Chapter XIV : ANTI-AIRCRAFT GUNS. MOBILE WEAPONS

When the airship and the aeroplane became accepted units of warfare it was only natural that efforts should be concentrated upon the evolution of ways and means to compass their destruction or, at least, to restrict their field of activity. But aircraftappeared to have an immense advantage in combat. They possessvirtually unlimited space in which to manoeuvre, and are able toselect the elevation from which to hurl their missiles ofdestruction.

There is another and even more important factor in their favour. A projectile fired, or even dropped, from a height, say of 5,000feet, is favourably affected by the force of gravity, with theresult that it travels towards the earth with accumulating energyand strikes the ground with decisive force.

On the other hand, a missile discharged into space from a weaponon the earth has to combat this action of gravity, whichexercises a powerful nullifying influence upon its flight andvelocity, far in excess of the mere resistance offered by theair. In other words, whereas the projectile launched fromaloft has the downward pull of the earth or gravitational forcein its favour, the shell fired from the ground in the reversedirection has to contend against this downward pull and itsdecelerating effect.

At the time when aircraft entered the realms of warfare verylittle was known concerning the altitudes to which projectilescould be hurled deliberately. Certain conclusive informationupon this point was available in connection with heavy howitzerfire, based on calculations of the respective angles at which theprojectile rose into the air and fell to the ground, and of thetime the missile took to complete its flight from the gun to theobjective. But howitzer fire against aircraft was a sheerimpossibility: it was like using a six-inch gun to kill a fly ona window pane at a thousand yards' range. Some years ago certainexperiments in aerial firing with a rifle were undertaken inSwitzerland. The weapon was set vertically muzzle upwards anddischarged. From the time which elapsed between the issue of thebullet from the muzzle until it struck the earth it was possibleto make certain deductions, from which it was estimated that thebullet reached an altitude of 600 feet or so. But this wasmerely conjecture.

Consequently when artillerists entered upon the study of fightingair-craft with small arms and light guns, they were compelled tostruggle in the dark to a very pronounced extent, and thisdarkness was never satisfactorily dispelled until the presentwar, for the simple reason that there were no means of gettingconclusive information. The German armament manufacturersendeavoured to solve the problem by using smoking shells ormissiles fitted with what are known as tracers. By following theascensional path of the projectiles as revealed by the smoke itwas possible to draw certain conclusions. But these were by nomeans convincing or illuminating, as so many factors affected theissue.

Despite the peculiar and complex difficulties associated with theproblem it was attacked some what boldly. In this trying fieldof artillery research the prominent German armamentmanufacturers, Krupp of Essen and Ehrhardt of Dusseldorf, playeda leading part, the result being that before the airship or theaeroplane was received within the military fold, theanti-aircraft gun had been brought into the field of appliedscience. The sudden levelling-up serves to illustrate theenterprise of the Germans in this respect as well as theirperspicacity in connection with the military value of aircraft.

Any gun we can hope to employ against aircraft with some degreeof success must fulfil special conditions, for it has to dealwith a difficult and elusive foe. Both the lighter-than-air andthe heavier than-air craft possess distinctive features andvarying degrees of mobility. Taking the first-named, thefacility with which it can vary its altitude is a disconcertingfactor, and is perplexing to the most skilful gunner, inasmuch ashe is called upon to judge and change the range suddenly.

On the other hand, the artilleryman is favoured in certaindirections. The range of utility of the airship is severelylimited. If its avowed mission is reconnaissance and conclusiveinformation concerning the disposition of forces, artillery andso forth is required, experience has proved that such work cannotbe carried out satisfactorily or with any degree of accuracy at aheight exceeding 5,000 feet, and a distance beyond six miles. But even under these circumstances the climatic conditions mustbe extremely favourable. If the elements are unpropitious theairship must venture nearer to its objective. These data werenot difficult to collect, inasmuch as they were more or lessavailable from the results of military observations with captiveballoons, the conditions being somewhat similar. With theordinary captive balloon it has been found that, in clearweather, a radius of about 3 3/4 miles at the maximum elevationconstitutes its range of reliable utility.

With the aeroplane, however, the conditions are very dissimilar. In the first place the machine owing to its diminutive size ascompared with the airship, offers a small and inconspicuoustarget. Then there is its high independent speed, which is farbeyond that of the airship. Furthermore its mobility is greater. It can wheel, turn sharply to the right or to the left, andpursue an irregular undulating flight in the horizontal plane,which renders it well nigh impossible for a gunner to pick it up. The machine moves at a higher relative speed than that at whichthe gun can be trained. It is the rapid and devious variationwhich so baffles the gunner, who unless he be highly skilled andpatient, is apt to commence to fire wildly after striving for afew moments, and in vain, to pick up the range; he trusts to luckor depends upon blind-shooting, which invariably results in awaste of ammunition.

A gun, to be of tangible destructive efficiency when directedagainst aircraft, especially those depending upon the gas-bag forequilibrium, must be of special design. It must be capable offiring at an angle only a few degrees less than the absolutevertical, and in order to follow the rapid and involved movementsof its objective, must be so mobile that it can be trainedthrough a complete circle at any angle of inclination less thanits maximum. At the same time, if the weapon is being used infield operations it must be mounted upon a carriage of adequatemobility to enable it to follow the airship, and thereby keeppace with the latter, so that the aerial craft may be sorelyharassed if not actually hit. The automobile is the obviousvehicle for this duty, and it has accordingly been extensivelyused in this service.

The automobile and the gun mounted thereon follow widelydifferent lines. Some vehicles are designed especially for thisduty, while others are improvisations, and be it noted, inpassing, that many of the latter have proved more serviceablethan the former. Still, the first-named is to be preferred,inasmuch as necessarily it is designed to meet the all-roundrequirements imposed, and consequently is better able to stand upto the intended work, whereas the extemporised vehicle is onlyserviceable under favourable conditions.

The Krupp Company has evolved many designs of anti-aircraftmotor-driven guns--"Archibalds" the British airmen term them withemphatic levity. They are sturdily-built vehicles fitted withheavy motors, developing from 40 to 50 horse-power, with thechassis not widely dissimilar from that adopted for motor-omnibustraffic. Consequently, they are not necessarily condemned to thehigh-roads, but within certain limits are able to travel acrosscountry, i.e., upon fields or other level expanses, where thesoil is not unduly soft.

But the very character of the problem rendered the evolution ofthe vehicle a somewhat perplexing matter. There were manyfactors which had to be taken into consideration, and it waspossible to meet the imposed requirements only within certainlimits. In the first place, the weight of the gun itself had tobe kept down. It was obviously useless to overload the chassis. Again, the weight of the projectile and its velocity had to beborne in mind. A high velocity was imperative. Accordingly, aninitial velocity varying from 2,200 to 2,700 feet per second,according to the calibre of the gun, was determined.

Moreover, as mobility was an indispensable condition, the gun hadto be so mounted that it could be fired from the motor-car evenif the latter were travelling at high speed. This requiremententailed another difficulty. The gun had to be mounted in such amanner as to enable the gunner to train it easily and readilythrough the complete circle and through its complete range ofvertical inclination. As the result of prolonged experiments itwas ascertained that the most suitable arrangement was a pedestalmounting, either within a turret or upon an open deck. To meetthe weight of the gun, as well as the strains and stressesincidental to firing, the chassis was strengthened, especially over the rear axle near which the mounting is placed.

The heaviest gun of this type is the 10.5 centimetre (4 1/4-inch)quick-firer, throwing a shell weighing nearly forty pounds, withan initial velocity of 2,333 feet per second. This "Archibald"is totally unprotected. The gun is mounted centrally upon thecarriage over the rear axle, and occupies the centre of the deckbetween the driver's seat and that of the gun crew behind. Thewhole of the deck is clear, thereby offering no obstruction tothe gunner in training the weapon, while the space may be widenedby dropping down the wings of the vehicle. At the rear is a seatto accommodate the gun crew, beneath which the ammunition isstowed. When travelling and out of action, the gun lieshorizontally, the muzzle pointing from the rear of the car.

To reduce the strains arising from firing, the arm is fitted withwhat is known as the "differential recoil." Above the breach isan air recuperator and a piston, while there is no hydraulicbrake such as is generally used. The compressor is kept undercompression while the car is travelling with the gun out ofaction, so that the arm is available for instant firing. This isa departure from the general practice in connection with suchweapons. When the gun is loaded the bolt which holds thecompressor back is withdrawn, either by the hand for manualfiring, or by the action of the automatic closing of thebreech when the arm is being used as a quick-firer. In firingthe gun is thrown forward under the pressure of the released airwhich occurs at the moment of discharge. The energy of therecoil brings the gun back and at the same time recharges thecompressed air reservoir.

The gun is so mounted upon its pedestal as to enable a maximumvertical inclination of 75 degrees to be obtained. The mountingsystem also enables the weapon to be trained in any desireddirection up to the foregoing maximum elevation throughout acomplete circle, and it can be handled with ease and celerity. Asmaller "Archibald" is the 7.5 centimetre (3-inch gun) throwing a14.3 pound shell at an initial velocity of about 2,170 feet persecond.

The turret anti-aircraft gun carried upon a motor-car differsfrom the foregoing very considerably. This is a protected arm. The gun of 7.1 centimetres--approximately 2.75 inches--is mountedin the same manner upon the car-deck and over the driving axle,but is enclosed within a sheet steel turret, which is proofagainst rifle and machine-gun fire. This turret resembles theconning-tower of a battleship, and is sufficiently spacious tohouse the whole of the gun crew, the internal diameter beingabout seven feet. Access to the turret is obtained through arear door. This gun has a maximum elevation of about 75 degrees,while its operation and mechanism are similar to those of theunprotected weapon.

The vehicle itself is practically identical with the armouredmotor-car, which has played such an important part during thepresent campaign, the driver being protected by a bullet-proofsteel screen similar in design to the ordinary glass wind-screenfitted to touring automobiles. This is carried sufficiently highto offer complete protection to his head when seated at thewheel, while through a small orifice in this shield he is able toobtain a clear view of the road. The engine and its vital partsare also adequately protected. The ammunition is carried in acupboard-like recess forming part of the driver's seat, encasedin bullet-proof steel sheeting with flap-doors. This deviceenables the shells to be withdrawn readily from the side of thecar and passed to the crew within the turret. The caisson is ofsufficient dimensions to receive 69 shells.

The Ehrhardt airship fighting ordnance is similarly adapted tomotor-car operations, one type being especially powerful. Thewhole of the vehicle is encased in armour-plating impervious torifle and machine-gun fire. The driver is provided with a smallorifice through which he is able to obtain a clear uninterruptedview of the road ahead, while the armouring over the tonneau iscarried to a sufficient height to allow head-room to the gun crewwhen standing at the gun. All four wheels are of the disk typeand fashioned from heavy sheet steel. The motor develops 40-50horse-power and, in one type, in order to mitigate the risk ofbreakdown or disablement, all four wheels are driven. The gun, asmall quick-firer, is mounted on a pedestalin a projectingconning-tower. The mounting is placed behind the driver's seat,and is trained and operated from the tonneau. The maximumelevation is 75 degrees, and like the gun carriage bearing thetube guide it can be moved through a complete circle, being freeto rotate in the fixed pivot jack to enable this end to beattained.

The foregoing may be said to represent the most powerful types ofmobile anti-aircraft weapons used by the Austro-German forcesto-day. Arms of similar design, roughly speaking, have also beenintroduced into the French and Russian services. In additionmany semi-armoured weapons of this character are in operation,some specially built for the work, while others have beenimprovised. In the semi-armoured motor-car the carriage followsthe usual lines; it has an open top, the armouring comprising thebody of the tonneau and the diskwheels, which are made of lightbullet-proof steel. Here again the prevailing practice is tomount the gun as nearly above the rear axle as possible, andto work it from the tonneau. The maximum elevation is also 75degrees, with training throughout the entire circle.

Another type comprises a very light machine gun of rifle calibre,and this is intended for attachment to an ordinary motor car. There is a pedestal mounting which can be set within the tonneau,while the weapon is pivoted in an outrigger, the latter beingfree to rotate in its pivot jack. This arrangement enables thearm to cover a wide range,while it also admits of trainingthrough an extensive angle of elevation.

The Allied forces improvised travelling anti-aircraft offencesby mounting the latest types of Vickers, Hotchkiss, and othermachine guns in armoured motor cars. Some of these have thedomed turret form, with the gun projecting through the roof,while others are protected against hostile attack from the sideonly, the carriage being panelled with bullet-proof steelsheeting. While such weapons are useful, inasmuch as they canmaintain a hot fire ranging up to 750 shots per minute, they arenot to be compared with the "Archibalds," which are able to throwheavy shrapnel and incendiary shells, and have a vertical rangeof about 6,000 to 8,000 feet.

The improvised motor-gun has not proved a complete success,except in those instances when the hostile aircraft has venturedto approach somewhat closely to the ground. The more formidableweapons cannot be mounted upon ordinary vehicles, inasmuch as theincrease in weight, which is appreciable, impairs the efficiencyof the vehicle, and at the same time enhances the possibility ofbreakdown at a critical moment. For such arms a special andsubstantial chassis is imperative, while the motive power andgearing must be adapted to the circumstances.

Motor-mounted anti-aircraft weapons, however, have not proved anunqualified success. The fact that the vehicles are condemned tothe high roads, or at least to comparatively smooth and levelground, constitutes a severe handicap. Again, when travelling athigh speed, and this is essential when pursuing a fast aeroplane,the accurate laying of the weapon is extremely difficult, owingto the oscillation of the vehicle itself, especially if the roadsurface is in a bad condition. The sighting arrangements are ofa wonderfully complete character, as described elsewhere, but theirregular rolling movement arising from high speed is anullifying quantity. It is tolerably easy for the aircraft,especially an aeroplane, to evade successful pursuit, either byrising to an elevation beyond the range of the gun, or bycarrying out baffling evolutions such as irregular undulatingflight, wheeling, and climbing. According to the reports of theBritish and French airmen the "Archibald" has failed to establishthe glowing reputation which was anticipated, for the simplereason that, unless it has a clear straight road and can maintainits high speed, it can easily be out-distanced by the fleet humanbird.

The motor-car suffers from another serious disability. It cannotmanoeuvre with sufficient celerity. For instance, if it isnecessary to turn round in a narrow lane, valuable time is lostin the process, and this the airman turns to account. In hillycountry it is at a still greater disadvantage, the inclines,gradients, and sinuosities of the roads restricting itseffectiveness very pronouncedly. It must also be rememberedthat, relatively speaking, the "Archibald" offers a better targetto the airman than the aeroplane offers to the man behind theanti-aircraft gun on the motor below. A few well-placed bombsare sufficient to induce the pursuers to cease their activities. Even if the missiles fail to strike the motor-car itself they canwreak disaster in directly by rendering the road impassable ordangerous to negotiate at high speed. On the whole therefore,the "Archibald" is a greatly exaggerated weapon of offenceagainst aircraft, and, so far as is known, has failed to fulfilexpectations. In fact, the Germans have practically abandonedthe idea of using it in the manner of a pursuing arm; they workthe weapon as a fixture, depending upon the car merely as a meansof moving it from point to point. Thus, in reality, it has beenconverted into a light field-piece, and may almost be included inthe category of fixed weapons for combating aerial operations.

[ << Back to Aeroplanes and Dirigibles of War]

Chapter XIV : ANTI-AIRCRAFT GUNS. MOBILE WEAPONS

Categories