DARPA
Phoebus 2A, the most powerful space nuclear reactor ever built, was fired up on June 26, 1968, at the Nevada Test Site. The test lasted 750 seconds and confirmed that it could carry the first humans to Mars. But Phoebus 2A never flew anyone to Mars. It was too big, it cost too much, and it didn’t fit with Nixon’s idea that we shouldn’t go anywhere beyond low Earth orbit.
But it wasn't NASA that first asked for nuclear rockets. It was the military that wanted them for intercontinental ballistic missiles. And now the military wants them again.
Nuclear powered ICBMs
Work on nuclear thermal rockets (NTRs) began with the Rover program initiated by the U.S. Air Force in the mid-1950s. The concept was simple on paper. Take tanks of liquid hydrogen and use turbopumps to feed the hydrogen through a nuclear reactor core to heat it to very high temperatures and force it out a nozzle to generate thrust. Instead of heating and expanding the gas by burning it in a combustion chamber, the gas was heated by coming into contact with a nuclear reactor.
Tokino, vectorized by CommiM on en.wikipedia
The main advantage was fuel efficiency. “Specific impulse,” a measurement similar to a rocket’s fuel consumption, could be calculated from the square root of the exhaust gas temperature divided by the molecular weight of the fuel. This meant that the most efficient fuel for rockets was hydrogen, because it had the lowest molecular weight.
Chemical rockets required hydrogen to be mixed with an oxidizer, which increased the total molecular weight of the fuel but was necessary for combustion to occur. Nuclear rockets did not require combustion and could operate on pure hydrogen, making them at least twice as efficient. The Air Force wanted to deliver nuclear warheads efficiently to targets around the world.
The problem was that keeping stationary reactors running on Earth was one thing, but making them fly was another story.
Space Reactor Challenge
Fuel rods made of uranium 235 oxide dispersed in a metal or ceramic matrix form the core of a standard fission reactor. Fission occurs when a slow-moving neutron is absorbed by a uranium 235 nucleus, splitting it into two lighter nuclei, releasing enormous amounts of energy and excess, very fast neutrons. These excess neutrons normally do not cause further fission, because they are moving too fast to be absorbed by other uranium nuclei.
To start a chain reaction that keeps the reactor going, you have to slow it down with a moderator, such as water, which “moderates” its speed. This reaction is kept at moderate levels using control rods made of neutron absorbing materials, usually boron or cadmium, which limit the number of neutrons that can cause nuclear fission. Reactors are raised or lowered by moving the control rods in and out of the core.
Translating all of this into a flying reactor is a challenge. The first problem is the fuel. The hotter you make the exhaust, the more you increase the specific impulse, so NTRs needed the core to operate at temperatures of up to 3,000 K, nearly 1,800 K higher than ground-based reactors. Producing fuel rods that could survive such temperatures proved extremely difficult.
Then there was the hydrogen itself, which is extremely corrosive at these temperatures, especially when combined with the few materials that are stable at 3000 K. Finally, the standard control rods had to go, because on the ground they would have been pulled into the core by gravity, and that wouldn't work in flight.
Los Alamos Scientific Laboratory proposed some promising NTR designs in 1955 and 1956 that addressed all of these problems, but the program didn’t really get going until it was turned over to NASA and the Atomic Energy Commission (AEC) in 1958. There, the idea was renamed NERVA, or Nuclear Engine for Rocket Vehicle Applications. NASA and AEC, blessed with a virtually unlimited budget, got to work building space reactors—lots of them.
