Encyclopedia Astronautica is the most comprehensive online catalog of spacecraft, rockets, missiles, and rocket engines. It has extensive writeups on all the well known systems (frex Apollo, Gemini, Soyuz, etc) as well as thousands of obscure systems, including thousands that never made it off the drawing board. My favorites in this category include the Sea Dragon, the proposed X-15 orbital flight (!!) using boosters from the Navaho intercontinental cruise missile, and the X-20 Dynasoar.
It can be found at http://www.friends-partners.org/mwade/spaceflt.htm

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