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Aviation History
1960
1960 - 0814.PDF
822 All non-atomic anti-tank missile warheads rely for their penetrative effect upon the intense jet produced from the detonation of a hollow- shaped charge. In this diagram the detonator A is activated by the impact fuze B by way of the fuze cord C; D is the critical stand-off distance for maximum penetration; the explosive is shown shaded Missiles and Spaceflight . . . S S 10 design was frozen at least eight years ago inevitably means that it can be bettered, and by a substantial margin. The follow- ing notes indicate the trend of present thinking. Training Experience has shown that, although the first costof these small and simple missiles is relatively low, the task of training operators to the required degree of proficiency is veryhigh Most existing anti-tank systems presuppose the use ot a simulator which can place representations of moving targets infront of an operator for engagement by a system with the same control-response characteristics as the actual weapon. Such devicesare relatively cheap to run, but no soldier can be allowed in battle without actual experience of several live launchings, and currentestimates suggest that it may cost £10,000 or more per man per year to keep operators on the top line. Certain manufacturers, such as Oerlikon, have produced recover-able versions of their missiles in order to help to minimize this cost The technique involves firing a missile through a frangibletarget and thereafter pulling it up into a steep climb, triggering a switch to release a parachute and then recovering the round torre-use Unfortunately, although this appears attractive, in prac- tice it has been found that a recovered missile may require anexpenditure almost equal to its first cost before it can again be made operational. Powder-driven gyros and many other close-tolerance parts would almost certainly have to be replaced, and the skilled maintenance needed is likely to be considerable. Airframe In all missiles the shape and construction of theairframe is a compromise. In the anti-tank missile the conflicting factors have a mutual relationship which differs from the situationpresented by almost all other guided weapons, and prime import- ance must be attached to minimizing size and weight, and easingthe problems of cheap manufacture. Flight loads are relatively modest, and sheer performance is unnecessary (practically allweapons in this category are subsonic). On the other hand, any- thing taken into a battlefield must be able to withstand beingkicked about and possibly being misused severely; accordingly it must be resistant to impact, immersion in water and the adverseeffects of vibration, high temperatures and dust. Major portions of the airframe (and in many cases almost theentire missile) are today manufactured from various types of reinforced plastics, frequently with an interior stabilized by aninner core of honeycomb or foamed material. Dimensional stability is of prime importance, in view of the fact that performancebetween successive rounds must have a high degree of repeat- ability and that all portions of the dismantled missile must cliptogether readily in a matter of seconds In conclusion it is pertinent to note that practically all anti-tankmissiles may be carried and launched by aircraft. Where wire guidance is employed this imposes a restriction on the speed atwhich the aeroplane or helicopter may fly; obviously the launching aircraft must not catch up on the missile and allow the wiresto trail too far behind the slipstream. Warhead Without the invention of the hollow charge nocheap infantry anti-tank missile would have much chance of piercing anything. Precise details of contemporary types of war-head are naturally classified secret, but an idea of the sort of configuration chosen is afforded by the diagram. . The principle of the hollow charge was first demonstrated in - ;- .--y .,-••--.-^-:;' FLIGHT, 17 June 1960 the 1820s, but was largely neglected until comparatively recenttimes. Although specific designs of hollow-charge head are much more sophisticated than is apparent from superficial inspection,the basic modus operandi can be explained simply. The explosive charge of the warhead is hollowed out at thefront in the form of an involute cone symmetrically disposed about the longitudinal axis of the missile. The charge is detonated by afuze triggered mechanically by the impact of the forward-pointed probe in order to ensure detonation at the correct stand-off distance(see diagram). Detonation of the charge produces an intense jet of gas which, by virtue of the profile of the involute cone, is concen-trated in a forwards direction along the axis of the missile. This jet can pierce any type of armour, forming a hole of rela-tively small—and usually progressively diminishing—diameter. Experiments show that, according to the design of the warhead,a balance may be struck between penetration ability and resultant killing power within the vehicle. For example, a given weight ofcharge can, with suitable profiling, be made to punch through exceedingly thick armour; but from the emergent side will issuea jet of perhaps only 0.125in diameter with little residual energy. On the other hand, the same charge weight can be made to wreakhavoc inside a tank, but at the expense of reduced maximum penetration One can visualize warheads clever enough to forma non-tapering hole through which the missile can insert some- thing highly destructive or obnoxious—such as an additionalexplosive charge or even a toxic material. Most of the major western nations are studiously exploring thepotentialities of the hollow-charge, and the USA are also well advanced in the field of miniature nuclear devices. Already atomicwarheads of less than 6in diameter are now in production—for Davy Crockett, for example—and it would appear that thedetonation of such devices in the immediate proximity of any armoured vehicle would be more than sufficient. Control There are really two basic types of control systemin common use in missiles of this type. The most obvious arrange- ment is the velocity-type control, in which the demand ot theoperator produces a proportionate change in missile heading; or, put another way, it causes the missile to have a lateral velocityacross the operator's field of view exactly proportional to, and in the same direction as, the demand signal. In the other system,the acceleration-type control, the signal produces a lateral accelera- tion There are many who feel that the former method ispreferable. It is certainly a demonstrable fact that the human brain can deal more naturally with a velocity system. It is relativelyeasy to estimate an angular displacement almost instantly, and with little difficulty one can appreciate a crossing velocity with tairaccuracy It becomes more difficult to predict or govern future situations when the crossing motion must be regarded as anacceleration, involving mental integration. To an observer on the touch-line the acceleration and velocitytypes of control both appear to have a roughly equal number of advantages and disadvantages—although it would be foolish todeny that there seems to be a trend towards the simpler velocity type of control, largely owing to the fact that it comes more natur-ally to the operator. For manipulation for a missile with relatively modest performance the combination of trailing wires andflip/flop or trembler spoilers is adequate; but, as flight speed is increased, spoilers may no longer be tolerated. One of the most popular ideas is simply to transfer controlfrom the free airstream to the propulsive jet from the motor. 1 he reasons for this are manifold. Wing controls become lesseffective as the flight speed rises and at speeds of the order ot M0 6 (characteristic of later missiles, such as the S.S.I 1) separatedflow occurs across the wing so that, to obtain proper response, spoilers would have to project an undesirable amount Moreover,aeroelastic problems and the shape of a swept wing militate against wing-mounted surfaces at these high speeds Typical of the jet-deflection control systems is that of the S.S.ll, in which four vibrating spoilers are positioned by solenoids. In their neutralposition they lie entirely clear of the rocket jet, and as the missile rolls each in turn may be brought into action to steer it. In the case of the British Vigilant, each wing carries onits trailing edge a simple hinged surface which, according to tne transient roll position of the missile, governs attitude in P«cn^yaw The position of each of the four surfaces is governed by miniature actuators, pressurized by gas bled from the mainBWtM-
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