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Aviation History
1951
1951 - 0009.PDF
FLIGHT, 4 January 1951 Two of the four 20 mm guns in the Glottcr Meteor.' Attention is drawn to (A) the gun barrels, with their blast tubes removed, (B) pneumatic cocking valves, (C) link chutes, (D) case chutes, (£) rear mountings and adjusiors. is considered less reliable, due to the riskof mechanical seizure caused by solid residues, in the powder gases; but it islighter "and will normally allow a high rate of fire. German experts hold the view thatthe gas-operated airborne gun deserves pre- ference for all purposes because of its lowweight and because of the smaller number of moving parts; and they consider as ex-ceptional the mass-locking system preferred in the case of the M.K.108 and M.K.112weapons. Rate of fire is directly related to muzzlevelocity. On our assumptions, the greater propellant charge needed for high muzzle-velocity demands a longer cartridge case, and, consequently, increased stroke of the breech-block anda longer breech; also, a longer barrel is needed, and the recoil is higher. High muzzle-velocity, therefore, increasesthe reciprocating masses and thus slows the rate of fire. In addition, the gun with the higher muzzle-velocity islonger and heavier; the larger ammunition requires more stowage space and is heavier; and a more substantialmounting is needed. For a 20 mm aircraft shell-gun (with mechanically lockedbreech and recoil-operated mechanism), a 50 per cent in- crease in muzzle velocity reduces the shell's time of flightover 1,000 yards by 20 per cent, while the trajectory is flatter and the impact penetration greater. But this is achieved atthe cost of a gun weighing not less than two-and-a-half times as much, of twice the length, and firing at half thespeed. The cartridge is longer by 44 per cent. In general, the trend in air combat is to rely on moderatemuzzle velocities. With fixed-gun installations, the flying speed adds to the muzzle velocity: as modern interceptershave high speeds, especially with power-boost during attack, sufficiently flat trajectories result, particularly at highaltitudes. Moreover, a deficient muzzle velocity- can be compensated by automatically computing sights. A defi-ciency in fire density, however, compels us to multiply the weapon installation, and this is usually an unacceptableremedy. Methods of Use.—Interception can be accomplished byfour methods of discharging projectiles: — (a) Single shot: automatically triggered, or guided, ordirected. Burst: i.e., a sequence of several projectiles.(b) (c) (d) Salvo: i.e.,jectiles. simultaneous discharge of several pro- Salvo-burst: a combination of (b) with (c). Destruction of the target is effected either by a directhit (or hits), or by nearby detonation. In the latter case, however, it must be realized that the lethal range of even thevery effective German 88 mm Flak shell (with time fuze) was actually less than five yards distance from the target,and that the lethal sphere formed by a detonating 3.7in A.A. shell has a diameter of only about 50 feet, effectiveduring about one-fiftieth of a second. The alternative between splinter and blast action has beenreferred to in an earlier instalment of this article. Explosive shrapnel, i.e., the ejection of small, separate explosivemissiles from a detonating shell is possible, and was eagerly studied in Germany; but its operational effectiveness hasyet to be established. A modern trend is towards single-shot action by meansof substantial-sized guided or directed, homing, air-to-air missiles which produce lethal spheres large enough todestroy an enemy even with the aid of proximity fuzes, and which can be discharged over ranges exceeding one milewithout proper taking of aim. The operational similarity to the naval torpedo is obvious. The traditional but by no means*bbsolete method is action by salvo-bursts, from weapon batteries firing at high rateover ranges not exceeding about 1,000 yards. The proba- bility of a hit is not related to the single-round dispersion,i.e., the difference between aiming point and average point of impact; burst-dispersion alone is essential. The formercharacteristic is more usually found in salvo fire; in bursts, projectiles follow each other and cause departure deflectionsnot present in single-shot fire. Moreover, apart from effects of barrel vibration, etc., the target and the attacker movewhilst the actual projectiles are under way. Since a modern bomber easily covers 300 yards during one second, rangeand bearing are subject to considerable variations. Salvo fire from rocket batteries offers good prospects forthe use of small, supersonic rocket or ramjet missiles. If the nose of the fuselage is available for such " honeycomb " bat-teries as in the Natter (illustrated in Part V), full benefit can be derived, but with launching devices distributed along awing the fire-density produced might not be good enough. For supersonic intercepters having automatically triggeringsights, salvo fire of rockets should be satisfactory. In such aircraft a prone position of the pilot in the nose of thefuselage might be preferred. As to salvo-bursts, it must first be remembered that the" curve of pursuit" is the shortest path along which an intercepter can get into position to open fire. Fixed-guninstallations compel the intercepter to follow something approaching a " ramming course "; ihus the axis of the inter-cepter must point towards the target or, in view of the target's motion during the time of flight of the projectiles,towards the point which the target will reach when the pro- jectiles complete their flight. The " lead " required for therelative movement during projectile flight may be consider- able. If the target proffers defensive fire, the curve of pur-suit will be adhered to solely during the short interval between taking aim and firing; this may be anything be-tween, say, three and six seconds, and during it the inter- cepter is helplessly exposed. Operational experience indicates that, when the target isnot fired at from directly aft or whilst moving in the same vertical plane as the intercepter, the probability of hits ismuch reduced (" lead dispersion"). Angles exceeding 20 deg in flight courses can give prospects of desruction onlywith the aid of automatically computing (i.e, gyro-moni- tored) sights. However, in an attack from above or below,at an angle of less than 30 deg, whilst flying on the same course as the target, the latter presents a larger area, andthe chances of a hit are greater. Automatically computing sights (first suggested andexperimented with during the First World War, by this country) are a necessity for an intercepter. They must atleast provide the angle of deflection (or lead), while auto- matic ranging, also, is most desirable. AH this was includedin German Luftwaffe experimental devices, upon which the German experts set high hopes. The final (and, for totalair defence, absolutely necessary) development is an auto- matically triggering (" blind-aiming ") sight, in combinationwith radar stalking. Ii is hard to see how, otherwise, night
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