WHY DOES THE SHUTTLE ROLL JUST AFTER LIFTOFF?
The following answer and translation are provided by Ken Jenks (kjenks@gothamcity.jsc.nasa.gov).
The "Ascent Guidance and Flight Control Training Manual," ASC GandC 2102, says:
"During the vertical rise phase, the launch pad attitude is commanded until an I-loaded V(rel) sufficient to assure launch tower clearance is achieved. Then, the tilt maneuver (roll program) orients the vehicle to a heads down attitude required to generate a negative q-alpha, which in turn alleviates structural loading. Other advantages with this attitude are performance gain, decreased abort maneuver complexity, improved S-band look angles, and crew view of the horizon. The tilt maneuver is also required to start gaining
downrange velocity to achieve the main engine cutoff (MECO) target in second stage."
This really is a good answer, but it's couched in NASA jargon. I'll try to interpret.
1) We wait until the Shuttle clears the tower before rolling.
2) Then, we roll the Shuttle around so that the angle of attack between the wind caused by passage through the atmosphere (the "relative wind") and the chord of the wings (the imaginary line between the leading edge and the trailing edge) is a slightly negative angle ("a negative q-alpha"). This causes a little bit of "downward" force (toward the belly of the Orbiter, or the Z direction) and this force "alleviates structural loading." We have to be careful about those wings -- they're about the most "delicate" part of the vehicle.
3) The new attitude (after the roll) also allows us to carry more mass to orbit, or to achieve a higher orbit with the same mass, or to change the orbit to a higher or lower inclination than would be the case if we didn't roll ("performance gain").
4) The new attitude allows the crew to fly a less complicated flight path if they had to execute one of the more dangerous abort maneuvers, the Return To Launch Site ("decreased abort maneuver complexity").
5) The new attitude improves the ability for ground-based radio antennae to have a good line-of-sight signal with the S-band radio antennae on the Orbiter ("improved S-band look angles").
6) The new attitude allows the crew to see the horizon, which is a helpful (but not mandatory) part of piloting any flying machine.
7) The new attitude orients the Shuttle so that the body is
more nearly parallel with the ground, and the nose to the east
(usually). This allows the thrust from the engines to add velocity in the correct direction to eventually achieve orbit. Remember: velocity is a vector quantity made of both speed and direction. The Shuttle has to have a large horizontal component to its velocity and a very small vertical component to attain orbit.
This all begs the question, "Why isn't the launch pad oriented to give this nice attitude to begin with? Why does the Shuttle need to roll to achieve that attitude?" The answer is that the pads were leftovers from the Apollo days. The Shuttle straddles two flame trenches -- one for the Solid Rocket Motor exhaust, one for the Space Shuttle Main Engine exhaust. (You can see the effects of this on any daytime launch. The SRM exhaust is dirty gray garbage, and the SSME exhaust is fluffy white steam. Watch for the difference between the "top" [Orbiter side] and the "bottom" [External Tank side] of the stack.) The access tower and other support and service structure are all oriented basically the same way they were for the Saturn V's. (A side note: the Saturn V's also had a roll program. Don't ask me why -- I'm a Shuttle guy.)
I checked with a buddy in Ascent Dynamics. He added that the "roll maneuver" is really a maneuver in all three axes: roll, pitch and yaw. The roll component of that maneuver is performed for the reasons stated. The pitch component controls loading on the wings by keeping the angle of attack (q-alpha) within a tight tolerance. The yaw component is used to determine the orbital inclination. The total maneuver is really expressed as a "quaternion," a grad-level-math concept for combining all three rotation matrices in one four-element array.