Even without Epstein drives there were the less efficient torch engines which are in the books and came before the Epstein. Humans had access to those engines to colonize Mars and the start of the belt I believe (Finishing up Leviathan Wakes currently). Even today NASA has researched and experimented with NERVA which is a nuclear rocket engine design. They also recently from my knowledge spun that research back up so maybe we will see this new engine in space in the near future.