Section Iv Motor

5-13. General detonating fuze. Generally, the motor consists of the following major components (fig. 5-2):

a. A motor, which propels the rocket, is assembled to the rear of the warhead or base- (1) Motor body (combustion chamber)

Figure 5-2. Major components of rocket motor.

(2) Propelling charge

(3) Igniter assembly

(5) Trap assembly (optional)

(6) Resonance rod assembly (optional)

The base of the motor is constricted to form the throat of one or more nozzles. Flight of the rocket is stabilized by fins attached to the rear of the motor, or by spin of the rocket about its axis. This spin is produced by highvelocity gases passing through canted nozzles in the base of the motor. In some larger motors, an assembly of spin rockets provides the initial thrust required to rotate the rocket.

b. The motor body, usually a hollow metal cylinder fitted with a nozzle at the rear, is either closed at the forward end or threaded for assembly with the warhead. The body houses the propelling charge and the igniter assembly.

c. 'The propelling charge consists of one or more grains of solid propellant, either double-base or composite. Double-base propellant consists principally of nitrocellulose and nitroglycerin. Composite propellant is a mixture of an organic fuel, an inorganic oxidizing agent and a binding agent. To control the burning rate, propellent grains may be coated with sheets of a slower burning inhibitor material. However, if the propellant has center perforated grains, resonance rods running through these perforations serve the same purpose by dampening pressure waves created by the burning propellant.

d. The igniter assembly consists of a charge of black powder housed in a cylindrical plastic container, and one or more electric squibs.

e. The nozzle is convergent-divergent (Venturitype) in shape to eliminate turbulence and to provide a relatively frictionless flow of escaping gas. The throat (constricted portion) of the nozzle may be lined with a refractory substance, such as graphite. This prevents heat of the propellent gases from changing dimensions of the throat. A small change in throat area affects functioning of the motor by altering flow rate and direction of the escaping gases. Nozzles on most rocket motors are canted (scarfed). However, motors used with highspeed aircraft rockets have straight nozzles. Nozzle closures or seals prevent moisture from entering the motor. In some cases, the closure or seals aid ignition of the propellant by causing pressure to build up within the chamber when the igniter is 'fired. Special safety devices (pressure relief valves, etc.) limit pressure and prevent rupture of the chamber.

f. A grid-like trap assembly may be located on the approach side of the nozzle to prevent ejection of unburned portions of the propellant. Some propelling charges are suspended in the motor body in a manner which eliminates the need for a trap.

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