Rich,
Yes, the Shuttles engines (SSME's) actually run all the way through
launch until the shuttle reaches orbit. The SSME's ignite a few seconds
before the solid rocket boosters (SRB's) ignite.
The SSME's burn liguid hydrogen and liguid oxygen for fuel. Water vapor
is the by-product of burning hydrogen and oxygen. The majority of the
billowing white clouds that you see around the shuttle at launch is simply
steam and condensed water vapor, as well as the exhaust from the SRB's. The
water vapor exhaust is mostly transparent, except when it condenses into
water droplets. Also at launch the exhaust from both the SRB's and the
SSME's are directed through channels that are sprayed with liquid water to
help control the sonic shock of the engines. This is done to help reduce
damage to the orbiter stack and the launch tower that can be caused by the
noise of the engines. This sonic blanket of water is immediately flashed
into steam by the hot rocket exhaust. Lower in the atmosphere, the
atmospheric air pressure is great enough so that water vapor can condense
into droplets and be seen along with the SRB exhaust. You won't see a
"smoke" trail after the SRB's seperate, because at that altitude the air
pressure is so low that the water vapor exhaust from the SSME's cannot
condense into droplets.
I once read (and I cannot say for sure if this is true or not) that the
solid rocket boosters thrust is simply used to cancel the incredible weight
of the entire shuttle stack, and the SSME's thrust is what is used to
accelerate the stack to orbital velocity. Hope this helps...

Randy L.
--
Remember: Any landing that you can
walk away from, is a landing that you
can be fined, sued, or prosecuted for.

"Rich" wrote in message
ps.com...
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?

Thanks,


Rich