It's the Holy Grail of spacecraft propulsion - an engine that has both very high thrust and very high efficiency. Chemical rockets have high thrust but very poor efficiency, ion engines have very high efficiency but low thrust, and nuclear thermal rocket engines are somewhere in between with middling efficiency and thrust. Is an engine that has both very high thrust and very high efficiency possible?

Actually they already designed one - in the 1950's. Project Orion was a project to develop a nuclear pulse detonation engine, a rocket that is actually propelled by atomic bombs. They actually tested the idea with conventional explosives but never got around to a full sized test vehicle for reasons that will become apparent.

The short version of explaining this is, you chuck an atom bomb behind your rocket, hide behind a huge ass shield, and ride the shock wave. Simple. Crazy, but simple. The long version is that you project what are in essence very small shaped atomic explosives behind a pusher plate that has shock absorbers behind it. The shaped nuclear charges explode, they are contained for a microsecond by a depleted uranium vessel and the X-rays are projected into a medium that absorbs the X-rays and becomes really really really hot. This medium vaporizes a disk of tungsten which it then blasts outward in a cigar-shaped plume aimed at the pusher plate. The pusher plate is pushed forward, the shock absorbers receive that thrust and then transmit it to the rest of the ship, and you repeat that as often as needed to get where you are going. There are other small details, but that's basically it.

In order to withstand these blasts, an Orion-type spacecraft would have to be very large. Huge in fact, like a battleship pretty much. Which means that getting an Orion-type spacecraft into orbit without using its engines and sprinkling fallout like pixie dust over the landscape is very very difficult. So they planned to lift it up just far enough to keep the explosions from reflecting off the ground and scrambling the crew, and then they would use the Orion drive to get to orbit and beyond. Sprinkling radioactive fallout, as I mentioned, all over the landscape.

They showed the plans for Orion to President Kennedy who reportedly was... appalled. And canceled the project. It would also have been in violation of a couple nuclear test ban treaties, so it never got any farther. Also the aforementioned radioactive pixie dust was kind of a drawback for some. If they could have made Orion operational however, it would have been a spacecraft of unparalleled performance. A trip to Mars would not take a year as is currently envisioned, but a few weeks. Trips to Jupiter in less than a year were envisioned. Really, no currently planned propulsion system has anything like that performance.

But you know, radioactive pixie dust and the Russkies being nervous about hundreds of atom bombs orbiting over their heads and all that. ;)

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