Jul 23, 2024 07:50 PM
https://arstechnica.com/science/2024/07/...-for-real/
EXCERPTS: Phoebus 2A, the most powerful space nuclear reactor ever made, was fired up at Nevada Test Site on June 26, 1968. The test lasted 750 seconds and confirmed it could carry first humans to Mars. But Phoebus 2A did not take anyone to Mars. It was too large, it cost too much, and it didn’t mesh with Nixon’s idea that we had no business going anywhere further than low-Earth orbit.
But it wasn’t NASA that first called for rockets with nuclear engines. It was the military that wanted to use them for intercontinental ballistic missiles. And now, the military wants them again.
Nuclear-powered ICBMs
The work on nuclear thermal rockets (NTRs) started with the Rover program initiated by the US Air Force in the mid-1950s. The concept was simple on paper. Take tanks of liquid hydrogen and use turbopumps to feed this hydrogen through a nuclear reactor core to heat it up to very high temperatures and expel it through the nozzle to generate thrust. Instead of causing the gas to heat and expand by burning it in a combustion chamber, the gas was heated by coming into contact with a nuclear reactor.
The key advantage was fuel efficiency. “Specific impulse,” a measurement that’s something like the gas mileage of a rocket, could be calculated from the square root of the exhaust gas temperature divided by the molecular weight of the propellant. This meant the most efficient propellant for rockets was hydrogen because it had the lowest molecular weight.
In chemical rockets, hydrogen had to be mixed with an oxidizer, which increased the total molecular weight of the propellant but was necessary for combustion to happen. Nuclear rockets didn’t need combustion and could work with pure hydrogen, which made them at least twice as efficient. The Air Force wanted to efficiently deliver nuclear warheads to targets around the world.
The problem was that running stationary reactors on Earth was one thing; making them fly was quite another.
[...] It took over 40 years before NASA brought up nuclear propulsion again, first in the short-lived Jupiter Icy Moon Orbiter project and then in the design reference architecture for human exploration of Mars. Powering the latter missions with a compact reactor could cut down Mars transit by more than half, to three to four months versus the six to nine months predicted for chemical rocket engines. Less time in space meant less exposure to radiation for the astronauts and fewer supplies for the trip.
So, in 2017, NASA started a small-scale NTR research program. The budget was just a hair above $18 million, but it was something. Two years later, Congress passed an appropriation bill that granted $125 million for developing NTRs. Things were progressing, but they were mostly paper studies, followed by more paper studies, followed by even more paper studies.
And then on June 17, 2020, DARPA entered the chat and said, “We want a nuclear rocket.” Not just another paper study—a demonstrator... (MORE - missing details)
EXCERPTS: Phoebus 2A, the most powerful space nuclear reactor ever made, was fired up at Nevada Test Site on June 26, 1968. The test lasted 750 seconds and confirmed it could carry first humans to Mars. But Phoebus 2A did not take anyone to Mars. It was too large, it cost too much, and it didn’t mesh with Nixon’s idea that we had no business going anywhere further than low-Earth orbit.
But it wasn’t NASA that first called for rockets with nuclear engines. It was the military that wanted to use them for intercontinental ballistic missiles. And now, the military wants them again.
Nuclear-powered ICBMs
The work on nuclear thermal rockets (NTRs) started with the Rover program initiated by the US Air Force in the mid-1950s. The concept was simple on paper. Take tanks of liquid hydrogen and use turbopumps to feed this hydrogen through a nuclear reactor core to heat it up to very high temperatures and expel it through the nozzle to generate thrust. Instead of causing the gas to heat and expand by burning it in a combustion chamber, the gas was heated by coming into contact with a nuclear reactor.
The key advantage was fuel efficiency. “Specific impulse,” a measurement that’s something like the gas mileage of a rocket, could be calculated from the square root of the exhaust gas temperature divided by the molecular weight of the propellant. This meant the most efficient propellant for rockets was hydrogen because it had the lowest molecular weight.
In chemical rockets, hydrogen had to be mixed with an oxidizer, which increased the total molecular weight of the propellant but was necessary for combustion to happen. Nuclear rockets didn’t need combustion and could work with pure hydrogen, which made them at least twice as efficient. The Air Force wanted to efficiently deliver nuclear warheads to targets around the world.
The problem was that running stationary reactors on Earth was one thing; making them fly was quite another.
[...] It took over 40 years before NASA brought up nuclear propulsion again, first in the short-lived Jupiter Icy Moon Orbiter project and then in the design reference architecture for human exploration of Mars. Powering the latter missions with a compact reactor could cut down Mars transit by more than half, to three to four months versus the six to nine months predicted for chemical rocket engines. Less time in space meant less exposure to radiation for the astronauts and fewer supplies for the trip.
So, in 2017, NASA started a small-scale NTR research program. The budget was just a hair above $18 million, but it was something. Two years later, Congress passed an appropriation bill that granted $125 million for developing NTRs. Things were progressing, but they were mostly paper studies, followed by more paper studies, followed by even more paper studies.
And then on June 17, 2020, DARPA entered the chat and said, “We want a nuclear rocket.” Not just another paper study—a demonstrator... (MORE - missing details)