I referred the Roll program in space shuttles. But, I can't understand the mechanism. So, They rotate during the launch in order for proper alignment. I know that only tilting, gimbaling or using additional vernier thrusters would align the space shuttles in proper direction. But how does this rotation of a space shuttle help in alignment? What's the mechanism behind this..?
The barbecue roll and the roll program are not related.
The former is for passive thermal control when the spacecraft is exposed to the Sun and the latter is a maneuver early in the launch sequence to orient the launch vehicle to the proper heading.
For a number of reasons, the space shuttle launch vehicle needs to be oriented "heads down" (the shuttle is positioned "underneath" the external tank) during most of the ascent. It's explained pretty well here:
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