There's plenty of talk out there about a certain type of propulsion system, but I don't know the proper name for it.
- You gather up / buy / otherwise acquire some kind of non-rocket material, like water or metal or rock: this is the reaction mass for your spacecraft.
- You propel it out the back of your spacecraft at high speed using some kind of propulsion system.
- When you're running low on reaction mass, you gather up / acquire some more.
Ion engines would be a subset of this category.