I know that ordinary people can sustain up to 4 gs for short bursts (The tower of terror pulls about 4 gs in Dreamworld, I think the Space Probe pulls about 3). I know that fighter pilots can pull 9 g without blacking out, but they use special anti-g suits, and are trained to brace themselves, tense up and are generally stronger. I rememeber watching a documentary where they launched this guy in a rocket sled, and he ended up pulling about 19g. Nothing happened to him though. BUt yeah i think space shuttle would not be more than 4 most of the time.