This is where I saw VirginGalactic's idea of reusability heading, though I didn't think it'd be nearly copy/pasted. Neat concept, but I'm with y'all's lot so far as not seeing it go much farther. Can't imagine the pitch being anything other than "Remember WhiteKnight? Lets just make that bigger! With more engines!"
Don't know why the he'll my brain short circuit ed when I read this. You're absolutely right with regard to airfoil + air frame. I can't find my notes from the specific year of study in HS which is a damn shame, but my idea of fluid dynamics as the aircraft enters the upper atmosphere is hazy, but my thought process is skeptical of the influences of reduced air density being helpful for nav/stability. Even if I am wrong, though, I'm more than certain the team is leaps and bounds more educated on the matter and taking appropriate precautions.
The tricky bit is the tensile strength of your structural members goes up linearly with cross-sectional area while your surface area goes up as the square and your volume goes up as the cube. The same thing that keeps ants smaller than elephants rewards aircraft that remember they're not whales. It's not so much a "stability" problem per se it's the fact that a big plane has to deal with more forces and less unity of strength which requires more care.