This article is from the Space FAQ, by Jon Leech firstname.lastname@example.org and Mark Bradford email@example.com with numerous contributions by others.
The following answer and translation are provided by Ken Jenks
The "Ascent Guidance and Flight Control Training Manual," ASC G&C 2102,
"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
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
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
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