Re: Why does the Shuttle roll

From: Doug... (dvandorn1_at_NOSPAM.mn.rr.com)
Date: 02/10/05 

Date: Wed, 9 Feb 2005 21:57:31 -0600

In article <3Z-dnf7CBcE7XJffRVn-gg@comcast.com>, nospam@atcomcast.net
says...
> 180 degrees shortly after liftoff?

It doesn't roll exactly 180 degrees. It rolls until, as the stack tips
onto its trajectory, the orbiter is "heads-down", under the stack. For
example, if the orbiter is going to climb into an orbit 46 degrees to
the equator, it will roll until a line through the orbiter and external
tank centerlines is 46 degrees north of east. Then, as the stack
ascends and moves from vertical to lateral flight, the orbiter will
"hang" underneath the ET.

I can't tell you exactly what that centerline-line compass orientation
is on the pad, and I suspect it *may* be different depending on whether
they launch from 39A or 39B. But the stack rolls from that starting
orientation to the trajectory's target orientation, so the amount of
roll can vary from launch to launch. (This whole discussion disregards
the use of "dogleg" trajectories on some missions, where the beginning
of powered flight follows one trajectory and, at a certain point, shifts
to achieve a slightly different orbital inclination.)

Now, the *real* question may well be "Why do they roll until the orbiter
is heads-down?" I'm not completely positive about this -- it may be a
center-of-mass issue, the angle at which the shuttle's main engines have
to fire to thrust through the stack's center of mass may make the heads-
down orientation advantageous from a performance standpoint. However,
all previous American spacecraft flew into orbit in a heads-down
position, as well. I know that, in the cases of Mercury, Gemini and
Apollo, they did it so that the pilot(s) had a good view of the horizon
and landmarks to use in case they needed to manually guide the rocket,
or determine attitude change rates when weighing abort options. So,
there may be similar operational reasons for the shuttle to fly heads-
down -- I'm actually not sure. (It could also be one of the ways in
which they deal with the lift generated by the orbiter's wing during
ascent. A lot of the shuttle's powered flight regime is based on
managing the lift from the wing and the stresses it puts on the stack.)

On some of the more recent shuttle flights, the entire stack has rotated
back to a heads-up position well after SRB sep, near the end of powered
flight. I believe that had more to do with dynamics of ET sep than any
other reason, but would appreciate it if anyone has more solid data on
this. 

-- 
"The problem isn't that there are so   |   Doug Van Dorn
many fools; it's that lightning isn't  |   dvandorn1@NOSPAM.mn.rr.com
distributed right."  -Mark Twain

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