Dwayne Day has published another scoop, this one on a partially declassified effort called ISINGLASS from the late 1960s that would have replaced the SR-71 with a spaceplane seven times as fast. The idea came out of the CIA, the Air Force never cottoned to it, and it died, though only after McDonnell Douglas had put significant work into it.
Would it have flown as designed? Seems unlikely - they were asking for a gosh-awful amount of energy from something that could not be much larger than the X-15 (and thus couldn't have much onboard fuel to play with.) But those guys in the 60s, man, they thought BIG.