OTV

Orbital Transfer Vehicle, an unbuilt space tug from the 80s. The OTV was designed to ferry payloads to geosynchronous orbit and beyond. Once done, it would fall back to Earth, skim the planet's atmosphere to slow down, and return to a space station to refuel for the next mission. NASA investigated a variety of configurations for the OTV as well as different uses for the concept. This included a study for a mission to Mars that would use two OTVs to form an interplanetary spacecraft. Ultimately the OTV was never built.