The shuttle actually throttles back twice during ascent. The first time
is at max Q. This is not a 3 G situation for the vehicle. After the
solids are dropped, the shuttle again throttles back in the last 30
seconds or so before main engine cut-off, so as to not exceed 3 G's of
forward acceleration when the fuel is almost gone. Whether this is due
to a structural or other reason, I'm not sure. But structural makes the
most sense for several reasons, weight being the primary one.

Bud


Jim wrote:
Thanks Danny, I had a good belly laugh from that one.
Jim
wrote in message
ps.com...

Danny Deger wrote:
Why does the shuttle throttle to 3 Gs on ascent?

Danny Deger

Because structurally the shuttle vehicle is only stressed for 3 g's. If
they didn't throttle back in the late stages of the ascent when the
fuel is almost gone and the vehicle is light, it would exceed this
level of acceleration. Since the engines are liquid fueled, they can
hold full thrust until the fuel is gone.

Bud