a. When the propellant in the rocket motor is ignited, pressure in the chamber generally rises within 0.005 to 0.05 second to a maximum value.

This value is determined by the burning rate of the propellant and the diameter of the nozzle orifice. Depending on the design of the propellent grain and the presence of ballistic modifiers, the charge burns at a nearly constant rate. This steady state pressure is maintained constant or decreases (tails off) very slowly until the propellant is completely consumed. Complete consumption takes from a fraction of a second to a minute or more. At very high operating temperatures, the burning of the propellant is usually completed before the rocket leaves the launcher. At low operating temperatures, burning may continue after the rocket is launched and a phenomenon known as afterburning will occur. With all rockets at all temperatures a blast from the rear of the launcher (backblast) occurs.

b. Each type of propellent composition has a critical pressure. Below this pressure, the composition burns nonuniformly and gives erratic ballistic effects. In addition, composite propellant, burning below the critical pressure, burns until it is extinguished by ashes which form on the surface of the grain. When the ashes sluff off, the propellant reignites and the process is repeated, creating an erratic sound pattern called "chuffing." c. The critical pressure is also affected by cracks in the propellent grain. Cracks increase the burning surface and, subsequently, the pressure. This can result in an overpressure sufficient to rupture the rocket motor.


5-15. General

The rocket launcher holds the rocket and provides initial guidance and electric contacts for firing. In some launchers, the source of electricity for ignition is integral with the launcher, as a magneto or batteries; in others, electrical energy is derived from an outside source, such as an aircraft's electrical system.

5-16. Types a. Tube. The launcher proper, as distinct from the mount, consists of a tube or a set of tubes with a means of holding the rocket in place and a mechanism for igniting the rocket motor. Some tube launchers are expendable (c below) ; that is, they are used for only one firing and discarded. This type launcher may be used as the shipping container for the rocket. Singletube launchers can be fired from the shoulder in standing, kneeling, or sitting position. A bipod and rear monopod are normally used for firing in a prone position. Multiple-tube launchers consist of a number of tubes in a cluster mounted on a carriage, vehicle, or aircraft.

b. Rail. These launchers are equipped with railtype launching beams traversed and elevated to provide guidance and spin to the rockets. Rockets fired from this type of launcher are generally stabilized by a combination of fins and spin.

c. Expendable. Typically, an expendable launcher consists of a plastic or metal alloy tube in which the complete rocket is shipped. The launcher is used once and discarded. Some expendable launchers have mounting hardware (tripods, azimuth and elevation adjustment devices, etc.) attached. Shoulder-fired, expendable launchers are complete with sights and firing mechanism.

Section VI. FUZES

5-17. General

A fuze is a device used to function a rocket at the time and under the circumstances desired. Rocket fuzes are classified according to location in the warhead as point detonating (PD), base detonating (BD), or point initiating, base detonating (PIBD). They are classified according to method of functioning as time, proximity, or impact.

5-18. Types a. Time fuzes function a preselected number of seconds after the round is fired. Impact fuzes function upon impact with superquick, delay, or nondelay action.

(1) In the case of superquick action, the warhead functions almost instantaneously on impact, initiated by a firing pin driven into a detonator.

(2) In delay action fuzes, the warhead functions a fixed time after impact to permit penetration of the target before the warhead explodes. The amount of delay, usually between 0.025 and 0.15 second, depends on the delay element incorporated in the fuze. Arming may be accomplished by mechanical means utilizing gear trains, air stream (air arming), spring action, centrifugal force or inertia, gas pressure (pressure arming), or a combination thereof.

(3) Nondelay action, somewhat slower than superquick, occurs in delay-action fuzes when the black powder normally contained in the delay element has been removed.

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