Aircraft technical Basics: TM 1-409, Aircraft Armament and Pyrotechnics, 1941: I. - Explosives

Section I - Explosives

1. General.-a. Purpose and scope.-This publication is designed to provide a text for students of Army Extension Course 30-9. It is introductory and general in its nature and is intended to supply exactly the students' need of a t ext for this course. Being introductory, it is presumed that after completion of the extension course the student will be afforded the opportunity of receiving additional advanced training. A simple statement has been made of the basic principles of aircraft armament so as to furnish to one unacquainted with that subject a background that will suffice as a foundation for acquirement in the future of skill and knowledge when further study is undertaken and experience obtained.

b. Definitions.-(1) Explosive.-An explosive may be defined in general as being a gaseous, a liquid, or a solid substance, or mixture of substances, which upon application of a blow to a small portion of its mass, or by a rise in temperature is converted in a small space of time into other substances more stable which are mainly gases or vapors but may include solids. The chemical changes thus produced develop a sudden rise in pressure in the surrounding medium, and the word "explosive" is applied as a general term to the different types of phenomena occurring in this sort of reaction. Coincident with this evolution of gas, a considerable amount of heat is evolved which produces a flame. A high explosive is one in which this change occurs within the shortest space of time and the phenomenon accompanying this decomposition is termed a detonation. Detonation therefore applies more properly to the chemical reaction incident to the almost instantaneous decomposition of the high explosive. However, it is not properly employed when discussing the analogous reaction incident to combustion of a slower burning explosive as smokeless powder. The general term explosive which finds universal acceptance applies alike to high explosives, black powder, and smokeless powder. It will be seen, therefore, that the accepted distinction between the different classes of explosives is based primarily upon their relative speed of decomposition, and only those explosives are classified as high explosives in which this chemical change is considered practically instantaneous.

(2) Explosion.-Marshall, in his work on "Explosives," Vol. 1, 1917, pages 135 and 136, published by Blakistons & Co., defines an explosion thus : "When gas or vapor is released so suddenly as to cause a loud noise, an explosion is said to take place as, for instance, the explosion of a steam boiler or of a cylinder of compressed gas." The barrel of the gun may be considered at the time of the explosion as being a cylinder of compressed gas. Walls of the gun constitute walls of the cylinder, the breech mechanism one end, and the projectile the other.

2. Low explosives.-The oldest common low explosive is black powder which has been known since 1250 A. D., at which time it was made of a mixture of saltpeter, sulphur, and charcoal. The present composition of black powder used for military purposes is a mixture of potassium nitrate or sodium nitrate, charcoal, and sulphur in the approximate proportions of 75, 15, and 10-varying percentages being used with varying effects. Depending upon the use to which the particular grade is to be put, the. percentage composition of the finished material is altered as necessity demands. It has been found, for instance, that by increasing the percentage of saltpeter, the rate of burning is increased; by increasing the percentage of charcoal, the rate of burning is decreased.

3. Use.-a. The present military use of black powder is practically confined to

(1) Ignition charges.

(2) Base charge or expelling charge for shrapnel shell.

3) Manufacture of primers and fuzes.

(4) Saluting and blank-fire charges.

(5) Time-train rings and combination fuzes.

b. Black powder formerly used altogether as a propellant has been superseded almost entirely, being replaced by smokeless powder. Among some of the factors which may be mentioned as responsible for effecting this change are

(1) Large quantity of solid residue after ignition of charge.

(2) Volume of smoke caused by pressure of a large quantity of noncombustible material.

(3) Relatively great speed of erosion of the gun barrel due to high temperature of combustion.

(4) Rapid deterioration when exposed to atmospheric conditions due to hygroscopic nature.

4. Smokeless powder.-The term "smokeless powder" when considered from the exact meaning of the word is a paradoxical misnomer, for the substance is not entirely smokeless and it is not a powder. Smokeless powder is manufactured in the form of small flakes, strips, pellets, sheets, or perforated cylindrical grains. The last mentioned form of grain is the one most commonly used in the military powders manufactured in the United States. These cylindrical grains are made with diameters varying from 0.032 inch for the caliber .30 rifle to 0.947 inch for the 16-inch gun, and in corresponding lengths varying from 0.085 inch to 2.170 inches. In general, the smaller caliber weapons are provided with cylindrical powders having a single perforation running lengthwise through the grain, while powders having seven perforations are used in the larger caliber weapons. Triperforated powders have been used experimentally and a rosette or so-called sliverless grain has been used to some extent; such forms of grains are resorted to only when special ballistic properties are required.

5. Composition.-Smokeless powder consists of cellulose obtained from the lint of cotton treated with a mixture of nitric and sulphuric acids. This product is then called a cellulose nitrate or "pyrocotton." After purification of pyrocotton from all traces of free acids and lower nitrates, pyrocotton is mixed with ether-alcohol and pressed into the form of a colloid. This colloid is then pressed through steel dies to form the shape and size of the grains. After granulating, the final step is the removing of all excess solvent and drying and blending of the powder to such a point that it will give the proper burning characteristics desired from that lot of powder.

6. Compound propellants.-The ideal powder for any given weapon is a powder which builds up the pressure in the chamber progressively, producing the most pressure just as the projectile leaves the muzzle of the weapon. However, in actual practice, the maximum compression in the gun barrel is never developed at the muzzle, a condition which would cause bursting, according to our present design in which maximum pressure is exerted just after the projectile starts moving from its original position; usually maximum pressure occurs when the projectile is only one foot or so ahead of its original position.

a. Thus it may be seen that for short-barreled weapons, such as a pistol, a powder must be developed which will burn more rapidly than that used in a long-barreled weapon. For this purpose such a powder has been developed known as a compound propellant or "double base" powder. Compound propellants are characterized by their fast burning; being composed of two bases, namely nitrocellulose and nitro-glycerine. While nitroglycerine is in itself a high explosive, its action when mixed in small quantities with nitrocellulose is to speed up the burning of the nitrocellulose product. These powders as used by various military services usually contain from 60 to 80 percent of nitrocellulose and from 40 to 20 percent nitroglycerine. Small percentages of inorganic salts, such as potassium or barium nitrate, are often used in double base powders, these salts serving the purpose of reducing flash or rendering the powder more ignitible. Frequently the grains of compound propellants are in the form of thin strips or flakes, sometimes in the shape of small balls, to add more surface and thereby further hasten the burning time.

b. The present tendency is to draw a sharp distinction between two classes of explosive bodies in accordance with the use to which they are put and also in accordance with the speed of reaction whereby combustion or decomposition takes place.

(1) Propellants.-Those substances or mixtures whose rate of combustion is such as to permit their use for propelling projectiles from guns are termed propellants.

(2) High earplosives.-Those substances whose rate of decomposition is so very high as to preclude their use as propellants but which on the other hand bring about very powerful disruptive action are known as detonating explosives, disruptives, or more commonly, as high explosives.

7. High explosives.-High explosives differ from low explosives in that, when initiated, instead of burning progressively through the mass or grain, the entire mass decomposes instantaneously, thereby causing considerable heaving or explosive effect. This decomposition is called detonation and the progress of the detonation throughout the mass of explosive is called the "detonating wave."

8. Explosive train.-a. Of all high explosives used by the military services, mercury fulminate is the most sensitive. It may be initiated by very slight shocks and/or by a very small amount of heat. The heat of the hand has been said to sometimes explode a mercury fulminate cap. For this reason mercury fulminate is used for all priming charges, both in small arms ammunition and in aerial bombs. Since the filler charge for aerial bombs is quite insensitive to shock, it has been found necessary to have what is known as an explosive train, whereby initial shock explodes a small amount of very sensitive explosive which in turn sets off a larger amount of less sensitive explosive which produces sufficient detonating wave to initiate the main explosion. In small arms ammunition the priming charge of mercury fulminate is mixed with other ingredients, thus altering it. from a high explosive to a low explosive. A primer composed of mercury fulminate constitutes the first link in the explosive train.

b. The second link comprises the delay element in cases where it is desired to delay the explosion of a bursting charge for a period of time after the bomb has struck its objective. This delay is brought about by the introduction of a column of pressed black powder.

c. The third link in the explosive train is called the detonator.

NOTE.-The detonator may also he the first link in cases where no delay is desired, when it is caIled a "primer-detonator." As in primers so also in detonators the explosive charge is mercury fuIminate. The explosion of the mercury fulminate transmits sufficient energy to detonate the next link in the explosive train which is the booster charge. Tetryl is the explosive used by military services for booster charges because of its greater sensitivity to shock and heat than TNT and other bursting charges, while at the same time it is more stable and less sensitive to shock or heat than is mercury fulminate. Normally the booster contains more explosive than the detonator (or primer), and it imparts sufficient shock to the bursting charge to detonate the entire mass.

d. The final link in the explosive train is the bursting charge. Several explosives have been found suitable for use as bursting charges by this country. The most important and the most used, however, are TNT, explosive "D", and amatol. Amatol is a mixture of ammonium nitrate and TNT and has approximately the same properties as TNT. Its use during the World War was brought on by shortage of toluene, and it is listed at present as a substitute for TNT in case such a shortage ever occurs again.

e. Trinitrotoluene, or TNT is the standard bursting charge for bombs, mines, and blasting work of the military forces. In aerial bombs it has a disadvantage in that penetration of the bomb by a rifle bullet may cause an explosion of the charge. This is also true of amatol, which is used extensively in drop bombs.

f. Ammonium picrate called explosive "D" is also used as a bursting charge for armor piercing shell and delayed action demolition bombs on account of its insensitiveness to shock, which permits the shell or bomb to pass through armor without exploding.