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.{{Cite web |last=Waid |first=Jack |date=June 21, 2021 |title=History in Two: Manned Nuclear Aircraft Program |url=https://www.afmc.af.mil/News/Article-Display/Article/2664365/history-in-two-manned-nuclear-aircraft-program/ |access-date=May 15, 2024 |website=Air Force Materiel Command}}{{Cite web |last=McMillan |first=Tim |date=October 4, 2023 |title=Air Force research lab takes giant leap into the new space age with plans for nuclear-powered spacecraft systems |url=https://thedebrief.org/air-force-research-lab-takes-giant-leap-into-the-new-space-age-with-plans-for-nuclear-powered-spacecraft-systems/ |access-date=May 15, 2024 |website=thedebrief.org}} 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. The ANP Program was canceled in March 1961 after investing $1 billion.

Using nuclear energy for space travel reportedly has also been discussed since the 1950s among industry experts. Freeman Dyson and Ted Taylor, through their involvement in Project Orion, aimed to create an early demonstration of the technology. Ultimately, the project received backing from Wernher von Braun, and reached the test flight stage of development, but the project ended early due to environmental concerns.{{Cite web |title=Nuclear Pulse Propulsion: Gateway to the Stars |url=https://www.ans.org/news/article-1294/nuclear-pulse-propulsion-gateway-to-the-stars/ |access-date=2024-04-13 |website= ans.org |language=en}}

In 1955, the Air Force partnered with AEC to develop reactors for nuclear rockets under Project Rover.{{Cite web |last=Butz Jr. |first=J. S. |date=June 1, 1961 |title=Rover—the Nuclear Way to Space |url=https://www.airandspaceforces.com/article/0661space/ |access-date=May 14, 2024 |website=airandspaceforces.com}} In mid-1958, NASA replaced the Air Force and built Kiwi reactors to test nuclear rocket principles in a non-flying nuclear engine. With the next phase's Nuclear Engine for Rocket Vehicle Application (NERVA), NASA and AEC sought to develop a nuclear thermal rocket for "both long-range missions to Mars and as a possible upper-stage for the Apollo Program."{{Cite web |title=Nuclear Rockets |url=https://www1.grc.nasa.gov/historic-facilities/rockets-systems-area/7911-2/ |access-date=May 14, 2024 |website=NASA}} Due to funding issues, NERVA ended in 1973 without a flight test.

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:{{cite report|author= National Academies of Sciences, Engineering, and Medicine| date= 2021| title= Space Nuclear Propulsion for Human Mars Exploration |url= https://doi.org/10.17226/25977| publisher= The National Academies Press| location= Washington, DC| doi= 10.17226/25977| isbn= 978-0-309-68480-4| url-access= subscription}}

• 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." Nevertheless, the committee's report concluded "An aggressive program could develop an NTP system capable of executing the baseline mission [a crewed mission to Mars during opposition] in 2039."

In April 2021, DARPA announced the start of DRACO by awarding 18-month Phase 1 contracts to General Atomics for the nuclear reactor concept design ($22 million), and to Blue Origin ($2.5 million) and Lockheed Martin ($2.9 million) for their competing operation system and demonstration system concept designs.{{Cite web |last=Erwin |first=Sandra |date=2021-04-12 |title=DARPA selects Blue Origin, Lockheed Martin to develop spacecraft for nuclear propulsion demo |url=https://spacenews.com/darpa-selects-blue-origin-lockheed-martin-to-develop-spacecraft-for-nuclear-propulsion-demo/ |access-date=2024-06-02 |website=SpaceNews |language=en-US}}{{Cite web |date=April 12, 2021 |title=DARPA Selects Performers for Phase 1 of Demonstration Rocket for Agile Cislunar Operations (DRACO) Program |url=https://www.darpa.mil/news-events/2021-04-12 |access-date=June 3, 2024 |website=darpa.mil}}

In January 2023, NASA and DARPA announced their collaboration on DRACO, dividing the $499 million program between them for Phases 2 and 3. NASA is to be responsible for the propulsion system and nuclear reactor, and DARPA is to lead the vehicle and integration requirements, mission concept of operations, nuclear regulatory approvals and launch authority. The U.S. Space Force plans to launch DRACO on either a SpaceX Falcon 9 or a United Launch Alliance Vulcan Centaur.{{cn|date=June 2024}}

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.{{Cite web |last=Hitchens |first=Theresa |date=2023-07-26 |title=DARPA, NASA tap Lockheed Martin to design, build DRACO nuclear rocket for deep space missions |url=https://breakingdefense.sites.breakingmedia.com/2023/07/darpa-nasa-tap-lockheed-martin-to-design-build-draco-nuclear-rocket-for-deep-space-missions/ |access-date=2024-05-09 |website=Breaking Defense |language=en-US |archive-date=16 March 2024 |archive-url=https://web.archive.org/web/20240316085306/https://breakingdefense.com/2023/07/darpa-nasa-tap-lockheed-martin-to-design-build-draco-nuclear-rocket-for-deep-space-missions/ |url-status=dead }}{{Cite news |last=Davenport |first=Christian |date=2023-07-27 |title=NASA, Pentagon award contract to build nuclear-powered rocket engine |url=https://www.washingtonpost.com/technology/2023/07/26/nuclear-rocket-contract-nasa-darpa/ |access-date=2024-05-10 |newspaper=Washington Post |language=en-US |issn=0190-8286}} BWXT is slated to design and build the reactor, manufacture the fuel and deliver the complete subsystem for integration into the DRACO vehicle.{{cite web | url= https://www.bwxt.com/news/2023/07/26/BWXT-to-Provide-Nuclear-Reactor-Engine-and-Fuel-for-DARPA-Space-Project | title= BWXT to Provide Nuclear Reactor Engine and Fuel for DARPA Space Project |date=26 July 2023 |website=BWXT.com |publisher=BWX Technologies, Inc. |access-date=10 June 2024}}

The main design features of DRACO include the following:{{citation |last= Norris|first= Guy|date=14 August 2023 |title=Going Nuclear|magazine= Aviation Week and Space Technology}}{{Cite web |last=Krywko |first=Jacek |date=2024-07-22 |title=We're building nuclear spaceships again—this time for real |url=https://arstechnica.com/science/2024/07/were-building-thermonuclear-spaceships-again-this-time-for-real/ |access-date=2024-07-23 |website=Ars Technica |language=en-us}}

• 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,{{cite book |last=Connolly |first=Thomas J. |date=1978 |title= Foundations of Nuclear Engineering | location=New York |publisher=John Wiley & Sons |page=288 |isbn=9780471168584}} 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.{{cn|date=September 2024}}

According to a 2019 presidential memorandum,{{cite document|date=20 August 2019 |title= Launch of Spacecraft Containing Space Nuclear Systems | publisher=National Security Presidential Memorandum-20}} 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.{{cite conference |url=https://nets2020.ornl.gov/wp-content/uploads/2020/09/TRACK-3-Full-submission.pdf |title=Improvements to the Nuclear Launch Approval Process and Opportunities for New Missions | last=Mccallum |first=Peter | date= 6–9 April 2020 |conference= Nuclear and Emerging Technologies for Space 2020: Track 3: Mission Concepts and Policy for Nuclear Space Systems | publisher=Oak Ridge National Laboratory}}

• 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.{{cn|date=September 2024}}
• 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.{{cn|date=September 2024}}
• While details of the design thrust level have not been released, the design goal is said to be a specific impulse in excess of 800 seconds. (This is the length of time that the rocket can accelerate its own initial mass at a constant 1 gravity.{{cite book |last=Goodger |first=E. M. |date= 1970|title=Principles of Spaceflight Propulsion |url=https://shop.elsevier.com/books/principles-of-spaceflight-propulsion/goodger/978-0-08-013884-8 |publisher=Elsevier |isbn=9781483158600}}) 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.https://rocket.com/sites/default/files/documents/Capabilities/PDFs/RL10_data_sheet.pdf
• 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.{{cite book |last= Celnikier|first=L. M. |date=1993 |title=Basics of Space Flight |location=Gif-sur-Yvette, France |publisher=Editions Frontieres |isbn=9782863321317}} 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.

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.{{Cite web |title=Demonstration Rocket for Agile Cislunar Operations (DRACO) |url=https://www.darpa.mil/program/demonstration-rocket-for-agile-cislunar-operations |access-date=May 12, 2024 |website= darpa.mil}} The U.S. Department of Energy will provide HALEU metal to BWX Technologies for processing into low-enriched fuel.{{Cite web |date=July 26, 2023 |title=DARPA Kicks Off Design, Fabrication for DRACO Experimental NTR Vehicle |url=https://www.darpa.mil/news-events/2023-07-26 |access-date=May 15, 2024 |website=darpa.mil}} The amount of HALEU utilized for the vehicle has sparked some safety concerns among industry experts and the science community.{{Cite web |last=Krywko |first=Jacek |date=2024-06-10 |title=DARPA's planned nuclear rocket would use enough fuel to build a bomb |url=https://arstechnica.com/science/2024/06/planned-nuclear-fuel-has-higher-proliferation-risks-than-thought/ |access-date=2024-06-16 |website=Ars Technica |language=en-us}}

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.{{Cite web |date=July 28, 2023 |title=Leading DRACO to launch: An interview with DARPA's Tabitha Dodson |url=https://www.ans.org/news/article-4842/leading-draco-to-launch-an-in/|access-date=June 28, 2024 |website=ans.org/news/}}

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. Once in space, DRACO's reactor is not planned to be activated until it is established in a safe orbit. The minimum orbital altitude is determined by the estimated time it would take for the fission products to decay to the radioactivity level present at launch. In the case of the DRACO reactor, that is about 300 years, which requires an orbit above about 700 km if the orbital decay time is to exceed that value.

According to a timeline in NASA's FY 2025 Budget Estimate document presented to Congress, the project aimed to begin the implementation phase in September 2024.{{Cite web |title=FY 2025 Budget Estimates |url=https://www.nasa.gov/wp-content/uploads/2024/03/nasa-fy-2025-congressional-justification.pdf }} However that date has passed, and more recently it was reported

{{Cite web |title=Researchers eye space nuclear thermal propulsion for space maneuver warfare and to cut transit time to Mars |url=https://www.militaryaerospace.com/power/article/55250237/space-nuclear-propulsion-for-space-maneuver-warfare-and-exploration |access-date=January 1, 2025 |website=militaryaerospace.com|date=18 December 2024 }} 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.{{cite web |last=Machi |first=Vivienne |date=January 17, 2025 |title=Nuclear Reactor Test Requirements Put DRACO Launch Plans On Hold |url=https://aviationweek.com/space/launch-vehicles-propulsion/nuclear-reactor-test-requirements-put-draco-launch-plans-hold |website=Aviation Week & Space Technology |publisher=Gregory D. Hamilton |access-date=4 May 2025}}

{{Reflist}}

• [https://www.youtube.com/watch?v=SlTzfuOjhi0 Nuclear Propulsion in Space - NASA documentary on nuclear thermal rockets]

{{US government sources}}

Category:Nuclear spacecraft propulsion

Category:Research and development in the United States

Category:Corporate spin-offs

Category:DARPA projects

Category:Military technology