EXPLOSIVES.
As a matter of practical interest, explosives may be divided into three classes, namely:
(1) Progressive or propelling explosives called low explosives.
(2) Detonating or disruptive explosives, termed high explosives.
(3) Detonators or exploders, known as fulminates.
The first includes all classes of gun powders used in firearms of all kinds; the second, explosives used in shell, torpedoes, and for demolitions; the third, those explosives used to originate explosive reactions in the two first classes. Corresponding names are given to the phenomena characteristic of each class of explosives, (1) explosions proper, of low order, progressive, or combustions, (2) detonations, of high order, (3) fulminations, this last possessing exceptional brusqueness.
The explosion of low order is marked by more or less progression; the time element is involved as a controlling factor, the time required to complete the explosive reaction being large compared with that of the other forms of explosion.
The second class of explosion is of a different nature. The explosive reaction is not limited or confined to the surfaces exposed but appears to progress in all directions throughout the mass radially from the point of initial explosion. It has been determined experimentally that the velocity of propagation of the explosive wave throughout a mass of guncotton is from 17,000 to 21,000 feet per second.
Fulmination is a class of explosion still more brusque than the last. The abruptness of their explosion and the consequent sharpness of the blow and the concentration of heat on the point of ignition constituting their efficiency as originators of explosions of the first two classes.
Methods of Exploding. Explosives may be exploded by three methods; in reality but two, by heat and by application of energy as by a blow. The heat may be applied directly by friction, by electricity and detonating cap, these two methods of applying the heat giving rise to the three practical methods above mentioned. As it is not practical to apply heat directly to the charge, small charges of special explosives are made up into primers and these are exploded in one of the ways above mentioned and so communicate the explosion to the main charge. Fulminate of mercury is one of the high explosives fulfilling the requirements and it is readily exploded by any one of the methods mentioned. It is used in all detonating caps. Primers for cannon also contain an additional charge of black powder to increase the flame. For this purpose also igniting charges of black powder are attached to the smokeless powder charges for the larger calibers.
Uses. The chief use of low or progressive explosives is as a propelling charge in guns and for blasting where it is desired to exert a pushing effect rather than a blow. High explosives are used when it is desired to exert a high pressure and shatter the container, as in a shell, mine, etc. This class is not satisfactory as a propelling charge for the reason that its rapidity of action is so great that the pressure exerted would burst the gun before the projectile could start. Low explosives are not satisfactory shell fillers for the reason that their action is so low that the shell would break at its weakest point before all the explosives had exploded and what remained would be wasted. With a high explosive, all or most of the charge explodes before the shell can break up. The greater the rapidity of action of an explosive the finer the fragmentation of the projectile. With too rapid action the pieces are too small; with too slow action they are too large. Experience teaches the proper rapidity of action to attain the fragmentation most efficient against animate and material targets.
Propelling Charges. Up to the present time nitrocellulose powder has complied better with the requirements of a suitable, smokeless powder than any other that has been proposed and is used in our service for propelling charges in guns. The danger of manufacture is also less than that of nitroglycerine powders. Moreover the latter, which was formerly used in our service and still is in the British and some others, causes too much erosion of the tubes due to the greater heat of explosion. It has the advantage of requiring a smaller charge for the same muzzle velocity and therefore a smaller powder space and consequent lighter weight of gun.
Shell Fillers. High explosives for shell fillers. Up to the present explosive “D,” trinitrotoluol and picric acid are the principal high explosives which fulfill the requirements as shell fillers. Explosive “D” on account of its great insensitiveness to shock is used in armor piercing projectiles and also in field gun and howitzer shell. It is detonated by a fuze. Trinitrotoluol is used in submarine mines and in general demolition work as it is much easier to explode than explosive “D.”
Table of Explosives. The following table gives a good idea of the principal explosives in use in our service and the characteristics of each:
| Purpose. | High or Low. | Name. |
| Propelling charge in guns. | L | Nitrocellulose, smokeless powder. |
| Bursting charge, projectiles. | H | Picric acid, explosive “D” (powder form) Trinitrotoluol. |
| Blank, saluting charges. | L | Black Powder. |
| Re-inforce charges, primers. | L | Black Powder. |
| Base charge, shrapnel. | L | Black Powder. |
| Time trains, fuzes. | L | Black Powder. |
| Igniting charges, cannon. | L | Black Powder. |
| Charges, submarine mines. | H | Trinitrotoluol wet guncotton. |
| Igniting elements, fuzes and primers. | H | Fulminate of mercury, chlorate of potash (potassium chlorate). |
High Explosives. The principal high explosives used as shell fillers in our service are: picric acid, explosive “D” and trinitrotoluol, or more popularly known at TNT. The picric acid and picrates used as shell fillers are secret compositions. Mellinite, essentially picric acid alone or with some other substance is used as a shell filler by the French. It is poured into the shell in a fused state and allowed to harden, thus giving a very compact charge and one easily handled. It has the disadvantage however of forming unstable compounds with the metal of the shell and great care must be exercised in coating the interior of the shell with a protective coat before pouring in the fused mellinite. Lyddite is the English equivalent of mellinite. Picric acid was also used by the Japanese or it may be a mixture of picric acid and some nitro compound. The most successful explosive of this type is explosive “D” invented by Colonel Dunn of our Ordnance Department and sometimes known as “Dunnite.” It is not fusible and must be compressed for use as a shell filler, being forced into the shell by compression. This is a disadvantage as compared to mellinite as the density of loading is less and weight for weight therefore less efficient. It is little sensitive to shock and therefore not very dangerous to load even under great pressure. Trinitrotoluol is also used as a shell filler but its chief use is in demolition work and as the charge for submarine mines.
Nitrogen Compounds. It may be interesting to note that all of the principal explosives with which we have been dealing are compounds containing nitrogen. In fact the war has been fought with fixed nitrogen which explains the great interest taken in the various attempts to fix the free nitrogen of the air which is the world’s great storehouse of free nitrogen. As nitrogen is also a necessary ingredient in the various fertilizers, the result to the world of a commercial process for speeding up the cycle of changes through which nitrogen passes in its life giving mission from free nitrogen in the air to its various compounds in the nitrogeneous animal and vegetable tissues is almost limitless and as usual war has been the incentive to speed up a process which will result in incalculable value to mankind.
Classification. Guns are loaded with three kinds of ammunition: fixed, semi-fixed and separate loading ammunition. In fixed ammunition the round is complete and projectile and powder loaded into the chamber at the same time. In semi-fixed the projectile is separate from the powder charge, which however is put up and loaded into the chamber in a container. In separate loading ammunition the powder is loaded into the chamber in bags. In the first two cases the cartridge case furnishes the means for sealing the rear of the powder chamber against escape to the rear of the powder gases. In the last case some form of obturating device is made a part of the breechblock furnishing a gas check to seal the rear of the powder chamber.
Fixed Ammunition. All of our field guns below 5 inches in calibre use fixed ammunition. The powder is placed loose in the cartridge case, the space not filled with powder being stuffed with packing paper, excelsior, or felt wadding next to the projectile so as to hold the powder in contact with the primer, in some fixed ammunition a brass diaphragm is soldered to the inside of the case for the same purpose and to keep out moisture, (4.7” Gun). An igniting charge of black powder is a part of the primer and in some cases an additional charge is placed at the forward end of the powder space in the cartridge case to insure rapid ignition of the smokeless powder. In this case it is held in place between two quilted disks of crinoline.
Semi-fixed ammunition is employed in our 6” and 4.7” field howitzers. The cartridge case contains three weights of propelling charge for firing in the three zones designed to give a high angle of all with these weapons. Access to the charge is had by tearing off the brass diaphragm closing the forward end of the cartridge case. By removing the first charge the remaining charge is that prescribed for the second zone, and by removing the top two charges the remaining charge is that of the first zone. The three charges are tied together and the middle charge has an igniting charge of black powder attached. The removal of charges is facilitated by the separate container for the powder charge and the round is more easily handled in the two parts especially in the case of the six-inch howitzer, where the projectile weighs 120 lbs. The same primer is used as in fixed ammunition, the cartridge case performing the function of an obturator.
BREECH MECHANISM OF 155 GUN SHOWING
DE BANGE OBTURATOR.
Separate Ammunition.—Obturation. The 155-mm Filloux gun and 155-mm howitzer use separate ammunition. In such guns there must be provided some form of a gas check which will prevent the powder gases from rushing to the rear into the threaded portion of the breechblock, as this would soon erode the thread sectors and render the gun useless beside losing a large amount of pressure in the bore. The device used as a gas check is called an obturator. There are two systems of obturation in use, named after their inventors:
The DeBange and the Freyre. The former is used in the 155’s. It consists of a steel mushroom head closing the rear of the powder chamber, the spindle of which passes through a central hole in the breechblock. Between the mushroom head and the face of the breechblock is a pad of asbestos, paraffine and tallow, pressed into shape by a hydraulic press and covered by canvas or asbestos wirecloth. Split rings having hardened outer surfaces are fitted, one just behind the mushroom head and one just in front of the face of the breechblock. Their diameter is slightly greater in the free state than the conical surface of the bore where they bear when the breech is closed so that they always close the rear of the powder chamber. The pressure of the powder gases forces the mushroom head to the rear and this compresses the asbestos pad which in turn forces the split rings to bear with greater force against the walls of the powder chamber thus securely closing the rear opening of the powder chamber. For more details of this device see pages 302 to 306 Tschappat’s O & G.
Powder Bags. Cartridge bags for separate loading are made of raw silk, and are sewed with silk thread. Other materials are apt to produce flare-backs or premature explosions because they are not entirely consumed in the bore or continue to burn if not consumed. The raw silk however either is entirely consumed or if not, the parts ignited immediately go out as soon as the flame is removed and do not smoulder. Specially treated cotton fibre bags have been tried but so far as I know have not as yet superseded the raw silk for the purpose. The gases remaining in the bore after the discharge of a charge of smokeless powder are explosive and with air form an explosive mixture, hence the danger upon opening the breech if any smouldering particles remain in the bore.
21 GRAIN PERCUSSION PRIMER MARK II-A
Primers. The devices for initiating explosions of propelling charges in military guns are called primers. With fixed and semi-fixed ammunition the primers are seated in the base of the cartridge case. In the case of separate loading ammunition the primers are inserted separately in the breechblock, the expanding gases of the detonated primer forcing the walls of the primer case tightly against the bore through the breechblock and thus sealing this channel of escape for the gases from the powder chamber. This necessitates a much larger and stronger case for separate loading primers than for those inserted in the base of a cartridge case.
Classes of primers. Primers are divided into three classes according to the method by which they are fired: (a) friction primers, (2) electric primers, (3) percussion primers. Combination primers are made which may be fired by any two of these methods, usually electric and one of the others. The characteristics of a good primer are, certainty of action, safety in handling, no deterioration in storage, simplicity in construction and be cheap to manufacture. They are also divided into obturating and non-obturating depending upon whether they close the vent during discharge or not.
Primer pressing. Primers for fixed ammunition are inserted in the base of the cartridge cases by means of a special press for this purpose. The primer body is a trifle larger than the seat in the cartridge case provided. This seat is rough bored to a diameter less than the finished size and then mandreled to finished dimensions with a steel tapered plug. This process toughens the material of the case around the primer seat and prevents the expansion of the primer seat under pressure of the expanding gases.
Percussion primers. Except for very heavy siege guns and railroad artillery the guns handled by the Field Artillery use percussion primers. The 110-grain percussion primer is the one in use in our service and as typical will be described. The charge consisting of 110 grains of compressed black powder makes the charge burn like a torch rather than explode, which facilitates the ignition of the charge of smokeless powder, with which the flame comes in contact. The diametral holes spray the flame in several directions thus insuring ignition at many points simultaneously. The percussion element consists of a percussion primer cup, the percussion composition and an anvil, all of which are assembled together in a cup in the rear face of the primer case. The percussion composition is made up of chlorate of potash, sulphide of antimony, ground glass and sulphur. A blow upon the cap by the firing pin detonates the percussion composition and the flame from this detonation ignites the black powder which in turn explodes the charge of smokeless powder.