Project Rho is a good one-stop-shop for all things interstellar travel. Here's their page on the Orion: http://www.projectrho.com/public_html/rocket/enginelist

Project Rho is a good one-stop-shop for all things interstellar travel. Here's their page on the Orion: http://www.projectrho.com/public_html/rocket/enginelist3.php#boomboom http://www.projectrho.com/public_html/rocket/enginelist3.php... The numbers there indicate that the Orion design doesn't have the sort of exhaust velocity you'd want to get up to 0.95c That site's also useful reading for all the OTHER downsides: for instance, traveling at 0.95c through space means that everything in your way is hitting the ship at 0.95c. Even stray hydrogen atoms are no picnic when they're arriving at that energy...