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.
In a compartment at Jackass Flats, Nevada, pumped down to about one pound per square inch to fake the vacuum of space, a rocket engine hung nozzle-down and ran on hydrogen and fission. The XE-Prime started twenty-eight times and accumulated 115 minutes of running in 1969. It was the second and last complete nuclear rocket engine the United States ever assembled, and its engineers reported that no component or system problem stood between them and a flight engine.
The point was exhaust velocity. A good hydrogen–oxygen stage tops out near 450 seconds of specific impulse; the NERVA flight baseline was 825, from a reactor of just over 1,000 megawatts heating hydrogen and throwing it through a nozzle with no combustion involved at all. Doubling specific impulse is the only move that genuinely changes the delta-v arithmetic of a crewed Mars trip; everything else rearranges the same ledger. 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 how six of these engines would push two ships out of Earth orbit toward Mars in November 1981.
None of it was clean. Reactors at Jackass Flats fired into the open desert air, on the reasoning that fission products scoured from the core would disperse. On 12 January 1965 Los Alamos deliberately destroyed a Kiwi reactor to see what a worst-case runaway looked like, releasing about 1.6 million curies. Three more were wrecked in ordinary testing: two shed fuel elements to flow-induced vibration, and one core overheated after a procedural error ran it dry of the hydrogen that was also its coolant.
What killed it was money, and not from the branch usually blamed. Congress kept trying to save NERVA. Senator Clinton Anderson held hearings, twenty-one members wrote to Nixon in May 1971, and Congress appropriated $81 million for nuclear rockets — whereupon the Office of Management and Budget refused to release 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 was announced in January 1973.
Twenty reactor tests, two engine tests, about seventeen hours of accumulated running, $1.4 billion since 1955, and not one gram of it ever left the ground. Every Mars architecture since has been chemical, which is why the modern plans end where they do: Starship closes its case only by refuelling in orbit and then making propellant on the surface. The hardware that would have removed that constraint was built, run and taken apart while Viking was still being bolted together.