Between 1955 and 1972, the United States quietly built and successfully test-fired the most efficient rocket engine ever operated. It ran on liquid hydrogen and a graphite-uranium reactor core, produced roughly twice the specific impulse of any chemical rocket, and was ready to fly. Nixon cancelled it anyway.

The program. NERVA — Nuclear Engine for Rocket Vehicle Application — grew out of Project Rover at Los Alamos in 1955. Westinghouse built the reactors, Aerojet built the engines, and every test happened at Jackass Flats on the Nevada Test Site, where operators simply vented the exhaust straight into the desert sky. A reactor called KIWI-A first went critical in July 1959. By 1968, Phoebus 2A ran at 4,082 megawatts thermal — still the most powerful nuclear reactor ever built for propulsion. Peak calculated thrust: 930,000 lbf, roughly 60% of a Saturn V F-1.

What actually worked. The flight-configured XE Prime engine fired 24 times in 1969, hanging from a test stand in a downward-firing orientation to simulate flight. It hit 55,000 lbf of thrust at 1,140 MW and delivered a specific impulse of about ~800–850 seconds. For comparison, the RS-25 Space Shuttle Main Engine — the best hydrogen chemical engine ever flown — tops out at 452 s. That factor-of-two efficiency is not marginal. It cuts Mars transit time from 8–9 months to roughly 4, and it lets you throw twice the payload from the same tank of propellant.

NRX-A6 ran at full power for 60 continuous minutes in December 1967 without failure. Pewee, a compact version, tested in 1968 at 503 MW and demonstrated fuel elements that survived 2,550 K. Twenty-three reactors were built and fired over the program's life.

Why it died. Cost was the surface answer — around $1.4 billion in 1972 dollars over 17 years — but the real reason was mission. NERVA existed to send crews to Mars in the 1980s under the post-Apollo Space Task Group plan Nixon received in 1969. Nixon rejected the Mars plan in favor of the Shuttle. Without Mars, there was nothing that needed a nuclear thermal upper stage. Congress zeroed the budget in January 1973. The RIFT flight-test vehicle, which would have launched an NRX engine on a Saturn IB, was five months from hardware integration.

Why it works now. Three things have changed:

• Fuel. The 1960s reactors used weapons-grade highly enriched uranium (93% U-235). Modern designs use HALEU (High-Assay Low-Enriched Uranium, under 20%), which sidesteps most nonproliferation objections and can be exported to allies.
• Materials. The old graphite composite fuel eroded in hot hydrogen — the "mid-band corrosion" problem. Ceramic-matrix composites and tungsten-cermet fuels (developed for Gen-IV fission) now hold up to 3,000 K without shedding uranium into the exhaust.
• Manufacturing. Additive manufacturing lets BWXT print reactor cores with cooling channels that Aerojet couldn't have machined in 1969.

DARPA and NASA are now funding DRACO — Demonstration Rocket for Agile Cislunar Operations — with BWXT building the reactor and Lockheed Martin the vehicle. First on-orbit firing is targeted for 2027. It will be smaller than XE Prime, produce roughly 12,500 lbf of thrust, and demonstrate the same physics NERVA proved on the ground 58 years earlier.

The uncomfortable part: DRACO's Isp target is ~900 s. NERVA hit 850 in 1969. We are, essentially, redoing the demonstration.