Space tug

A space tug is a type of spacecraft used to transfer payloads from low Earth orbit (LEO) to higher-energy orbits such as geostationary transfer, lunar transfer, or escape trajectory. The term is often used to refer to reusable, space-based vehicles, such as NASA's 1970s STS proposal or the proposed Russian Parom, and sometimes used to refer to expendable upper stages, such as Fregat, or Spaceflight Inc.'s Sherpa.

The space tug was first envisioned in the post-World War II era as a support vehicle for a permanent, Earth-orbiting space station. It was used by science fiction writer Murray Leinster as the title of a novel published in 1953 as the sequel to Space Platform, another novel about such a space station.

A reusable space tug was studied by NASA in the late 60s and early 70s as part of a reusable Space Transportation System (STS). This consisted of a basic propulsion module, to which a crew module or other payload could be attached. Optional landing legs could be added to land payloads on the surface of the Moon. This, along with all other elements of STS except the Space Shuttle, was never funded after cutbacks to NASA's budget during the 1970s in the wake of the Apollo program.

The Shuttle program filled the role of high-energy orbital transfer by the development of a solid-fueled single-stage Payload Assist Module and two-stage Inertial Upper Stage.

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