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.