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Aviation History
1956
1956 - 1716.PDF
880 FLIGHT MISSILES THAT THINK . . . Fig. 1. Direct command wire guidance: A, "beep box"; B, wire; C, reel; D, guidance unit. Fig. 2. Direct radar guidance: A, "beep box"; B, radar command set; C, receiver and guidance package. ,.'&" ^v^''^/:;'|;- Fig. 6. Fully-active homing: A, radar transmitter; B, pencil beam on target; C, receiver; D, strike. Fig. 7. Passive homing: A, plot of temperature, sound intensity or other parameter in jet from target; B, radiation from target; C, sensitive receiver in missile; D, guidance system. Fig. 3. Radar command guidance: A, lock-on tracking set; B, computer; C, command set; D, reflected radiation from target. Fig. 4. Beam rider: A, twin radars, one producing a pencil beam directed at the target; B, gathering path in wide-angle radar; C, riding the pencil beam. Fig. 5. (below) Semi-active homing (collision-course is depicted): A, lock-on "lamp" radar; B, computer; C, launcher; D, return from target; E, enlarged view of homing head; F, position of strike. The numbers are successive positions of missile and target. t6 5 4 3 2 '/</'//»/•'• on course in the event of the star being obscured for any reason(for example, by high cloud or icing of the tracking-unit window); it is also needed to provide temporary control in turbulent airwhen the tracking head might be knocked off its target. The missile must also carry a programming unit which, when thestar reaches a pre-calculated angle, puts the missile into an opti- mum dive on to the target by smothering or replacing the signalssent from the tracking head to the autopilot. Inertial. This is an "ultimate" guidance system fully worthyof the "ultimate weapon," the ICBM. Completely self-con- tained, it needs no external source of information, neither does itsend out any emission which might be detected by the enemy. Foundation of the whole system is the basic integral calculus.A pre-requisite to the operation of an inertial system is a precise and correct knowledge of the geographical position of the launch-ing point and the target. The missile is fired with this vital information locked up inside it and the inertial system then con-tinuously calculates the speed and direction, and produces steer- ing instructions to ensure that the missile will arrive at the latitudeand longitude of the target. This is done by sensing, with extreme accuracy, all accelerations imparted to the missile (sometimes it is possible to ignore accelerations in the vertical plane) and thenapplying the double integral to determine the distance gone and track followed. Measurement of the accelerations experienced by the missileare obtained from two or three accelerometers. These must be of extraordinary accuracy, yet capable of measuring an extremelywide range of accelerations, from a maximum of perhaps lOg down to as little as l/200,000g. Each accelerometer is mountedin one of the missile's major axes, in a gyro-stabilized platform. It is essential that, apart from being sensitive to true linear accelera-tions, the accelerometers should not be disturbed by other per- turbations of the missile. This can be ensured only by tying thegyro-stabilized platform to a pendulum with an effective length equal to the radius of the earth (so that, in effect, the pendulumbob remains at the centre of the earth and is therefore unaffected by movement of the point of suspension). Such a pendulum hasa natural period of about 84 minutes, and the inertial-guidance system is accordingly stabilized by an arrangement which simu-lates the behaviour of a pendulum with such a period. It will readily be appreciated that errors from an inertial systemare cumulative. To reach sufficiently close to a target after a flight of an hour or more demands a standard of accuracy andrepeatability of components which is virtually impossible to achieve—it is certainly impossible to manufacture such systemsin quantity. Fortunately, it is possible to relax the tolerances, and bring the problems roughly into line with those found in themost precise research laboratories, by employing a hybrid system which is not wholly dependent on information fed in before thestart of the flight. Of such hybrid systems, the combined inertial and celestial is perhaps the most popular—it is used in severallong-range American missiles—since, although sufficiently accur- ate and capable of mass production, it remains self-contained,unjammable and undetectable. Propulsion. There are certain classes of guided missile whichhave no propulsion at all, the most prolific example of these being the guided bomb which falls freely under its own weight. Inaddition there are several very important missiles, principally short-range AAMs, which blast through to full speed in fromone to three seconds and then discard all their propulsive systems and carry on under their own considerable momentum. Gener-ally, however, some form of internal motor is provided. Many designs for long-range missiles bear a close resemblanceto aircraft. Snark, a U.S.A.F. strategic cruise-type (i.e., non- ballistic) missile is a typical example. Power for such vehiclescan be provided by turbojets. The chief requirement for the powerplants is 100-per-cent-dependable operation on one occa-sion only, and it is possible to make them relatively simple and cheap to manufacture, since they do not have to perform variable-
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