Aerospaceplane - 1961

[Generic Username]

Martin "Astroplane" - liquid air cycle plus nuclear thermal:

[Generic Username]

The Douglas Model 2235 "MARS," which was a sorta-SSTO that required meeting up with *two* tankers at 100,000 feet and Mach 6+.

[Vahe231991]

This thread is 13 years old and thus I may feel tempted not to post anything on it, but I should emphasize that Douglas and Martin weren't the only contenders for the USAF's Aerospaceplane requirement. Other designs for this competition were the Boeing Model 879, a Convair design, the Lockheed CL-510, and a Republic design study for a Mach 25 spaceplane. 

Links:
https://www.secretprojects.co.uk/threads/aerospaceplane-1958-1963.1079/
https://www.up-ship.com/eAPR/ev2n5.htm