NERVA runs in Nevada, then the money stops
A nuclear engine ran 28 times at Jackass Flats at twice the exhaust velocity of any chemical stage. The budget office zeroed it, and the programme ended in January 1973.
The XE-Prime engine started twenty-eight times and ran 115 minutes at Jackass Flats, Nevada, in 1969, hanging nozzle-down in a chamber pumped to about one pound per square inch to simulate vacuum. It was the second and last complete nuclear rocket engine the United States assembled; its engineers reported no component or system problem standing between them and a flight engine.
The point was exhaust velocity: a hydrogen–oxygen stage tops out near 450 seconds of specific impulse, the NERVA flight baseline 825, from a reactor of just over 1,000 megawatts heating hydrogen and throwing it through a nozzle, no combustion at all. Doubling specific impulse changes the delta-v arithmetic of a crewed Mars trip. The design engine stood 22 feet tall and made 75,000 pounds of thrust. On 4 August 1969, eleven days after Apollo 11 came home, Wernher von Braun told Vice President Agnew's Space Task Group that six of them would push two ships out of Earth orbit toward Mars in November 1981.
Testing was dirty. Reactors at Jackass Flats fired into the open desert air, on the reasoning that fission products would disperse. On 12 January 1965 Los Alamos deliberately destroyed a Kiwi reactor to model a worst-case runaway, releasing about 1.6 million curies. Three more were wrecked in ordinary testing: two shed fuel elements to flow-induced vibration, one overheated after radiation made its hydrogen-tank gauges read wrong and it ran dry of the hydrogen that was also its coolant.
Congress tried to save it: Senator Clinton Anderson held hearings, twenty-one members wrote to Nixon in May 1971, and $81 million was appropriated for nuclear rockets. The Office of Management and Budget released no more than $30 million. Acting administrator George Low, carrying White House orders, told the Senate that "NERVA needs the Shuttle, but the Shuttle does not need NERVA." The last tests ran in June and July 1972, termination in January 1973.
Twenty reactor tests, two engine tests, about seventeen hours of running and $1.4 billion since 1955 produced no flight hardware. NASA's Mars plans kept choosing nuclear stages; none flew: Starship closes its case only by refuelling in orbit and making propellant on the surface.