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Aviation History
1950
1950 - 2107.PDF
536 FLIGHT, 7 December 1950 FIGHTER ARMAMENT . . . poses). Consequently, the modern interceptor will have toopen fire at long range, with high fire-density and with the certainty of a kill, even if only one of the shells scores a hit;no " repeat performance " will be practicable. So far as ordinary shell-guns are concerned, such demands upon armament arecontradictory. Calibre increase is opposed to greater rate of fire, and the latter property cannot be condoned with a highmuzzle-velocity. Technically, this necessitates turning to other weapons. Similar arguments hold good in respect of inter-ception at supersonic flying speeds. At such speeds, the quick manoeuvring of the gun into attack position presents technicaland physiological difficulties. Attacks at close range can, there- fore, be deemed highly improbable. The target will allow onlyglimpses for taking aim and firing and, because of reduced manoeuvrability, a repeated attack is unlikely. Experiments in Destruction.—During the late war, theGermans found cause to develop adequate interceptor arma- ment of the kind indicated. Proceeding methodically, they beganby establishing experimentally how the destruction of a bomber could best be effected. The ideal was, of course, to achievethis purpose by a single hit, out of a number of shells fired— or, better, by a dose miss. The need to approach this idealhad become obvious from actual fighting experience in air combat: photographic evidence collected during attacks byFwl90 fighters against B-17 bombers had proved that only two per cent of the shells fired produced hits, against a minimumof five per cent which could be reasonably expected from calcu- lations based on maximum dispersion. Moreover, the Germansearly realized that with jet and rocket interceptors under development, the curves of pursuit would allow the pilot tohold his target in the sights for an extremely short time only. They also had to reckon with the " snaking " characteristicof high-speed aircraft, referred to in Part IV of this article. In addition, the experts were painfully aware that the speedsuperiority of the interceptor was vanishing quickly. The close " box" formations in which Allied day-bomberforces had finally learned to operate, and their substantial defen- sive fire-power in all directions, obliged the Germans to attackfrom ranges as great as 800 or even 1,000 yards. This, too, demanded appropriate armament. Even the 30-mm Mk. 108shell-gun with a muzzle velocity of 1,650 ft/sec had too curved a trajectory to guarantee a sufficient number of probable hits.Obviously, a larger calibre with high muzzle-velocity was needed. Experiments by Burgsmueller at the Rechlin establishment,conducted on captured four-engined bombers (of smaller size than the present-day "medium" types) proved that, to bringdown such an aircraft, direct hits by H.E. shells were required in the numbers shown in the table at the foot of col. 2. The minimum amount of H.E. shell-filling (" Hexogen-AL")necessary to produce destruction was established at between 15 and 18 oz (420 and 500 grammes). As a matter of comparison ii is interesting that, according toR.A.F. assessment in 1939, no fewer than 850 hits by bullets fired from light machine guns (Lewis) were considered neces-sary to bring down an aircraft in A.A. practice—" lucky " hits, of course, excluded.Operational evidence collected by the Germans confirmed that in order to produce twenty direct hits by 20 mm shells inair combat, no fewer than 1,000 rounds would have to be fired, all most carefully aimed. Using the otherwise excellent MauserMG. 151/20 non-synchronized eauipment (four guns firing at 700 rounds/min each, with a muzzle velocity lower by only5.5 per cent than that of the British Hispano Mk. 5 gun), this meant 21.5 seconds' continuous firing during a single attack—well-nigh an impossibility. The Sum of Experience.—This suggests that, basically, thepresent British interceptor armament of four 20 mm shell-guns is, a priori, insufficient, even against obsolete bombers of 1941design; it also tallies with the operational experience collected by the Luftwaffe during 1943 and 1944. According to air-combat statistics covering the period between January and October, 1944, the Luftwaffe lost one of its 20 mm-gun fightersfor each Allied aircraft (of any type) shot down. The argument that Allied bombers were indeed shot down by20 mm-gun fighters, contrary to the conclusions drawn from the experiments and statistics, does not hold good, simply forthe reason that the experiments related to probabilities, and not to lucky lethal hits. Likewise, aircraft have been "bagged"by riflemen with a single pot-shot. Not so long ago, a light aircraft making an airfield circuit was even brought down bya high-kicked football! Yet it would be futile to reason that rifles or footballs should be considered as A.A. weapons. More-over, the German conclusions intentionally neglected all destruc- tion arising from secondary causes, e.g., by the incendiary effect of the H.E., by subsequently initiated explosions, and so forth.Another conclusion derived from the German experiments was that no type of ammunition was required for successfulair combat other than " mine " shells. Even before the outbreak of the war, the German experts had been partial to such H.E.shells; now their superiority was established once more, with pointers clearly indicating the path of future progress. H.E.produces sufficient incendiary effect in contact with fuels, coolants, etc., while against such armour as is practicable withaircraft, the large-calibre "mine" shell proves sufficient. As for tracer ammunition, the period of its usefulness has passed.The experiments rendered absolute evidence that the mini- mum practical calibre to destroy piston-engined bombers is30 mm; and taking into account further technical and tactical development (expected for late 1945 had the war continued),the need for 50 to 55 mm calibre guns was foreseen, and designs of equivalent calibre (2in to 2.17in bore) subjected tointense development. Such calibres, however, even when com- bining medium muzzle-velocities with a reasonable rate of fire,result in rather cumbersome and heavy weapons. An urgent need was thus felt to arrive at more appropriate types. The experience collected with airborne recoilless guns hadnot been encouraging enough to suggest this variety as a quick solution. The self-propelled shell, fired from low-velocitymortars, was still in the first stages of development, although not unpromising in larger calibres. But with rocket missiles,a makeshift attempt (which maimed an American attack on Schweinfurth) gave surprisingly good results. Consequently, the development of a small supersonic rocketmissile, equivalent to a 55 mm shell and of the same calibre, was taken in hand directly by an energetic and capable arma-ment group, of the German Air Ministry. The resulting R.4/M rocket (of 7.75 lb launching weight) had an immediate andamazing success in the few weeks during which it was opera- tionally tried. An automatic launcher was developed to dis-charge missiles at a rate of 500 to 600 rounds/min; it weighed only 55 lb and the automatic feed was very ingeniously arranged.It saw, however, no further operational service- Comparative data for this novel armament and guns havealready been given (Part III of this article, Ocrober 5th). The aircraft designer, of course, has no wish to change from four20 mm guns to a single 55 mm weapon—unless the Christmas- tree tradition be revived; but he may well be able to changeover to rocket-missile armament. In respect of flatness of trajectory, the 20 mm Hispano gunscores. The R.4/M rocket is, however, only the beginning of a new development, so the comparison is not quite fair. Inthe opinion of former Luftwaffe experts who had operational experience in air combat, a high rate of fire should rank superiorto flatness of trajectory, provided that gyro sighting devices are used. There, too, is overwhelming proof that a greater calibreis preferable to high muzzle-velocity. Luftwaffe Rocket-missile Experience.—On the strength of theexperimental evidence and of the tactical conclusions derived therefrom, Col. G. M. Gollob, then in charge of Luftwaffefighter requirements and operationally highly experienced, urged a speeding-up of R.4/M development and basic modification of-interceptor armament. For operational trials, Me262 twin-jet fighters (modificationsof the original day-bomber version first used in the summer of 1944) were equipped with four 30 mm guns—of the Mk. 108type, then already considered obsolescent—and also with 48 of the first series of R.4/M rockets, primitively mounted inexterior racks underneath the wing. In February, 1945, some 60 interceptors of this kind became available, all substantiallyslower than 500 m.p.h. at optimum altitude and afflicted with engines of short life. Judged as a weapon-carrier, the aircraftwas undoubtedly the best then in production, yet its operational characteristics were reputed to be far from ideal: it couldoperate for about 50 minutes only, required a full-load take-off run of nearly 2,000 yards, lost its controllability when divedto more than 590 m.p.h., and was exceptionally throttle- sensitive: at falling revolutions approaching 6*000 r.p.m., theturbojets were likely to stop, and could not be restarted in flight. In all, the Me262 fighter was far from easy to fly, quiteapart from its somewhat haphazard conversion to a fighter. Yet on April 7th, 1945, a group of these interceptors brought SHELL EFFECTIVENESS, THEORETICAL AND PRACTICAL Calibre of Shell Rifle calibre or 12.7 mm. (0.5in) 20 mm ... 30 mm ... 55 mm Minimum Number of Hits Required (H.E. Mine-typ? Shell)— Test Results. Not investigated 20 4 1 Hits Actually Required—Combat Statistics against B-I7s 50—60 20 7 Not established
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