FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1950
1950 - 1987.PDF
460 FLIGHT FIGHTER AH3MAMENT . . . fuzes correctly during air combat means accurate range- determination, which is not easily accomplished. Never- theless, it would seem possible, with the aid of fully auto- matic ranging and fuze-setting devices, to reach practical solutions. An automatically following-up fuze-setter (such as was used in the 1918 Szakatz-Eisenach gun) and monitor- ing by radar ranging might be a step in the right direction. Like the proximity fuze, time fuzing has the advantage that narrow misses can have effect upon the target; with impact fuzes such misses are simply waste of ammunition. More- over, in a time fuze, a separate (secondary) self-destruction device becomes unnecessary; when aircraft are attacking in formation, wasted projectiles are less apt to present danger to the user's side in a melee. In design, the "burning-composition" fuze is probably simplest and safest; yet it is possible that other types might be superior. Percussion fuzes still predominate. In view of die " dissolved " character of modern airframe structures, they need exceptional sensitivity combined with nearly in- stantaneous action. For "mine" shells, some delay in the fuze action is advisable, because blast destroys stressed-skin structures more effectively when acting from the interior outwards. The reliability of small percussion fuzes is still the cause of many headaches. Fuzes reacting to changes in attitude are of use only as secondary devices to cause self- destruction of a spent shell. The most intriguing principle is that of a fuze which detonates the shell when it is at a minimum distance from the target. The Admiralty (prodded by Mr. Churchill) was leading in the development of a photo-electric proximity fuze for A.A. projectiles. Later, A.A. shells had British- invented fuzes based on radar ranging. Such fuzes, with their miniature valves and batteries, require rather large calibre because of their bulk. Detonation takes place when the time interval between emitted and echoed impulses is at a minimum. In view of the very short ranges which characterize air combat, shell fuzes of this kind might be too delicate as to be practical and/or safe. There exist, of course, other possibilities for action at close quarters—as, for instance, the use of electrostatic triggering on the capacity principle. Though it is foreseen that the present method of destroy- ing enemy aircraft in combat by direct hits only will even- tually be superseded by the use of proximity-bursts, it must not be overlooked that such development is closely linked up with die calibre question. Quite apart from obvious design-difficulties, proximity fuzes present little advantage for calibres of less than, say, 40 mm, because close detonation will then do little harm to the target. Even with a splinter-forming 3.7in (94 mm) A.A. shell, the lethal sphere has a radius of approximately 30ft only. Hence, for the 30 mm shell-gun to which this country seems to have pledged the immediate future, the adoption of proximity fuzes would offer no tactical gain worth achieving. As far as bigger calibres are concerned, fuzes of this kind would widen die prospects of attacks with short bursts over wider ranges, and ease the problem of ammunition wastage. Another important question is whether action by splinters or by blast is more effectual. Any detonating shell or rocket, of course, produces both; yet it is in the hands of the weapon designer whether one or the other shall predominate: for extensive splinter-formation, his shell will have thick walls of tough steel; for blast, a thin-cased ("mine-type") con- tainer with much high explosive will be essential. In this country, the preference is towards splinter forma- tion as being most effective against aircraft This view is held because a shell designed primarily for action by splinters possesses, by reason of higher section-density, better ballistic properties. Moreover, the "lethal zone" (i.e., the space adequately covered by splinters of sufficient kinetic energy for certain destruction) is bigger. Even before the late war die Germans decided in favour of thin-cased mine shells, with predominantly blast action. For the 20 mm Oerlikon MG.FF such ammunition (carrying 19.5 grammes of H.E. per shell) was introduced. During the war, the Luftwaffe found no cause to change this policy, while raising calibres to 30 mm and, later, to 50 and 55 mm. The last German systematic experimentation in ammunition development fully confirmed the superiority of action by blast. The reason is that, although blast produces a smaller lethal zone around the point of detonation, stressed-skin structures are far more prone to collapse under action of blast than when pierced by splinters. The time is past when a few, relatively small, vital members—such as spars, struts, longerons and cables—made a vulnerable main structure sensitive to hits by splinters but practically immune from blast. Even when one of the massive main spars of a modern stressed-skin aircraft is pierced by splinters, the additional stringers and die skin connected therewith will still retain sufficient strength. When, however, intense blast waves react upon exposed fields of thin-gauge, stress-carry- ing metal sheet, a general collapse of the structure often follows. The proper mechanics of die blast action in such circumstances are complicated and not easily understood. During the war, the author suggested the exploitation of die self-frequency of free sheet fields (which does not vary gready so far as unsupported fields of identical material are concerned) for easier destruction by an appropriate selec- tion of explosive and shell; it was, however, rightly held against die proposal mat die frequency spectrum of all ex- plosives was too large. Also, a fuel tank is more likely to discharge its contents by splitting open under blast than when pierced by splinters. For the future, anodier aspect in favour of blast-action is the strong incendiary effect of high explosives. Turbojet and rocket-propelled aircraft carry plenty of ignitible or even explosive fuel in a number of places, and are, hence, more vulnerable to incendiary action. The use of special incen- diary shells can thus be dispensed with. Ballistic Properties.—Once the shell leaves the barrel, no further energy is imparted to it. Because of the action of gravity (which is not a priori counteracted by a lift force on the projectile) trajectory drops, Le., the path described becomes inclined earthward. Since the shell also suffers air resistance, die subsequent parabola foreshortens; and the higher the drag the greater is this foreshortening. Hence, when a gun is fired at great altitude, die shell's trajectory becomes flatter than when it is fired near the ground. Con- sequently, for air combat at high altitude, a lower muzzle- velocity may produce a trajectory comparable widi that of a high-velocity gun fired near sea level. The characteristic shape of die trajectory, too, is affected by shape, section density, and stability of the shell. The shape decides the air resistance. Section density is a para- meter akin to, but somewhat different from, the wing loading of an aircraft; the higher the section density, the greater the kinetic energy of the shell in respect to air drag. The sta- bility of the shell in flight not only affects the trajectory on account of the magnitude of air drag, but may alter it by producing deflecting forces. Shells fired successively by one and die same gun with an identical charge always vary in their trajectories, and hence cause dispersion of bursts. The flight-path of a shell is not so regular as some text- books will have us believe: because of the spin, and be- cause of the action of the air, a variety of additional forces and moments develop: the shell does not remain in the same vertical plane, and it does not fly truly with its longi- tudinal axis tangential to the trajectory. In fact, a shell's trajectory is a rather complicated affair, full of kinks and bends. To hit a target in air combat, two qualities are para- mount: "drop" and "dispersion." In die same gun, different ammunition produces variations in both. H.E. shells, for instance, commonly have greater dispersion and drop than does armour-piercing or ball ammunition (high explosive is of about one-fifdi the weight of steel). Single-shot dispersion is a measure to show the deflec- tion of a single shell from the point it should have reached in accordance with the aim taken. Burst dispersion is commonly given as the ratio, to the range, of the circle through which half of the trajectories of all shells pass. To consider only half of the shells fired is arbitrary but practical. The circular area thus delineated (normal to the line of sight) is defined as the lethal area covered by a gun
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
