Rich wrote:
Saw a footage of liftoff last July 4th. When the boosters separated
from external tank, did the shuttle's engine take over or had it been
running since launch? There is no smoke/trail coming from the
shuttle's engine, unlike the boosters. Why is there no smoke?

The solid fuel in the SRB's is comprised of:

69.83% ammonium perchlorate
16% aluminum
12% rubber
2% curing agent
0.17% iron oxide catalyst

This combination produces the "smokey" exhaust that you see during
launch. Basically, the SRBs are the largest firework ever built (and
if you live in the US, you know how smokey it tends to get outside
after Indepenance Day!)

On the other hand, the space shuttle main engines burn liquid hydrogen
and liquid oxygen as fuel. As you've probably guessed by now, the
byproduct of this reaction is basically water vapor, which produces
little or no visible exhaust.

Craig