The Orion Service Module is a stage in the Orion assembly, along with the Orion Crew Capsule. It is a lightweight, but high-capacity engine, and solar panels to provide power, and is commonly used to either travel long distances, or for station building. This stage has a fairing built in, which unfortunately this prevents it from being used on the bottom of a rocket.

This is a good substitute for the Medium Tug as the Orion Service appears to carry more fuel.