SMOKELESS CANNON POWDER IN THE LIGHT OF

THE MOST RECENT DISCOVERIES.

As originally made, gunpowder consisted of a loose mixture of pulverized sulphur, charcoal, and saltpetre. It then actually existed in the form of a powder, hence the name. The idea of granulation probably arose from the admixture of bituminous matter with the powder, to retard the combustion. The first methodical granulation of gunpowder recorded was in France in 1825. The pulverized ingredients were mixed thoroughly and pressed into a hard cake, which was then broken up into irregular fragments or grains.

The next important improvement in granulation was made by General T. J. Rodman, the inventor of prismatic gunpowder, who in 1854 had presses made for moulding the grains separately, giving to them a definite and uniform shape. He was also the first to make multi-perforated powder grains with a view to securing progressive combustion. (See Fig. 1.) The improvements which followed those of General Rodman related mainly to composition and density, having for their object the retarding of combustion. Brown prismatic powder was the result. There was no further improvement until the advent of smokeless powder.

Black gunpowder, being but a mechanical mixture caked together by pressure, was not well adapted to the multi-perforated form invented by Rodman, which, after much experimenting, was abandoned for the form having a single central perforation. (See Fig. 2.) Notwithstanding the obvious advantages of multi-perforations, the material did not possess sufficient tensile strength to render those advantages available.

In order to understand the action of gunpowder, we must bear in mind that there are two forms of combustion known as explosion. One is what is termed detonation, where the action takes place throughout the mass at practically the same instant. This form of combustion or explosion applies to what are known as high explo

sives. Gunpowder is consumed by surface combustion only. This requires time, and although the time is relatively short in the usual sense, it is long when compared with the infinitely quicker action of high explosives. From the pulling of the lanyard until the projec

tile leaves the muzzle of a cannon only about a sixtieth of a second intervenes. Short as is this period it gives the projectile time to move forward in the bore and provide space for the reception of the powder gases as fast as they are set free; whereas, if the charge should be consumed instantly, as by detonation, the projectile would not have time for any appreciable movement, and the whole rear portion of gun would be blown to fragments from the enormous pressure.

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Obviously, an ideal gunpowder should produce a pressure which would be so well maintained behind the projectile in its flight through the bore of the gun that a curve representing the pressure would coincide with a curve representing the working tangential

strength of the piece throughout its length. This result can be attained only by means of multi-perforations, giving the powder grains such form that they will be consumed by rapidly accelerating combustion. The powder grain which has been adopted by the United States Government for both branches of the service is the longitudinally multi-perforated cylinder shown in Fig. 3, the usual number of perforations being seven, though frequently as many as nineteen are employed. The diameter of the cylinder and the thickness of the material between the perforations is made greater or less according to the size of the gun in which the grain is to be used.

The dense colloid of which smokeless powder is composed, when properly made, is free from pores and wholly impervious to the hot gases with which it is enveloped in the gun. Ignited in the air, this material burns with comparative slowness, requiring several seconds for the consumption of a grain of ordinary size. When burned under

FIG. 4.

pressure, however, the action is much more rapid. Under service pressure in a gun-about 35,000 pounds to the square inch-the same grain is consumed in about the sixtieth of a second.

Fig. 4 represents a grain of smokeless cannon powder burning in the air. The expulsion of the products of combustion from the perforations generates a pressure there in excess of that upon the outer surfaces of the grain. This causes a more rapid rate of burning within the perforations, and accounts for the strong blast of flame being thrown out of the perforations at each end, as shown in the figure. As this difference between external and internal pressures is increased in proportion to the increase in the rate of combustion under service pressures, it is obvious that the tensile strength of grains becomes a very important factor in preventing their disruption or blowing up in guns. In fact, the tangential strength of a grain of multi-perforated powder is quite as important as that of the gun.

If the powder grains be made too long, or the perforations too small, they will explode even when burned under atmospheric pressure. The higher the pressure, the shorter must be the grains. The disruption of powder grains in guns might be very disastrous, because the pressure would suddenly mount to a point where a gun would burst, owing to the enormous increase of burning areas presented to the flame by the large number of small fragments.

FIG. 5.

In the manufacture of smokeless powder there should be as little solvent employed as possible to effect gelatinization of the guncotton, in order to reduce shrinkage to the minimum and prevent warping and cracking in drying. The drying of cylindrical powder grains may be compared to the cooling of a piece of ordnance, the stresses set up being similar. General Rodman discovered in the manufacture of cast-iron cannon that when allowed to cool from the outside such internal stress was set up that the piece was capable of standing much less pressure than when the same was cooled from the inside. When externally cooled the outer portion of metal becomes solid and unyielding, assuming a definite and final shape, while the interior is still soft and yielding. As the inner portions of the metal also shrink in cooling, more and more stress is set up toward the bore. Although the metal may be strong and elastic' enough to prevent cracking, still the stress is such that the gun is to a large extent under a strong bursting strain while in a normal state, deducting just so much from the tangential resistance of the piece to the pres

sure of the powder charge. Fig. 5 is a cross-sectional diagram of a gun made by General Rodman, indicating the tendency to crack from internal stress when a cannon is cooled from the outside. When cooled from the inside, the exact reverse condition is produced; the inner portion of metal becoming first solidified, the outer in cooling shrinks upon it, with the result that the stress set up materially strengthens instead of weakening the piece.

Of course, it is impossible to dry powder grains from the inside; hence the advantage of using the smallest possible quantity of solvent to secure minimum shrinkage. However, there must always be more or less internal stress; and though no actual cracks may appear, still, as in the case of an externally cooled cannon, the grains may be so nearly brought to the point of cracking as to require but little internal pressure to effect their disruption. Doubtless, many erratic pressures which have occurred from time to time in the early experiments with smokeless powders have been due to the disruption of the grains from the excess of pressure within the perforations, and were in many cases attributable to initial stress already existing in the grains, although no visible faults or cracks might have appeared.

Fig. 6 is an end view of a grain of United States multi-perforated smokeless cannon powder, perfect in form. Fig. 7 is a simi

lar grain, but made with so much solvent that it has become distorted in drying. Although no cracks may be visible, yet the tangential strength of the grain is very much lessened. When the powder composition is of a less tough material than that used in making the grain shown in Fig. 7, the cracking effect shown in Fig. 8 is the result,