As we explained in two earlier questions about max q, or maximum dynamic pressure, the Shuttle reaches a point about one minute after launch when the pressure force of the atmosphere rushing past the rapidly accelerating rocket reaches a peak. The roll maneuver is performed shortly before max q is reached because this "heads-down" orientation helps alleviate the stresses that the dynamic pressure loads cause on the vehicle's structure.
The second factor we need to consider is that for each mission, the Shuttle must launch at a certain azimuth angle in order to be inserted into the correct orbital plane. Since the launch pad (and therefore the Shuttle) sits in a fixed position, the Shuttle must perform a roll maneuver during ascent in order to orient itself to achieve the desired launch azimuth angle. If it were possible to rotate the launch pad prior to launch, the pad could simply be rotated to accomodate the launch azimuth angle, and the Shuttle could launch into a heads-down orientation while gradually pitching over during ascent.
Finally, the Shuttle orbits such that its cargo bay faces towards the Earth. The heads-down position assists in
communications with the ground and allows instruments within the cargo bay to be pointed back towards Earth, which
is required for many of the experiments carried within the bay. There is probably also some psychological benefit
to the crew since they are given spectacular views of home rather than staring into the cold darkness of the great
void of space.
- answer by Aaron Brown, 8 June 2003
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