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The habitable modules of US sector of the ISS didn't have any propulsion systems to rendezvous the ISS by themselves. They were intended to be brought there by the Space Shuttle. Most of the Russian heavy modules of Mir and the ISS have arrived to the stations by themselves.

To bring 11 tons Kvant-1 module to Mir, Russia has used the disposable FSM space tug, based on TKS.

Has NASA announced any realistic space tug projects to be able to bring a habitable module, such as, say, Node 4 or a similar payload to the ISS?

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At NASA, the type of vehicles you are describing go by the name Orbital Transfer Vehicles (OTVs). While none are currently funded or planned for, there were studies underway as recently as the 80s.

Upper stages are sometime considered "space tugs" but I don't think that is a good description as they are only used once and are really just, well, an upper stage.

NASA delivered the ISS modules it was responsible for delivering directly via the STS. Also, there are no additional modules planned for the ISS as it is at "assembly complete."

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The Orbital Sciences entry in the CRS program is the Cygnus vehicle. It has two components a cargo module and an orbital module. While not officially offered as a tug, it functions as a tug to bring the cargo module to the station.

While the Cygnus payload module is not that big, it seems likely that Orbital could provide such functionality, should the serious need arise (I.e. Someone willing to pay for it).

Whereas the thrusters and systems for orbital maneuvering on a Dragon capsule are integrated in the capsule, as the trunk is mostly empty space.

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Googling around, I couldn't find any mention of a space tug program being officially announced by NASA.

From Wikipedia:

A reusable space tug was studied in 1969 as part of NASA's Space Transportation System, but went unfunded, as did every other component of that system except the Space Shuttle.

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