What it was: A large spacecraft concept intended for long-term missions developed in response to two goals in the NASA Authorization Act of 2010: developing a crewed exploration vehicle capable of operating beyond Low Earth Orbit, and incorporating new technologies into NASA programs.

Details: Long-term readers may have noticed that, even for a blog devoted to space projects that didn’t happen, few of the ships discussed here are very big. Contrast this with the regular images of spacecraft in science fiction, even the ones with pretensions to realism like 2001: A Space Odyssey and Mission to Mars. Gravity is a harsh mistress and real spacecraft like Soyuz and Apollo are miracles of miniaturization because the cost of launching anything larger is prohibitive.

The notable exceptions are vehicles that were proposedcame in January 2011. The NAUTILUS-X (supposedly a tortured acronym for Non-Atmospheric Universal Transport Intended for Lengthy United States eXploration, but almost certainly instead a shout out to the Nautiluses of Rickover and Verne as well as the X-planes of the past) was floated as a concept by NASA’s Technology Applications Assessment Team as part of a general program to build infrastructure and spacecraft that either developed or integrated new technology.

Prior to the building of the craft two main projects would be undertaken: the development of cryogenic propellant depots for placement in LEO and at the Earth-Moon L1 point, and the building and testing of an inflatable centrifuge ring for attachment to the ISS. Once these were in place, the NAUTILUS-X would refuel at one and incorporate a