Assuming proton-proton fusion, the speed limit is about 12% c, relative to the interstellar medium (http://www.projectrho.com/public_html/rocket/slowerlight.php

Assuming proton-proton fusion, the speed limit is about 12% c, relative to the interstellar medium (http://www.projectrho.com/public_html/rocket/slowerlight.php http://www.projectrho.com/public_html/rocket/slowerlight.php). That depends, however, on the assumption that the initial kinetic energy of the incoming hydrogen is completely wasted- that the ramjet does not have "regenerative braking", so to speak, that can recover some of the pre-existing kinetic energy. That's a pretty reasonable assumption from an engineering point of view, but physics theoretically allows us to do better.