Sign up for our Newsletter! Mobile Newsletter banner close. Mobile Newsletter chat close. Mobile Newsletter chat dots. Mobile Newsletter chat avatar. Mobile Newsletter chat subscribe. Figure 5D7. Displacement signals alone would give the missile a tendency to over-correct its errors, and yaw or pitch about its desired course.
The displacement and rate of change signals minimize over-correction, and ensure stability. A "pickoff" is a device that produces a useful signal from the intelligence developed by a sensor.
The sensing devices for missile control generally indicate angular or linear displacement, measured with respect to some fixed quantity. The pickoff must be able to measure the amplitude and direction of the sensor displacement, and produce a signal that represents both quantities. Electrical pickoffs use phase relation or polarity difference to indicate direction. The ideal pickoff should have a linear output and minimum friction loss.
Altimeters An altimeter measures altitude. Since the atmospheric pressure at a given altitude is predictable, it is possible to calibrate a pressure-sensitive instrument in terms of altitude. A pressure altimeter is a form of aneroid barometer. Its mechanism includes a bellow s-like chamber from which most of the air has been removed. The pressure of the atmosphere tends to collapse the bellows. The surface of the bellows is connected to a scale pointer through a mechanical linkage, which magnifies the bellows surface movement.
As the pressure does not remain constant at any one level, this type of altimeter may have an error due to variable atmospheric conditions. It indicates altitude above the ground, rather than above sea level. It is actually a form of radar, since it measures the time required for a radio pulse to reach the ground, be reflected, and return. The transmitter antenna sends an FM signal straight down. The reflected energy is picked up by a separate antenna. A detector combines the reflected signal with a sample of the transmitted signal and generates a difference frequency.
This frequency is determined by the height above the ground. The detector output is amplified and fed to an indicator such 88 as a cathode-ray tube, a meter, or a discriminator that operates a control circuit. This system accurately indicates height above the ground, but it can not indicate height above sea level. Air-speed transducers A guided missile may use a transducer to measure ram air pressure, and provide an output voltage that indicates missile air speed.
One type of airspeed transducer uses bellows coupled to the shaft of a potentiometer. As the bellows is actuated by ram air pressure, it turns the potentiometer shaft and thus changes the circuit resistance.
Another type of airspeed transducer uses a synchro generator. The rotor of the synchro is so connected that expansion or contraction of the bellows causes the rotor to turn. This type of sensor will be described in the following section. Pickoffs 5E1. Function The pickoff device is important to the missile control system because it produces a signal from the intelligence developed by a sensor unit. Requirements The signal produced by the pickoff must be suitable for use in the control system it is serving.
The pickoff must have an output sense. That is, it must be able to determine the direction of displacement and then produce a signal that indicates the direction. In electrical systems the indication may be a phase or polarity difference. The ideal pickoff should have a considerable change in output for a small movement of the pickoff. It should also have minimum torque or friction loss since these losses would be reflected to the sensor element and affect its operation.
Small physical dimensions and light weight are additional requirements for pickoffs used in missiles. The null point no output should be sharply defined. Type Electrical pickoffs in common use fall into four categories. Each has some characteristic that makes it suitable for certain applications. A synchro pickoff device is normally composed of a pair of synchro units wired as a generator and synchro motor. When an exciter voltage is connected to the pair, movement of the generator rotor will produce a corresponding movement of the synchro motor rotor.
The generator rotor shaft can be turned by a mechanical connection to the sensor unit, or by a motor. Regardless of the method used to turn the generator shaft, the synchro motor shaft will move the same amount.
The synchro motor does not develop enough power to operate missile flight control surfaces. Therefore, it is used to operate other parts of the system which in turn operate the control surfaces. To get better action as the null point is approached, a differential synchro system is sometimes used. In this system, two inputs-one electrical and the other mechanical-are fed to a synchro differential generator unit, which then furnishes a voltage equal to the sum or difference between the two inputs.
Synchro pickoffs are sometimes called selsyns, autosyns, or microsyns. A potentiometer is a variable resistance that is normally used as a voltage divider. The resistance element is formed into a circular shape and a moving arm makes contact with the element. By connecting leads to the ends of the strip from a voltage supply and then connecting a load to the moving arm and one end of the strip, the source voltage may be divided by varying the position of the arm on the strip.
The resistance used for many electronic applications is composed of a thin film of carbon deposited on an insulating material. This type of resistance element is not suitable for servo applications because the resistance changes with temperature, humidity and wear. These disadvantages are overcome by using a wire-wound resistance strip, as shown in figure 5E1.
Figure 5E2 shows how a potentiometer divides voltage. The source voltage is applied 89 Figure 5E1. One side of the load is also connected to B. The section of the resistance strip between the moving arm and A acts as a resistance in series with the load, and there is less voltage at the load that is being furnished by the source. If the moving arm is all the way down to B, the load will get no voltage.
Thus, the position of the moving arm determines the amount of voltage. It is also possible to use the variation in resistance as a control medium. Since the resistance between A and the moving arm and between B and the moving arm vary as the arm is moved, a null can be indicated when the two resistances are equal. If the shaft of the potentiometer is mechanically coupled to the sensor, the output voltage will vary according to the moving arm displacement.
However, the voltage does not change smoothly with this type construction. The jumpy output is due to the voltage Figure 5E2. In order to remove this objection, the resistance element is sometimes wound in the form of a helix. Potentiometers are often used in bridge circuits.
The potentiometer forms two arms of the bridge, and fixed resistors form the other two. It is also possible to use two potentiometers to comprise all four arms of the bridge. A variable reluctance pickoff and gyro rotor are shown in figure 5E3. The gyro rotor is the cylindrical object in the drawing. The rotor is made of ferrous material, which has been slotted lengthwise and the slots filled with brass.
The pickoff element is an E-shaped metal mass, of which the center arm is a permanent Alnico special alloy of aluminum, nickle, and cobalt magnet.
The outer arms, and the long side of the E, are made of soft iron. Coils are wound on each leg and connected in series opposition. As the gyro rotates, it causes regular variations in the magnetic flux paths as the brass and ferrous strips pass the end pieces. This establishes regular variations in flux density and induces an a-c voltage in the coils.
However, because of the opposing connection of the coils, the induced voltages cancel. The gyro spin and precession axes are shown by dashed lines. As the rotor precesses, the air gaps at each end vary oppositely one increases and the other decreases in proportion to the precessing force, and cause different induced voltages in the pickoff coils. Since the two voltages are different they no longer cancel, and the output voltage is that produced by acceleration. The output voltage is rectified by the crystal diode and filtered by the condenser.
The resulting d-c voltage can be used as an acceleration signal. The polarity of the signal voltage depends upon which coil has the greatest induced voltage, and therefore indicates the direction of the acceleration. Figure 5E4 shows another type of reluctance pickoff. The stator has four coils divided into two pairs.
One pair is supplied with a constant-amplitude a-c voltage from a reference oscillator. Voltage from one pair of coils is induced in the second pair through an armature. The O 90 Figure 5E3. Figure 5E4. As the gimbal moves, it causes the armature position to change, and alters the coupling between the two sets of coils.
The amplitude of the induced voltage changes in proportion to the gimbal movement. This change produces a phase shift that depends on the direction of shift in missile position. As shown in figure 5E5, a capacitance pickoff is composed of two outer plates that are fixed in position. A movable plate is centered between the two fixed plates and connected to the sensor. The capacity between the center plate and the two outside plates is equal when there is no output from the sensor.
If, however, a signal from the sensor causes the center plate to move toward the bottom plate, the capacity between these two plates will increase and the capacity between the top plate and the center plate will decrease. This change in capacity can be used to vary the tuning of an oscillator. The change in oscillator frequency is then used for sense control. This type of pickoff is the most sensitive of all, since a very slight change in plate spacing will cause a large change in frequency.
Computing Devices 5F1. General Computers appear in missile systems in a variety of forms. The computer maybe a simple mixing circuit in a missile, or it may be a large console type unit suitable for use at ground installations only.
Function and requirements One important function of a computer is the coding and decoding of information relating to the missile trajectory.
It is necessary to code and decode control information in order to offset enemy countermeasures and to permit control of more than one missile at the same time. Another function of the computer is the mixing of signals from sensor and reference units to produce error signals. Figure 5C1 shows, in block form, how the computer is linked with other sections of the complete system. The signals from the sensor and reference units may be mixed in a preset ratio, or they may be mixed according to programmed instructions.
The error signals produced by mixing are amplified and passed to the control actuating system and the followup section. The output of the followup section is then fed back to the computer for reprocessing. The purpose of feedback is to reduce over control that would cause the missile to oscillate about the desired attitude.
The computer section may also compare two or more voltages to produce error signals. For this purpose, voltage or phase comparator circuits are added. The synchro units discussed in the previous section are used in computers to convert signal voltages into forms that are better suited for processing. Airborne computers are generally classified according to the phase of missile flight in which they are used.
The computers may be separate units or they may be combinations of prelaunch computer, launch computer, azimuth computer, elevation computer, program computer, and dive-angle computer.
Types of computers In a missile control system, computer elements are of general types-mixers, integrators, and rate components. As you will recall from the first part of this chapter, a mixer is basically a circuit or device that combines information from two or more sources.
Most systems use electronic mixers. However, mixers may also use mechanical, pneumatic, or hydraulic principles. Electronic mixers may use a vacuum tube as a mixing device. Probably the most common type of tube mixer is the one used in conventional superheterodyne radio sets. Here a tube mixes an incoming RF signal with the signal of a local oscillator to produce a difference frequency. It is also possible to use a network composed of inductors, capacitors, and resistors for mixing.
Regardless of the type of mixer, the signals to be combined are represented by the amplitude and phase of the input voltages. Voltages from such sources as pickoffs, rate components, integrators, followup generators, and guidance sources may be combined by the mixer section to form control signals.
Mechanical mixers consisting of shafts, levers, and gears can also be used to combine information. Figure 5F1 shows how lateral signals from two sources can be combined by using plain levers.
To see how this works, assume that shafts 1 and 2 operate independently, and that their positions represent information that must be combined. The three connections pivot freely. The position of shaft 3 represents a weighted average of the other two shafts, because the vertical lever arms from shaft 3 are not of the same length. The direction of shaft movement gives sense information. The output of shaft 3 may be used to operate an electrical pick-off, such as a potentiometer.
Another mechanical mixer uses gears to combine position or angular velocity information. The gear arrangement is similar to that of an automobile rear axle differential. If the input shafts contain position information, they will move slowly and maintain approximately the same average position.
The position of the output shaft constantly indicates the difference between the two shaft positions. If the information is represented by the speed of the shaft rotation, the angular velocity of the output shaft represents the difference between the two input shaft speeds.
It is possible to arrange the input shafts so that the output represents the sum of the inputs rather than the difference. Weighting factors can be controlled by changing the gear ratios in the differential.
Sometimes information is transferred through air or hydraulic tubes. The signals are created by varying the pressure inside the tube. Two signals can be combined by joining two tubes into one. An integrator performs a mathematical operation on an input signal. The Figure 5F1. Assume that the integrator output is four volts when the duration of the constant input signal is one minute.
Then if the same input signal had lasted for one-half minute, the output would have been two volts. But, an actual missile error signal is not constant, as we assumed in the above example. The amplitude and sense of the error change continuously. Even so, the integrator output is proportional to the product of the operating time and the average error during that time. Should the sense of the error change during the integration period, a signal of opposite sense would cause the final output of the integrator to decrease.
The integrator can be considered as a continuous computer, since it is always producing a voltage that is proportional to the product of the average input voltage and time. Therefore, the integration of an error with respect to time represents an accumulation of intervals of time and errors over a specified period.
Any integrator has a lag effect. To see why this is true, let us visualize a situation like that shown graphically in figure 5F2. The solid lines forming the rectangles represent on-off signals plotted with respect to time. The polarity is represented by the position of the rectangle above or below the time reference line. The heavy white lines represent the integrated output signal. Although the input signal goes from zero to maximum with zero time lag, there is no output at that instant.
The graph shows the lag effect; note that time is required before the output reaches an appreciable amplitude. Approximately the same length of time is required for the output amplitude to drop to zero after the input pulse ends. The figure also shows the additive effect of two successive negative pulses.
This action is made possible by the time lag, and is used to give more precise control action. The output signal from the integrator is used to support the proportional error signal, to make sure that enough correction will always be made by the control system.
Keep in mind that the degree of control exerted by a pure proportional unamplified signal is limited. Over-control, or undercontrol, cause excessive movement of the missile about the desired trajectory.
There are times when proportional control alone is not enough to overcome a strong, steady force that is causing the missile to deviate from the correct path. In a case of this kind, the proportional error signal will have a steady component that affects the integrator. The error signal sense remains constant, so that the integrator output increases with time as shown at the right of figure 5F2. This output increase reinforces the proportional signal until correction of the flight path takes place.
Integration may be performed by a motor, the speed of which is proportional to the amplitude of the input signal. The motor drives a pickoff, and the distance the pickoff moves is proportional to the integral of the input signal. The direction of motor rotation will depend on the polarity or phase of the input signal. The amplitude of the error signal varies irregularly; the sense of the signal may reverse, causing reversal of the motor rotation. Other types of integrators use ball-and-disk mechanical arrangement, resistance-capacity R-C circuits, resistance-inductance R-L circuits, and thermal devices.
The preceding section showed that a time lag is present in integrator circuits. It is this time lag that makes rate circuits necessary. Missile deviation cannot be corrected instantly, because the control system must first detect an error before it can begin to operate. The ideal control system would have zero time lag, thus permitting zero deviation during the missile flight.
All design efforts are toward a control system with this degree of perfection. Control surfaces are designed to correct missile flight deviations rapidly. The control surfaces are moved rapidly by actuators, which are operated by amplified error signals.
But it is possible to have a signal so large that the missile is driven beyond the desired attitude, and an error occurs in the opposite direction. This error drives the missile back in the first direction.
The end result is a series of swings back and forth across the desired trajectory. These unwanted swings are known as oscillation or hunting and the addition of a rate signal has the effect of damping retarding 94 Figure 5F2. The end effect of a rate signal is a reduction in the time between the initial control pulse and the output action.
To reduce this time, the rate signal is combined with the proportional signal to produce a resultant signal that leads the original proportional signal. However, there is output from the rate device only when the missile deviation is changing.
The amount of output is dependent on the rate of change. By combining the rate signal and the error signal, the system can be made to respond to a constant error. It is also possible to combine an attitude rate signal with a guidance signal. Perhaps the most common method of producing a rate signal is by using a separate sensor unit, such as a rate gyro. As explained previously, the rate gyro construction is such that it can precess only a few degrees and in only one plane.
Precession is restrained by a spring that tends to return the gyro to the midpoint. Any precession in this plane is caused by a force acting on the gyro gimbals. Such a force would be developed by any angular movement of the missileframe.
The magnitude of the force would be proportional to the rate of movement. The gyro displacement is detected by a pickoff, and the output of the pickoff is the rate signal. Amplifiers 5G1. Both are used in missile control systems to build up a weak signal from a sensor so that it can be used to operate other sections of the control system. These sections normally require considerably more power or voltage than is available from the sensor. Most amplifiers use electronic tubes, but in this section we will discuss some of the less conventional amplifiers.
Operating principles Some functions in missile control systems require a series of flat-topped pulses, called square waves, at a definite frequency. It is possible to convert other wave shapes to square waves with vacuum tube amplifiers and clippers. It is also possible to accomplish the same result with a mechanical device known as a chopper. The chopper is a mechanical switch designed to operate a fixed number of times per second.
A cutaway view of a mechanical chopper is shown in figure 5G1. This unit has the contacts arranged for single-pole double-throw switching, center OFF position.
The coil is excited by an a-c voltage that causes the vibrating arm to move at the frequency of the exciting voltage. Normally, the reed would vibrate at twice the a-c frequency-once each half-cycle.
This can be prevented by incorporating a permanent magnet in the structure. Then, on one half of the a-c cycle the a-c field about the coil is reinforced by the permanent magnet field, and on the other part of the a-c cycle the field about the coil is opposed by the permanent magnet field. As a further aid to operation on the desired frequency, the vibrating reed is tuned for that frequency by weighting.
The contact arrangement is shown near the bottom of the drawing. Leads are brought out separately from each of the two fixed contacts and the vibrating reed to pins on the base. These pins are arranged so that the chopper can be plugged into a conventional radio tube socket. In order to reduce operating noise, the entire mechanism is enclosed in a sponge rubber cushion before it is placed in the metal can.
By using the chopper in connection with a conventional transformer, amplification can be obtained at the pulse frequency. Vacuum tubes can be used as electronic choppers. Other amplifiers, known as saturable reactors, are used for a-c motor control.
This type of amplifier may sometimes be used in combination with vacuum tubes. Controller Units 5H1. Function A controller unit in a missile control system responds to an error signal from a sensor. In certain systems an amplifier which is furnishing power to a motor serves as a controller. In this section we will discuss controller units other than amplifiers.
Types There are several types of controllers and each type has some feature that makes it better suited for use in a particular missile system than other types.
A solenoid consists of a coil of wire wound around a nonmagnetic hollow tube; a movable soft-iron core is placed in the tube. When a magnetic field is created around the coil by current flow through the winding, the core will center itself in the coil. This makes the solenoid useful in remote control applications, since the core can be mechanically connected to valve mechanisms, switch arms, and other regulating devices. Two solenoids can be arranged to give double action in certain applications.
Figure 5H1 shows an application in which two solenoids are used to operate a hydraulic transfer valve. When 97 Figure 5H1. Transfer valve closed. Hydraulic transfer valve and actuator. If S2 receives more energy, the center part of the valve section is pulled to the right, and the actuator is caused to move. The converse is true if more current flows thru S1. The actuator can be used to physically position a control surface. Relays are used for remote control of heavy-current circuits. The relay coil may be designed to operate on very small signal values, such as the output of a sensor.
The relay contacts can be designed to carry heavy currents. Figure 5H2 shows a relay designed for controlling heavy load currents. When the coil is energized, the armature is pulled down against the core. This action pulls the moving contact against the stationary contact, and closes the high current circuit. The relay contacts will stay closed as long as the 98 Figure 5H2. Figure 5H3. The relay just described has a fixed core. However, some relays resemble a solenoid in that part of the core is a movable plunger.
The moving contacts are attached to the plunger, but are electrically insulated from it. Figure 5H3 shows a form of relay that can be used in a penumatic control system.
Two air pressure lines are connected to the air input ports. The relay operates when its arm is displaced by air pressure. A modified design of this type relay might be used in a hydraulic-electric system in which case the diaphragm would be moved by hydraulic fluid pressure.
An amplidyne can be used as a combined amplifier and controller, since a small amount of power applied to its input terminals controls many times that amount of power at the output. Figure 5H4 shows an amplidyne. The generator is driven continuously, at a constant speed, by the amplidyne drive motor. The generator has two control field windings that may be separately excited from an external source. When neither field winding is excited, there is no output from the generator, even though it is running.
It follows that no voltage is then applied to the armature of the load driving motor. The field winding of the motor is constantly excited by a d-c voltage. The control field windings of the generator are arranged so that the polarity of the excitation voltage from the sensor will determine the polarity of the generator output voltage.
The generator output is connected to the load driving motor armature through the latter's commutator. Since the field of the motor is constantly excited by a fixed polarity, the polarity of the voltage applied to the armature will determine the direction of armature rotation.
Actuator Units 5I1. Function In a missile control system, any error detected by a sensor must be converted into mechanical motion to operate the appropriate control device. The device that accomplishes this energy transformation is the actuator unit.
The actuator for a specific control system must be selected according to the characteristics of the system. The actuator must have a rapid response characteristic, with a minimum time lag between detection of the error and movement of the flight control surfaces. At the same time, the actuator must produce an output proportional to the error signal, and powerful enough to handle the load.
Principal types Actuating units use one or more of three energy transfer methods: hydraulic, pneumatic, or electrical. Each of these has certain advantages, as well as certain design problems. We will discuss each system briefly. Hydraulic actuators Pascal's Law states that whenever a pressure is applied to a confined liquid, that pressure is transferred undiminished in all directions throughout the liquid, regardless of the shape of the confining system.
This principle has been used for years in such familiar applications as hydraulic door stops, hydraulic lifts at automobile service stations, hydraulic brakes, and automatic transmissions. Generally, hydraulic transfer units are quite simple in design and construction. One advantage of a hydraulic system is that it eliminates complex gear, lever, and pulley arrangements. Also, the reaction time of a Figure 5I1.
A hydraulic system does, however, have a slight efficiency loss due to friction. Figure shows that equal input piston displacement will produce the same output piston movement. Actually, this statement is not wholly true because of slight friction and compressibility losses. But for all practical purposes, the motion can be considered to be the same for equal piston displacements.
A different condition is shown in figure , where the output piston force has been increased. Because we can't get something for nothing, increased force results in a decrease in piston travel. Keep in mind also, that the output piston could be used as the input piston, and vice versa. Hypothetically then, a small gyro displacement acting on a piston can be made to produce a large control surface motion if the gyro is connected to the large piston and the control surface connected to a much smaller piston.
In the practical application of hydraulics, something must be done to keep fluid in the proper lines. To accomplish this a circulating system is used. This requires pressure, which is furnished by a pump. The pump used in a hydraulic system must be driven by some power source, usually an electric motor, within the missile.
Pumps used in missile systems generally fall into two categories-gear and piston. A gear type pump is shown in figure It consists of two tightly meshed gears enclosed in a housing. The clearance between the gear teeth and the housing is very small. Figure 5I2. One gear is driven by an external motor. The other has an idler-type mount, and turns because its teeth are meshed with those of the driven gear.
In operation, the intake port top of figure is connected to a hydraulic fluid reservoir, and the output port is connected to the high-pressure delivery line. As the gears turn past the intake port, fluid is trapped between the gear teeth and the housing. This trapped fluid is carried around the housing to the output port.
Because the fluid then has no place else to go, it is forced into the high-pressure delivery line. A double-action piston pump is shown in figure This arrangement is called double action because fluid is pumped from the reservoir as the piston moves in either direction.
To see how this happens, assume that the piston is at the extreme right of the cylinder, and that it has started to move to the left. A slight vacuum will be created as the piston moves, and this will reduce the pressure on valve No.
At the same time, system pressure will force valve No. Atmospheric pressure, which is admitted to the reservoir through regular inlets, will then act on the fluid which opens valve No.
At the same time, fluid to the left of the piston is being compressed. This forces valve No. The pump pressure forces valve No. When the piston reaches the left side of the cylinder, it reverses direction. This creates pressure against valves Nos. At the same time, valve 3 closes and valve 2 opens.
The reservoir shown in figure is a storage compartment for hydraulic fluid. Fluid is removed from the reservoir by the pump, and forced through the hydraulic system under pump pressure. After the fluid has done its work, it is returned to the reservoir to be used again. The reservoir is actually an open tank because of the atmospheric pressure inlets. The valves in the illustrated piston pump are of the flap type, which operate with very small changes in pressure.
Another type of valve used in hydraulic systems is the pressure relief valve. As its name implies, it is used to prevent damage to the system by high pressures. Some combination systems use hydraulic pressure regulating switches instead of pressure relief valves. A typical hydraulic relief valve is shown in figure It consists of a metal housing with two ports. One port is connected to the hydraulic pressure line and the other to the reservoir return line.
The valve consists of a metal ball seated in a restricted section of Figure 5I4. Figure 5I5. The ball is held in place by a spring, the tension of which is adjusted to the desired lifting pressure. This pressure is chosen so that it will be within the safe operating limits of the system. Should the system pressure become greater than the spring pressure, the ball will be forced away from the opening, and fluid will flow into the port that leads to the reservoir return line.
Connect and share knowledge within a single location that is structured and easy to search. Fins : Most missiles use tail fins which give extreme maneuverability and high angles of attack.
Often they are fitted with wings to give extra lift and improved range. Wings : Generally, wings play the same role as for an aeroplane by providing lift, and are the oldest type of control surface.
Moreover, wings cannot work independently and in most cases are used as vortex generators to improve the efficiency of the fins.
The problem is that the wings must be long enough to produce necessary lift. Canards : They work more effectively with great maneuverability on low angle of attack, and work the same way as fins. However, located near to the nose of a missile, canards can cause a missile to stall at high angles of attack, which limits them to short range.
Almost all smart bombs and laser-guided missiles use canards. Furthermore, the split canard is a modern development found in missiles like Python-4 and AA which uses two canards, one of which is fixed, and the second is located just behind the first set and is movable.
The role of the fixed canard is to develop high power vortices for the movable canards, for better performance at high angle of attack. Some also use thrust vectoring of their rocket motor exhaust gases. The control system is typically driven by input from inertial measurement units accelerometers, rate sensors and gyros , seekers and sensors such as radar and laser, and GPS.
A unique variation on the control surfaces is the Starstreak missile :. Each dart consists of a rotating fore-body with two canard fins attached to a non-rotating rear assembly which has four fins. The rear assembly of each dart also houses the electronics that guide the missile, including a rearwards facing sensor. The sub-munitions steer by briefly decelerating the rotating fore-body with a clutch. The front wings then steer the missile in the appropriate direction.
You look at an airplane, you see big honking wings and tails. You look at a missile and you see stubby little things. What you don't realize is that they're the same thing. Airplanes don't need big honking wings to cruise, they need them to fly at low speed for takeoff and landing. A missile, however, will be launched from an aircraft already moving along rapidly, it never needs to be able to fly at low speed and thus the fact that the wings are utterly inadequate for low speed flight doesn't matter.
Most missiles have minimum launch velocities that are far above normal runway speeds. Yes, some missiles are capable of taking off from the ground, this is done by boosting them into the sky on a separate rocket booster that then falls away and allows them to fly normally. Missiles steer either by commands from internal computers, for Air-to-Surface missiles with GPS or Inertial navigation systems , by commands from a ground radar station over a datalink SAMs such as SA-2, SA-3, use this technique or, for missiles designed against moving Air-to-Surface or Air-to-Air targets , by using Proportional Navigation.
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