Vapour pile

A type of engine often found in heavier spacecraft.

Uses electrical elements (or in some cases a small fusion reactor) to rapidly heat and vapourise water. Water is heavy, but can be found on most planets even including gas giants. Vapourising the water faster gives more thrust for the same mass of water. These engines (and the ships that use them) are also known as 'steamers' among vetran engineers. Most commonly seen on tugs.