Why does the shuttle throttle on ascent?

"Henry Spencer" wrote in message
...
In article ,
MichaelJP wrote:
chosen to *avoid* having the wings generate lift. The wings are not
strong enough to provide any useful amount of lift during ascent, and
the dominant concern is to avoid tearing them off by overloading them.

Is it also the case that the zero-lift trajectory you describe is very
similar to the optimum flight path for orbital insertion? Or is a lot more
fuel used because of it?

Yes and no. :-)

If memory serves, the ascent trajectory is pretty close to what a wingless
rocket with similar mass and propulsion characteristics would fly. Flying
even slightly sideways at supersonic speeds is very hard on lightweight
structures; even jet fighters, built for violent maneuvering, can handle
only a very little bit of this. Rockets normally take considerable pains
to fly pretty much(*) straight "into the wind" until clear of most of the
atmosphere. The shuttle trajectory isn't *exactly* what a wingless rocket
would use, because the trajectory that minimizes loads on the orbiter
wings isn't exactly the trajectory that would minimize structural loads in
general -- the wings have priority. But the penalty for this is small.

(* There are minor exceptions, in which lift can be of some use after the
air thins out, plus some complications for air-launched rockets like
Pegasus. But this is still basically correct. )

*However*, there is a more general caveat: even the wingless-rocket
trajectory actually isn't optimal. For one thing, an optimal ascent would
tip over toward the horizontal much more quickly. On Earth, the early
ascent has to be close to vertical, to get the rocket up out of the
atmosphere before the speed builds up too much. For another thing, even
disregarding that, the straight-into-the-wind trajectory isn't exactly
optimal, although it's not too far off.

The only rocket ascent that was ever able to use a truly optimized
trajectory was the Apollo LM ascent stage's departure from the Moon. On
Earth, you inevitably pay some price for the necessities of getting clear
of the atmosphere quickly and pointing straight into the wind while you
do. It's not huge, but it's significant. This is one of the two big
technical advantages of air launch -- starting from even 30,000ft means
you're dealing with considerably thinner air, reducing the price tag
noticeably. (The other is also related to thinner air: rocket engines
are more efficient with less back pressure. The forward speed of the
aircraft is a relatively minor gain by comparison, unless it's a pretty
unusual aircraft.)
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Thanks for the extra detail!