The invention of smokeless powder marks an advance in the science of artillery ranking in importance with that of the breech-loading cannon. Measured by muzzle energy, the change in ordnance construction from cast-iron to steel has not contributed more to the improvement of artillery.
In 1852, seven years after the invention of gun-cotton, Captain Von Lenck of the Austrian army first made use of it as a propellant in guns. So far as smokelessness was concerned, he was of course successful, and its use continued in Austria for about ten years. In 1862, the gun-cotton factory was destroyed by an explosion and the manufacture ceased, notwithstanding the fact that the new powder had come into use to a considerable extent.
The gun-cotton as made at that time was unstable; no efficient means had been discovered for entirely freeing it from the residual acids used in the process of nitration; this difficulty was overcome by Frederick Abel who invented the process of grinding the cotton fiber in a paper mill after nitration, thus reducing the minute tubes to short length and making it possible to remove the acid from their interior.
Von Lenck spun the gun-cotton and laid it up into ropes; this prevented its burning too rapidly, and when Abel overcame the difficulty of instability the material was no longer in a form that would permit its rate of burning to be controlled. About this time Captain Schultz of the Prussian Artillery nitrated wood cut into thin sheets and this powder has been used ever since for sporting purposes.
In 1886, Vieille produced the first practical military smokeless powder, since however abandoned; this consisted of gun-cotton dissolved in ethyl ether and mixed with picric acid. In 1888, cordite was invented in England and has been used ever since. Cordite is a mixture of 58 parts of nitroglycerine with 37 parts of high-grade gun-cotton and 5 parts of Vaseline. The gun-cotton is soluble in nitroglycerine, but with difficulty, and to overcome this acetone is used as a solvent for both and then dried out. Ballistite, which is used in Italy and Austria, is very similar to cordite, only a different solvent is employed.
Cordite is squeezed out in a vermicelli press in smooth cords of varying thickness, depending upon the size of gun it is intended for. Its color varies from a light yellow to brown and the cords bend like vulcanized rubber and are about as hard to the touch as stiff beeswax.
There are only two general classes of smokeless powder in use; those consisting of a mixture of nitroglycerine and gun-cotton as above described, and those consisting of gun-cotton alone, dissolved in a suitable solvent pressed into the desired shape and then having nearly all of the solvent removed by evaporation. The latter class are generally referred to as "gun-cotton powders." It is to this class that the Navy smokeless belongs.
The Navy powder is a solution of three parts by weight of soluble nitro-cellulose in two parts of ethyl ether and one part of ethyl alcohol. When mixed in a mechanical bread-kneader, the substance has the appearance of damp cornmeal. Under a pressure of a couple of hundred pounds per square inch, the solution is completed and the mixture has the consistency of stiff dough, small masses of it being translucent. From this stage it is pressed into suitable grains and nearly all of the solvent evaporated off. About one or two percent of alcohol remains in the powder, it being extremely difficult to expel the residual solvent to a lower point than that.
In the original Navy smokeless it was specified that the nitration should be in the neighborhood of 12.45 percent; in other words, the nitrated cotton should consist of cellulose nitrates the mean nitrogen contents of which should be 12.45 percent. Furthermore, all but two percent should be "soluble." This word "soluble" is used in speaking of gun-cotton in a special sense. It means soluble in a mixture of ether and alcohol.
Prof. Mendeleef has demonstrated that a gun-cotton containing 12.44 percent nitrogen will give a greater volume of gas than any other. Experiments at Indian Head have proved that the nitrogen may be increased to as much as 12.80 without adversely affecting the velocity. Mr. G. W. Patterson has shown that owing to the impossibility of getting rid of all of the solvent the nitrogen should never be as low as 12.44. In the very smallest grains there remains never less than one-half percent. This is probably nearly all alcohol and to provide for the oxidation of this it is necessary to increase the nitration, the amount of increase depending upon the size of grain, for the reason that the quantity of residual solvent is a direct function of the wall-thickness. However, about 12.80 is the highest nitrogen that can be obtained with a resulting solubility within the 98 percent demanded by the specifications; therefore, nitrations between 12.45 and 12.80 are permitted.
The amount of nitration depends principally upon the strength of the nitric and sulphuric acid used. With the very strongest nitric acid the result produced will reach as high as 13.1 in ordinary practical working. That would be insoluble military gun-cotton and only useful so far as we are concerned for mines and torpedoes. Such gun-cotton is easily soluble in acetone but the dried colloid where acetone is used as a solvent is brittle. The colloid produced by ether-alcohol is decidedly tougher and therefore much less likely to break up in the gun. For obvious reasons it is important that this should not take place.
In the powder manufactured abroad so far as we have been permitted to observe them, no effort is made to have the gun-cotton all soluble. Nor is it certain that such a result is necessary. With that condition thrown out it is easy to obtain high nitrations.
Physical Characteristics.—The Navy Smokeless powder varies in color from a light lemon to a deep brown, almost black. These variations in color depend generally upon variations in the process of manufacture, although the region in which the fiber is grown may have an influence. If pure clear water is used in the various processes of washing and boiling the gun-cotton the powder is clear or at least translucent. If the water is contaminated with iron salts the powder is brown and opaque.
The grains are about as hard as horn but not so tough. They can be readily turned in a lathe and take a fine finish. Minute particles can be exploded by striking them with a hammer, but only the portion struck will explode.
The powder ignites at about 180° C. which is a considerably lower temperature than that required to ignite black or brown powder. Notwithstanding this low ignition point, the material is much less inflammable than black powder and this constitutes a serious disadvantage which will be referred to later.
Ballistics.—It is certain that the powder is absolutely progressive in its burning. When small charges are used with consequent low pressures and rates of burning, it frequently happens that partially burned grains are recovered in front of the muzzle of the gun. In all cases it has been observed that the outside diameter has been reduced and the inside diameter of the perforations increased in exactly equal amounts. The walls may be reduced to the thinness of paper, but the general structure of the grain remains intact.
The two great advantages pertaining to smokeless powder are first, that the entire weight of the charge is reduced to gas and becomes useful in propelling the projectile, and second, that the target is not obscured by smoke. I have given second place to the latter quality advisedly. As an example of the importance of the first point, let us compare the weights and velocities obtained with the 6-inch 40-cal. gun using brown powder and smokeless. A charge of 45.5 pounds of brown-prismatic gives in this gun a velocity of 2000 f. s. with 15 tons pressure; twenty pounds of smokeless gives the same velocity with 10.8 tons pressure. If the pressure curves for the two powders were the same in general form, this equality in velocity of the two powders would be unaccountable since the maximum pressure with the new powder is 28 percent less than the old. As a matter of fact the curves are very similar and the area embraced by the smokeless curve is less than that contained in the curve of brown powder-pressures. In the latter case, however, it must be remembered that but about 35 percent of the weight of powder becomes gas useful in propelling the projectile; there remains 65 percent in the form of smoke and residue left in the bore. Suppose so percent consists of smoke, that allows for about seven pounds of residue (which is undoubtedly an overestimate) and we then have the useful gas propelling a projectile weighing 100 pounds, plus about 23 pounds of inert products of combustion in the shape of smoke, making a virtual projectile of 123 pounds weight.
The relations of weight, pressure and velocity given in the single instance above do not always hold good. The powder must have a wall-thickness exactly suitable to the size of the chamber and length of the bore or else all of the powder will not be consumed in case the powder is too thick, or in case it is too thin the powder will burn rapidly and the pressure will be too high. The one case makes the chamber end of the gun do excessive work and the other throws the strain on the chase and muzzle. Thus, it is quite as important to the safety of the gun not to have too low a maximum pressure for a given velocity as to have one that is too high.
In guns fifty calibers in length with chambers large enough to accommodate a charge having about 45 percent of the weight of the shell, a velocity of 3000 foot-seconds is obtained with a pressure of 16.5 or 17 tons per square-inch. These results have been obtained with a 6-inch gun of 46 calibers. A velocity of 3380 f. s. was produced in a 5-inch 50-calibre gun, the shell weighing 50 pounds, the resulting pressure having been 15.8 tons. With a 60 pound shell the velocity was 3200 f. s. using a charge equal to that employed in producing the higher velocity with the lighter shell. In the latter case the pressure was 18 tons and the smaller amount of energy produced in the former can only be accounted for by the presumption that the powder was not all burned up, a result that would be produced when the pressure was too small to keep up the rate of burning. This of course was due to the lighter projectile moving forward under a lesser impulse than was required for the heavier.
Considerable disappointment is felt that the discharge of a gun using smokeless powder produces so much smoke. The powder-charge proper is really smokeless, the only smoke produced coming from the ignition charge. The lack of inflammability of the powder renders the employment of the ignition charge indispensable. For a 6-inch charge 100 grams of fine-grained black powder is about the least amount that will insure proper ignition. For a 13-inch charge weighing 300 pounds, a 14-pound ignition charge is used. Relatively the 13-inch charge requires something like seventy times as much black powder as the 6-inch. With the latter, the smoke does not obscure the target at all, but the smoke from the 13-inch is considerable. Fortunately it is so intermixed with the large volume of colorless gas that it dissipates readily enough not to seriously hamper the aiming. At the Proving Ground it was found that a point four miles distant was visible in five seconds after firing the 13-inch loaded with smokeless; with brown powder the time taken was twenty-five seconds; this was in calm weather; with anything of a breeze the five seconds would of course be lessened.
All attempts to reduce the amount of black powder result in decreased velocities, long hang-fires and increased pressures. The first is probably caused by not having all the powder consumed; it can be remedied by increasing the smokeless charge, but this not only increases the expense but probably burns powder in the wrong part of the gun. The hang-fires could not be overcome and they would be long enough to interfere with good shooting. The increased pressures are due to igniting a portion of the charge first, and setting up "wave-motion" by the released gases balloting back and forth from one end of the chamber to the other thus getting the dynamic effect of their weight.
It is necessary to fill the entire chamber with flame so as to instantaneously ignite all portions of the charge at once to avoid these effects and, so far, fine black powder is the only substance that has afforded the necessary inflammability to accomplish the desired result. It naturally occurs to one that smokeless powder so finely divided as to have a large amount of burning surface would be sufficient. This has been tried using all varieties of fine-grain smokeless including shot-gun powder, pistol-powder and even powder dust.
In the 6-inch the substitution of 50 grams of 60 mm. smokeless for the regular 100 grams of black powder has resulted in an increase from the normal pressure of 16 tons to the high one of 20 tons; and this with a perceptible hang-fire.
Stability.—The powder being nothing but gun-cotton with a small percentage of alcohol and a trace of ether included in the mass, its stability depends on the stability of the gun-cotton. That of course in turn depends on the purity of the gun-cotton. This substance has a long and favorable history, and when it has failed in keeping qualities the failure has always been due to imperfect preparation.