Raymond Chuang wrote:

Actually, there is still a better solution: a horizontally-launched
spaceplane. Has anyone been following what Burt Rutan is doing with the
White Knight/SpaceShipOne combo? That could become the technology
demonstrator for a true spaceplane that could be launched from the top of a
modified Boeing 747-200 fitted with a de-rated Shuttle main engine (which
will allow the 747 to do a steep climb to 45,000 to 50,000 feet, the
altitude of the spaceplane launch).

Mach 4 and Mach 25 are very different goals. SS1 is an interesting
step, but don't overstate the size of that step.

Paul