HIGH AND LOW EXPLOSIVES
Military explosives are divided into two general classes, high explosives and low explosives, according to their rate of decomposition.
High explosives are usually nitration products of organic substances, such as toluene, phenol, pentaerythritol, arnines, glycerin, and starch, and may be nitrogen-containing inorganic substances or mixtures of both. TNT is an example of a high explosive. Ahigh explosive may be a pure compound or a mixture of several compounds with additives, such as powdered metals (aluminum), plasticizing oils, or waxes, that impart desired stability and performance characteristics. A high explosive is characterized by the extreme rapidity with which its decomposition occurs; this action is known as "detonation." When initiated by a blow or shock, it will decompose almost instantaneously in a manner similar to an extremely rapid combustion or with rupture and rearrangement of the molecules themselves. In either case, gaseous and solid products of reaction are produced. The disruptive effect of the reaction makes these explosives valuable as a bursting charge but precludes their use as a low-explosive propellant.
Low explosives are mostly solid combustible materials that decompose rapidly but do not normally detonate. This action is known as "deflagration." Upon ignition and decomposition, low explosives develop a large volume of gases that produce enough pressure to propel a projectile in a definite direction. The rate of burning is an important characteristic that depends upon such factors as combustion gas pressure, grain size and form, and composition. Under certain conditions, low explosives may be made to detonate in the same manner as high explosives.
CHARACTERISTICS OF EXPLOSIVE REACTIONS
The most important characteristics of explosive reactions are as follows:
1. VELOCITY An explosive reaction differs from ordinary combustion in the velocity of the reaction. The velocity of combustion of explosives may vary within rather wide limits, depending upon the kind of explosive substance and upon its physical state. For high explosives the velocity, or time of reaction, is high (usually in feet per second), as opposed to low explosives, where the velocity is low (usually in seconds per foot).
2. HEAT An explosive reaction of a high explosive is always accompanied by the rapid liberation of heat. The amount of heat represents the energy of the explosive and its potential for doing work.
3. GASES: The principal gaseous products of the more common explosives are carbon dioxide, carbon monoxide, water vapor, nitrogen, nitrogen oxides, hydrogen, methane, and hydrogen cyanide. Some of these gases are suffocating, some are actively poisonous, and some are combustible. For example, the flame at the muzzle of a gun when it is fired results from the burning of these gases in air. Similarly, solid residues of the explosives remaining in the gun have been known to ignite when brought into contact with air as the breech is opened. The ignition may come from high temperature of the gas or from the burning residue in the gun bore. The resulting explosion may transmit
flame to the rear of the gun, producing what is called a "flareback." This danger has led to the adoption of gas-expelling devices on guns installed in enclosed compartments or mounts.
4. PRESSURE: The high pressure accompanying an explosive reaction is due to the formation of gases that are expanded by the heat liberated in the reaction. The work that the reaction is capable of performing depends upon the volume of the gases and the amount of heat liberated. The maximum pressure developed and the way in which the energy of the explosion is applied depend further upon the velocity of the reaction. When the reaction proceeds at a low velocity, the gases receive heat while being evolved, and the maximum pressure is attained comparatively late in the reaction. If in the explosion of another substance the same volume of gas is produced and the same amount of heat is liberated but at a greater velocity, the maximum pressure will be reached sooner and will be quantitatively greater. However, disregarding heat losses, the work done will be equal. The rapidity with which an explosive develops its maximum pressure is a measure of the quality known as "brisance." A brisant explosive is one in which the maximum pressure is attained so rapidly that a shock wave is formed, and the net effect is to shatter material surrounding or in contact with it. Thus brisance is a measure of the shattering ability of an explosive.
5. STABILITY The stability of an explosive is important in determining the length of time it can be kept under normal stowage conditions without deterioration and its adaptability to various military uses. A good, general explosive should stand a reasonable exposure to such extremes as high humidity in a hot climate or cold temperatures of arctic conditions.
6. IGNITION TEMPERATURES: There is no one temperature of ignition or detonation in an explosive for its behavior when heated depends on two factors: the reamer of confinement and the rate and manner of heating. It is usually possible, however, to find a small range of temperatures within which a given explosive will ignite or detonate. These so-called ignition temperatures, or explosion temperatures, are useful in setting limits near which it is certainly unsafe to heat an explosive. When an unconfined explosive is heated sufficiently, it may detonate or simply catch fire and burn. Detonation can occur either immediately or after an interval of burning. In general, the likelihood of detonation rises rapidly with increasing confinement because of the resultant rise in pressure.
7. LOADING PROPERTIES: The adaptability of an explosive to loading requirements is an important factor in fixing its range of usefulness. When projectiles are press-loaded, it is necessary to fill them with a
granular explosive. On the other hand, cast-loaded munitions require either an explosive having a relatively low melting point or a thermosetting plastic to act as a casting medium.
8. SENSITIVITY: The amount of energy necessary to initiate an explosion is the measure of the sensitivity of the explosive. Sensitivity is an important consideration in selecting an explosive for a particular
purpose. For example, the explosive in an armor-piercing projectile must be relatively insensitive; otherwise, the shock of impact would detonate it before it had penetrated to the point desired