The great increase of power due to great increase of initial velocity of the projectile, which has been obtained by the use of smokeless powders in guns which were designed to use brown or black powder, has naturally led to much speculation in regard to the possibilities of the use of smokeless powders in guns constructed to withstand the pressures obtainable with heavy charges of the new explosive. The maximum pressure with charges of smokeless powder, in the guns in use at the present time, is limited to about 16 tons, the same as that obtained by brown or black powders; and the increase of power with these guns is due, mainly, to the absence of a solid residue when smokeless powder is used, giving a larger volume of gas (at the instant of maximum pressure, to expand through the remaining length of bore) than was possible with brown or black powders.
The development of the gun of the present type of construction, by increasing the length of the gun and thus increasing the length of the travel of the projectile in the bore, has a practical limit, for naval guns at least, and the logical development would seem to be in the direction of increasing the resisting power or the elastic strength of the present length, since the proportional increase of velocity, by increasing the travel of the projectile, is small as compared to the possible increase if a safe maximum pressure of 24 tons, say, could be used instead of 16 tons, as is the rule at present with the present form of construction and the present quality of steel when used in the form of masses.
With a maximum pressure of 24 tons instead of 16 tons, velocities of from 3000 f.s. to 3400 f.s. would be the rule with guns of the present length, instead of 2000 f.s. to 2400 f.s., and the energy of projectiles of the same weight would be more than doubled. The attention of inventors, then, has been directed toward the use of steel in some other form than that of tubes, and successful efforts have been made to utilize the very high elastic strength of steel obtainable in the form of wire or riband.
Mr. Thomas Willson, of New York, the electrical engineer, has invented a process of construction which will allow the very highest grades of cold rolled steel in small thicknesses to be used in building up the gun and which does not involve any of the possible uncertainties of the use of steel in the form of wire or riband. His process of construction has some very novel features, is certainly simple and, so far as it has been tested in the form of a trial cylinder, has fulfilled the expectations of the inventor. The gun built according to Mr. Willson's plan may be described as follows: It consists of a tube of the size and thickness, approximately (extending the whole length of the bore), now used with guns of the ordinary design. Over this tube are shrunk by heat discs of steel of the highest grade (prepared by a process of cold rolling), of an elastic strength of 100,000 lbs. per square inch, a value which steel makers will contract to furnish. These discs are shown in the plates, and may extend, if considered necessary, all the length of the gun tube to the muzzle. These steel discs, together with the gun tube, are designed to resist the principal stress produced by powder-pressure, that of hoop tension. The elastic strength of the gun in this direction being limited to the elastic strength of the steel in the tube from compression to extension.
Without going into the mathematical details of computing the elastic strength of such a gun, it may be said of it that an elastic strength of 32 tons per square inch can be obtained by a proper proportion of the dimensions. Thus far, resistance to but one stress has been provided for, and Mr. Willson provides for the other, the longitudinal stress, by an outside jacket of aluminum bronze cast in one piece over the assembled tube and discs.
The success of this method of providing for the longitudinal strength will depend upon the certainty of a sound casting, and upon an assured elastic strength of aluminum bronze of 30,000 lbs. per sq. in., while the elastic strength of the assembled tube and discs must not be materially changed by temperature of the molten bronze in the process of casting. If these things can be assured, then the plan of using an outside jacket of aluminum bronze affords a simple and easy solution of the problem.
Manufacturers having sufficient plant are willing to contract to assure sound castings and an aluminum bronze of the required strength. Steel makers of undoubted responsibility are also willing to furnish a steel for the discs which, after the casting of the outside jacket of bronze, will not show a reduction in elastic strength below the fixed limit of 100,000 lbs. per sq. in., claiming that the effect of the casting of the bronze jacket outside the steel discs will be to anneal them, and not to destroy the high qualities obtained by cold rolling or other processes of working. By experiments with a cylinder of 2-in. bore constructed on this plan, this has proven true. Aluminum bronze has been chosen as the material for the outside jacket, because of the simplicity of manufacture combined with low cost; the outside jacket might, however, be made of steel, in two or more lengths, shrunk over the discs and with a locking band to lock all parts together. The system of construction does not, in other words, forbid the use of steel throughout, but with aluminum bronze of the proper strength, and a sound casting, the longitudinal strength will be ample.
The breech-plug is housed in a short steel tube which screws over the rear end of the gun-tube in rear of the seat of the gas-check, and which is itself gripped by the outside jacket.
This system of construction, so far as the position of the metals of different strength is concerned, is essentially the same as the wire-wound system, so long advocated by Professor Longridge, of England, and which in principle is eminently sound; that is, to place the strongest metal, or that which will stretch most within its elastic limit, as near to the surface of the bore as possible, utilizing the principle of varying elasticities as well as that of initial tensions.
If it were possible to place the discs at the surface of the bore the strongest possible gun with the metal used would be the result. A tube is a necessity, however, to allow for the rifling and also to prevent the gas from entering between the discs. Each disc is rolled cold from a block of high grade steel, the block being thicker in the middle than at the outside edges, and the effect of the cold rolling, aside from the working of the steel, is to produce a state of initial compression of the interior layers and an initial tension of the exterior layers of the metal in each disc. The same condition is thus produced in a solid state in each disc as must exist in a coil of wire in a wire gun, with the advantage that there are no wire ends to secure, and the difficulty of securing the proper tension of winding for each layer of wire is avoided.
This gun is essentially a weapon designed for high pressures, such as are obtainable with smokeless powders, and is in the line of the rational development of the power of the gun. It is thought also that the gun can be built as cheaply as guns of the present design, and certainly the mechanical features seem to offer no difficulties. The weight of the gun, as compared to the present built-up gun, is not greater than the latter, calibre for calibre.