INTRODUCTORY
In an essay written in the first part of 1914 by Captain H. C. L. Cock, R. A., published in The Journal of the Royal Artillery, we read:
No limit can well be placed to the ultimate development of airships and aeroplanes as new discoveries are made and applied to their construction. To look too far ahead, to anticipate problems which we may some day have to face, but which are not yet with us, is only to complicate an already sufficiently difficult question; and it may well be that by thinking too much of the possibilities of the future we may overlook some of the actual problems of the present. . . .
For the present it may be said generally that troops in the field have little to fear from aircraft. The offensive power of airships will be employed against materiel rather than against personnel. These costly and vulnerable crafts will not be risked except to do decisive damage; they will not be used to drop a few bombs on troops below them. Aeroplanes have no means of attack or defence other than rifles, or possibly machine-guns, and these are intended principally if not solely for use against hostile aircraft. Bombs, if carried, are intended for the enemy’s airships. Aircraft are the eyes of an army, and as such are too valuable to be used for other tasks for which they are by no means so well qualified. It follows then that defence against aircraft requires no special consideration since the best defence will be provided by the best method of attacking them.
The present European war shows conclusively that Captain Cock was right in calling aircraft “the eyes of the army,” but he made a mistake in stating that aircraft will not be used against personnel. Aircraft are used against the materiel and personnel of the army and navy, and even against non-combatants. We have not to look ahead for anticipated problems—the fourth arm exists, it has come to stay and, although yet in its infancy, is already able to work terrible havoc, hence the necessity of sparing no effort to develop the efficiency of anti-aircraft guns for the navy as well as for the army.
The problem of firing against aircraft is not an easy one; the difficulties of hitting a moving target on the water are sufficiently great. There are not only the ordinary difficulties of line and range, but the speed of the target as well. In the case of an aircraft always moving in any plane the difficulties increase enormously; we might say they vary, not as the square, but as the cube of the factors. Later on we shall investigate those difficulties and we may propose a method of solving the problem, but the principal object of this paper is to settle for once and all the question of automatic compensating device, to correct the angle of elevation, used by some American and foreign builders.
THE ANTI-AIRCRAFT GUN AUTOMATIC COMPENSATING DEVICE
The automatic compensating device for elevation, used by some builders, rests on the principle that the angle of elevation for a given range must vary in inverse relation to the angle of position of the target: for an altitude zero, that is, the target being situated on the same horizontal plane as the gun, the angle of elevation is that given by the range table, while for the same range, the target being at the zenith of the gun, the angle of position being 90°, the angle of elevation should be zero.
It would not be an exaggeration to affirm that almost everybody that has anything to do with gunnery, and has not investigated thoroughly the problem of firing against aircraft, would say that such a device is absolutely necessary. For ourself, we have accepted for granted such a statement until the day we started the computation for the requisite of such a device, and then we discovered that the compensating arrangement is not only unnecessary, but that it introduces an error at the time the accuracy of the gun is most needed.
This being said, we shall now prove our statement:
- To convince the gun builder that he is justified to dispense with a complicated and very expensive apparatus.
- To give full confidence in the gun and its sight; to the gunner and, above all, to the battery commander, so that, when the target is missed, which will be often the case, he would not put the blame on the lack of compensating device, and would in conscience be compelled to look somewhere else for the cause of the miss,
To that effect the trajectories for every five degrees, from 10° to 85° for a 3-inch projectile fired with an initial velocity of 2100 f.-s., were-computed step by step. These trajectories have been plotted on cross-section paper and then, by interpolation, the trajectories for every degree were drawn. From the center o, muzzle of the gun, were described arcs of circles representing ranges in yards. A photographic reduction of the drawing, completed .by a protractor, is given at the end of these notes.
The notation to be used is:
R = range, distance, in yards, from the aircraft to the muzzle of the gun.
M = position of the aircraft at the time of firing.
M' = position of the aircraft at the time the projectile reaches the altitude of M, assuming that during the time of flight of the projectile the aircraft has flown in the plane of fire toward the gun.
M" = same as above, the aircraft flying in the opposite direction.
φ = angle of position,
α = angle of elevation.
Θ = angle of departure.
x = projection orthogonal of R on the axis of X.
y = projection orthogonal of R on the axis of Y.
t = time of flight of the projectile.
v = velocity of the aircraft in feet per second.
MW = tv, MM"— t'v.
v = 88 for 60 miles an hour.
v = 44 for 30 miles an hour.
PROOF
Assuming that the angle of departure corresponds exactly to the angle of position plus the corrected angle of elevation of the target at the time of firing, when the projectile will reach that point the aircraft will be away from it fit a distance equal to the product of the velocity of the flying machine per second by the time of flight of the projectile. The aircraft flying in any direction, it is clear that, at the time the projectile will arrive at the point aimed at, the machine will be somewhere on the surface of an imaginary sphere having the projectile for center and for radius a distance equal to the product mentioned above.
It would be an endless undertaking to try to analyze the problem in relation to the innumerable positions that the machine may occupy at the time the projectile reaches the point aimed at, but that is not necessary to show the difficulty of hitting a flying machine, and to demonstrate definitely the uselessness of a compensating device it will be sufficient to treat the problem in assuming that the machine is flying in a horizontal plane toward the gun or in opposite direction, at least during the time of flight of the projectile.
Let us at first explain Figs. 1-4 and the range tables.
Figs. 1 and 2 represent graphically the information given by the range table for 3000 yards.
Fig. 1 shows portions of trajectories from 10° to 24°. The heavy arc of circle represents the 3000-yard range from o to 3000 feet altitude, the thinner arcs are drawn 100 yards apart. The numbers near the trajectories 10°, 15° and 20° give the time of flight, that is, the position occupied by the projectile upon the trajectory at the given time.
N, intersection of the uncorrected trajectory with the horizontal of the altitude y.
MM', space traveled by the aircraft during the time of flight of the projectile from o to M' (o the gun); the aircraft traveling horizontally in the plane of fire toward the gun at a velocity of 60 and 30 miles an hour.
MM", the same as above, the aircraft going in the opposite direction.
RANGE TABLE
y = projection orthogonal of R (in feet) on the axis of Y.
x = projection orthogonal of R (in feet) on the axis of X.
φ = the angle whose tangent is y over x; it is the angle of position of a target situated at the altitude y and at a distance from the gun of 3000 yards.
α = the angle of elevation, from the range table, of a 3-inch projectile, the initial velocity being 2100 f. s., and the range 3000 yards.
Θ = the uncorrected angle of departure; it equals φ + α.
Θ1, = the corrected angle of departure; it is the angle of departure of the trajectory containing the point M, at the intersection of y and R.
Θ — Θ1, gives the amount of correction to be made by the compensating device.
MN, the two numbers have no relation to 30 and 60 miles; the number on the same line as the angle gives the distance from M, at which the uncorrected trajectory cuts the horizontal through j\I. The other number indicates the nearest distance the same trajectory passes from M.
MM' = distance in yards, which has been already defined.
R' = range corresponding to M'.
Θ' = angle of departure of the trajectory containing M'.
Θ' — φ = angle of elevation that should be used to strike M' while aiming at M.
MM", R", Θ" and Θ — φ, same as above except that the aircraft is flying in opposite direction.
Θ' — Θ" = variation in the angle of departure of M' and M".
Fig. 2 gives the variations in the angle of elevation to hit M or M"—the angle of position being that of M.
The vertical line a represents the angle of elevation used for altitude zero, which is constant when there is no compensating device.
The curve N represents the corrections, in minutes, to be made by the compensating device.
The curves M'-30 and M'-60 give the variation of the angle of elevation when the machine is flying toward the gun; M"-30, M"-60, the variation when the machine is flying away.
Following M'-60 we see that at o altitude the angle of elevation is smaller than α, which is clear since R becomes R — vt, while φ is always 0. But when φ increases from 0 to φ of 3000 feet, though R' is always smaller than R, the angle of elevation varies from α — 0° 19' to α +0° 48.9'. For M'-30 we have α +0° 9.6' to α + 0° 18.5'.
Following M"-60 at altitude o the angle of elevation is greater than α, which is right, since R becomes R + vt and φ is 0. But when φ increases from 0 to φ of 3000 feet, though R" is always greater than R, the angle of elevation varies from α + 0° 23.3' to α —1° 00', and for M"-30 the variation is α +0° 11.2' to α —0° 33.9’.
Naturally all that we have said for 3000 yards range applies equally to 2500, 2000, 1500 and 1000 yards.
We have now all that is necessary to treat thoroughly the problem. We will analyze one firing at 3000-yard range and 3000- foot altitude, and one at 1000-yard range and 2500-foot altitude.
Let us take again the range table for 3000 yards with Figs. 1 and 2.
Assuming that the point M at 3000-foot altitude represents an aircraft traveling horizontally, in the plane of fire, toward the gun at a velocity of 60 miles an hour. The range and altitude being exactly known, the firing is made with an angle of departure of 22° 49' or 22° 55.2', according as the angle of elevation is or is not corrected. Everything happening as expected, the projectile will reach the point M after 6.5 seconds; during the same period the aircraft has also traveled and at that time it is 572 feet or 191 yards from the point M.
Before going further we have to give a short word of explanation. For the range table we have made the computation, not with the time of flight to M, but with the time of flight on the trajectory meeting the aircraft after it has traveled from M, in the stated conditions, during a period equal to the time of flight of the projectile. Fig. 1 shows clearly that the time of flight, for 60-mile velocity, is not 6.5, but about 6 seconds. This explains why the range table gives 176 yards instead of 191 and it explains also the difference between MM' and MM".
Now let us go back to our problem. The round being fired with an angle of departure of 22° 49' or 22° 55.2', the projectile passes either at M or N while the aircraft is at M'. Fig. 1 shows that M' is farther from M than from N, and consequently the compensating device, instead of bringing the projectile nearer the target, sends it farther away.
The angle of departure to hit M' is 23° 44.1'; the angle of position being 19° 28.3', since the gun was aimed at M, consequently to hit M' the angle of elevation should have been 4° 15.8' instead of 3° 20.7' or 3° 26.9', and we see that the compensating device works in the wrong way, since the difference between 4° 15.8' and 3° 20.7’ is greater than the difference between 4° 15.8' and 3° 26.9'.
When the machine is flying away from the gun the point is M". The angle of departure for M"-60 miles is 21° 55.2', the angle of position is 19° 28.3', since the gun is still aimed at M, hence to hit M" the angle of elevation should be 2° 26.9' instead of 30 20.7' or 3° 26.9'.
Let us take now the range table for 1000 yards and Figs. 3 and 4.
The point M at 2500-foot altitude represents an aircraft traveling horizontally, in the plane of fire, toward the gun at a velocity of 60 miles an hour. The range and altitude being exactly known, the firing is made with an angle of departure of 56° 50.6' or 57° 11.8', according as the angle of elevation is or is not corrected. Everything happening as expected, the projectile passes at M or N when the machine is at M'. Fig. 3 shows that M' is farther from M than from N, and again, as with the 3000-yard range, the correction is made in the wrong direction. The angle of departure to hit M' is 58° 54.3', the angle of position being 56° 26.5', since the gun was aimed at M, consequently to hit M' the angle of elevation should have been 2° 27.8' instead of 0° 24.1' or 0° 45.3'; we see once more that the compensating device works in the wrong way, since 0° 24.1' is the corrected angle. The same thing can be verified for every range at any altitude, hence the assertion, made previously, that the automatic compensating device works in the wrong way at the time the accuracy of the gun is most needed, is well proved.
When the machine is flying away from the gun the point is M". The angle of departure for 60 miles is 54° 44', the angle of position is 56° 26.5', since the gun is still aimed at M, hence to hit M" the angle of elevation should be —1° 42.5' instead of 0° 24.T or 0° 45.3'.
Figs. 3 and 4 represent graphically the above reasoning, and the range tables of 2500, 2000 and 1500 yards give the variations between those limits.
The conclusion is that it is impossible to know, with any degree of accuracy, the angle of elevation that should be used to hit an aircraft, even should the range and altitude be known exactly at the time of firing.
In those conditions it would be, certainly, senseless to build a complicated and expensive apparatus to make a ridiculously small correction, correction that cannot be computed without making a false assumption and, besides, a correction that the automatic device would make in the wrong direction at the time the accuracy of the gun is most needed.
ANTI-AIRCRAFT PROBLEM
The question of automatic compensating device for elevation being set aside for good, we shall now propose a method of attack and defence against aircraft. Naturally a problem of this kind, which, when assumed, even in its simplest form presents difficulties that seem insuperable, can be treated in various ways, so we do not have the pretentions of giving the proposed method as the only one; from our view-point and from the limited data at our disposal, we think it applicable and as simple as the question permits for the present, but our aim is above all to present one solution of the problem, as far as the defence of the navy is concerned, and to offer material for discussion and improvements.
We do not expect to treat the question exhaustively; many points will be touched just enough to demonstrate the necessity of studying them thoroughly.
To attack an enemy, or to protect oneself against his blows, with any chance of success, the first requisite is to know him, to ascertain his strong and weak points and to discover his aim.
The armies and navies of the world have now at their disposal three classes of aircraft, which are more or less effectively used for offensive, as well as for defensive, purposes. These three classes of aircraft are the airship, the aeroplane and the seaplane.
The airship, as it is known, derives its lifting power from the hydrogen gas enclosed within one or more gas-tight bags or envelopes. If we remember that it requires about 13 cubic feet of hydrogen to lift one pound of weight into the air, we can easily realize that airships, if they have to carry a numerous crew besides the requisite machinery, must of necessity be very bulky. The latest Zeppelins are 525 feet long with a beam of 46 feet in diameter and a displacement of 820,000 cubic feet. The greatest size of the airship is naturally a disadvantage; it is very visible by day, at the ranging distance of an anti-aircraft gun it makes a good target; besides, it is comparatively easily destroyed—a single hit, by a suitable shell, on the envelope would probably ignite the gas inside and cause the complete destruction of the whole vessel.
Aeroplanes and seaplanes, on the other hand, deriving their sustentation from the action of the air on their wings, are much smaller, more compact and stronger than airships; they are comparatively small and by no means easy to see, their constant and rapid motion, in vertical as well as horizontal plane, tends to make the range and altitude taking, the application of fire and observation very difficult.
In the present war, airships and aeroplanes are used indiscriminately for scouting and bomb dropping on land and harbor. Up to the present we have not heard of any attempt made against war ships, but such a thing must be expected at any time. Very likely aircraft will not undertake to attack singly a squadron, especially if, as we suppose, most of the ships are equipped with guns that can fire at a great angle of elevation. The target is too small; to drop a bomb on a ship with any chance of success would necessitate to fly very low and consequently to be too much exposed to the firing of an enemy relatively free. But when a squadron is actively busy against foes on the surface and under the water, it is clear that conditions would be changed, and if the ships are not especially prepared against aircraft, any daring air pilot would have a chance to do some damage. That assumption, which may become an accomplished fact before long, proves that it is of necessity now to equip fleets with anti-aircraft guns.
ANTI-AIRCRAFT GUN VELOCITY
The initial velocity of the projectile should be the greatest possible. Figs. 1-4 show that the displacement of the aeroplane during the time of flight of the projectile is function of that time; the shorter the time of flight the smaller the displacement; the shorter the time of flight the greater the number of rounds that can be fired during the time the target is in sight, time that will never be long, as the following example will show. We never had the opportunity of testing ourself at what distance an aeroplane becomes visible. Some writers say that an aeroplane is barely visible three miles away. Let us assume that an aeroplane is discovered at 6000 yards; if flying toward the observer at a rate of 60 miles an hour it will be overhead in 3.4 minutes; if flying at the rate of 100 miles an hour the time is reduced to 2 minutes.
Should an airman wish to drop a bomb on a ship he is likely to travel at a high altitude and great velocity until he reaches the point where he must come down in altitude and velocity to accomplish his mission. It would not be wise to count too much on hitting the machine at that time, because though the range will be reduced the variation in angle of departure will be greater. To make this clear, let us look at the range table of 3000 yards and that of 1000 yards, for the 2500-foot altitude. In the former table we have Θ' — φ = 4° 5.9', and in the latter Θ' — φ = 2° 27.8'; 4° 5-9'-3° 26.9'= 0° 39'; 2° 27.8' —0° 45-3'= 1° 42-5'- Hence if firing at 3000 yards, with the angle of elevation of 3000 yards, we have an error of 0° 39', while if firing at 1000 yards, with the angle of elevation of that range, we have an error of 1° 42.5'; even should the machine drop to 1500-foot altitude, the error would be still 1° 14.7'. This shows that the farther the machine the smaller the error in the angle of departure and the greater the chance of hitting the machine; consequently, the time of 3.4 minutes should be reduced in notable proportion if we wish to have a chance of stopping the machine before it accomplishes its purpose. The very short time at our disposal proves clearly that the velocity of the projectile should be as great as possible, and it proves also that the use of complicated instruments entailing intricate calculations are quite out of place in the present problem, and we estimate that a few rough and ready rules would be more practical.
CALIBER
The caliber of the gun, as well as other points, offers a good field for discussion. Some artillerists propose a small caliber— 1 inch or not more than 2 inches—and they would use a handy quick-firing gun to be able to maintain a rapid rate of firing; others, and we are of that number, would not use anything smaller than a 3-inch projectile.
We have already stated that to fire with percussion fuse against aircraft is time and money lost. If the percussion fuse is not used, we do not see the necessity of a quick-firer; there cannot be much difference in the rate of firing between the 3- and 2-inch guns, when firing time fuse, unless many fuses are set in advance at an estimated time.
Though the period of time for firing may, in many cases, be very short, is a great rate of fire very desirable? Assuming that we are firing at 3000 yards, Fig. 1 shows that the projectile must travel between 6 or 7 seconds before bursting; then if the range or time estimation is wrong, all the rounds fired with the same time, before a correction is made, will be lost unless, by chance, the aircraft is traveling in a direction that will bring it in the path of one of those projectiles.
Anti-aircraft projectiles should carry “ tracers ”—the smoke trails of which being, for the present, the only means of observation of fire. We fail to see how a small 2-inch shrapnel of about 5 pounds can have sufficient capacity to carry a tracer that should burn at least 7 or 8 seconds and contain at the same time a charge of explosive large enough to burst the projectile.
The advantages we can see in the use of a small caliber are in the facility of handling light fixed ammunition in great quantities in a short time and in the greater number of rounds in a given weight.
From the little we hear from the seat of war, we can take for granted that the rifle is a failure against the flying machine, and that the anti-aircraft gun is not much of a success up to the present. That is not surprising if the methods of firing are the application of what we have read in the technical papers on the subject.
In a previous essay (not published), using a single gun, we have proposed a method of firing which might be practical against airships, but which is neither simple enough nor quick enough to be of great value against aeroplanes. We have since arrived at the conclusion that to succeed against aeroplanes we must adopt an entirely different method; one which discards both chart and computation at the time of firing.
PROPOSED METHOD
Single shot should never be fired against aircraft; a salvo of four rounds should be a minimum. Double gun mount should be used; the two guns having a permanent angle of divergence of 1 degree in elevation and 20 minutes in deflection. The divergence of the two guns, the variation in the aim of the two pointers, the dispersion of the projectiles and the irregularity in the working of the time fuses, would insure the bursting of the four projectiles in a space in which the aircraft will be more exposed than if 12 rounds were fired successively at it.
The second salvo to be fired only after the battery commander has made the corrections suggested by the first one, and so on for the other salvos.
The sighting apparatus should be made in such a way that, by a simple arrangement, the angle of elevation could be that of the near gun or the far gun. We call the near gun the one which, when placed horizontally, has the other making with it an angle of elevation of I degree; if fired in that position the near gun will fire point blank and the far gun with I degree of elevation. Then if the machine is flying toward the gun the telescope will be set in relation to the near gun, and when flying away, in relation to the far gun.
To show clearly the simplicity of the method and to demonstrate all its advantages we shall give an example. An aeroplane is discovered flying toward the battery, the telescope is set for the near gun, the battery commander decides to fire when the machine will be at 3000 yards, he gives the exact angle of elevation of 30 26.9', the fuses are set at 6 seconds (see range table for 3000 yards and Fig. 1). On account of the arrangement of the two guns the battery commander has not to worry about the altitude of the aircraft; he has not even to think of it.
Assuming the aircraft to be at 3000 yards at the time the salvo is fired, the aircraft will be somewhere between two M' when the projectiles burst. The distance between M and the nearest M' is that covered, during the time of flight of the projectiles, by the aeroplane traveling 30 miles per hour and the distance between M and the farthest M' being that of 60 miles per hour.
Now let us assume the machine at an altitude of 3000 feet at the time of firing; the pointers give unconsciously the correct angle of position, and the angle of departure of the projectile from the near gun is 22° 55.2' and that of the far gun 23° 55.2'. When the projectiles reach 3000-foot altitude the machine is somewhere between the paths of 23° 13.7' and 23° 44.1' trajectories, consequently in the midst of the bursting projectiles if the fuses work at the proper time.
If the aeroplane is at 2500-foot altitude the angle of departure of the near gun is 19° 34.6', that of the far gun 20° 34.6', and the machine is between the trajectories of 19° 50' and 20° 13.6'.
If the aeroplane is at 2000-foot altitude the angle of departure of the near gun is 16° 17.3', that of the far gun 17° 17.3', and the machine is between the trajectories of 16° 28.3' and 16° 45.9'.
If the aeroplane is at 1500-foot altitude the angle of departure of the near gun is 13° 2.5', that of the far gun 14° 2.5', and the machine is somewhere between 13° 9.6' and 13° 19.4'.
This shows clearly that if an aircraft, traveling toward the battery, is at or near 3000 yards at the time of firing, no matter its altitude, it will be well covered by the bursting projectiles, if the fuses work at the right time.
If the aeroplane is flying away, the telescope is set for the far gun.
At 3000-foot altitude the angle of departure of the far gun is 22° 55.2', that of the near gun 21° 55.2', and the machine is somewhere between the trajectories of 22° 21.3' and 21° 55.2’.
If the aeroplane is at 2500-foot altitude, the angle of departure of the far gun is 19° 34.6', that of the near gun 18° 34.6', and the machine is between 19° 8.5' and 18° 48', and so on for 2000- and 1500-foot altitude. Consequently, what we have said above, for the machine flying toward the battery, is true also for the machine flying away.
Analyzing in the same way the 2500-yard range we find the same result as above, no matter the altitude, for the 2000-yard range we find the same up to 2500-foot altitude, and for the 1500-yard range up to 2000-foot altitude.
From that example we see clearly how simple and practical is the proposed method. The range being known, the target can be easily straddled, and probably hit, in discarding completely the perplexing question of the target’s altitude. This question of altitude has already caused much discussion, much expense for the construction of useless devices, and is still a stumbling block for many.
We have seen that at 3000-yard and 2500-yard ranges, no matter the altitude at which the aircraft is flying, if it is fired at with the angle of elevation corresponding to the range at the time of firing in all probability the four projectiles will burst around the aircraft. It is the same for 2000-yard range up to 2500-foot altitude, and for 1500-yard range up to 2000-foot altitude. Should we draw a straight line from 1500-yard range 2000-foot altitude to 2000-yard range 2500-foot altitude, and produce it to 3000-foot altitude, we see what a large portion of the chart is covered by the 1 degree divergence of the guns. From now, for clearness sake, we shall call that space the “ critical space.” It is evident that, had the chart been drawn for 6000-yard range and 6000-foot altitude, the 3000-yard range would not be entirely in the critical space, but probably some of the ranges would be in that space from o to 6000-foot altitude, and it is clear that the best time to hit will be when the aircraft is in that space.
If the method is adopted, the ballistician who will compute the range table will have to draw the critical space on the chart, and the battery commander will have to study it well to know, by memory, the ranges for which he has no correction to make.
The only instrument necessary, assuming that the range-finder of the ship is used for other targets, is a good, quickly operated range-finder, with a large field of vision, worked by a single man if possible.
The sighting apparatus should be so constructed that the pointer, while remaining seated on the carriage, could follow the target at all angles of elevation. To increase the rate of firing a special arrangement could be made to disconnect the guns from the sighting apparatus, so, when firing at a great angle of elevation, the guns could be loaded in a convenient position while the pointer keeps aiming at the aircraft.
To sum up the advantages of the proposed method:
- Great probability of sending at least one projectile of the salvo through the 525 x 46-foot airship, should it come within the ranging distance of the guns.
- Great probability of bringing down from the critical space in a few salvos any aircraft, and almost a certainty of at least preventing them from crossing a given zone.
- Great economy in ammunition. We can state conservatively that one salvo of four projectiles will be fully worth 12 rounds fired consecutively.
- Great simplicity of the sighting apparatus.
- During firing there is no need of chart, transit or any other cumbersome instruments, and no computation.
- The duty of the battery commander, instead of being a Chinese puzzle, becomes no more difficult than in direct firing.
Should the method be adopted, an instruction would be written covering all the various particularities of the problem. We would describe a practical method of target practice, which would enable the anti-aircraft crew to acquire the necessary experience. This we think could be accomplished in conditions almost similar to the reality and at a nominal expense.