In flight, any rocket will rotate about its center of gravity, a point which is the average location of the mass of the rocket. We can define a three-dimensional coordinate system through the center of gravity with each axis of this coordinate system perpendicular to the other two axes. We can then define the orientation of the rocket by the amount of rotation of the parts of the rocket along these principal axes. The roll axis lies along the rocket centerline. A roll motion is a circular movement of the fins of the rocket as shown in the animation.
The rolling motion can be caused by the deflection of the fins of the rocket. The fins can be hinged like the rear section of an airplane wing. Deflecting the rear section changes the aerodynamic shape of the fin and generates a lift force perpendicular to the flight direction. If all of the fins are deflected in the same manner, a net torque is generated about the roll axis. Some early rockets used this method to generate a stable rocket configuration.
Generating Roll Motion
Another way to generate a roll motion is to gimbal the rocket engines to again generate a net torque about the roll axis. The Space Shuttle normally executes such a maneuver early in its launch phase to properly position the crew for orbital insertion.