Like any moving body, a guided missile executes two basic types of motion-rotation and translation. In pure rotation, all parts of the missile pivot around the center of gravity. In movements of translation, or linear motions, the center of gravity moves along a line.

Missiles, like other aircraft, have six degrees or dimensions of freedom (movement). To describe these motions, we use a reference system of lines or axes. These axes intersect at the missile's center of gravity.

A missile can make three kinds of rotary movement-pitch, roll, and yaw (fig. 9-7). Pitch, or turning up and down, is rotation about the lateral axis. The lateral axis is the reference line in the horizontal plane and is perpendicular to the line of flight. The missile rolls, or twists, about the longitudinal axis. This axis is the reference line running through the nose and tail. The missile yaws, or turns left and right, about the vertical axis.

A missile can make three kinds of translation or linear movements. For example, a sudden gust of wind or an air pocket could throw the missile a considerable distance from its desired trajectory. This displacement could happen without causing any significant rotary or angular movements. Any linear movement can be resolved into three components-lateral, vertical, and along the direction of thrust.

The missile must sense and correct for each degree of movement to maintain an accurate and stable flight path. This stable flight path is often called "attitude" and refers to the position of the missile relative to a known (horizontal or vertical) plane. The control system contains various components used to maintain a proper flight attitude.

Figure 9-7.-Rotary movements of a missile: pitch, roll, and yaw.

Gyroscopes

Gyroscopes are very important control system components. Any spinning object (a top, a wheel, etc.) is fundamentally a gyro. It can be defined as a mechanical device containing a spinning mass. It is mounted in such a manner as to have either one or two degrees (directions) of freedom.

A gyro that has two degrees of freedom is referred to as a free gyro. Its rotor is mounted in gimbals so it can assume any position. View A of figure 9-8 shows a free gyro that can turn on two axes, Y and Z. View B shows a different type of gyro. It is called a rate gyro and has only one degree of freedom or axis.

Gyros have two useful characteristics in guided missiles. First, the gyro rotor tends to remain fixed in space if no force is applied to it. The idea of maintaining a fixed plane in space is easy to understand. When any object is spinning rapidly, it tends to keep its axis pointed in the same direction. A toy top is a good

Figure 9-8.-Gyroscopes: A. Free gyro has two degrees of freedom, B. Rate gyro has one degree of freedom.

example. As long as it is spinning fast, it stays balanced on its point. A gyro, like a spinning top, resists the tendency of gravity to change its spin axis. The resistance of a gyro against any force which tends to displace the rotor from its plane of rotation is called rigidity in space.

The second characteristic of a gyro is that its spin axis tends to move at right angles to the direction of an applied force. This action can be seen in figure 9-9. When a downward force is applied at point A, the force is transferred through pivot B. This force causes a downward movement at point C.

That movement, at a right angle to the direction of the applied force, is called precession. The force associated with this movement (also at right angles to the direction of the applied force) is called the force of precession.