Suppose you have a reaction wheel or a control moment gyroscope on your spacecraft for attitude control. This would be a self contained unit, with 3-axis control. Either device utilizes the conservation of angular momentum, trading angular momentum between the unit and the spaceship frame as needed for attitude control. http://en.wikipedia.org/wiki/Reaction_wheel http://en.wikipedia.org/wiki/Control_moment_gyroscope Does the placement of the reaction control unit matter? Should it be as close as possible to the spacecraft's center of mass for the best effect?