Tactical Missile Guidance

and Control

Notes

Contents

1 Missile Servos or Actuators 1

1.1 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Types of actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.3 Pneumatic Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3.1 Stored Cold Gas Servos . . . . . . . . . . . . . . . . . . . . . . . 3

1.3.2 Hot Gas Servos . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.4 Ram Air Servos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.5 Hydraulic Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.6 Electro-Mechanical Servos . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.7 Recent Developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

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Chapter 1

Missile Servos or Actuators

1.1 Requirements

The basic requirements of a servo or actuator used for moving the control surfaces

in a missile are as follows:-

(a) Minimum weight and volume.

(b) Good shelf life.

(c) Low cost.

(d) Reliability.

(e) Good dynamic performance.

1.2 Types of actuators

Based on the medium or method used, actuators can be of the following types:-

(a) Pneumatic

(i) Cold Gas

(ii) Hot Gas

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(b) Ram Air

(c) Hydraulic

(d) Electro-mechanical

1.3 Pneumatic Actuators

Pneumatic actuators are used when the force requirement is moderate/low and the

time of operation is short. Some of the advantages are of using pneumatic actuators in

missiles are as follows:-

(a) Low cost.

(b) Less number of components required.

(c) Freely available medium.

(d) Very good power to volume ratio as well as weight ratio.

(e) Self-cooling.

However some of the major limitations of pneumatic actuators are as follows:-

(a) Storage at high pressures.

(b) Leakage problems.

(c) Low efficiency.

(d) Low torque.

(e) Low stiffness due to compressibility of air.

(e) Slow response.

Based on the source of air, pneumatic actuators can be of two types namely:-

(a) Stored cold gas servos.

(b) Hot gas servos.

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1.3.1 Stored Cold Gas Servos

In this type of actuator, actuation power is obtained from stored gas, which is released

just before the firing of the missile by means of a solenoid operated start valve. A pressure

regulator is used to maintain a constant servo supply pressure. The movement of the

actuator arm is controlled by the signal given to the solenoid, which in turn operates

the valve, to move the actuator arm up or down. This in turn results in the movement

of the control surface to the desired degree. Helium or Nitrogen is used as the medium.

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1.3.2 Hot Gas Servos

In this type of pneumatic actuator, Cordite or Isopropyl nitrate is burnt to produce

hot gases, which in turn is used to operate the piston. A relief valve is employed when

cordite is used, since its burning rate is high. Used for short time flights only, but highly

reliable.

1.4 Ram Air Servos

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The salient features of ram air servos shown in figure above, are as follows:-

(a) They are used when the missile speed exceeds 1.5 Mach, below which the perfor-

mance is poor.

(b) The air is supplied to the actuator by a number of pitot intakes positioned around

the body and connected to a common manifold.

(c) The paddle valve is controlled by a switch motor, which controls the supply of ram

air to the vane actuator.

(d) Altitude of operation of the missile also plays a significant role, since pressure

decreases with height.

(e) The design is simple and gives advantage of size and weight.

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(e) Since ram system can be used only after certain speeds, a backup servo is required

during the boost phase.

1.5 Hydraulic Actuators

Hydraulic actuators are used when large actuation force is required. They are ide-

ally suited for long operation time, provides high stiffness and good speed of response.

Generally used for missiles having ’moving wing’ configuration. The hydraulic medium

can be ’recirculating’ or ’non-circulating’, where it is expelled into the atmosphere. The

pressure for the oil can be generated by means of a turbine operated by burning cordite

or iso-propyl nitrate. They are also used in thrust vector control (TVC), during boost

phase.

1.6 Electro-Mechanical Servos

In this system, an electric motor (DC) is used to generate the motion either through

gears or ball screw arrangement. Very low torque is available using gear, which can

control small sized control surfaces or seeker motion. Ball-screw arrangement provides

higher force and used in the ”flexible nozzles” of TVC. For flight times less than 20

seconds, the DC motor can withstand currents more than its ’rated’ value, to provide

higher torques.

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The construction and working principle of the electro-mechanical actuator shown in

figure above is as follows:-

(a) This electro-mechanical servo has two ’contra-rotating’ drums, in constant motion

from a direct drive from a motor.

(b) A spring is wrapped round each drum and is attached to the nut in one end and

one member of a clutch at the other.

(c) Actuation of one clutch provides positive torque to the final drive, the capstan

acting as the main power amplifier; actuation of the other clutch provides torque

in the opposite sense.

(d) The output end of each clutch spring is attached to the rotating nut of a screw

jack.

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(e) The translating screw jack drives the missile fin shaft through a rack and sector

gear.

(f) When the error is zero, the springs act as brakes, locking the output shaft.

(g) A mechanical interlock prevents simultaneous operation of both clutches.

1.7 Recent Developments

1.8 Conclusion

Hot or Cold gas servos provide very high performance standards. Hydraulic servos

are ideally suited for heavier medium range missiles. Missiles against stationary or slow

moving targets may use electrical servos for their autopilots.

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