[wcoenen]

Your references are about rocket designs, which have limits to how fast they can expel reaction mass before they blow themselves up. Orion isn't really a rocket as it does the violent reaction outside of the spaceship. With Orion, the limit is how fast the pusher plate can be cooled or how much momentum can be handled by the shock absorbers.

Granted, even with fusion bombs there is still a limit to how much energy and momentum you can extract from a certain mass, so the same concerns of diminishing returns apply. But according to Dyson's numbers, a 133 year trip to Alpha Centauri might be possible[1].

Even discounting interstellar travel, a trip time of only a few weeks to Mars with current technology sure sounds interesting to me.

[1] http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsi...

[jasonwatkinspdx]

Orion most definitely is still an action/reaction rocket. It works via radiation and particle pressure against the pusher plate. While its thrust is high, its overall efficiency is low because the majority of the fission energy result is not contained enough to influence forward motion of the spacecraft. High thrust is pointless for interstellar travel. High isp is essential. This is why fission or antimatter powered ion drives beat Orion. Orion is great for interplanetary however.

If you read the through the material on that site you will see they are familiar with Orion. While the public oriented site glosses over details, the companion site will provide you with papers with rigorous detail.

There is no escaping it: for practical interstellar travel, we must develop massless drive or some form of spacetime manipulation that changes the mass requirements.