American Projectiles and Explosives
Rocket Bodies Introduction
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 pressures 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 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 to the ignition point. Upon ignition, the ballistite burns evenly and relatively slowly; this type of burning is necessary to prevent sudden and excessive pressures being exerted against the thin walls of the motor tube. Rocket motors operate at much lower pressures than guns, and correspondingly longer times are required for the complete combustion of the rocket propellant. Burning times of American rockets range from about 0.15 seconds 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 few feet to several hundred feet at high velocities; hence, most of the burning of the rocket propellant 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 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 controlling to a fine degree the pressures 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 is 20 to 40 mils, while fin-stabilized rockets launched from aircraft have a mean deviation of about 5 to 10 mils. The increased accuracy of aircraft-launched rockets is attributed to the immediate stabilizing effects 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 a greater 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-stabilized rockets is not comparable to that of gun projectiles, they are generally more accurate 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 important is the absence of recoil against the launcher. Since there is no recoil action on the launcher, rockets may be launched from small trucks, amphibious ships, and aircraft which could not withstand the recoil forces exerted by equivalent projectiles fired from guns. Other advantages of rockets are cheapness, simplicity, and portability of the launchers as compared to guns.
Head: This is the part which is functionally similar to a projectile and which contains the payload and the initiating device. This payload may be solid shot, high explosive, chemical, incendiary, window, flare, or a special load.
Motor Tube: This contains the propelling charge and the igniter. It is a combustion chamber in which the propellant is burned to provide the motive power for the rocket. It generally threads to the rocket head and is usually shipped separate from the head and fuze. The diameter of the motor is generally less than the diameter of the body with which it is used.
Grid or Trap assembly: The Navy refers to the assembly which supports the powder grain as the grid. This grid supports the grain in such a position that sufficient clearance 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 than the Navy grid. The trap assembly consists of spacing discs and wires running between them, 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 with the type of motor and method of stabilization. The nozzle has several functions. It directs the gas jet in the desired direction and provides for expansion of the hot gas in the exit cone, thus giving additional thrust (about 33%) over the obtainable from a simple orifice. In spin-stabilized rockets, it imparts a clockwise rotation tot he rocket when launched.
Fins: During burning, the action of the air against the fins gives a restoring moment against side forces at the nozzle, thus improving the accuracy of fire. When there is a tail shroud, it supports the rear end of the rocket in the launcher and may also provide electrical contacts for firing.
Propellant and igniter: The igniter contains loosely packed black powder and an electric squib with a high-resistance bridge running through a match composition. The propellant is a double-base smokeless powder called ballistite, which burns slowly and uniformly. Production of ballistite differs somewhat for the Army and the Navy, the Army preferring the solvent extrusion process and the Navy specifying the solventless extrusion process. The solvent extrusion process is impractical for grains having a web of more than 1-1/4 inches.
Grain shapes also vary. Army rockets generally have several small cylindrical grains of ballistite, with an axial hole to increase the burning surface and uniformity of burning. The Navy rockets use either a single solid cruciform grain without perforation or a single cylindrical grain with an axial hole and radial perforations. The latter, used in Navy ground- or shipboard-mounted rockets, is characterized by three ridges 120 degrees removed and running longitudinally along the grain. Inhibitors are not used on this type. The cruciform grain, in Navy aircraft rockets, is a symmetrical cross with rounded ends. If all the exterior surface of this grain were permitted to burn, there would be a gradual decrease of area, and a regressive rate in burning. Hence, a number of slower burning cellulose acetate strips are bonded to parts of the area exposed on the outer curved ends of the arms, to give desired burning characteristics.
Storage: To decrease hazards in handling, rocket bodies and motors are generally shipped and stored separately. Motors with large grains are kept in a non-propulsive state until final assembly is necessary. The seals at both ends of the motors are light and easily displaced by pressure developed inside the tube. Should the igniter and grain igniter, the closures would fail quickly, relieving the pressure without more than a slight movement of the motor.
It is necessary that loaded motors be kept at moderate temperatures as much as possible. Event hough spontaneous ignition should not take place, the powder should not be stored where temperatures exceed 100 degrees Fahrenheit, because such conditions tend markedly to decrease the stable life of the propellant. Because of the electric squib, rocket motors should not be stored near radio apparatus or antenna leads.
Although there is very little possibility of a motor firing as a result of falling or rough handling, such treatment is likely to cause malfunctioning of the rounds. Ammunition should be kept in packing containers or ready boxes and should not be handled in a loose condition unless necessary.
Practice rockets are loaded with plaster of paris or other inert substances to simulate the explosive loads in service rounds. These rockets also have dummy fuzes.
The burning rate of propellent powders changes with temperature and pressure - the higher temperatures and pressures causing more rapid burning. If rockets are fired at temperatures higher than those for which they are designed, the pressure may build up faster than the nozzle can release it, perhaps bursting the round. At temperatures below the safety limit, there will be back blasts of flame with burning fragments of powder.
These were rockets designed to be fired aft from a fast moving ship or plane - the movement aft to compensate exactly for the movement forward of the launching vehicle, thus leaving gravity as the only effective force on the rocket.
2.25-inch A.R. Practice
General: The 2.25-inch sub-caliber rocket rocket for aircraft was developed for training purposes. Initially, two types were designed to approximate the trajectory of the 3.5-inch and 5-inch rockets; however, only the Motor Mk 11 and the Head Mk 3 Mod 2 will be used in future training.
The Mk 1, a California Institute of Technology production, was issued until adopted and issued by Bureau of Ordnance as the Mk 3 Mod 2. The Mk 2, a California Institute of Technology production, was designed as a slow sub-caliber rocket. The complete assembly for the latter is no longer available.
The 2.25-inch Motors Mk 10 and Mk 11 are similar to each other, as are the 2.25-inch Motors Mk 12 and Mk 13. The Motors Mk 10 and Mk 11 differ from the Mk 12 and Mk 13 in that the diameter of the nozzle on the latter is smaller and the weight of the propellant of the Mk 10 and the Mk 11 is 1.75 pounds, as compared to the weight of 1.12 pounds in the Mk 12 and Mk 13.
The external dimensions of the rockets are the same. For recognition purposes, the 2.25-inch motors Mk 10 and Mk 11 are painted white with black fins, while the Motors Mk 12 and Mk 13 are grey with black fins.
Motor Mk 11 and Head Mk 3 Mod 2: Overall length of the rocket is 29 inches. Two button-type lugs are provided ont he motor tube, spaced approximately 19 inches apart. Four fins are welded to the after end of the motor tube. The propellant is a cylindrical grain of ballistite weighing approximately 1-3/4 pounds.
General: As a target for anti-aircraft gunners, the rocket is projected with speeds approximating those of an aircraft. It consists of a rocket propulsive unit to which are attached large stabilizing fins, for maximum visibility. They all consist of a simple rocket motor with three large fins prepared from wooden frames and light-weight fiber board. The fins are 120 degrees apart, each attached by two lugs.
The 3.25-inch Rocket Targets Mk 1 and Mk 2 consist of a motor 36 inches long, to which fins 18 inches by 34 inches are attached. An electrical connection is made by a standard 110-volt plug. The 3.25-inch Target Rocket Mk 1 is standardized at 425 mph and the Mk 2 at 300 mph. On some models, a screamer is put over the nose end.
The Mks 3 and 4 differ from the Mks 1 and 2 in that the motor is heavier and the fins are held on by threaded studs instead of lugs. The ballistics are similar; Mk 3 is like Mk 1, and Mk 4 is like Mk 2.
Next Time: Navy Rockets (Part 2)