A space tug is a type of spacecraft or upper stage used to transfer payloads from low Earth orbit (LEO) to higher energy orbits such as geostationary transfer, lunar transfer, or escape trajectory. The term was used to describe NASA's design for a flexible, modular, reusable vehicle, and has often been applied to expendable upper stages used to ferry payloads out of LEO, such as the Russian Fregat, or other proposed vehicles used to transfer payloads between orbits.
Space tug proposals
NASA STS space tug
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 1970's in the wake of the Apollo program.
Space Shuttle Era
Orbital Maneuvering Vehicle
NASA studied another space tug design, termed the Orbital Maneuvering Vehicle (OMV), along with its plans for Space Station Freedom. The OMV's role would have been a reusable space vehicle that would retrieve satellites, such as Hubble, and bring them to Freedom for repair or retrieval, or to service uncrewed orbital platforms.
The Russian RKK Energia corporation proposed a space tug named Parom in 2005 which could be used to ferry both the proposed Kliper crew vehicle or uncrewed cargo and fuel resupply modules to ISS. Keeping the tug in space would have allowed for a less massive Kliper, enabling launch on a smaller booster than the original Kliper design.
Spaceflight Inc. has proposed an orbital tug for SpaceX's Falcon 9 launch vehicle named Sherpa that would be capable of ferrying small and secondary payloads to orbits other than that of the primary payload. Sherpa is scheduled to fly in 2017 and will host a secondary payload by DLR.
The VASIMR electric plasma rocket could be used as a high-efficiency space tug, using only 9 tons of Argon propellant to make a round trip to the moon, delivering 34 tons of cargo from Low Earth Orbit to low lunar orbit. The Ad Astra Rocket Company is working to utilize the technology to make a space tug. 
Indian Space Research Organisation is building an upper stage called Payload Assist Module (PAM-G) capable of pushing payloads directly to MEO or GEO orbits from low Earth orbits. PAM-G is powered by hypergolic liquid motors with restart capability. As of 2013, ISRO has realized the structure, control systems, and motors of PAM-G and has conducted hot tests. PAM-G would form the fourth stage of GSLV Mk2C launch vehicle, sitting on top of GSLV's cryogenic third stage.
|Wikimedia Commons has media related to Space Tug.|
- "Space Tug". Astronautix. Retrieved July 2014.
- "Fregat". RussianSpaceWeb.com. Retrieved July 2014.
- "Space tug". Astronautix. Retrieved July 2014.
- "The Space Shuttle Decision: NASA's Search for a Reusable Space Vehicle". nasa.gov. Retrieved July 2014. "Because a rising tide lifts all boats, NASA's flight rates during the 1960s had been buoyed powerfully by the agency's generous budgets. The OMB had no intention of granting such largesse during the 1970s."
- "Long-forgotten Shuttle/Centaur boosted Cleveland's NASA center into manned space program and controversy". Cleveland.com. Retrieved July 2014.
- "NASA's New Launch Systems May Include the Return of the Space Tug". SpaceRef. August 7, 2005. Retrieved July 2014.
- "Linking Space Station & Mars". Wired. December 2013. Retrieved July 2014.
- "Parom orbital tug". RussianSpaceWeb. February 9, 2010. Retrieved July 2014.
- "Lighter Kliper could make towed trip to ISS". Flight Global. Nov 2005. Retrieved July 2014.
- "Spaceflight Unveils SHERPA In-Space Tug". Parabolic Arc. May 7, 2012. Retrieved July 2014.
- "DLR Signs Launch Services Agreement with Spaceflight Inc.". Parabolic Arc. July 8, 2014. Retrieved July 2014.
- "VASMIR". Ad Astra Rocket Company. Retrieved July 2014.
- NASA Report, Technical Study for the Use of the Saturn 5, INT-21 and Other Saturn 5 Derivatives to Determine an Optimum Fourth Stage (space tug). Volume 1: Technical Volume, Book 1, Web Address when accessed: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810065609_1981065609.pdf
- Wade, Mark. "Space Tug". Encyclopedia Astronautica. Retrieved June 15, 2011.
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