Nuclear Thermal Propulsion: Status, Challenges, and Timeline

Nuclear thermal propulsion (NTP) is not a new idea. The Rover/NERVA (Nuclear Engine for Rocket Vehicle Application) program, run by the Atomic Energy Commission and NASA from the mid-1950s until January 1973, ground-tested a series of nuclear rocket reactors. The XE Prime engine was designed for about 247 kilonewtons of thrust, and test reactors measured specific impulses of up to about 869 seconds, close to twice that of the best chemical engines. NERVA was deemed ready for integration into a spacecraft but never flew; President Nixon cancelled the program in 1973.

Interest in NTP has returned with renewed ambitions for crewed Mars missions. DARPA's DRACO (Demonstration Rocket for Agile Cislunar Operations) program, run with NASA, aimed to demonstrate an NTP engine in orbit: General Atomics, Blue Origin and Lockheed Martin received Phase 1 contracts in 2021, and Lockheed Martin and BWX Technologies were selected for Phases 2 and 3 in July 2023. DARPA cancelled DRACO in 2025, citing a declining return on investment. NASA has funded parallel NTP development under its Space Nuclear Propulsion (SNP) initiative. Understanding what NTP can and cannot do, and what stands between current programs and operational hardware, requires looking closely at both the physics and the development challenges.

How Nuclear Thermal Propulsion Works

An NTP engine heats a propellant (almost universally hydrogen, chosen for its low molecular weight and therefore high exhaust velocity) by passing it through or around a nuclear fission reactor core. The hot hydrogen expands through a conventional rocket nozzle, producing thrust. Unlike nuclear electric propulsion (which uses reactor power to run an electric thruster), NTP is a direct thermal process: the propellant is the working fluid that removes heat from the reactor.

The Isp advantage over chemical rockets comes from the higher temperature achievable in a nuclear reactor (up to ~2,700 K for tested solid-core designs) compared to chemical combustion. Since Isp scales with the square root of exhaust temperature divided by molecular mass, the combination of very high temperature and very low molecular weight (hydrogen's molar mass is 2 g/mol, vs. ~18 for water in chemical H₂/O₂ engines) produces Isp values of 800–1,000 seconds for solid-core NTP.

The Core Technical Challenge: Fuel

The most significant technical challenge for modern NTP development is not the reactor physics or the engine design (NERVA solved those problems). It is the fuel. NERVA reactors used highly enriched uranium (HEU, >90% U-235), which is subject to strict export controls and dual-use weapons concerns that make it essentially unavailable for commercial space programs.

Modern NTP development has therefore focused on high-assay low-enriched uranium (HALEU), enriched to between 5% and 20% U-235. HALEU provides sufficient reactivity for compact NTP designs without the proliferation concerns of HEU. However, commercial HALEU production infrastructure in the United States is still maturing.

The fuel form is equally challenging. NTP reactors require fuel that can withstand temperatures above 2,500 K while in contact with hot hydrogen, which is a corrosive environment. NERVA used graphite-composite fuel elements; modern designs are exploring carbide-matrix fuels (uranium carbide in a zirconium carbide matrix) that offer higher temperature capability and better hydrogen compatibility. Fabricating and testing these fuel forms at scale is a significant manufacturing challenge.

What Happened to DRACO

DRACO targeted a cislunar flight demonstration rather than a Mars mission, with a launch once planned for 2027. That launch was put on indefinite hold, and the FY2026 budget request released on May 30, 2025 confirmed the program's cancellation. A DARPA official said that as launch costs fell, the efficiency nuclear thermal propulsion offers no longer looked worth its large research and development cost.

Timeline Realism

Historical nuclear propulsion development programs have consistently taken longer and cost more than initial estimates. The combination of nuclear regulatory requirements (under the Nuclear Regulatory Commission and Department of Energy), HALEU supply chain development, fuel form qualification testing, and flight system development is a formidable sequence of challenges.

Key milestones to watch include:

• HALEU production contracts and delivery timelines from commercial enrichment providers
• Fuel element fabrication and irradiation test results
• Presidential space policy directives that specifically authorize nuclear propulsion mission development
• NASA budget requests for SNP development funding year-over-year

Alternative Nuclear Propulsion Concepts

Beyond solid-core NTP, other nuclear propulsion concepts are at earlier development stages:

• Bimodal NTP: A reactor that provides both thermal propulsion and electrical power, eliminating the need for separate solar arrays on deep-space missions. Studied extensively but adds reactor design complexity.
• Nuclear pulse propulsion (Orion concept): Propulsion by sequential nuclear detonations behind a pusher plate. Extremely high performance (Isp potentially thousands of seconds) but faces the Partial Test Ban Treaty prohibition on nuclear detonations in space and other practical obstacles.
• Nuclear electric propulsion (NEP): Reactor-powered electric thrusters. Lower thrust than NTP but potentially higher Isp (for electric thrusters) with continuous operation. Kilopower and other fission surface power programs are advancing the reactor technology needed for NEP.

Nuclear propulsion represents one of the most technically significant areas to follow for anyone interested in long-duration human spaceflight. SpaceNexus tracks related government contract awards and regulatory filings at Market Intelligence.