Artillery Ammunition Design
It was a sport on a staminate specimen, and on account of its striking habit was at once propagated, and has since spread over all countries of the globe. This was in the first decade of the eighteenth century, and the tree has, under all conditions of soil and- climate, retainbd its peculiar char- acteristics unchanged through more than two centuries. The story of its appearance in continental Europe is as follows: “A mer- chant in northern Europe received a: shipment of fruit from Italy, packed in willow-twig baskets.
The merchant noticed that some of the twigs had a very light gray bark. A willow of this color he had never seen before. On close examination he found that the bark was yet green and the buds very little shriveled, so he carefully unwound the baskets and made cuttings of the newly discovered willow. Some of them grew, and proved to be a new and& interesting poplar.” From this small start the tree spread rapidly and soon appeared all over Europe, and finally, in less than one hundred years after its first discovery in Italy, was introduced in America. The striking contrast with other trees and its usefulness in the variation of the sky line made it a desirable material for group and specimen plantings in parks and gardens, and soon was extensively propagated by nurserymen. In 1872 I saw beautiful, large and healthy specimens in Pennsylvania and New Jersey.
Some of them were at least forty years old, perfectly sound and uninjured by wind and cold. But trees I planted in the Topeka parks since 1900 showed unmistakable signs of decay when not more than ten years old, and in a few years more commenced to die and break off. In examining the young trees when a year old I find that about 10 per cent have not covered the base cut with callus. The cutting turned black about an inch up and a few roots formed from the glands in the bark. By dissecting the plants I found that the decay had followed up the pith the full length of the cutting. The branches and roots were thin and the leaves smaller than those of the healthy plants.
Where great care is not exercised in selecting the cuttings it will be seen that this weakness or disease will be inherited by all the descendants of the weak ancestor. The only way to produce a healthier race of this valuable tree is by care- ful selection, using the healthiest wood of the healthiest trees, making both the upper and lower cut smooth, by using a sharp knife. Dip the upper cut in oil paint to exclude air and moisture, and protect the lower cut by rubbing powdered charcoal well over the surface, and if possible plant them in a sandy soil.
Problems in Artillery Ammunition Design.
Because of the very wide field covered by the subject as assigned to me (Scientific Engineering Problems in Ordnance Manufacture), I am limiting myself to one phase only of the subject, as indicated in the title of the paper; and the treatment of this phase is necessarily incomplete because of the limited time at my disposal and because of the complex nature of the subject. The mathematical details are eliminated in so far as possible, and an effort made to give a more or less popular presentation of the subject in order that you may not be wearied by the more technical details.Because of the very wide field covered by the subject as assigned to me (Scientific Engineering Problems in Ordnance Manufacture), I am limiting myself to one phase only of the subject, as indicated in the title of the paper; and the treatment of this phase is necessarily incomplete because of the limited time at my disposal and because of the complex nature of the subject. The mathematical details are eliminated in so far as possible, and an effort made to give a more or less popular presentation of the subject in order that you may not be wearied by the more technical details.
EARLY FORMS OF AMMUNITION.
The early forms of projectiles used in cannon were solid, irolf shot, grape and canister, the two latter being composed round shot of much smaller diameter than the bore of the held together by a can, or by rods and plates, to facilitate lastenings were made so light that they would not withstand discharge from the cannon, so that the effect was very much though the balls had been loaded loosely into the gun, the ing essentially a huge shotgun.
Grape and canister were ineffective except at very short damage done by the solid spherical shot was very small, even hit was made. An improvement was made when an explosive loaded with black powder was introduced, but this was still as compared with the modern high-explosive projectile.
MODERN FORMS OF AMMUNITION.
With the discovery of so-called high explosives it became greatly increase the effectiveness of projectiles against both fortifications. The modern high-explosive shell consists cylinder with a conical or ogival point, filled with a high explosive insensitive to resist the shock of discharge and capable of being a fuse fitted to the projectile. When used against materiel acts chiefly as a carrier for the high explosive, the damage being violent detonation of the explosive.
When used against personnel, the fragments into which the shell is blown by the high explosive jected with a high velocity and serve to materially increase the Early in the war it was found by experience that the effectiveness’ shell against materiel was in direct proportion to the amount plosive contained, so that the modern shell is made with walls will safely stand the shock of discharge, in order to carry as of explosive as possible without exceeding the permissible weight. In chemical shells–a new development of this war-the effect proportional to the amount of chemical which the shell carries, this, also, it is necessary that the shell itself be made with possible. Shrapnel, which before the war was considered to be one of the most ef- fective forms of ammunition, proved to be relatively unimportant except against massed troops in the open-a target not often found.
The shrapnel consists of a hollow steel cylinder with a pointed end, which contains a very large number of lead bullets. A pocket between the bullets and the base of the shell contains black powder, which is ignited by a time fuse while the shrapnel is still some distance from the target. This black powder blows the bullets forward and downward, spraying them over a wide area, each bullet being intended to have sufficient velocity to kill a man or horse on striking one. The shrapnel case is not blown to pieces, so that it is entirely ineffec- tive unless it should happen to strike a living target. It can, therefore, be seen that it is desirable to make the shrapnel cases with as thin walls as possible in order that the major portion of the weight of the projectile may consist of bullets.
STRESSES IN FIRING.
The design of these shell bodies and shrapnel cases involves some interesting problems in the strength of materials. With the improvement in the materials used in the manufacture the powder pressures used have increased until at the present time of 38,000 to 40,000 pounds per square inch are common. When is compared with that of about 100 to 150 pounds per square high-pressure steam boilers, or about 200 to 500 pounds per square engines, it can be realized what enormous forces are brought to,projectiles when these are being fired from the cannon. The powder sure acts on the base of the projectile, which in turn pushes forward side walls. The inertia of the walls and the forward part of the to resist this pressure, so that a very heavy compressive stress the walls. This is the most obvious stress on the projectile walls, one which until quite recently was used as a basis for the design jectiles.
The intensity of the stress can easily be computed by formula of mechanics–force equals mass times acceleration. This is not the only important stress, however, and when it was for the design of projectiles it was necessary to use a factor of more accurately, a factor of ignorance, which sometimes gave satisfactory sults and sometimes did not. The introduction of semisteel shells clearly the defects of this method of design, for these shells failed pressive stresses far below what the material could safely stand, the conclusion that failure was not due primarily to these compressive stresses, but to hoop tension developed in the walls due to the pressure contained charge. These stresses are brought to bear in the following The high explosives which are commonly used are solids, and the form of a powder pressed into the shell or may be melted into it. When the shell is fired from the cannon the inertia of tends to cause it to lag behind, while the particles to the rear force This sets up a heavy internal pressure in the charge, which is the base of the shell. In shells used in the United States army this of pressure frequently runs up to more than 10,000 pounds per Under such enormous pressures it is probable that the charge acts it were a fluid, in much the same way that the ice of glaciers acts under heavy pressures.
Consequently, the shell is in the condition low cylinder subjected to a very high internal fluid pressure, which burst the walls by hoop tension. In the chemical shells the shell liquid, and it is obvious that these shells are also subjected to this sion. This tensile stress in the walls of the shells frequently runs as higher than, the compressive stresses in the walls due to direct powder sure. As the tensile strength of semisteel is only from one-third fourth of the compressive strength, it can readily be seen why the design semisteel shells on the basis of compressive stresses gave factory results. The effect of the longitudinal compression and the sion acting simultaneously is much more severe than it vgould of these acting singly, and it is commonly the combined effect stresses which limits the ability of either steel or semisteel shells stand discharge from the gun. A number of other stresses also those due to centrifugal force and to the rapid angular acceleration shell, but these may usually be neglected without serious error.
HIGH QUALITY OF STEEL REQUIRED.
The combined stresses in shells used in our service frequently run as high 60,000 pounds per square inch, which makes necessary the use of a high quality of steel in their manufacture. Ordinary steels are not able to with- stand these stresses, and if used without special treatment would allow the shell to swell in the bore of the gun. This would be very likely to cause premature detonation of the charge, with the destruction of the gun and crew. On the other hand, high carbon steels of the quality which could safely withstand these stresses would be so hard as to make it difficult and ex- pensive to manufacture them.
It is therefore necessary to make use of an intermediate quality of steel and by the use of appropriate heat treatment after the shell is manufactured to give it the desired strength. When semi- steel is used for shell manufacture a radically different design must be used, with much lower tensile stresses. This considerably reduces the amount of charge which can be carried and renders the shell inefficient. The very high stresses developed, and the seriousness of a possible failure of the shell to stand up properly under firing, make necessary very careful inspection and testing of the shells during and after manufacture. The final test as to the acceptability of the shell is, of course, the firing test, and this is made upon a considerable percentage of all shells manufactured, a non- explosive charge of the same specific gravity as the high explosive being used. In order to give a margin for safety, a powder pressure about 12 per cent in excess of the normal is used. The shells are recovered after firing, and in- spected to observe whether any appreciable swelling has taken place. If this has occurred the entire lot from which the samples were taken must be rejected.
INCREASED RANGES USED.
One of the important developments of this war has been the range obtained with the artillery. The importance of being able the enemy is obvious. It was largely because the French 75 mm. ranged the German guns of a similar caliber that the French were the Germans in the early part of the war in spite of great disadvantages other respects. The Germans recognized this, and greatly improved ranges of their variouq calibers of guns during the progress of the An increase in range can be secured in one of three different combination of these, namely, (a) by increasing the powder pressure; increasing the length of the gun, so that the powder pressure projectile for a longer time; or (c) by improving the shape of Either of the first two methods will result in giving the projectile creased muzzle velocity, while the third method will result in resistance of the air to the flight of the projectile. Disadvantages attending the increase of powder pressures stresses in both gun and projectile are increased, erosion of creased, and its life is shortened. The gun must be made heavier increased stresses, and it consequently becomes less mobile.Increasing the length of the gun also increases the weight and makes gun less mobile. It is therefore highly desirable that the improvement shape of the projectile should be carried as far as practicable.
EFFECT OF SHAPE OF PROJECTILE ON RANGE.
In a vacuum the path or trajectory of a projectile would be the projectile would have the same velocity at the target that muzzle of the gun, assuming these to be in the same horizontal maximum range would be obtained with an angle of elevation and the range obtained for any given angle of elevation would proportional to the square of the muzzle velocity. It may be that in a vacuum the axis of the projectile would not remain projectile’s path, but would remain always parallel to its original At the high velocities used with guns the resistance of the air of the projectile becomes very important. With certain artillery used in our service the air resistance at service velocities is from ten to fifteen times the weight of the projectile. Due to the range is greatly decreased, the path is no longer a parabola, is no longer proportional to the square of the muzzle velocity. It is readily seen that the ability of the projectile to penetrate medium such as the air will depend on the pointedness of the that the more pointed the projectile the greater the range that with a given muzzle velocity. It can also be seen that for a of the air the retardation will be inversely proportional to the projectile. It follows, therefore, that increasing the length of a given size will increase the range, since this will increase without greatly affecting the air resistance.
This is one of the of the cylindrical-ogival form of shell over the spherical one. limit beyond which the increase of length must not be carried. latter becomes greater than about four times the diameter, comes unstable and will no longer move point forward, but thus present a very greatly increased area to the resisting air, will be short and very erratic. The length of projectile which may be increased somewhat by the use of a light, hollow throw the center of pressure forward without greatly disturbing of the center of gravity. Another method used to decrease the resistance of the air taper the rear end of the projectile, very much in the same way portion of the boat is tapered. It is because of this similarity ing is given the name of boat-tailing. Boat-tailing has been used on projectiles designed in the last few years.
OTHER FACTORS AFFECTING RANGE.
Another very important factor which affects the range of projectiles is air density. The retardation is directly proportional to the density of the and consequently an increase or decrease of one inch in the barometer change the retardation by about one-thirtieth of its value. It is an interesting fact not usually understood that a moderate change in the barometer change the range of a projectile more than a strong head wind. The modern long ranges are obtained by firing at high angles of elevation the maximum range being obtained at an angle of from 40 The projectile, therefore, rises to a considerable height above the air density at this altitude is much less than that at the earth, thus materially decreasing the retardation of the projectile.the maximum range being obtained at an angle of from 40 The projectile, therefore, rises to a considerable height above the air density at this altitude is much less than that at the earth, thus materially decreasing the retardation of the projectile.
POSSIBILITIES OF VERY LONG RANGES.
The firing of shells upon Paris by the Germans from a distance 75 miles has drawn attention to the possibility of securing very long It happened that I was in the ordnance department at Washington time the Germans began firing these projectiles, and the War Department called upon me to calculate the elements of the trajectory and whether it were possible that the Germans were really firing on such a distance as was reported. Ballistic calculations are usually made by the aid of tables, in very the same way that tables of the functions of the angles are used in tion of problems in trigonometry. A brief preliminary investigation that while the tables extended only to a velocity of 3,600 feet per second, velocity would be much too low to give the necessary range. It was fore necessary to resort to an analytical solution based on graphical and to solve the trajectory step by step. The process was a tedious the calculations gave the desired results. It was found that for the data regarding the projectile, and an angle of elevation of 50 degrees, velocity of about 5,000 feet per second was required; that the projectile rise to a height of about 25 miles above the surface of the earth; time of flight would be about 3 minutes; the angle of fall about and the striking velocity about 2,800 feet per second. These results satisfactorily with the latest information available. It should be noted that in making these calculations it was necessary extend both the laws of variation of air resistance with velocity, density with altitude, far beyond the limits of experimental data.
Very experimental work has been done with velocities greater than 3,000 second, while 25 miles above the earth’s surface is several times has ever been reached by man. It is interesting to compare these results with the values required 75-mile range in vacuo with an angle of elevation of 50 degrees. Very calculations show, for the latter case, that the muzzle velocity required about 3,600 feet per second, the maximum height above the earth’s about 22 miles, and the time of flight about 171 seconds. The only these values differing greatly from the corresponding one for flight the muzzle velocity. The reason for this is that the major portion actual trajectory was in what is practically a vacuum. About four-fifths range was covered at a height of over nine miles above the earth’s and hence in an air density less than one-seventh that at the earth’s while the density at the highest point of the trajectory is about the earth’s surface. The great decrease in velocity of the projectile the first few miles of its travel, while it is going through the dense near the earth. There is no question but that had the war continued there would have been a further development in the matter of very long ranges. How far the in- crease in range would have gone is problematical.
It may be interesting to note in this connection that calculations show that, neglecting the retardation of the air, it would require a muzzle velocity of only about seven miles per second to make a projectile leave the earth entirely and never return, while with a velocity of about five miles per second, only five times that reached in the German gun, the projectile would revolve around the earth as a satellite. Obviously, velocities somewhat short of these values would be sufficient to reach from any one point of the earth’s surface to any other point, if the resistance of the air could be neglected. To actually accomplish the result it would be necessary only to give a sufficient added velocity to the projectile so that it might have the velocity mentioned by the time it had risen above the earth’s atmosphere. Whether it will ever be pos- sible to design a gun capable of giving such a velocity to a projectile is a problem for the future.
Factors Influencing the Teaching of Science and Engineering.
The following factors contribute to efficient instruction: 1. ORGANIZATION. The duties of every person connected with the adminis- tration, instruction and research activities of an educational institution should be carefully worked out, showing lines of authority and of responsibility. A diagram should then be drawn up which shows at a glance to whom each individual in the organization is responsible, and the main duties, whether executive, teaching or investigational, every person is performing. This chart should be supplemented by departmental charts and by written instruc- tions, which should set forth details of organization. It should be the duty of the head of the institution to familiarize the heads of the various departments with the organization. The heads of de- partments should be held responsible for the quality of instruction in their departments. To correlate the work of the various instructors in any given department, frequent conferences should be held of all instructors teaching the same or related subjects. These conferences should be very informal and should aid in developing esprit de corps among the instructors, while improving teaching methods and bringing out defects in- textbooks, schedules of assignments, sub- ject. matter, etc. The head of the institution should also hold frequent conferences of all department heads in order to correlate the work of the various departments and to discuss administrative details. Matters affecting the entire teaching force should be discussed at general meetings, which should be attended by every person connected with the institution. When several instructors are teaching the same subject, but to different sections, the schedule of instruction should be planned by a committee in- cluding all such instructors, and in co6peration with the head of the depart- ment. If at all possible, where several instructors are handling the same sub- ject, the sections should be arranged so that men possessing similar qualifica- tions are assigned to the same section. Greatest aid-that is, better teachers and smaller sections-should be set aside for those students of lesser ability who show a desire to make most of their opportunity.