COMPONETS OF A ROCKET PROJECTILE

The components of a rocket projectile, and the functions which they perform are briefly outlined below. The exact construction of these components varies somewhat according to the particular rocket for which they are designed, and there is some diffe-rence between the character and nomenclature of Army and Navy designed compo-nents, but these differences will be noted in the discussions of the individual rockets.

Body: This carries the payload and generally the initiating device.

Head: This is a broader term than body and refers to that assembly to which the motor tube is threaded. This is standard terminology for the Navy; however as to the Army, if the head is of one piece construction, the terms head and body may be syno-nymous as contrasted to some rockets in which the head consists of the body, the ogive, and the body union (see drawing page _____).

Motor tube: This contains the propellant and igniter. It is a combustion chamber in which the propellant is burned to provide the motive power for the rocket. If generally threads to the rocket head (body) or an adapter, and is usually shipped separate from the head (body) and fuze. The diameter of the motor is generally less than the diame-ter of the body with which it is used. For details of particular rockets, refer to section on rocket heads.

Grid or Trap Assembly: The Navy refers to assembly which supports the powder grain as the Grid. This grid supports the grain in such a position that sufficient clear-ance is allowed between the grain and the motor tube to allow the gas to flow from the propellant to the nozzle. The Army uses a Trap assembly, which is somewhat more complicated then the Navy grid. The trap assembly consists of spacing discs and wires running between then on which the sticks of ballistite are supported. Such an assembly is necessary where numerous small grains are used.

Nozzle: The number of nozzles varies as to the type of motor and method of stabili-tation. The nozzle has two functions: (1) it directe the gas jet in the desired direction; (2) it provides for expansion of the hot gas in the exit cone, thus giving additional thrust (about 33%) over that obtainable from a simple orifice; and (3) in spin-stabilized rockets, it imparts a clockwise rotation to the rocket when launched.

Fins: These provide stability in flight, prevent tumbling, and insure head-on impact. During burning, the action of the air against the fins gives a restoring moment against side forces at the nozzle and improves the accuracy of fire. When there is a tail shroud, the latter supports the rear end of the projectile in the launcher and may also be utilized to provide electrical contacts for firing. Spin-stabilized rockets do not have fins and stabilization is achievid through the rotary motion of the round.

Propellant and Igniter: The igniter contains loosely packed black powder and an electric squib with a low resistance bridge running through a violent match composition. The propellant used by the United States is a double base smokeless powder called balistite, which burns slowly and uniformly. Production of ballistite differs somewhat for the Army and the Navy, the Army perferring the solvent extrusion process and the Navy specifying the solventless extrusion process. The solvent extension process is im-practical for grains having a web of more than 1-1/4".

Grain shapes also vary somewhat with Army rockets generally propelled by several small cylindrical grains of ballistite, with an axial hole and several small radial holes to increase the burning surface and uniformity of burning. The Navy rockets use either a single solid cruciform grain without perforations or a single cylindrical grain with an axial hole and radial perforations. The latter, usually used in Navy ground or shipboard moun-ted is characterized by three ridges 120° removed and running longitudinally along the grain. Inhibitors are not used on this type. The cruciform grain, usually found in Navy aircraft rockets in section is a symmetrical cross with rounded ends. If all of the exter-ior surface of this grain were permitted to burn, there would be a gradual decrease of area, and a markedly regessive behavior would result in the burning rate. Since this type of burning would be detrimental to the desired uniform burning rate, a number of slower burning cellulose acetate strips are bonded to certain parts of the area exposed on the outer curved ends of the arms. By inhibiting a suitable frection of the surfaces along the ends of the arms, the proper combination of increasing and decreasing sur-faces may be obtained to give desired burning characteristics.