Rockets cannor be classified exactly as either bombs or projectiles, since they utilize an entirely distinctive propulsive feature. Whereas bombs are dropped from aircraft and projectiles are fires by means of a charge placed in the gun, rockets are propelled by a charge which is carried with the rocket in its flight. Hence, through rockets often are launched from tubes which may resemble the barrel of a gun, the launchers are merely guiding devices to direct the initial flight of the rocket. Launchers also contain points whereby the electrical circuit through the igniter can be completed.

The propelling unit of the rocket is called the motor and contains the propellant charge. The motor is usually attached to the body or heads which contains the payload and the initiating device, by external or internal threads on the forward end and thread to the head or an adapter. The motor is closed on the forward end and partially open at the after end. The propellant is a relatively slow burning double-base smokeless powder called ballistite, which is discussed more completely further in the introduction.

The principle of the rocket is a simple one: expanding gases exert an equal pressure in all directions. 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 counter-balance each other; however, the pressure against the forward closed end of the tube is not counteracted by pressure against the aft end, since that end is partially open. The re-sultant force, then, is a thrust against the closed forward end of the motor, and hence the rocket is propelled in that direction. In order that the pressure of the gases will not be expended too rapidley, and that the propellant can be retained in flight, the aft 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 taner, 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 or charges, the initiating device for which is an eletric squib with a small bridge wire of low 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 gives off hot gases which raise the temperature of the ballistite to the ignition point. Upon ignition, the ballistite burns evenly and relatively slowly; this type of burning is neccesary to prevent sudden and excessive pressures being exerted against the thin walls of the motor tube. Rocket motor operate at much lower pressure than guns, and correspond-ingly longe time 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 thichness of the grain and the temperature of the pro-pellant; and burning distances range from a few feet to serveral hundred feet at high velocities. Hence, most of the burning of the rocket propellant usually 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 partically because of the inherent simplicity associsted which fin sta-bilization. Because of many factors such as the effect of temperature on the burning rate of the propellant, difficulties in controlling 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., rockets cannot be launched with that degree of accuracy characteristic of gun projectiles. The mean deviation for most stan-dard land or shipboard launched, fin stabilized rockets is on the average of 20 to 40 mils while those fin stabilized rockets launched from aircraft have a mean divisation of about 5 to 10 mils. The increased accuarcy of aircraft launched rockets is attributed to the immediate stabilizing effect given to the fins during the initial stages of flight by the rapid tarvel of the plane through the air. Fine on rockets exert an appreciable restoring force in flight only at a high velocity and thus a greater degree of accuracy is achieved if rockets are launched from aircraft or if the acceleration occure to a large extent on the launcher.

Spin stabilized rocktes are now in service use with stabilization being dependent on the rotation of the round. Although their accuracy is not comperable to that of gun projectiles, they are generally more accurate than fin-stabilized rockets at short rang-es. The use of spin stabilized rockets will be particularly advantageous to ground and amphibious forces inasmuch as they do not have fins, the rocket is shorter, and the launching gear is more compact, which facilitates the loading and stowage problems. Both fin-stabilized and spin-stabilized rockets have inherent advantages and disadvan-tages, and the method of stabilization employed will be dependent on the required characteristics for each individual case.

As against their disadvantages, rockets have many advantages over gun-propelled projectiles. The most important is the lack of recoil imparted to the launcher. Since the force of the expanding gases are utilized in the propulsion of the rocket, there is no re-coil action on the part of the launcher; hence, rockets may be launched from small trucks, amphibious ships, and aircraft which could not withstand the recoil forces exer-ted by equivalent projectiles fired from guns. Other davantages to rockets are cheap-ness, simplicity, and portability of the launchers as compared to guns. Rocket launchers in general are more easily replaceable and hence can be considered as more expendable than even light artillery pieces.