ON THE EXPLOSIVE FORCE OF GUNPOWDER.
Advantages of Gunpowder
The advantages of Gunpowder, as a propelling agent, over any other explosive material are, the comparative safety attending its manufacture and transport, and the gradual nature of its decomposition when compared with those materials, such as fulminating gold, silver, mercury, &c. &c. In gunpowder, the force resulting from the rapid evolution of gas in a confined space has sufficient time to overcome the inertia of the projectile, which is not the case with other explosive materials, the conversion of which gaseous products is so instantaneous that nothing can resist the intensity of their explosive action. Other advantages suggest themselves in the use of Gunpowder, such as the comparative cheapness of the ingredients composing it, and the ease with which they may be obtained; for the sulphur and saltpetre are very abundant productions of nature, and the charcoal can be manufactured cheaply and with great facility, and if care is taken in the process of the fabrication of powder, little deterioration will take place on its exposure to heat or moisture.
Air & Steam as propellants
Condensed air and steam have been used as propelling agents; but the great inconvenience attending their use quite preclude the possibility of adapting them to war purposes.
Force of Gunpowder.
As the force and effect obtained from Gunpowder is the foundation of all other particulars relating to Gunnery, we will briefly consider these points.
Upon what the action of powder depends.
The action of Gunpowder is dependent upon a purely chemical process. Mr. Robins proved that the force generated by the combustion of gunpowder, was owing to an elastic gas which was suddenly disengaged from the powder, when it was brought to a certain temperature, and further that this disengaged gas had its elastic force greatly augmented by the heat evolved by the chemical action.
Ingredients are charged with a large volume of heated gas.
The propelling power of Gunpowder is dependent on the rapid decomposition of the nitre into its component parts; the oxygen forms carbonic acid with the carbon in the charcoal, and the heat thus generated by ignition changes both this and the nitrogen into a large volume of heated gas. In a mixture of nitre and charcoal alone, the oxidation proceeds with comparative slowness; by the addition of sulphur, an augmentation of combustibility is gained, in consequence of its igniting at a very low temperature; the sulphur, also, by its presence, renders available for the oxidation of the carbon an additional amount of oxygen, viz: that which is united with the potassium, the latter being at once converted into sulphite upon ignition of the powder.
Weight of gas evolved.
It appears that the weight of gas generated is equal to three tenths of the weight of the powder which yielded it, Volume of gas evolved.and that its bulk when cold, and expanded to the rarity of Common air was 240 times that of the powder; the barometer standing at about 30 inches. From this Robins concluded that if the fluid occupied a space equal to the volume of the gunpowder, its elastic force, when cold, would be 240 times the pressure of the atmosphere, when the barometer stands as above. Heat of gas evolved.Mr. Robins also considered that the heat evolved was at least equal to that of red hot iron, and he found by experiments that air heated to this temperature had its elasticity quadrupled, and therefore, that the force of gas from powder is at least four times 240 = 960, or in round numbers 1,000 times as great as the elasticity of the air measured by its pressure on an equal extent of surface. Pressure of gas generated.From the height of the barometer it is known that the pressure of the atmosphere is about 143⁄4lbs. upon the square inch, so that the pressure of the elastic gas generated by the combustion of the gunpowder upon the same area would be 14.75 by 1,000 or 14,750lbs. at the moment of explosion. Strength of powder not affected by density of air, but by damp.He found that the strength of Gunpowder was the same whatever might be the density of the atmosphere, but that the moisture of the air effected it considerably, in fact that the same quantity of powder which would give a bullet an initial velocity of 1,700 feet per second on a day when the atmosphere was comparatively dry, would upon a damp day give no more than 1,200 or 1,300 feet.
Velocity of gas
The velocity of the expansion of the gas is a most important point, upon which depends, chiefly, the peculiar value of the substance as a propelling agent. Many of the warlike machines of the Ancients produced a momentum far surpassing that of our heaviest cannon, but the great celerity given to the bodies projected from guns by gunpowder cannot be in the least approached by any other means than by the sudden production of an elastic gas. Mr. Robins found that the flame of gunpowder expanded itself when at the muzzle of the gun with a velocity of 7,000 feet per second.
Dr. Hutton’s calculation as to:—Volume, Temperature, Pressure.
It has been calculated that one cubic inch of powder is converted into 250 cubic inches of gas at the temperature of the atmosphere, and Dr. Hutton states that the increase of volume at the moment of ignition cannot be less than eight times; therefore one inch of gunpowder, if confined, at the time of explosion exerts a pressure of about 30,000lbs. being 250 by 8 by 15 = 30,000lbs. on the cubic inch, or 5,000lbs. on the square inch; and which at once accounts for its extraordinary power. TemperatureThe value of the temperature to which the gases are raised, on the explosion of the powder, has been variously estimated and it may be concluded to rise as high as will melt copper, or 4,000° Fahrenheit. Expansion.All gases expand uniformly by heat, the expansion having been calculated with great precision, to be 1⁄480th for each degree of Fahrenheit. If therefore we take Dr. Hutton’s calculations of one volume of powder expanding into 250 volumes of gas at the temperature of the atmosphere, and if we suppose 4,000° Fahrenheit to be the heat to which they are raised on ignition, the expansion of gunpowder would be calculated. How to calculate expansionThus, suppose the gas to be at 60°, the temperature of the atmosphere, we must deduct 60° from 4,000°, which will give 3,940, being the number of degrees remaining to which it is raised, hence temp.1° : vol.1480 temp.3,940° : vol.3940480 = vol.8·2 that is, each volume of gas would at a temperature of 4000° be increased 8·2 in volume. Gunpowder when at the temperature of the air being expanded 250 times in volume; therefore 250 by 8·2 = 2,050 as the increased expansion for each volume of gas generated by the explosion of gunpowder at the temperature of 4,000° Fahrenheit. Lieut-Colonel Boxer calculates that the heat generated by good dry powder is not under 3,000° Fahrenheit. Absolute force of gunpowder cannot be determined.It appears with our present knowledge, the absolute value of the force of gunpowder cannot be determined. Still by careful and extensive experiments no doubt a near approximation to the truth may ultimately be arrived at, so that although much has already been done by various eminent philosophers, there is still more to be accomplished; and the importance of the subject ought to act as a stimulus to the exertions of those belonging to a profession the most interested in the question.
Loss of velocity by windage.
It has been found by experiments that in calculating the initial velocity of a projectile, one third of the whole force was lost with a windage of 1⁄10th inch with a shot of 1·96-in. and 1·86-in. in diameter. The bore of the gun being 2·02-in.
Definition of ignition and combustion.
By ignition we understand the act of setting fire to a single grain, or to a charge of gunpowder, and by combustion we mean the entire consumption of a grain or of a charge.
Quickness of combustion.
Upon the quickness of combustion mainly depends the applicability of gunpowder for Military purposes.
Ignition by heat.
Gunpowder may be inflamed in a variety of ways, but whatever be the method, one portion of the substance must in the first instance be raised to a temperature a little above that necessary to sublime the sulphur, which can be removed from the other ingredients, by gradually raising the compound to a heat sufficient to drive it off in a state of vapour. The heat required for this purpose is between 600° and 680° Fahrenheit.
Progressive combustion.
When a charge of powder is exploded in the bore of a gun, to all appearance there would seem to be an instantaneous generation of the whole force. But in fact it is not so, a certain time being necessary to the complete combustion of the substance. This gradual firing is of the utmost importance, for were it otherwise, the gun, unless of enormous strength, must be shattered in pieces, as well as the projectile; for in such a case, this great force being suddenly exerted upon one part only of the material, there would not be time for the action to be distributed over the particles, at any great distance, before those in the immediate vicinity of the explosion, were forced out of the sphere of action of the cohesive force, and consequently rupture must take place.
Substances which have a more violent action than powder.
The effect of such an action may be observed by exploding detonating powders, in which are contained chlorate of potash or fulminating mercury. The action of that peculiar substance the chlorite of nitrogen is still more remarkable. There is also another compound, containing three parts of saltpetre, one part of carbonate of potash and one part of sulphur, which when brought to a certain heat will explode with great violence, its destructive force being very considerable; and this is principally due to the rapidity of the evolution of the gas, for its amount is less than that produced from gunpowder, but the complete decomposition occurs in a much shorter time.
In a damp state less quickly fired, and why.
If gunpowder be in a damp state, the velocity of combustion will be less than when dry, and also a longer time will be necessary to ignite it, since the moisture upon its conversion into vapour, absorbs a certain amount of heat which remains latent, and of which the useful effects so far as igniting the powder is concerned, is entirely lost.
Ignition by percussion.
Gunpowder may be ignited by the percussion of copper against copper, copper against iron, lead against lead, and even with lead against wood, when the shock is very great. It is more difficult to ignite gunpowder between copper and bronze,[1] or bronze and wood than between the other substances. Again, out of ten samples which were wrapt in paper and struck upon an anvil with a heavy hammer, seven of grained powder exploded and nine of mealed.
[1] Bronze consists of 78 parts copper to 20 of tin. Bell metal—78 copper and 22 tin. Gun metal—100 copper to 8 to 10 tin. Brass—2 copper, 1 zinc and calamine stone, to harden and colour.
Influence of shape of grain on ignition.
If the part to which the heat is applied be of an angular shape, the inflammation will take place quicker than if it be of a round or flat form, on account of the greater surface that is exposed to the increased temperature.
The form of the grain influences the velocity of the transmission of flame.
If the grains are of a rounded form, there would be larger interstices, and a greater facility will be afforded to the passage of the heated gas, and therefore this shape is most favourable to the rapid and complete inflammation of each grain in the whole charge. On the other hand, particles of an angular or flat form, fitting into each other as it were, offer greater obstruction to this motion, and the velocity of the transmission of inflammation is thereby diminished.
Effect of size on the velocity of transmission of inflammation.
If the grains be small, the interstices will be small also, and the facility to the expansion of the gas thereby diminished. In the experiments with trains of powder, the increased surface exposed to the heated gas was found to more than compensate for the diminished facility to its expansion, and generally a train of small-grained powder laid upon a surface without being enclosed, will be consumed more quickly than a train of large-grained powder.
Large grain best suited for heavy ordnance.
But this is not the case in a piece of ordnance, a circumstance which amongst others will account for the diminished initial velocity given to the shot by a charge of small-grained musket powder, below that produced by the large-grained usually adopted for this service.
Velocity of the transmission of inflammation of the charge.
When a number of grains of powder are placed together as in the charge of a gun, and a few of them are ignited at one end of the cartridge, a certain quantity of gas is developed of a temperature sufficiently high to ignite those in their immediate vicinity. This has also such elasticity as to enable it to expand itself with considerable velocity. Again, the grains which are so ignited continue the inflammation to others in the same manner. The absolute velocity of expansion of this gas is very considerable; but the grains of gunpowder in the charge offer an obstruction to this motion, the gas having to wind its way through the interstices, and consequently the velocity is considerably diminished, but it is quite clear that it must be very much greater than the velocity of combustion. Estimate of Mr. Piobert.Mr. Piobert estimates the velocity of transmission of inflammation of a charge in a gun at about 38 feet per second, and in all probability even this is much under the mark.
Experiments made on this subject.
Many experiments have been made by observing the velocity of transmission of inflammation of trains of powder under various circumstances, but they do not show us what would be the velocity in a confined charge. The velocity increased with the section of the train, and further when at the end first lighted, there was an obstruction to the escape of gas, as in the case of a gun, a much shorter time was required for complete inflammation.
Time of decomposition depends upon form of grain.
When the charge of powder in a gun is ignited the grains being enveloped by the heated gas, we may consider that each grain is ignited over its whole surface at once. If the grains of powder were of equal or regular form, the time each would be consuming, might be easily calculated, but since in ordinary cases they are irregular in form, although the grains may be of the same weight, the time necessary for their complete decomposition will be very different.
Circumstances affecting combustion.
The quickness of combustion will depend upon the dryness of the powder, the density of the composition, the proportion of the ingredients, the mode of manufacture, and the quality of the ingredients.
Combustion of cubical grains considered.
Were a cubical grain to be ignited upon its whole surface, the decomposition may be supposed to take place gradually from the surface to the centre, and the original cubical form to remain until the whole is consumed, the cube becoming smaller and smaller. If, then, the rate of burning be the same throughout, the quantity of gas generated in the first half portion of the time will evidently be considerably more than in the latter half, as in the latter case there will be a much lesser surface under the influence of flame.
Elongated and cylindrical grains.
If the form of the grain be elongated, then will the quantity of gas generated in a given time from a grain of similar weight to that of the cube or sphere, be increased, on account of the greater ignited surface, and consequently the time necessary for its combustion will be diminished. If it be of a cylindrical form for example, this time must be reckoned from the diameter of the cylinder, its length not influencing it in the least, although as we have seen, it enters into the consideration of the quantity of the gas generated in a given time.
Large grain.
In the ordinary large-grain powder, the majority of the grains are of the elongated or flat form, from whence considerable advantage is derived, particularly in short guns, since it causes the greatest portion of the charge to be decomposed before the projectile is moved sensibly from its original position.
Mealed powder.
If the charge be composed of mealed powder a longer time is found to be necessary for the complete combustion of the whole than in the case where the substance is granulated, and the initial velocity of a shot is reduced about one third by employing the substance in that state.
The effect of granulating gunpowder.
A piece of pressed cake weighing 1·06oz., was put into a mortar, and a globe of some light substance, placed upon it, and the powder being consumed after ignition without ejecting the ball from the bore of the piece. When an equal quantity was divided into seven or eight pieces, the globe was thrown out of the mortar; breaking the cake into twelve pieces; the ball ranged 3·3 yards; being further increased to fifty grains, it ranged 10·77 yards; and when the ordinary powder was used, the ball was projected 56·86 yards.
Action depends upon size and form of grain.
It will appear from the above remarks, that the force generated from the charge of powder in a gun, will be greatly influenced by the size and form of the grains composing it.
Density of gunpowder.
In order to obtain a gunpowder which shall possess a proper amount of force, it is necessary that the ingredients should be thoroughly incorporated, and the process of incorporation will in great measure affect the density of the grains. After going through the process, it is subjected to a certain pressure, in order that the substance in travelling may not be reduced to a fine powder, which would cause the velocity of transmission of inflammation to be diminished. But there is a certain point beyond which it would not be advantageous to increase the density, and this seems to vary with the size of the grain. With large-grain powder the action in a musket, or in guns with small charges, is greatest with a low density; while with very small grain, the highest velocities are obtained generally with the gunpowder of great density; but in heavy guns with ordinary charges, the large-grained powder should be of considerable density in order to obtain the greatest effect, though still it must not be too great.
Advantages of glazing.
The principal advantages of glazing are; first, that the powder so prepared, will in travelling, owing to the smaller amount of destructive force consequent on friction, produce less mealed powder; and secondly, that in a damp country like England, the glazing imparts a preserving power to the powder, as the polished surface is less likely to imbibe moisture than the rough.
Disadvantages of glazing.
The disadvantages of glazing consists in its polishing the surface, and thus depriving it of those angular projections which cause the ignition and combustion to be carried on with greater rapidity, by rendering the interstices smaller, the consequence of which is, that there is not so much gas produced previously to the projectile leaving the gun, and in large charges a portion will be blown out unfired. There must be a limit then to glazing, which it would not be proper to exceed. Experiments as to glazing.At an experiment with glazed and unglazed powder, the ranges on the eprouvette were 75 for glazed, and 98 for unglazed. This loss of power, consequent on glazing, has caused it to be done away with in France and Russia. Glazing less hurtful to fine grains.With fine grain powder it is not of so much consequence, as it is, to a certain degree, corrected by the size of the grain.
Size of grain determined by size of charge.
The rapidity with which a charge of gunpowder is consumed will depend not only in a certain degree upon the size of the grain, but on the manner in which the charge is put together, for if a charge is closely pressed, the gases meeting resistance in their endeavours to escape between the interstices, will not propagate the ignition so rapidly. With large charges, there exists a positive advantage for the grains to be rather large, so that the most distant parts of the charge should be reached by the gases as quickly as possible; whilst with that of a rifle, the charge being small, the fineness of the grain does not interfere with the quantity of the gas developed. Whence it may rationally be concluded that the dimensions of the grains should increase in proportion to the quantity of the charges into which they are to enter, that is to say, in proportion to the interstices. Tight ramming bad.Ramming down a charge tightly must therefore interfere with the velocity of combustion.
Note—The foregoing on the explosive force of gunpowder was taken from Lieut-Colonel E. M. Boxer’s Treatise on Artillery.