In missiles the control function is to ensure stability of the missile and implement the guidance signals received from external sources or generated onboard. The control, after processing the guidance signals, actuates the aerodynamic surfaces or thrust vector to generate turn of the missile speed and direction as required.
The guidance system is to detect whether the missile is flying above or below, to the left or right, of the required path. It obtains these deviations or errors and sends signals to the control system to reduce these errors to zero. The task of the control system therefore is to manoeuvre the missile quickly and efficiently making use of these signals.
In order to appreciate controls we shall briefly describe the motion of the missile as a free body. The missile has a total of six degrees of freedom of movement. Out of this, three degrees are translational or linear about the three axes viz., x, y and z; while the other degrees are rotational movement about three axes termed as pitch, yaw and roll.
Pitch is the turn of missile when it climbs up or down. Yaw is its turn to left or right. The roll is when the missile rotates about its longitudinal axis, which is also called roll axis. The longitudinal axis is the one running from nose to tail. If a missile is resting horizontally then, the pitch axis is the one which is normal to longitudinal axis and parallel to the horizontal axis and pitch axis. Missiles can roll when in motion due to various reasons.
There are missiles in which roll is controlled. Roll can be sensed onboard using a free gyro sensor and eliminated through actuation of controls. Some missiles have roll induced by design to use it for stability. The other axes which are controlled for motion are pitch and yaw axes.
Control Force Generation
The required force to generate the turn in the missile can be produced by many methods like aerodynamic control, thrust vector control and reaction control system or vernier rockets.
This method can be used when the missile is moving in the atmosphere above a certain minimum speed. In this flat aerodynamic surfaces called control surfaces provided on the body of the missile are deflected relatively with respect to the body to generate local differential force leading to a moment acting on the body and resulting in its rotation about a particular axis.
Depending on the location of the control surface along the longitudinal &is of the missile, they are termed as canard control (nose end location), moving wing control (middle location), or tail control. Each has specific advantages and application. The control force generated IS a function of the dynamic pressure control surface size and shape; angle of deflection, where the dynamic pressure is further a function of velocity of missile; and density of air at the altitude at which the missile is flying. The turning moment will be a function of this control force, location of centre of gravity of missile mass and location of the resultant centre of pressure of the aerodynamic forces acting on the body. The control designer has to reckon with changing velocity, altitude, mass and location of' the centre of gravity of the missile during its mission.
Thrust vector control.
Here the control force and moment are generated by deflecting the thrust force vector either by gimballing the engine (in case of liquid propellant engines), by rotation of nozzle (used in case of solid rocket motors through a flexible nozzle) or by inserting in or out vanes or blades at the exit of the jet. This system is useful when there is not adequate velocity of missile (immediately after launch) or when the vehicle is in low density atmosphere or space. However, it cannot be used when the engine burning is over.
Reaction control system or vernier rockets.
This is also based on chemical propulsion system wherein a number of sets of independent small thrust body fixed engines are provided in addition to the main engine to provide control along various axes. These are generally liquid propellant systems and they can be switched on and off as and when required.
Elements of Control System
The major elements of control system are: autopilot (inertial sensors, altimeter and sensor associated along with electronics) and actuation.
Autopilot: An autopilot regulates the execution of a guidance command. For its function it gets feedback from the inertial sensors like accelerometers and gyroscopes mounted along the pitch and yaw axes.
The accelerometers give the feedback of the translational acceleration developed by the missile about pitch and yaw axes while the gyros give the rate of turn of the missile about pitch and yaw axes.
These feedbacks are suitably weighted anti used along with the incoming guidance signal to determine actuation. The electronic circuit for this is designed to perform this calculation. In missiles where height has to be controlled, an altimeter is also provided as a sensor, and helps generate actuation of controls to maintain the height. Free gyro is also an autopilot sensor which helps in determining the roll of the missile from a reference and helps to control or eliminate the roll by generating actuation controls to develop counter moments in the missile.
Actuation: The means of deflecting the aerodynamic surfaces or thrust vector is called actuation and the force to do this can be from many sources like pneumatic (high pressure air power), hydraulic (high pressure oil power), electrical or turbo-mechanical. All the four methods are prevalent and used depending upon the size or availability of expertise. Most of the anti-tank missiles and smaller SAMs have electrical actuation, while somewhat bigger missiles have electrical or pneumatic system. While pneumatic actuation depends upon high pressure air pre-stored onboard, electric type will draw its power from an onboard battery. For the hydraulic type, the power source is electrical and is used in most of the large tactical and all ballistic missiles. For some of the very big missiles and space booster’s actuation is realized through turbine mechanical power.