The primary function of a propellant is to provide a pressure that, acting against an object to be propelled, will accelerate the object to the required velocity. This pressure must be controlled so that it will never exceed the strength of the container in which it is produced, such as guns, rocket motor housing, or pyrotechnic pistols. In addition, propellants must be comparatively insensitive to shock Propellants maybe either liquid or
Figure l-2.-Typical case electric primer.
solid (Liquid propellants will not be discussed here, since only solid propellants are used in Navy gun ammunition.)
Propellants can be classified by such terms as single-base, double-base, triple-base, and composite. Single-base propellants contain only one explosive ingredient, NC. Double-base and triple-base propellants contain, respectively, nitroglycerine (NG) and nitroguanidine (NQ) in addition to NC. Composite propellants are compositions that contain mixtures of fuel and inorganic oxidants. There are combinations of composite and double-base propellants called composite double-base propellants.
Solid propellants are manufactured in the form of flakes, balls, sheets, cords, or perforated cylindrical grains. They are made in various shapes to obtain different types of burning actions. In large guns, 40 mm and over, a cylindrical grain with seven perforations is used, while 20-mm guns use a single perforation. Smaller calibers, including small arms, use flake or ball grains. The cylindrical grains are made in various diameters and lengths, but size is normally stated in web thickness. (See fig. 1-3.) The different types of burning actions are regressive, neutral, and progressive. A propellant is said to be "regressive burning" when the surface area of the grains decrease as they burn. An example of a "neutral burning" grain is a single perforated grain whose inner surface increases and whose outer surface decreases as it burns. The result of these two actions is that the total surface remains the same. As a multiperforated grain burns, its total burning area increases since it burns from the inside to the outside at the same time. Thus it is called "progressive burning."
The main charge for explosive projectiles, bombs, mines, torpedo warheads, and other bomb type of ammunition is always a high explosive. These substances must meet certain requirements for military use. In general, they must do the following:
Figure 1-3.-Solid propellant grains.
1. Be insensitive enough to withstand the shock of handling, of being fired from a gun, and of impact against armor (only in armor-piercing projectiles).
2. Have maximum explosive power.
3. Have stability to withstand adverse stowage conditions.
4. Produce proper fragmentation (only for fragmentation weapons).
5. Be inexpensive and easy to manufacture from readily available materials.
High-explosive charges are loaded into their containers by one of three methods--cast-loading, pressloading, and extrusion. Cast-loading is performed by pouring the substance as a liquid into a container and letting it solidify. Explosives having no liquid form must be press-loaded (pressed into their container) or incorporated into a liquid casting medium. The combining of certain explosives with plastic binders results in plastic mixtures that can be loaded either by casting or by extrusion. Specific compounds of these explosives are described in the following section.