06-22-2017, 10:18 PM
I think that 'internal' is probably correct - it refers to propulsion whether the fission or fusion reaction is contained, as opposed to occurring externally and pushing against a 'pusher plate' - as in a Project Orion external pulse system. There is a fairly arbitrary line between internal and external - it depends on the shape of the 'pusher plate' or containment field, I suppose.