Re: Shuttle on 747 discussion that occurred while I was asleep in my part of the world.
Another thing to consider also is that the shuttle has wings ( + lifting body ) so that would all assist in the lifting the combined weight at flight speed. I figure NASA know what they're doing re: setup of flying craft, so the shuttle's mounted position on the back of the 747 would bring the shuttle's own lift in to effect to produce the most efficient setup.
But the thrust, an important part of the equation, is all supplied by the 747's engines.