A space tug is a type of spacecraft used to transfer spaceborne cargo from one orbit to another orbit with different energy characteristics. An example would be moving a spacecraft from a low Earth orbit (LEO) to a higher-energy orbit like a geostationary transfer orbit, a lunar transfer, or an escape trajectory.
The term is often used to refer to reusable, space-based vehicles. Some previously proposed or built space tugs include the NASA 1970s STS proposal or the proposed Russian Parom, and has sometimes been used to refer to expendable upper stages, such as Fregat, or Spaceflight Industries 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. 
NASA Space Transportation System
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.
Space Shuttle era
Expendable upper stages
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. In 1984, the Orbital Maneuvering Vehicle (OMV) preliminary design studies were initiated through a competitive award process with systems studies conducted by TRW, Martin Marietta Aerospace, and LTV Corporation.
Twenty-first century proposals
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. developed SHERPA, which builds upon the capabilities of the Spaceflight Secondary Payload System (SSPS) by incorporating propulsion and power generation subsystems, which creates a propulsive tug dedicated to maneuvering to an optimal orbit to place secondary and hosted payloads. The maiden flight is scheduled for November 2018 on a Falcon 9 rocket, and it consists of two separate unpropelled variants of the dispenser. This flight will deploy more than 70 small satellites from 18 countries.
The VASIMR electric plasma rocket could be used to power 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. As of 2014[update], Ad Astra Rocket Company had put forward a concept proposal to utilize the technology to make a space tug.[needs update]
Indian Space Research Organisation has built an upper stage called PAM-G (Payload Assist Module for GSLV) capable of pushing payloads directly to MEO or GEO orbits from low Earth orbits. PAM-G is powered by hypergolic liquid motor with restart capability, derived from PSLV's fourth stage. 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.
Lockheed Martin made a concept proposal to NASA in 2015 for a design called the Jupiter space tug, to be based on the designs of two earlier Lockheed Martin spacecraft—Mars Atmosphere and Volatile Evolution Mission and the Juno—as well as a robotic arm from MDA derived from technology used on Canadarm, the robotic arm technology previously used on the Space Shuttle. In addition to the Jupiter space tug itself, the Lockheed concept included the use of a new 4.4 m (14 ft)-diameter cargo transport module called Exoliner for carrying cargo to the ISS. Exoliner is based on the earlier (2000s) ESA-developed Automated Transfer Vehicle, and was to be jointly developed with Thales Alenia Space. In the event, NASA did not agree to fund the Jupiter development, and Lockheed Martin is not developing the tug with private capital.
|Wikimedia Commons has media related to Space Tug.|
- Service module
- Multistage rocket
- Apogee kick motor
- 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.
- "Space Tug". Astronautix. Retrieved July 25, 2014.
- "Fregat space tug". RussianSpaceWeb.com. Retrieved July 25, 2014.
- Leinster, Murray (1953). Space Tug. Shasta Publishers.
- "The Space Shuttle Decision: NASA's Search for a Reusable Space Vehicle". nasa.gov. Retrieved July 25, 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 25, 2014.
- "NASA's New Launch Systems May Include the Return of the Space Tug". SpaceRef. August 7, 2005. Retrieved July 25, 2014.
- "Linking Space Station & Mars". Wired. December 2013. Retrieved July 25, 2014.
- Department of Defense appropriations for 1986, pt. 1, p. 242.
- "Parom orbital tug". RussianSpaceWeb. February 9, 2010. Retrieved July 26, 2014.
- "Lighter Kliper could make towed trip to ISS". Flight Global. Nov 2005. Retrieved July 26, 2014.
- Sorensen, Jodi (August 6, 2018). "Spaceflight prepares historic launch of more than 70 spacecraft aboard SpaceX Falcon9". Spaceflight Industries. Retrieved August 6, 2018.
- "VASMIR". Ad Astra Rocket Company. Retrieved July 24, 2014.
- Somanath, S. "ISRO's Current Launch Capabilities & Commercial Opportunities" (PDF). Archived from the original (PDF) on September 3, 2013. Retrieved July 8, 2014.
- N. Gopal Raj (2014-10-01). "Upgrading Indian rockets for future Mars missions". Thehindu.com. Retrieved 2015-03-17.
- "Annual Report" (PDF). Archived from the original (PDF) on February 25, 2014. Retrieved July 8, 2014.
- "Outcome Budget 2010-2011" (PDF). Archived from the original (PDF) on October 13, 2011. Retrieved July 8, 2014.
- "Outcome Budget of the Department of Space Government of India 2009-2010" (PDF). Archived from the original (PDF) on November 23, 2010. Retrieved July 8, 2014.
- "GSLV". Space.skyrocket.de. Retrieved 2015-03-17.
- "'Jupiter' Space Tug Could Deliver Cargo To The Moon". 12 March 2015. Retrieved 17 March 2015.
- Jeff Foust (13 March 2015). "Lockheed Martin Pitches Reusable Tug for Space Station Resupply". Space News.
- Avery, Greg (2015-03-12). "Lockheed Martin proposes building ISS cargo ship for NASA". Denver Business Journal. Retrieved 13 March 2015.
- Wade, Mark. "Space Tug". Encyclopedia Astronautica. Retrieved June 15, 2011.