Chapter XV : ANTI-AIRCRAFT GUNS. IMMOBILE WEAPONS

The immobile anti-aircraft gun, as distinct from that attached toa travelling carriage such as a motor-car, may be subdivided intotwo classes. The one is the fixed arm which cannot be movedreadily, mounted upon a permanent emplacement; the other is thefield-piece which, while fired from a stationary position, may bemoved from point to point upon a suitable carriage. Thedistinction has its parallel in ordinary artillery, thefirst-named weapon coinciding with the heavy siege gun, which isbuilt into and forms part and parcel of the defensive oroffensive scheme, while the second is analogous to the fieldartillery, which may be wheeled from position to position.

In this phase of artillery the Germans led the way, for thesimple reason that they recognised the military value of aerialnavigation years in advance of their contemporaries. Again, inthis field the Krupp Organisation has played a prominent part.It embarked upon actual construction of weapons while its rivalsin other countries were content to prepare their drawings, whichwere filed against "The Day." But it must not be thought thatbecause the German manufacturers of armaments were ahead oftheir contemporaries they dominated the situation. Far from it. Their competitors in the market of destruction were every whit askeen, as ingenious, and as enterprising. Kruppism saw acommercial opportunity to profit from advertisement and seizedit: its rivals were content to work in secret upon paper, to keeppace with the trend of thought, and to perfect theirorganisations so as to be ready for the crisis when it developed.

The first Krupp anti-aircraft field-piece was a 6.5 centimetre (29/16 inch) arm. It possessed many interesting features, the mostsalient of which was the design of the axle of the carriage. Therigid axle for the two wheels was replaced by an axle made in twosections, and joined together in the form of a universalcoupling, so that each wheel virtually possessed its own axle, orrather half-axle. This was connected with the cradle of the gunin such a manner that the wheels were laterally pivoted thereon.

The result is that each axle can be turned forward together withits wheel, and thus the wheels have their rims brought into lineto form an arc of a circle, of which the rear end of the spade ofthe gun carriage constitutes the centre. This acts as a pivot,about which the gun can be turned, the pair of wheels forming therunners for the achievement of this movement. The setting of theweapon in the firing position or its reversion to the travellingposition can be easily and speedily effected merely by therotation of a handwheel and gearing.

With this gun a maximum elevation of 60 degrees is possible,owing to the trunnions being carried well behind the breech incombination with the system of long steady recoil. The balancingspring which encloses the elevating screw is contained in aprotected box. The recoil brake, together with the springrecuperator, follows the usual Krupp practice in connection withordinary field pieces, as does also the automatic breech-closingand firing mechanism. In fact there is no pronounced deviationfrom theprevailing Krupp system, and only such modifications asare necessary to adapt the arm to its special duty. When the gunis elevated to high angles the shell, after insertioin thebreech, is prevented from slipping out by means of a specialdevice, so that the proper and automatic closing of the breech isnot impaired in any way.

In such an arm as this, which is designed essentially forhigh-angle firing, the sighting and training facilities requireto be carried out upon special lines, inasmuch as the objectiveis necessarily at a considerable altitude above the horizon ofthe gun. In other words, in firing at a high inclination,distance between the gun and the target cannot be utiliseddirectly for the back sight. On the other hand, it is essentialthat in proportion as the angle from the horizontal increases,the back sight should be lowered progressively in a mannercorresponding to the distance.

To assist the range-finder in his task of sighting it isnecessary that he should be provided with firing tables set outin a convenient form, which, in conjunction with the telemeter,serve to facilitate training for each successive round. In thisway it is possible to pick up the range quickly and to keep theobjective in the line of fire until it either has been put horsde combat, or has succeeded in retiring beyond the range of thegun.

The sighting arrangements of these Krupp anti-aircraft guns arecarried out upon these lines. Beneath the barrel of theback-sight is an observing glass with an eye-piece for theartillerist, while above and behind the observing glass isanother eye-piece, to be used in conjunction with themanipulation of the back-sight. The eye-piece of the observationglass is so made that it can be turned through a vertical planein proportion as the angle of fire increases in relation to thehorizontal. The determination of the distance from the objectiveand from the corresponding back-sight as well as the observationof the altitude is carried out with the aid of the telemeter. This again carries an observation glass fitted with an eye-piecewhich can be turned in the vertical plane in the same manner asthat of the fore-sight. By means of this ingenious sightingdevice it is possible to ascertain the range and angle of firevery easily and speedily.

The weight of the special Krupp anti-aircraft field-piece,exclusive of the protecting shield, is approximately identicalwith that of the ordinary light artillery field-piece. It throwsa shell weighing 8.8 pounds with an initial velocity of about2,066 feet per second.

Although the German armament manufacturers were among the firstto enter the field with an anti-aircraft gun of this characterthey were speedily followed by the French, who devised a superiorweapon. In fact, the latter represented such a decisive advancethat the German artillerists did not hesitate to appropriatetheir improvements in sundry essential details, and toincorporate them with their own weapons. This applies especiallyto the differential recoil system which is utilised in the smallanti-aircraft guns now mounted upon the roofs of high buildingsof cities throughout Germany for the express purpose of repellingaerial attack.

The French system is admitted by the leading artillerytechnicians of the world to be the finest which has ever beendesigned, its remarkable success being due to the fact that ittakes advantage of the laws of Nature. In this system the gun isdrawn back upon its cradle preparatory to firing. In someinstances the barrel is compressed against a spring, but in themore modern guns it is forced to rest against a cushion ofcompressed air contained within a cylinder. When first bringingthe gun into action, the barrel is brought into the preliminaryposition by manually compressing the air or spring by means of alever. Thereafter the gun works automatically. When the gun isfired the barrel is released and it flies forward. At a criticalpoint in its forward travel the charge is fired and theprojectile speeds on its way. The kick or recoil serves toarrest the forward movement of the barrel and finally drives itback again against the strong spring or cushion of compressed airwithin the cylinder to its normal position, when it is ready forthe introduction of the next shell.

The outstanding feature of this system is that the projectile isgiven a higher initial velocity than is possible with the barrelheld rigid at the moment of discharge, because the shell isalready travelling at the moment of firing.

The fixed anti-aircraft guns such as are stationed upon eminencesand buildings are of the quick firing type, the object being tohurl a steady, con tinuous stream of missiles upon the swiftlymoving aeroplane. Some of the weapons throw a one-pound shelland are closely similar to the pom-pom which proved so effectiveduring the South African war. Machine guns also have beenextensively adopted for this duty by all the combatants, theirrange of approximately 2,000 yards and rapidity of fire beingdistinctly valuable when hostile aircraft descend to an altitudewhich brings them within the range of the weapon.

The greatest difficulty in connection with this phase ofartillery, however, is not so much the evolution of a serviceableand efficient type of gun, as the determination of the type ofprojectile which is likely to be most effective. While shrapnelis employed somewhat extensively it has not proved completelysatisfactory. It is difficult to set the timing fuse even afterthe range has been found approximately, which in itself is noeasy matter when the aircraft is moving rapidly and irregularly,but reliance is placed thereon in the hope that the machine mayhappen to be within the cone of dispersion when the shell bursts,and that one or more of the pieces of projectile and bullets maychance to penetrate either the body of the airman or a vital partof the mechanism.

It is this uncertainty which has led to a preference for a directmissile such as the bullet discharged from a machine gun. Astream of missiles, even of rifle calibre, maintained at the rateof some 400 shots per minute is certain to be more effective,provided range and aim are correct, than shrapnel. But theordinary rifle-bullet, unless the objective is within very closerange, is not likely to cause much harm, at least not to themechanism of the aerial vessel.

It is for this reason that greater attention is being devoted,especially by the French artillerists, to the Chevalieranti-aircraft gun, a weapon perfected by a Swiss technicianresident in Great Britain. It projects a formidable missilewhich in fact is an armour-piercing bullet 1/2- to 3/4-inch indiameter. It is designed for use with an automatic machinegun,which the inventor has devised more or less upon the well-knownFrench system. The bullet has a high velocity--about 2,500 feetper second--and a maximum range of 6,000 to 8,000 feet at themaximum elevation. Should such a missile strike the motor orother mechanism of the vessel it would wreak widespread havoc,and probably cause the machine to come to earth. This arm hasbeen designed for the express purpose of disabling the aeroplane,and not for the subjugation of the airman, which is a minorconsideration, inasmuch as he is condemned to a descent when hiscraft receives a mortal wound.

Attempts have been and still are being made to adapt an explosiveprojectile to this gun, but so far the measure of successachieved has not proved very promising. There are immensedifficulties connected with the design of an explosive shell ofthis class, charged with a high explosive, especially inconnection with the timing. So far as dependence upon percussivedetonation is concerned there is practically no difficulty. Should such a missile strike, say, the motor of an aeroplane, oreven the hull of the craft itself, the latter would bepractically destroyed. But all things considered, it isconcluded that more successful results are likely to be achievedby the armour-piercing bullet striking the mechanism than by anexplosive projectile.

The Krupp company fully reahsed the difficulties pertaining tothe projectile problem in attacks upon aerial craft. So far asdirigibles are concerned shrapnel is practically useless,inasmuch as even should the bag be riddled by the flyingfragments, little effective damage would be wrought--the craftwould be able to regain its haven. Accordingly efforts wereconcentrated upon the perfection of two new types of projectiles,both of which were directed more particularly against thedirigible. The one is the incendiary shell--obus fumigene--whilethe other is a shell, the contents of which, upon coming intocontact with the gas contained within the gas-bag, set up certainchemical reactions which precipitate an explosion and fire.

The incendiary shells are charged with a certain compound whichis ignited by means of a fuse during its flight. This fusearrangement coincides very closely with that attached to ordinaryshrapnel, inasmuch as the timing may be set to induce ignitionat different periods, such as either at the moment it leaves thegun, before, or when it strikes the envelope of the dirigible. The shell is fitted with a "tracer," that is to say, uponbecoming ignited it leaves a trail of smoke, corresponding withthe trail of a rocket, so that its passage through the air may befollowed with facility. This shell, however, was designed tofulfil a dual. Not only will it fire the gaseous contents out ofthe dirigible, but it has an explosive effect upon striking anincombustible portion of the aircraft, such as the machinery,propellers or car, when it will cause sufficient damage to throwthe craft out of action.

The elaborate trials which were carried out with the obusfumigene certainly were spectacular so as they went. Two smallspherical balloons, 10 feet in diameter, and attached to 1,000feet of cable, were sent aloft. The anti-aircraft gunsthemselves were placed about 5,1OO feet distant. Owing to theinclement weather the balloons were unable to attain a height ofmore than 200 feet in a direct vertical line above the ground. The guns were trained and fired, but the one balloon was not hituntil the second round, while the third escaped injury until thefifth round. When struck they collapsed instantly. Though thetest was not particularly conclusive, and afforded no reliabledata, one point was ascertained--the trail of smoke emitted bythe shell enabled its trajectory to be followed with ease. Uponthe conclusion of these trials, which were the most successfulrecorded, quick-firing tests in the horizontal plane were carriedout. The best performance in this instance was the discharge offive rounds in eight seconds. In this instance the paths of theprojectiles were simple and easy to follow, the flight of theshell being observed until it fell some 18,670 feet away. Butthe Krupp firmhave found that trials upon the testing ground witha captive balloon differ very materially from sterntests in thefield of actual warfare. Practically nothing has been heard ofthe two projectiles during this war, as they have proved anabsolute failure.

Some months ago the world was startled by the announcement thatthe leading German armament firm had acquired the whole of theinterest in an aerial torpedo which had been evolved by theSwedish artillerist, Gustave Unge, and it was predicted that inthe next war widespread havoc would be wrought therewith. Remarkable claims were advanced for this projectile, the foremostbeing that it would travel for a considerable distance throughthe air and alight upon the objective with infallible accuracy. The torpedo in question was subjected to exacting tests in GreatBritain, which failed to substantiate all the claims which wereadvanced, and it is significant to observe that little has beenheard of it during the present conflict. It is urged in certaintechnical quarters, however, that the aerial torpedo will proveto be the most successful projectile that can be used againstaircraft. I shall deal with this question in a later chapter.

During the early days of the war anti-aircraft artillery appearedto be a much overrated arm. The successes placed to its creditwere insignificant. This was due to the artillerymen beingunfamiliar with the new arm, and the conditions which prevailwhen firing into space. Since actual practice became possiblegreat advances in marksmanship have been recorded, and theaccuracy of such fire to-day is striking. Fortunately the airmanpossesses the advantage. He can manoeuvre beyond the range ofthe hostile weapons. At the moment 10,000 feet represents theextreme altitude to which projectiles can be hurled from the armsof this character which are now in use, and they lackdestructiveness at that range, for their velocity is virtuallyexpended.

Picking up the range is still as difficult as ever. The practicefollowed by the Germans serves to indicate the Teutonthoroughness of method in attacking such problems even if successdoes not ensue. The favourite German principle of disposing anti-aircraft artillery is to divide the territory to beprotected into equilateral triangles, the sides of which have alength of about six miles or less, according to the maximumeffective range of the pieces at an elevation of 23 1/2 degrees.

The guns are disposed at the corners of the triangles asindicated in Figs. 13-14. Taking the one triangle as an example,the method of picking up the range may be explained as follows. The several guns at the comers of the triangle, each of which canbe trained through the 360 degrees in the horizontal plane, arein telephonic touch with an observer O stationed some distanceaway. The airman A enters the area of the triangle. Theobserver takes the range and communicates with the gunner B, whofires his weapon. The shell bursts at 1 emitting a red flame andsmoke. The observer notes the altitude and relative position ofthe explosion in regard to the aircraft, while gunner B himselfobserves whether the shell has burst to the right or to the leftof the objective and corrects accordingly. The observer commandsC to fire, and another shell is launched which emits a yellowflame and smoke. It bursts at 2 according to the observer, whilegunner C also notes whether it is to the right or to the left ofthe target and corrects accordingly. Now gunner D receives thecommand to fire and the shell which explodes at 3 throws off awhite flame and smoke. Gunner D likewise observes whether thereis any deviation to right or left of the target and corrects in asimilar manner. From the sum of the three rounds the observercorrects the altitude, completes his calculations, andcommunicates his instructions for correction to the threegunners, who now merely train their weapons for altitude. Theobjective is to induce the shells hurled from the three cornersof the triangle to burst at a common point 4, which is consideredto be the most critical spot for the aviator. The fire is thenpractically concentrated from the three weapons upon the apex ofa triangular cone which is held to bring the machine within thedanger zone.

This method of finding the range is carried out quickly--two orthree seconds being occupied in the task. In the early days ofthe war the German anti-aircraft artillerymen proved sadlydeficient in this work, but practice improved their fire to amarvellous degree, with the result that at the moment it isdangerous for an aviator to essay his task within an altitude of6,000 feet, which is the range of the average anti-aircraft gun.

The country occupied by a belligerent is divided up in thismanner into a series of triangles. For instance, a machineentering hostile territory from the east, enters the triangleA-B-C, and consequently comes within the range of the guns postedat the comers of the triangle. Directly he crosses the line B-Cand enters the adjacent triangle he passes beyond the range ofgun A but comes within the range of the gun posted at D, andwhile within the triangular area is under fire from the gunsB-C-D. He turns and crosses the line A-C, but in so doing entersanother triangle A-C-E, and comes range of the gun posted at E.

The accompanying diagram represents an area of country divided upinto such triangle and the position of the guns, while the circleround the latter indicate the training arc of the weapons, eachof which is a complete circle, in the horizontal plane. Thedotted line represents the aviator's line of flight, and it willbe seen that no matter how he twists and turns he is alwayswithin the danger zone while flying over hostile territory. Themoment he outdistances one gun he comes within range of another.

The safety of the aviator under these circumstances depends uponhis maintaining an altitude exceeding the range of the gunsbelow, the most powerful of which have a range of 8,000 to 10,000feet, or on speed combined with rapid twisting and turning, orerratic undulating flight, rendering it extremely difficult forthe gun-layer to follow his path with sufficient celerity toensure accurate firing.

At altitudes ranging between 4,000 and 6,000 feet the aeroplanecomes within the range of rifle and machine-gun firing. Theformer, however, unless discharged in volleys with the shotscovering a wide area, is not particularly dangerous, inasmuch asthe odds are overwhelmingly against the rifleman. He is notaccustomed to following and firing upon a rapidly movingobjective, the result being that ninety-nine times out of ahundred he fails to register a hit. On the other hand theadvantage accruing from machine-gun fire is, that owing to thecontinuous stream of bullets projected, there is a greaterpossibility of the gun being trained upon the objective andputting it hors de combat.

But, taking all things into consideration, and notwithstandingthe achievements of the artillerist, the advantages areoverwhelmingly on the side of the aviator. When one reflectsupon the total sum of aircraft which have been brought to earthduring the present campaign, it will be realised that the numberof prizes is insignificant in comparison with the quantity ofammunition expended.

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Chapter XV : ANTI-AIRCRAFT GUNS. IMMOBILE WEAPONS

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