Nuclear Thermal Propulsion: Status, Challenges, and Timeline

Nuclear thermal propulsion (NTP) is not a new idea. The NERVA (Nuclear Engine for Rocket Vehicle Application) program, conducted jointly by NASA and the Atomic Energy Commission from 1955 to 1973, demonstrated NTP engines producing up to 334 kilonewtons of thrust with a specific impulse of approximately 825 seconds, roughly twice that of the best chemical engines available today. Twenty successful reactor tests were conducted, with the technology declared ready for flight before the program was cancelled following the end of the Apollo era and shifting budget priorities.

Interest in NTP has returned with renewed ambitions for crewed Mars missions. In 2023, DARPA selected three companies (General Atomics, Lockheed Martin, and BWX Technologies) for the DRACO (Demonstration Rocket for Agile Cislunar Operations) program, intended to demonstrate an NTP engine in orbit. 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 DRACO program's performance requirements were set to be achievable with HALEU-fueled designs.

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.

DRACO Program Status

DARPA's DRACO program is structured in two phases. Phase 1 (completed) funded concept development and early design work from the three selected vendors. Phase 2 funds detailed design and component testing, leading toward a flight demonstration. The program target is a cislunar demonstration mission, not a Mars mission; the goal is to demonstrate NTP technology in an operationally relevant environment and retire key uncertainties.

A cislunar NTP demonstration has significant advantages over Earth-orbit testing: it avoids the political complications of operating a nuclear reactor in LEO (where orbital debris and atmospheric reentry are concerns), tests the system in a relevant mission environment, and produces a useful operational demonstration for the defense community's interest in rapid cislunar maneuver capability.

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.

Most independent assessments suggest that an operational NTP system for crewed Mars missions, as opposed to a technology demonstration, is unlikely before the mid-2030s at the earliest, and more probably the late 2030s or 2040s depending on budget continuity. Key milestones to watch include:

• DRACO Phase 2 design review completions and any schedule changes
• 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.