Nuclear Thermal Propulsion: Why NASA and DARPA Are Reviving 1960s Rocket Science for Mars

Here is a number worth sitting with: a Mars mission using conventional chemical propulsion takes roughly seven to nine months one-way. A mission using nuclear thermal propulsion could do the same trip in three to four months. Halving transit time does not just save astronaut patience — it dramatically reduces radiation exposure (deep space radiation is one of the most serious medical risks of a Mars mission), reduces the mass of consumables required, and opens a trajectory window that chemical rockets simply cannot reach. The physics has been known since the 1960s. The engineering has been demonstrated. The question is why we stopped, and why we are now starting again.

## NERVA: The 1960s Engine That Actually Worked

The Nuclear Engine for Rocket Vehicle Application program — NERVA — ran from 1955 to 1972 under NASA and the Atomic Energy Commission. It was not theoretical. It produced real hardware that was tested in the Nevada desert. The final NERVA designs achieved a specific impulse of around 800 to 900 seconds — roughly twice the performance of the best chemical rocket engines, which top out around 450 seconds with liquid hydrogen and liquid oxygen.

Specific impulse (Isp) measures how efficiently a rocket uses propellant, expressed in seconds. Higher is better. The reason nuclear thermal beats chemical is not about energy release — nuclear fission produces vastly more energy per kilogram than chemical combustion — but about exhaust velocity. A nuclear thermal rocket heats a propellant (typically liquid hydrogen) by passing it through a nuclear fission reactor, heating the gas to around 2,500 Kelvin, and then expelling it through a nozzle. The light hydrogen molecules leave the nozzle at very high velocity. High exhaust velocity equals high Isp. High Isp means you need less propellant to achieve the same change in velocity, which means your spacecraft can be lighter, or carry more payload, or go faster.

NERVA completed twenty-three engine tests. The NRX/XE engine ran at full power for over 109 minutes in a single test in 1969. The technology worked. What killed it was not engineering failure but politics: the Nixon administration cancelled the Apollo program's future Moon and Mars missions, and without a mission requiring nuclear propulsion, there was no case for the program's cost. The hardware was mothballed.

## What NERVA Left Behind and What Changed

The core challenge that NERVA identified — and that has not been fully solved — is the nuclear fuel element. The reactor core must withstand extreme thermal cycling: it heats hydrogen propellant to thousands of degrees while itself remaining structurally intact under intense neutron flux. The original NERVA used uranium oxide fuel elements in a graphite moderator matrix, which worked but suffered from cracking under thermal stress and from hydrogen reacting with the graphite at high temperatures.

Two main approaches to improved fuel elements have emerged:

**Cermet (ceramic-metal composite)**: Uranium dioxide particles embedded in a tungsten or molybdenum metal matrix. Cermet elements are denser and more resistant to hydrogen corrosion than graphite. They can potentially operate at higher temperatures, which means higher exhaust velocity and better Isp. The tradeoff is manufacturing complexity — tungsten is notoriously difficult to work with.

**Carbide-based fuels**: Uranium carbide or uranium nitride ceramics that can withstand higher temperatures than oxide fuels. Several research programs are developing these. Higher operating temperature directly translates into higher Isp — potentially pushing toward 1,000 seconds — which would further reduce Mars transit times.

## DRACO: The 2026 Programme

DARPA's Demonstration Rocket for Agile Cislunar Operations — DRACO — is the current flagship nuclear thermal propulsion development programme. NASA joined as a partner in 2023. The goal is to demonstrate a nuclear thermal rocket engine in Earth orbit by 2027, with development and manufacturing contracts awarded to Lockheed Martin (for the spacecraft) and BWX Technologies (for the reactor).

BWX Technologies, formerly Babcock and Wilcox, is one of the few companies with the regulatory approvals, manufacturing infrastructure, and nuclear materials expertise to actually fabricate a nuclear reactor for this purpose. Their involvement is as much a practical constraint as a design choice — the number of organisations in the United States capable of building flight-qualified nuclear rocket fuel is genuinely small.

The 2027 timeline has been under pressure. Nuclear propulsion development involves not just aerospace engineering but nuclear licensing, environmental review, and a launch campaign for a fission reactor into Earth orbit that requires regulatory approvals from multiple agencies. Delays are not surprising; what would be surprising is if the timeline held exactly.

## The Radiation Shielding Problem for Crew Missions

For robotic or cargo missions, a nuclear thermal rocket can be designed without heavy radiation shielding around the crew — because there is no crew. For human missions to Mars, the reactor must be physically separated from the crew compartment by distance, shadow shielding, or both.

The standard approach is a "shadow shield" — a mass of hydrogen-rich material and heavy metals placed between the reactor and the crew habitat to attenuate neutron and gamma radiation to acceptable dose levels. This shield adds mass, which partially offsets the propellant savings from the high Isp. The engineering design optimisation — how much shielding against how much propellant saving — is a specific impulse versus mass problem that detailed mission design has to solve case by case.

Interestingly, the propellant itself — liquid hydrogen — provides radiation shielding, because hydrogen nuclei are effective neutron moderators. Missions can be designed to use the propellant tanks as part of the shielding geometry, reducing the dedicated shield mass.

## The Specific Impulse Arithmetic of Mars Transit

Let us run the numbers simply. To send a spacecraft from low Earth orbit to Mars requires a delta-v (change in velocity) of roughly 4 km/s using a Hohmann transfer orbit during a typical launch window. The Tsiolkovsky rocket equation relates this to the required propellant mass fraction:

mass ratio = e^(Δv / (Isp × g₀))

For a chemical rocket (Isp = 450 s): mass ratio = e^(4000 / 4414) ≈ 2.47. For every kilogram of payload and structure, you need 1.47 kg of propellant.

For a nuclear thermal rocket (Isp = 900 s): mass ratio = e^(4000 / 8829) ≈ 1.57. For every kilogram of payload and structure, you need only 0.57 kg of propellant.

The reduction in propellant requirement is dramatic. Less propellant means a smaller, lighter launch stack. Alternatively, the same launch stack carries more payload. For a human Mars mission, "more payload" translates directly into more food, water, radiation shielding, scientific equipment, and return propellant — everything that makes the mission survivable and productive.

## Why Now?

The combination of renewed Mars mission interest, advances in additive manufacturing for complex refractory metal components, better nuclear fuel fabrication capabilities, and sustained government investment — including Space Force interest in nuclear propulsion for rapid cislunar manoeuvring — has created the conditions for a genuine revival. The 1960s ended NERVA for political reasons, not engineering ones. The engineering has improved. The political will, cautiously, appears to have returned. Whether a nuclear thermal rocket reaches orbit by the late 2020s or slips into the 2030s, the direction of travel is clear. The sixty-year detour through chemical-only deep space propulsion may be coming to an end.

Nuclear Thermal Propulsion: Why NASA and DARPA Are Reviving 1960s Rocket Science for Mars

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