In article ,
says...
So far, subsonic air launch hasn't been terribly successful due to the
need for a *very* large carrier aircraft for what I'd consider to be a
"reasonable" sized payload to orbit. Stratolaunch plans to build its
subsonic carrier out of two Boeing 747's.

http://en.wikipedia.org/wiki/Stratolaunch_Systems

I wasn't very clear about their aircraft plans. They're pulling major
*parts* from the 747's, but the aircraft itself (fuselages, wings, etc.)
will be new composite based structures.

Jeff
--
"the perennial claim that hypersonic airbreathing propulsion would
magically make space launch cheaper is nonsense -- LOX is much cheaper
than advanced airbreathing engines, and so are the tanks to put it in
and the extra thrust to carry it." - Henry Spencer