[roywiggins]

It's left intentionally vague, but at some point they do refuel the drive. It's meant to be an improbably efficient fusion drive.

The alien forces demonstrably have a reactionless capability though (see: Eros) that utterly outmatches the Epstein drive.

[jerrysievert]

you can read the novella Drive online: http://www.syfy.com/theexpanse/drive/prequel.php where it appears to be some sort of issue that occurs to his failing fusion drive, so definitely a more efficient fusion drive.