The Apollo Service Module is a small rocket stage and is the lower portion of the Apollo CSM. Most any small payload or capsule can be mated to it, and the module can act like a second stage.
The Apollo Service module was used in various missions and programs such as the Apollo Moon Landings and the Apollo-Soyuz program. It was also included in the construction of the Saturn 1B and the Saturn V.
The Service Module is powered by batteries, running using batteries in-game.
It is the smallest object that can be used as a first stage.