Late 1970’s depictions of “realistic” starships as understood at the time. These include an Orion vehicle (which, despite claims to the contrary, would make a terrible starship, since the specific impulse of a reasonably conceivable Orion is an order of magnitude or two too low for practical interstellar craft), two Bussard ramjets, and a “golden globe” minimum weight starship proposed by Robert L. Forward, whose operating principles I am currently a bit fuzzy on.

Bussard ramjets would use magnetic fields to collect interstellar hydrogen. The hydrogen would be compressed in a fusion reactor, preferably a steady-state one, and used to provide thrust to the starship. For a number of years this concept promised great things, but in recent decades it has been pretty much discounted. On one hand, the magnetic fields are not very likely going to work well at a reasonable mass, and they tend to not form open-mouthed funnels, but rather closed-mouthed “cups,” thus preventing the hydrogen from getting into the engine. Whoops. Second, thrust is unlikely to exceed drag much above maybe a percent or two of lightspeed, meaning a Bussard ramjet might serve as a decent “anchor” or drag brake, but not as an accelerator to relativistic velocities.