An explosive reaction is initiated by the application of energy. The preferred method of initiation depends on the characteristics of the individual explosive. The most commonly used methods of initiation are the following:

1. By heat: Low explosives are commonly initiated by the application of heat in some form. High explosives will react when sufficient heat is applied, especially if heat is applied suddenly throughout the mass. Initiation by percussion (direct blow) or by friction is simply initiation by heat derived from the energy of these actions.

2. By shock (detonation): High explosives in general, such as the main charges of gun projectiles, require the sudden application of a strong shock to initiate the explosive reaction. This shock or detonation is usually obtained by exploding a smaller charge of a more sensitive high explosive that is in contact with or in close proximity to the main charge. The smaller charge can readily be exploded by heat or shock.

3. By influence: It has frequently been: demonstrated that detonation of an explosive mass can be transmitted to other masses of high explosive in the vicinity without actual contact. It has been generally accepted that such transmission is caused either by the passage of an explosive percussion wave from one mass to the other or by fragments. The second explosion occurring under these conditions is said to be initiated by influence. The result is called a "sympathetic" detonation or explosion. The distance through which this action may take place varies with the kinds of explosive used, the intervening medium, and certain other conditions.

CLASSIFICATION

The classification of explosives thus far has been based on characteristics. A more practical classification, from the standpoint of the GM, is based on military uses of the explosives. These classifications are

initiating explosives,

booster explosives and igniters,

propellants, and

the main charge.

Initiating Explosives

"Initiating explosives" are those explosives that

serve to initiate the ignition of propellants and the reaction of high explosives. Initiating explosives function when subjected to heat, impact, or friction.

Initiating explosives may function by themselves, as does the primer cap in a small-arms cartridge. However, inmost instances the initiating charge is the lead element in what is known as an "explosive train." An explosive train uses the impulse of an initiating explosive to initiate the chain reaction that leads to the detonation of a main burster charge or ignition of a propellant.

Booster Explosives

Relatively insensitive disrupting explosives require an intermediate charge to increase the shock of the initiating explosive to ensure proper reaction of the main explosive charge. A booster increases the shock of the initiating explosive to a degree sufficient to explode the disrupting explosive.

The basic high-explosive train consists of the initiator, the booster, and the main (burster) charge. However, high-explosive trains are often compounded by the addition of intermediate charges and time delays. (See fig. l-l.) An intermediate charge functions

Figure 1-1.-High-explosive train.

between the initial charge and the booster to ensure the detonation of the booster.

The large quantity and relatively slow burning rate of gun propellant requires the use of another type of exposive train. The small flame produced by the initiating charge is insufficient to ignite the propellant grains thoroughly to produce an efficient burning rate of the entire charge. An explosive train consisting of an initiating charge, usually a mixture of lead styphnate and nitrocellulose (NC), detonates to ignite a small black powder booster which, in turn, ignites the larger black pow&r igniter. The black powder igniter is contained in the extension tube. Figure 1-2 shows a typical electric primer used in a propelling charge. The primer

extension tube of a 5"/54 powder charge is approximately 20 inches long, contains 52 grams of black powder, and has 32 vent holes.

The devices that use initiating and booster explosives to ignite a propelling charge or detonate a projectile burster charge are called primers and detonators, respectively. Remember, the terms primer and detonator describe a device that contains explosives, not the explosives themselves.

PRIMERS.- A primer is a device used to initiate the burning of a propellant charge by means of a flame. Its explosive train normally consists of a small quantity of extremely sensitive primary high explosive which, when detonated ignites a small black powder booster which, in turn, ignites the black powder igniter. Primers are classified according to the method of initiation (normally percussion or electric). All primers function in a similar manner when initiated.

DETONATORS.- Detonators are used in initiating high-explosive bursting charges. They are similar to primers in that they also contain a small quantity of extremely sensitive initiating explosives. However, a detonator will use a high-explosive booster usually made of a more stable substance than the initiating charge but less stable than the main burster. Detonators are also classified according to the methods of initiation, usually electric or percussion.