. According to a 2019 presidential memorandum, approval for the launch of a spacecraft using uranium having enrichment below 20% (a so-called “Tier 2” vehicle) is required only by the head of the sponsoring agency (in this case, the Secretary of Defense) rather than the White House. • The propellant will consist of liquid hydrogen (LH2) stored in a cryogenic tank. The hydrogen will be heated by the reactor in less than a second from a temperature of about 20K (-420F) to around 2,700 K. For comparison, typical water temperatures of a modern pressurized water reactor are around 600 K. • The reactor will be integrated with an expander cycle rocket engine. In this design, a turbopump directs high-pressure liquid hydrogen down two paths. The first cools the engine’s nozzle and pressure vessel. Liquid hydrogen in the second path first cools the core support assemblies, then drives the turbopump assembly, the exhaust from which is routed back to the reactor pressure vessel where it absorbs energy from the fission reaction. The superheated gas is then expanded out through the nozzle to provide thrust. • While details of the design thrust level have not been released, the design goal is said) This would represent an increase of about 350 seconds compared with the specific impulse of the RL10, a liquid-fuel cryogenic rocket engine built in the United States by Aerojet Rocketdyne and which is used for Centaur upper stage of the Atlas V. • Currently it is uncertain how difficult it might be to maintain the hydrogen propellant in a liquid state for long periods of time, as would be required for trips to Mars. In-space liquid cryogenic propellant transfer has not yet been demonstrated, but Lockheed Martin is developing a refueling vehicle to support Blue Origin’s Blue Moon lunar lander, and discussions are said to be ongoing about the possibility of installing a refueling port on DRACO. ==Development and testing==

Phase 2 of the DRACO program will involve a test of the NTR engine without nuclear fuel, while Phase 3 will include assembly of the fueled NTR with the stage, environmental testing, and space launch to conduct experiments on the NTR and its reactor. The U.S. Department of Energy will provide HALEU metal to BWX Technologies for processing into low-enriched fuel. The amount of HALEU utilized for the vehicle has sparked some safety concerns among industry experts and the science community. In Phase 2, the engine will be evaluated in a cold-flow test with a nonnuclear engine mock-up to assess the mechanical integrity of the core. Such tests were conducted during the Rover/NERVA program in order to study ways to prevent the core from being destroyed from the pressure and high mass flow rates due to the engine’s turbomachinery. Phase 3 will address launch and space environments testing, assembly integration and testing of the host platform, loads testing, and learning how to interface and command the engine before it is sent to space. During the Phase 3 demonstration, the spacecraft will be launched into a high orbit around Earth, between 435 and 1,240 miles (700 to 2,000 kilometers) above the surface. However, that date has passed, and more recently, it was reported that Lockheed Martin Corp. will demonstrate its technology "as early as 2027", but that launch date has since been put on hold by nuclear reactor test requirements. == Program termination ==

DARPA did not include funding for nuclear thermal or electric propulsion programs in its FY2026 budget, whose cuts are being implemented within NASA itself in accordance to the White House goals. DARPA canceled its five-year-old DRACO project, citing a declining return on investment. According to DARPA Deputy Director Rob McHenry, the original rationale for DRACO—high launch costs and the projected efficiency of nuclear thermal engines—has been undermined by significant reductions in launch expenses driven by SpaceX and the potential of Starship. As these launch costs dropped, the benefits of nuclear thermal propulsion no longer justified its high research and development costs. In 2026, the proposed Space Reactor‑1 Freedom was seen as the spiritual successor to the project. ==References==