[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.