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Aviation History
1955
1955 - 0108.PDF
108 FLIGHT GUNS FOR FIGHTERS . . . The T-17 had a high muzzle velocity, but it fired only solidprojectiles, and was therefore abandoned. Corresponding developments in ammunition took place. Quiteearly it was decided that, in spite of the increasing use of armour, the high-explosive round was the most effective type of projectilefor normal combat use. Efforts were made to increase the ratio of weight of explosive to the weight of the case, and startling resultswere achieved in the development of thin-walled "mine ammuni- tion." For example, the original German 20 mm shell contained18 grammes of explosive out of a total weight of 90 grammes, and the 30 mm H.E/traccr contained 72 grammes out of a total weightof 330 gm. It was not found possible to make any significant decrease in the thickness of the case, but lengthening the shellmade possible an increase of as much as 30 per cent in explosive content, beyond which point muzzle velocity decreasedinordinately. (The fineness ratio of a projectile is denoted by expressing its length as a multiple of the calibre.) By this means, 20 mm ammunition with a length of 5.2 calibreswas developed, with an explosive charge of 24 or even 27 grammes. By eliminating the tracer the revised 30 mm shell could be madeto carry 85 grammes of explosive, and later developments—which were not employed in combat—envisaged increasing the charge toas much as 110 grammes (almost J lb). At the same time, improve- ments in explosives and propellants, the development of incendiaryH.E. and the use of special delayed detonators made possible the achievement of a maximum effective action for each calibre. Evenin 1955 it is doubtful if these results have been significantly improved upon. By 1942, the Germans had decided that even 20 mm was theminimum useful calibre for air-to-air combat. Already, a 30 mm weapon (the MK-101) was in service and, although it was slow-firing and generally uninspired in conception, it was an effective weapon. Moreover, it served as a basis from which the consider-ably better MK-103 and 108 (see table) were developed. Some 12 years ago, the standard armament for the then-projected Me 262twin-jet fighter was chosen as four MK-108s, representing a "punch" which has only recently been exceeded with theintroduction of our latest fighters. During the winter of 1942-43, the Mauser company wereactively developing the MG-213A, which was a high-performance cannon embodying all the combat experience amassed up to thattime. The MG-213A embodied no revolutionary principles and was merely a very good weapon in 20 mm calibre. Nevertheless,the Mauser engineers knew that they could do a lot better working along fundamentally new lines. In particular, it seemed clearthat guns of conventional pattern were already very close to the practical limit of cyclic firing rate* imposed by considerations ofinertia, mechanical strength and the difficulty of effecting a positive control of the ammunition as it passed through the gun. At this point, it should be made clear that, during World War 2and later, several countries produced a number of highly uncon- ventional weapons, including recoil-less guns firing projectiles ofup to 5in calibre; a gun with three distinct breech assemblies spaced at 120 deg around a rotating cylinder; and another in whicha number of projectiles were discharged under the influence of a single propellant charge. But none of these was really successful,and all the important cannon of today have stemmed from the weapon which will shortly be described. The MG-213A was a gas-operated weapon, and a notable featurewas the inclusion of a pivoted multiplying lever in the linkage between the gas piston and the recoiling portions of the mechanism. *Cyclic rate: the rate at which an automatic weapon fires successive rounds. A cyclic rate of 1,000 rounds per minute does not necessarily mean that in one minute 1,000 rounds can be fired. =3: General arrangement of the Oerlikon 302 RK 30 mm gun. The overall length is 117in, the maximum width 11 in and the weight 398 Ib. This threw the breech block to the rear at more than twice thespeed of the gas piston and made possible a cyclic firing rate exceeding 1,000 rounds per minute, albeit at some expense in wearand tear. The Mauser company completely redesigned the 213A, utilizing a principle which was by no means new but which CHARACTERISTICS Country and designation Great Britain Browning O.3O3in Mk 2 ... British Hispano20mm Mk5 Aden 30mm France D.E.F.A Hispano Model 606 Germany Rheinmetall MG-81 MG-131Mauser MG-151/20 Rheinmetall MK-101 MK-103 MK-108 ... MK-115 ... Mauser MG-213A MG-213C/20 ... MG-213C/30 ... Rheinmetall MK-412 Switzerland Oerlikon 206 RK 302 RK U.S.A. Colt-Browning M-3 T-17 M-39 OF SOME MODERN AIRCRAFT GUNS Calibre mm 7.7 20 30 30 30 7.92 13 20 30 30 30 55 20 20 30 55 20 30 12.7 15.2 20 in 0.303 0.787 1.181 1.181 1.181 0.311 0.5110.787 1.181 1.181 1.181 2.165 0.787 0.787 1.181 2.165 0.787 1.181 0.5 0.60.787 Weight of gun (Ib) 21.9 106 n.a. n.a. n.a. 13.9 4093.5 323 289 135 390 113 165 170 608 199 398 52 109 170 Weight of pro- jectile (Ib) 0.022 0.27 n.a. n.a. n.a. 0.028 0.130.3 0.728 0.728 0.728 3.4 0.33 0.42 0.73 3.2 0.276 0.662 0.106 0.16n.a. Max. cyclic rate of fire (r.p.m.) 1,200 650 1.200» n.a. n.a. 1,500 700750 250 420 450 300 1,100 1,500 1,200 250 1,800 1,200 1,100 1,200 1,500 Muzzle velocity (ft/sec) 2,600 2,750 n.a. n.a. n.a. 2,675 2,3702.500 2,900 1,657 2,900 2,000 3,280 3,000 1,805 1,900 3,510 3,610 2,500 3,500 3,400 * According to foreign publications. Pneumatic cocking equipment tor the 30 mm Aden. Developed by Dunlop, this unit works at 1,200 Ib/sq in (max. working pressure, 4,000 Ib/sq in), and moves a two-stage ram. A complete Oerlikon 302 RK weapon, show- ing pneumatic and electric connections, cables and cockpit control box. This gun has an unsurpassed combination of rate of fire, calibre and muzzle velocity. The business end of a 20 mm Oerlikon 206 RK. This gun, which fires at cyclic rates exceeding 1,700 r.p.m., ejects cases through the large pipe and belt links through the slot just above the pipe.
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