Source: http://motherboard.vice.com/read/the-747...

Designed but never built by the now-defunct Rockwell International, the proposed Star-Raker would load its cargo at a regular airport, fly to a spaceport near the equator, fuel up on liquid oxygen and hydrogen, and take off horizontally using its ten supersonic ramjet engines. A 1979 technical paper lays out its potential flight plan: At a cruising altitude of 45,000 feet, the craft would then dive to 37,000 feet to break the sound barrier. At speeds of up to Mach 6, the Star-Raker would jet to an altitude of 29km before the rockets kicked in, propelling it into orbit.