In connection with the present wide-spread interest in powders and explosives in use in the great war, a few brief and non-technical notes on military explosives may prove timely.
The subject will be considered under the headings of propellent powders, high explosives, and containers.
Propellent Powders
Since the last decade of the nineteenth century all the military powers have adopted as propellent powders for all classes of guns smokeless powders of some form or another.
The base of all such smokeless powders is nitrocellulose, which may or may not be compounded, in manufacture, with nitroglycerine. The majority of the powers use a so-called “straight” nitrocellulose powder, containing only nitrocellulose, with its necessary solvent in comparatively small quantities, up to 7 or 8 per cent, and usually a very small chemical constituent known as a stabilizer. The solvent used, while volatile and combustible, is not essentially an explosive substance, and is generally acetone, ether, alcohol, or a combination of any of them. Such powders are used by Germany for her army, France, Russia, Austria, Turkey, and the United States.
The nitroglycerine powders differ in that with the nitrocellulose is admixed nitroglycerine, which acts doubly as a solvent and as an additional explosive substance. Such powders are employed by England under the name of cordite, Italy (ballistite), Japan, and the German Navy. The proportion of nitroglycerine runs from 55-60 per cent in the older British cordites to 25-30 per cent in the later, with ballistite lying between the two, originally near 50 per cent, but now somewhat modified.
The manufacture of both powders is similar up to the point of the admixture of the solvent. Cellulose, in the form of cotton
usually, is thoroughly washed, dried, and nitrated by treatment with a mixture of strong sulphuric and nitric acids; the complex cellulose molecule (C6H10O5)x thereby acquires, by substitution, a number of the NO, radicals of the nitric acid and becomes nitrocellulose or guncotton (though the technical “ guncotton ” is cotton slightly more highly nitrated than that for smokeless powder). After a subsequent thorough series of washings and boilings to remove free acids, the nitrocellulose is ready for the solvent. For the straight powder the solvent is mechanically mixed with the nitrocellulose; the stabilizer, such as diphenylamine or amyl alcohol, being in solution in the solvent, and the whole, now a gelatinous mass, is pressed in dies, under strong pressure, into a continuous rope, which is then cut into grains of proper length. The diameter, length, and perforations of the grains vary for different calibers and according to the practice of the country. Powder for guns is made in cylindrical perforated grains; for small arms in small flat flakes. After the graining of the powder it is dried out, in drying chambers and drying houses, until the percentage of volatiles (or remaining solvent) is reduced to the desired; this process is tedious and delays greatly the delivery of the powder. For the nitroglycerine powder, the nitrocellulose is mixed directly with the nitroglycerine and then “grained” similarly as above. The process must be very carefully executed, owing to the extreme sensitiveness of the nitroglycerine. The grains, for both great guns and small arms, are in the form of cords (hence “cordite”) and vary in diameters according to the size of gun for which intended and are cut in length to suit the length of the powder charge or section of charge in which they are to be made up. A less lengthy period of drying is necessary for this powder.
Both powders give progressive burning, with a continuous impulse, in properly designed powders, behind the shell until its expulsion from the gun at a high initial velocity, without at any time undue pressure in the gun. The chief difference in the ballistic properties of the two powders lies in the higher temperature and chamber pressure of the cordites of a large proportion of nitroglycerine. To this is ascribed, by some authorities, the greater erosion caused by such powder, but it is now claimed that the erosion caused by the recent low-percentage-of-nitrogly-cerine cordites is no greater than with nitrocellulose powder. The controversy between the advocates of the two types is by no means ended, as will be seen by the adherents among the nations to each type. When carefully made, both give regular and uniform ballistic results, the costs differ little, and the advantage in weight of charge possessed by the earlier cordites is lessened in the later. The two terrific explosions in the French Navy, the Jena and the Liberte, have cast a doubt upon the stability of the nitrocellulose type, but cordite explosions have also been known, and the introduction of the stabilizer—a substance designed to counteract the first stages of decomposition—and periodical reworking of the powder have almost removed the danger of spontaneous combustion of the nitrocellulose type in properly cooled magazines. The tendency of the nitroglycerine in the cordite to freeze and to sweat has been decried. Neither type will stand a continued heat of over 100° F. in magazines for a length of time, and the old cry of “Trust in God and keep your powder dry” has been changed to “----- and keep your powder cool.”
High Explosives
Just as the development of smokeless powder has led to one substance, nitrocellulose, with or without nitroglycerine, for the base of all, so military high explosives, under whatever name of “D,” lyddite, melinite, shimose, T. N. T., etc., may all be traced, with the single exception of guncotton, to one source, the derivatives of the hydrocarbons of the aromatic or benzene series. These hydrocarbons have hitherto only been obtained commercially from coal tar, the distillation product of the manufacture of coke from coal, but now the discoveries of Dr. Rittman, of the U. S. Bureau of Mines, promise us an adequate supply, both for the explosive and for the aniline dye industries, obtained directly from crude petroleum.
Unlike the progressive burning of propellent powders, a high explosive must be capable of instant and complete detonation, with resultant great bursting pressure, and yet, for shell charges, must be insensitive to the shock of being fired from a gun. For mine and torpedo charges, insensitiveness, though desirable, need not be so high. The search for such a substance has resulted in the almost universal adoption of picric acid, C6H20H(N02)S. This is the trinitration product made by treating phenol (monohydroxy-benzene), known to the layman as carbolic acid, with concentrated nitric acid. It is a yellowish, bitter-smelling and acrid-tasting crystalline powder, insensitive to shock, burns freely on application of a flame, but detonates instantaneously under the blow of a proper “ detonator,” such as mercury fulminate. It is chemically quite stable, and will not deteriorate, but forms sensitive salts, “ picrates,” with most metals; hence it must be prevented from attacking the walls of the shell cavity by lacquering them. On detonation, the substance is instantaneously, not progressively as in propellent powders, changed into its constituent, or rather resultant, gaseous products, mostly oxides of carbon and nitrogen, pure nitrogen, and water vapor, with very high temperature evolved. The confinement of the quantity of gas at a high heat produces an immense bursting force within the shell.
Of the various powers, France uses almost pure picric acid under the name of melinite; the Germans use it, as also its related T. N. T. (which see below); the Japanese shimose is picric acid; the English lyddite nearly pure picric acid, having a small percentage of dinitrobenzene and vaseline.
A second important but perhaps less widely used high explosive is tri-nitrotoluene, or T. N. T. This is the tri-nitrated product of toluene, methyl-benzene (CH3C6H5), and bears the formula CH3C8H2(N02)3, and is made by the action of fuming and concentrated nitric acid on toluene. It is being much used in Germany for mines and torpedoes, possibly for shells also, and has been advocated for adoption by the United States. It is understood also that the British and Italians are adopting it. Like picric acid, it is a crystalline powder, nearly white (in commercial quality; dead white when pure), insensitive except to proper detonators. It is chemically very stable, does not form, in contact with metals, sensitive compounds, and possesses the mechanical advantage that it may be melted and poured into a mold, as for shell fillers, torpedo charges, mine charges, etc., thus doing away with the laborious and somewhat dangerous packing and tamping by hand necessary for picric acid charges. Its detonation products are of the same nature as those of picric acid.
Chemically closely akin to picric acid is tri-nitro-cresol, the “cresylite” of the French and the base of the Austrian “ecrasite.” It is simply picric acid, in which one of the H atoms has been replaced by the radical CH3, giving CH3C0HOH(NO2)3, so that the product is comparatively more rich in carbon and hydrogen and less rich in oxygen percentage than picric acid. It has much the same characteristics. It is little used, even in France and Austria.
Totally different, however, from these aromatic hydrocarbon explosives is guncotton, used largely everywhere, except in Germany, for mine and torpedo charges. This is made as described for nitrocellulose for smokeless powder, but with slightly stronger treatment by the reacting acids, with resultant higher nitration. It is then formed into cakes and blocks suitable for loading into torpedo war-heads and mine cases. The bulk of such charges is wet guncotton—guncotton saturated with water to 25 per cent (approximately, differing for various countries) of the total resultant weight—and only a small priming charge of dry guncotton is carried. The wet has the advantages of great insensitiveness to shock, being detonated only by the detonation of dry guncotton, and of good chemical stability, as long as its water content is not allowed to evaporate. The dry, however, is liable to deterioration and is very sensitive to shock; defects which, since the dry guncotton must be carried as a detonating charge, militate against the advantages of the wet.
Nor must be forgotten the father of all explosives, black powder. Originally used primarily as a propellent powder, it is now used only as a bursting charge, and that only in shrapnel and in small caliber shells; for these purposes it is used by all nations. It is a mechanical mixture (while all other explosives heretofore mentioned are chemical compounds) of carbon (as charcoal), sulphur, and potassium nitrate, made in varying proportions in different countries, but in the general approximate ratio of 15 per cent C, 10 per cent S, 75 per cent KNOa. This powder is incapable, under ordinary circumstances, of instantaneous detonation, but when confined in a small space burns or explodes so rapidly that a bursting effect is produced almost as great as that of a detonation. The decomposition products are oxides of carbon, nitrogen, and potassium sulphide, a solid. This too-rapid burning, with great heat and pressure, and the presence of a solid residue, were, as well as the smoke (mostly caused by the solid), reasons for the abandonment of black powder in favor of, as a propellant, “smokeless” powder. Black powder needs no special detonator, any flame- producing fuse igniting it, and is chemically stable, except that it loses much of its power when damp or wet. It is interesting to note that its smoke of explosion is white, and that of the high explosives on detonation is black, so that the white puffs of smoke described in accounts of present-day battles may all be ascribed to black-powder shells, usually shrapnel, while the black puffs or clouds show the larger shells charged with high explosives.
Mention has been made of the detonation of these high explosives. Detonation, as opposed to burning, a progressive conflagration, is the complete instantaneous transformation of the entire mass of the explosive into its resultant explosion-products, all gaseous in a well-chosen explosive. If the explosive is sufficiently insensitive to resist the shock of firing, naturally the hardly greater shock of striking will be insufficient to cause detonation. Consequently, the detonation is accomplished by means of a small quantity of some other explosive, contained in the fuse of the shell, or in the “exploder” or “detonator” of the mine or torpedo. This substance is generally mercury fulminate, a very powerful but expensive and sensitive explosive—hence not suitable for the bulk of the charge—secured in the fuse so that not until impact will it be exploded. It is a gray crystalline powder, of formula Hg(OCN)2 made by the action of concentrated nitric acid on metallic mercury in the presence of alcohol. For large explosive charges, however, a heavy impulse or shock is necessary to detonate, requiring a dangerous amount of fulminate; this is avoided by the use of a “booster” charge, contained in the fuse, of picric acid, T. N. T., or some other such material, which is detonated by the fulminate and, by its own detonations, in turn detonates the main charge. For wet guncotton charges, the booster, in this case known as a primer, is dry guncotton. Another detonating substance, less sensitive than mercury fulminate, and as powerful, is lead trinitride, Pb(N3)2, but to the present it has been adopted, it is believed, by but one country, Brazil.
Note.—An idea of the terrific effect of the instantaneous detonation of a high explosive may be shown by a brief, rough calculation. Assume an explosive carge of 48 lbs. of picric acid, such as might easily be carried in a thin-walled shell, with shell cavity volume about 2/3 cubic feet, from the 11-inch German siege howitzer. This is entirely converted into gas according to the approximate equation (owing to internal reactions and the phenomenon of “disassociation’’ the exact equation will differ, but this reaction as given will err rather on the side of too little than of too great a volume of the resultant gases),
C6H2OH (NO2) 2 = 4 CO2 + CO + H2O + N2 + HCN, giving eight volumes of gas. For eight cubic feet of such gases, at 0° C. and atmospheric pressure, the weight would be .64047 lbs.; but since all the explosive is converted into gas, there are 48 lbs. of gas, or the total volume is
48/.64047 = 600 cu. ft. (about).
That is, if unconfined at normal temperature and pressure, the volume would be 600 cu. ft. But this volume is confined in a space of about 2/3 cu. ft. in the shell cavity, and also it is subjected to the extremely high temperature caused by the chemical action of detonation. This temperature has been estimated at nearly 4000° C. Then to find the pressure in the shell by Charles’ law,
PVT' = P'V'T, or
14.7 X 600 X 4273 = P' X 2/3 X 273, or
atmos. pres. X normal vol. X abs. temp, of explosion, equals pressure of explosion X volume of shell X normal (0° C.) temp.
Then P' = 14.7_X 600 X 4273 X 3 / 2 X 273
= 202,270 lbs. per square inch
= 100 + short tons per square inch, showing the tremendous disruptive pressure within the shell at the moment of detonation.
Containers
It is hardly the province of this brief description to go deeply into the subject of containers, but the principal types, such as shells, torpedoes, mines, and bombs, will be noted in respect to their explosive content and its arrangement. (For propellent powders, of course, the container is the gun, of which no description is necessary.)
Shells are of three general types, armor-piercing, common, and shrapnel. The armor-piercing, used by all navies and in siege gun operations against armored forts, are special steel, thoroughly heat-treated to hardness and toughness, thick-walled and contain a small cavity and necessarily small charge of high explosive, detonated generally by a delayed action fuse, to allow passage through armor before detonation. Common shell, in use mainly in land operations against field works and troops, are ordinary steel, thin-walled, non-heat-treated (to any extent), containing a larger quantity of high explosive, detonated immediately upon impact by the fuse. Shrapnel are very thin-walled shell, containing a number of spherical balls of about .3 to .5 inch in diameter, with a small bursting charge either of high explosive or of black powder, and fitted with a fuse to explode the shell either on impact or after a certain time of flight determined by the artillerist’s setting of the fuse before the shell is fired from the gun. Except for the black powder in shrapnel, the explosive used in shells is some form, as noted above, of the hydrocarbon high explosives. The weight of the explosive content varies, of course, with the caliber and type of the shell.
Torpedoes carry a large charge in the thin steel war-head, the forward compartment of the torpedo. This charge is immediately on impact exploded by an “exploder” or “detonator,” carried in the extreme nose. The explosive in general use is wet guncotton, although Germany uses T. N. T. The normal explosive content of a torpedo is 200-250 lbs., but it is rumored that the German torpedo for submarine boats carries a charge of as much as 400 lbs.
Marine mines are thin steel cases, generally spherical, almost entirely filled with high explosive, either wet guncotton or T. N. T. They are exploded either by action of a percussion plunger on a fulminate exploder (a type now almost obsolete), or by the firing of a fulminate detonator by the completion of an electric current; this last effected either by the impact of a vessel, or by closing a key in an observation station. The explosive content of a large mine may run to 500 pounds, of the smaller as low as 200. Little has been made public of the nature of the land mines used in this war. It is probable, however, that they are chambers dug out of the earth, and filled with explosive, either packed directly into the chamber, or preferably in light metal cases. The explosive may be either black powder or high explosive, probably the former, as it is cheaper and nearly as efficient for the purpose; the ignition may be accomplished either by a slow-match fuse, or by closing an electric circuit from a distance.
Bombs are similar to mines in that they are thin-steel cases, spherical or pear-shaped, packed with high explosive, the exact nature of which is not known, but is probably in each case the hydrocarbon explosive favored by the country making and employing the bomb; and fitted with a fuse to detonate the contents on impact. Hand-grenades are similar, though sometimes of cylindrical shape. The explosive content varies, of course, with the size of the bomb, which may be of the small type provided for aeroplanes, carrying 5-35 lbs. of explosive, to the air-ship bombs containing as much as 200 lbs. Hand-grenades carry about 2-3 lbs. of explosive.