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Aviation History
1950
1950 - 1846.PDF
FLIGHT, 5 October 1950 381 using solid propellants of similar character also suffer from the consequences of temperature sensitivity. It is not, however, difficult to ensure that all missiles have identical propellant temperatures prior to firing, that this tem- perature is neither too low nor too high, and that immediate trigger response is retained. Too low a temperature leads to intermittent burning or "chuffing." A significant difference between gun and rocket propellant is that, in the gun, the burning of the charge should produce maximum gas-pressure shortly before the projectile reaches the muzzle. With a rocket, maximum thrust is desirable immediately after commencement of burning; subsequently, the charge should produce a jet of substantially uniform rate. Air resistance quickly dissipates the kinetic energy after a shell leaves the barrel. For obvious reasons, all air-combat projectiles should travel at supersonic speeds on their way from the attacker to the target, and short vanes only are required. Hence, there is no need to provide for projectile shapes to give reduced drag at transonic or subsonic velocities, such as the "boat tails" of certain shells. As supersonic drag is high, and because considerations of stability and feeding-stroke pro- hibit shapes of minimum shock drag, the speed of a shell is quickly expended, and the trajectory described assumes increasing curvature. A well-shaped rifle bullet which starts with 2,900 ft/sec, muzzle velocity has after a 2.4 sec. flight (i.e., at 1,100 yards range) a speed of only 925 ft/sec., its kinetic energy, hence, has dropped to one-tenth of the original figure. Larger-calibre shells sustain a little less loss, on account of greater section density (as the mass increases more quickly with the calibre than does the cross-sectional area); on the other hand, they have less fineness in shape. Since armour- penetration by impact depends so much upon the range, focused blast-energy (by "shaped" charges of high explosive) must be utilized for long combat ranges or low-velocity guns. The great rejection of heat to the barrel imposes restraint upon the duration of fire bursts. Fortunately, short bursts are tactically adequate. But barrel erosion under heat also limits the rate of fire. Though there are certain prospects that erosion may be substantially reduced, for the present a multiplicity of guns is necessary to provide the requisite fire density. Experimentally, as much as 2,500 rounds per minute have been achieved from single-barrelled rifle-calibre machine guns. In interceptor attacks, fire bursts lasting less than 3 sec seem to rule, and 1.5 sec bursts have been recorded during suc- cessful engagements. With British installations of eight rifle- calibre guns the aggregate firing time amounted to 25 sees (all guns firing), and this permitted eight to nine full-size bursts. Contemporary Luftwaffe fighters usually had less—perhaps 17 sees total firing time). Present-day British fighters seem to have ammunition sufficient for total firing time of 15 sees. In view of the weight and bulk of more substantial ammunition, the permissible firing time may in future be even more COMPARISON OF MODERN FIGHTER ARMAMENT Type of weapon Gun weifht, or weifhtof launching device Weifht of single round mt firinf Projectile weifht at im- pact Majctrajectorial velocity H.E. charge per projec-tile Rate of fire par fun or launching deviceLength of weapon Mean recoil force per fun R-4/M Rocket (55 mm) Supersonic air- to-air rocket 551b 7.751b 5.85 1b 1.800 ft/sec I.I Ib 500-600 rounds/ min55 in nil MK.II2 Shell-gun (55 mm) Gas - operated automatic fun 6061b 5.31b 3.27 Ib 1.970 ft/sec 0.92 1b 300 rounds/min 79.8 in 13,800 Ib Weifht* and Performance* of s Four-unit Installation, per gun or launching device, respectively :— Complete weifht at take-off Projectile weifht fired H.E. weifht fired Projectile impact persecond, in relation to R.4/M Installation weifht (com- plete with ammuni- tion), in relation to R.4/M Hits required for the destruction of a four- enfined medium bom- bar 2.075 Ib 234 Ib/sec 75 Ib/see 100 per cant 100 par cent 1 3,475 Ib 65 Ib/sec 18.5 Ib/sec 41 par cant 178 per cent 1 20 mm Hi.pano Mk. V Recoil-operated automatic fun 1061b 0.925 1b 0.27 Ib 2,750 ft/sec 0.022 1b 650 rounds/min 72.6 in. 3.3O0lb with 60 rounds 6501b 10.9 Ib/sec 0.95 Ib/sec 13 per cent 31 per cent 20 N.B.—The above data refer to the original version of R.4/M as operationally employed in the spring of 1945. Heavy " spinner " rockets leaving the nose of a U.S. Navy PBJ (Mitchell) bomber during tests at Inyokern, California. restricted. This emphasizes the need for fully automatic sighting and triggering devices, to exclude ail waste of ammunition. Rockets for Air Combat.—The history of the rocket- propelled war missile is so ancient that it has been forgotten. War rockets existed even before bright lads tried hollow tree trunks as guns and founded an artillery tradition. Centuries later, unpleasant experience by British forces in India—where sly Tippu Sahib used bamboo tube rockets (Battle of Seringa- patam, 1799)—caused an Army engineering officer, William Congreve, to develop large sheet-iron-cased war rockets at Woolwich Arsenal (1804). During the Napoleonic Wars and the American War of Independence (notably, in the Battle of Bladensburg, 1812) these Congreve rockets, launched from ground batteries, special barges and other vessels in amazing quantities, met with such success that most countries copied them. The U.S. national anthem immortalizes a reference to this weapon. From the middle of the nineteenth century, how- ever, the rifled gun began to surpass the simple gunpowder war rocket in performance and accuracy, notwithstanding the introduction of the spinning rocket by William Hale in 1846. Britain employed rockets operationally until 1859 (China and Afghanistan). Nor are air-to-air rockets a novelty. In 1916, R.N.A.S. fighter squadrons successfully used electrically fired, stick- stabilized Le Prieur incendiary rockets against kite balloons; Capt. Ball and his fellow-pilots even tried them against enemy aeroplanes. Britain, also, built the first intercepter specially designed for rocket weapons—the 200 h.p. Vickers F.B.25 single-seater, to carry the Crayford rocket gun. All this British experience, also, forgotten by the air- armament experts until the Russians and the Germans rendered evidence that airborne rockets had useful applications. Not until September, 1941, did our officials deign to consider proposals to discharge adapted subsonic A.A.—available in quantity—from aircraft. This was after news had been received of the success of the Russian Stormovik with subsonic rockets against tanks. The gross oversight was the less justi- fied as Woolwich Arsenal had indeed created the basis for modern rocket missiles by accomplishing the dry (i.e., solventless) extrusion of large cordite grains. Moreover, the Army as well as the Navy had conducted many experiments with cordite-propelled A.A. rockets and had introduced weapons of this kind. Thennodynamically, the rocket is a simple converter of heat energy into velocity energy. A jet produced by high-pressure expanding gas creates a reaction upon the vessel from which it emerges; this is the propulsive thrust. Consequently a rocket consists of a combustion chamber to produce not gas at high pressure, and a nozzle to convert the pressure into a high- speed gas flow by expansion. In solid-propellant rockets the propellant is stored in the combustion chamber; and as it is self-contained, feeding devices are not necessary. Liquid- propellant rockets have separate containers and feeding devices; the chemical reaction, and thus the thrust, can there- fore be accurately regulated during operation. With solid propellants some measure of regulation is possible by means of grain shape and burning inhibition, but it is less easy to achieve with reliability. Also, liquid propellants are less subject to temperature sensitivity. Continued on page 388 " Mighty Mouse " air-to-air R.P.s being test-fired from a Douglas Skyraider. These projectiles have folding fins which automatically extend on firing. \ I
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