General

The propelling unit of the rocket is called the motor and contains the propelling char-ge. The motor is attached to the head, which contains the payload and the initiating de-vice. The motor is closed on the forward end and partially opened at the after end. The propellant is a relatively slow-burning double-base smokeless powder called ballistite.

As the ballistite is burned, hot gases are generated which expand and exert pressure against the confines of the motor tube. Since the hot gases exert an equal pressure in all directions, the pressure against the side walls counterbalance each other; however, the pressure against the forward closed end of the tube is not counteracted by pressure against the after end, since that end is partially open. The resultant force, then, is a thrust against the closed forward end of the motor, and the rocket is propelled in that direction. In order that the pressure of the gases will not be expended too rapidly, and that the propellant can be retained in flight, the after end of the motor tube is partially closed by the nozzle attachment, which is built into the inside of the tube. This nozzle restricts the ejection of the hot gases and also, by means of its rear taper, furnishes a canted surface against which the rapidly expanding emitted gases may act to increase the forward thrust of the rocket.

The ballistite propellant is ignited by a black-powder charge, the initiating device for which is an electric squib with a small bridge wire of high resistance which, when heated by an electrical current, ignites a violent match composition. The black-powder charge sends a flash over the entire surface of the ballistite and raises the temperature of the ballistite to the ignition point. Upon ignition, the ballistite burns evenly and relatively slowly; this type of burning is necessary to prevent sudden and excessive pressure being exerted against the thin walls of the motor tube. Rocket motors operate at much lower pressures than gungs, and correspondingly longer times are required for the complete combustion of the rocket propellant. Burning times of American rockets range from about 0.15 second to as much as 1.5 seconds, depending on the web thickness of the grain and the temperature of the propellant; and burning distances range from a frew feet to several hundred feet at high velocities; hence, most of the burning of the rocket propel-lant occurs after the projectile has left the launcher.

The early productions of rockets were of the fin-stabilized type because of their use by the British and because of the inherent simplicity associated with fin stabilization. Rockets cannot be launched with that degree of accuracy char-associated with fin sta-bilization. Rockets cannot be launches with that degree of accuracy characteristic of gun projectiles. This is a result of many factors, such as the effect of temperature on the burning rate of the propellant, difficulties in controling to a fine degree the pressure exerted by the expanding gases inside the motor tube, the effect of the expansion of emitted gases against the rear taper of the nozzle, etc. The mean deviation in deflection for most standard land- or shipboard-launched fin-stabilized rockets in 20 to 40 mils, while fin-stabilized rockets launched from aircraft have mean deviation of about 5 to 10 mils. The increased accuracy of aircraft-launched rockets is attributed to the immediate stabilizing effect given to the fins during the initial stages of flight by the rapid travel of the plane through the air. Fins on rockets exert an appreciable restoring force in flight only at a high velocity, and thus agreater degree of accuracy is achieved if rockets are launched from aircraft or if the acceleration occurs to a large extent on the launcher.

A later development, the spin-stabilized rocket, is now in service use. Stabilization of this rocket depends on the rotation of the round. Although the accuracy of spin-stabili-zed rockets is not comparable to that of gun projectiles, theay are generally more accu-rate than fin-stabilized rockets at short ranges. The use of spin-stabilized rockets will be particularly advantageous to ground and amphibious forces, inasmuch as the rocket is shorter and the launching gear is more compact, facts which facilitate the loading and stowage problems.

As against their disadvantages, rockets have many advantages over gun-propelled projectiles. The most importatnt is the absence of recoil against the launcher. Since there is no recoil action on the launcher, rockets may be launched from small trucks, am-phibious ships, and aircraft which could not withstand the recoil forces exerted by equi-valent projectiles fired from guns. Other advantages of rockets are cheapness, simplicity, and portability of the launchers as compared to guns.