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
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
is a circular movement of the fins of the rocket as shown in the
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
perpendicular to the flight direction. If all of the fins
are deflected in the same manner, a net
is generated about the roll axis.
Some early rockets used this method to generate a stable
Another way to generate a roll motion is to
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
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