After the first men stepped on the Moon, many thought it wouldn’t take too long to get mankind on Mars and even further. More than 40 years later, even sending astronauts back to the Moon seems to be a problem… Interestingly, although I may sometimes sound pessimistic about our chances to travel to other planets or stars, we actually already have all we need go there. So why are we still stuck on our planet?
The main problem with today’s chemical rockets (all current spacecrafts use them for launch) is that they are costly, and not very efficient; in fact, most of the propellant is needed to lift… the propellant. Being realistic, chemical rockets will be able to take astronauts to Mars, but they won’t take anyone much further. Some research is constantly being done in order to improve these rockets, and alternative means are also studied. While most of these alternatives require technological advances not yet available, one of them could be developed right now, and it was designed more than 50 years ago: the Project Orion.
Behind Project Orion was a spacecraft intended to be directly propelled by a series of explosions of atomic bombs behind the craft. This mode of propulsion, known as nuclear pulse propulsion, was first proposed by Stanislaw Ulam in 1946. Project Orion was initiated by Ted Taylor at General Atomics and physicist Freeman Dyson in 1958.
The architecture of the vehicle is pretty “simple”: nuclear explosives are thrown behind a pusher-plate mounted on the bottom of the spacecraft, and exploded. The shock wave and radiation caused by the explosion would then impact against the pusher-plate; in order to smoothly transmit the acceleration to the rest of the spacecraft, the pusher-plate is mounted on two-stage shock absorbers. The pulse units (bombs) also had a specific design, so that the wave of plasma debris is cigar-shaped, for a better efficiency. I won’t go into further details about Orion’s architecture, as we’re only interested in the general aspects of the project.
Most spacecraft propulsion drives can achieve either a very high exhaust velocity, or a huge amount of thrust. Nuclear pulse rockets are the only proposed technology that could potentially deliver both. A considerable advantage is that for an Orion craft, weight is not a limitation anymore. Accelerations up to 100g could be tolerated, however manned spacecrafts would have to use the damping systems described previously to smooth the acceleration to a level tolerable for humans: about 2 to 4g.
Various sizes for the craft were considered by the designers of the project, ranging from 20 meters up to 400 meters in diameter! The bigger design, called “Super Orion” would have weighed 8 million tonnes, carrying 1,080 bombs weighing 3,000 tonnes each. Most of these 3,000 tonnes would actually be inert material used to transmit the force of the propulsion units detonation to the Orion’s pusher plate, and absorb neutrons to minimize fallout.
Because the Orion nuclear pulse rocket design has extremely high performance, both interplanetary and interstellar missions would be possible. Interplanetary missions for an Orion vehicle included a trip to Mars and back, as well as a trip to one of the moons of Saturn. In the case of interstellar missions, hydrogen bombs are used rather than fission bombs. The best pusher-plate design would allow a trip to Alpha Centauri in about 50 years, reaching a speed that is about 10% the speed of light (that’s 30,000 km/s). In other words, a trip to another star would be possible within a lifetime.
All the numbers given here come from calculations made in the 60’s, with a spaceship made from materials available then. With today’s technology, the maximum size and speed are likely to be even higher.
Of course, the use of nuclear weapons comes with a few potential problems. Ablation of the pusher plate as well as spalling were investigated: if the pusher-plate was sprayed with oil, there was no ablation at all, and with a surface layer of fiber glass spalling was significantly reduced and thought to be acceptable. The main unsolved problem was nuclear fallout, especially for a launch from the surface of the Earth. Using conventional explosives for the first detonation in a polar area would considerably reduce fallout. If a pure fusion explosive could be constructed, there wouldn’t be any fallout. Another option is a launch from Low Earth Orbit (LEO): the electromagnetic pulse generated would then be problematic as it could cause significant damage to computers and satellites. Once again, the problem might be solved by launching from very remote areas.
So, if such a spaceship could have been created then, why doesn’t it even exist now? You might think that danger to human life was the reason… It was not. The problems were of political origin. During its 5 years of existence, from 1958 to 1963, the project Orion was financed by DARPA, USAF and NASA. In 1963, the Partial Test Ban Treaty put an end to the project: nuclear weapon tests in the atmosphere, in outer space and under water were banned (you’ll notice that you can still blow nuclear bombs underground, but that won’t take any spaceship anywhere).
Finally, I was also “surprised” by the reaction of a DARPA member in 1959: « We use bombs to blow things into pieces, not to make them fly »… That’s the problem with our world. As long as there will be someone to think that bombs should be used for killing and destruction, we won’t go too far… Actually, we won’t go anywhere. Even if political difficulties were overcome, I’m not sure the public reaction would be enthusiastic: mention the word “nuclear”, and most of the public will react violently. If a project like Orion was to become reality in the near future, what would you say?