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Projectile Design

Since the first projectile was manufactured, the demand for greater accuracy and greater range has influenced projectile design. Without specifically constructed shapes and exterior parts, there would be no standard ballistic characteristics for any group or type of projectiles. A lack of ballistic standardization would prevent the computation of firing tables. Modern projectiles are designed for maximum stability and minimum air resistance in flight.

Critical diameter is the smallest casing diameter that is needed to sustain a detonation. The critical diameter for an explosive is the minimum diameter mass of that explosive that can be detonated without being heavily confined. Two examples of these insensitive main charge explosives are PBXW-122 and PBXW-124. The composition of PBXW-122 by weight is 47% 3-nitro-1,2,4-triazol-5-one (NTO), 5% cyclotrimethylenetrinitramine (RDX), 20% ammonium perchlorate (AP), 15% aluminum, and 13% binder. PBXW-122 has a critical diameter of 7 inches. PBXW-122 has a sensitivity of 130K bars (ELSGT). The composition of PBXW-122 by weight is 27% NTO, 20% RDX, 20% aluminum, 20% ammonium perchlorate, and 13% binder. PBXW-122 has a critical diameter of between 3 and 4 inches. The composition of PBXN-110 by weight is 86% cyclotetramethylenetetranitramine (HMX) AND 14% Binder. PBXW-122 has a critical diameter of 7 inches, which means that it cannot be detonated in less than a 7 inch diameter mass unless heavily confined. Future underwater and bombfill explosives will have critical diameters greater than one inch.

Eyebolt Lifting Plugs and Fuze Well Plugs. A separate-loading projectile has an eyebolt lifting plug. Other types of projectiles have metal hex-head or plastic closing plugs. The plug is for lifting; to keep the fuze well clean, dry, and free of foreign matter; and to protect the fuze well threads. The plug is removed, and the appropriate fuze is inserted at the firing position. Some special-purpose semifixed projectiles are issued with the fuzes already assembled in the projectile.

Ogive. The ogive is the curved portion of a projectile between the fuze well and the bourrelet. It streamlines the forward portion of the projectile. The curve of the ogive usually is the arc of the circle, the center of which is located in a line perpendicular to the axis of the projectile and the radius of which is generally 6 to 11 calibers. The ogival head is that particular part of a projectile from the forward end of the section of even diameter to the point, or from the beginning of the forward slope to the point. The purpose of the ogival head is, that it offers less resistance to the air in the flight of the projectile than any other shaped head, and at the same time masses a sufficient amount of metal at the point to give desired penetration when it strikes.

Bourrelet. The bourrelet is an accurately machined surface that is slightly larger than the body and located immediately to the rear of the ogive. It centers the forward part of the projectile in the tube and bears on the lands of the tube. When the projectile travels through the bore, only the bourrelet and the rotating band of the projectile bear on the lands of the tube. It is at the forward end of the section of even diameter or cylinder of the projectile, and is of slightly enlarged diameter over that of the rest of the projectile. It serves the purpose of providing a bearing surface on the forward part of the projectile and enables its more accurate seating in the gun bore; also it concentrates the spinning or revolution about the long axis of the projectile.

Body. The body is the cylindrical portion of the projectile between the bourrelet and the rotating band. It is machined to a smaller diameter than the bourrelet to reduce the projectile surface in contact with the lands of the bore. The body contains most of the projectile filler.

Rotating Band. The rotating band is a cylindrical ring of comparatively soft metal that is pressed into a knurled, or roughened, groove near the base of the projectile. It mates with the forcing cone of the tube to eliminate gas wash (blow-by) and to provide forward obturation. The rotating band, in conjunction with the rifling of the tube, imparts spin to the moving projectile. A properly rammed separate-loading projectile is held in the tube at all angles of elevation by the wedging action of the rotating band against the forcing cone. The diameter of the band is equal to that of the base of the grooves of the rifling in the gun tube or bore. The function of the band is to give to the shell, as it travels down the gun bore, the rotation or rotary motion required and which is secured by the lands of rifling cutting into the soft copper band. Chips cut from the rotating band by the rifling fall into small grooves around the circumference of the band. The forward edge of the rotating band is beveled, or slanted down, so that the projectile will start easily and there will not be too much strain on the rifling. When the projectile is seated in the gun, the bevel of the rotating band rests into and matches the bevel on the lands of the rifling. When the projectile is traveling down the gun bore, the joint between the lands of the rifling and the rotating band forms an effectual gas-cheek to prevent any gases from getting around in front of the projectile.

Obturating Band. On some projectiles, there is a nylon obturating band below the rotating band to help in forward obturation. Two examples of 155-mm projectiles with this type of a band are the illuminating round and the high-explosive rocket-assisted round.

Base. The base is that portion of the projectile below the rotating band or obturating band. The most common type is known as the boattail base. This type of base streamlines the base of the projectile, gives added stability in flight, and minimizes deceleration by reducing the vacuum-forming eddy currents in the wake of the projectile as it passes through the atmosphere.

Base Cover. The base cover is a metal cover that is crimped, caulked or welded to the base of the projectile. It prevents hot gases of the propelling charge from coming in contact with the explosive filler of the projectile through possible flaws in the metal of the base.

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