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Flechettes

Common usage distinguishes between a gunshot wound and a shrapnel wound. More precise usage would term the later a fragment wound. Strictly defined, shrapnel means preformed fragments (the fragments exist already made within the explosive munition). Thus, fragmenrs from a random-fragmentation shell are not shrapnel. Also note that by strict definition, flechettes are shrapnel.

A flechette is a common military missile which is shaped generally similar to a dart or arrow, i.e. it has a relatively long slender body, at one end of which are disposed guiding fins. Flechettes are launched individually toward a target from a launcher having a bore. Flechettes are fin stabilized steel projectiles similar in appearance to arrows. Flechettes have a performance criteria very different from the conventional rifle bullet. Typical modern flechettes are small light weight steel projectiles, and the velocity lost to air resistance is generally 375 fps. per 100 Meters of flight. Unlike rifle bullets, flechettes are not spin stabilized, but use fins to achieve level flight. The flechette's long body looses rigidity on target impact and bends into a hook, often breaking off the fin portion creating an additional wound.

William C. Ingram, of Grand Rapids, Michigan was granted patent 1,340,317 for a Shrapnel-shell on Sept. 18, 1917. This shell was an early embodiment of a flechette warhead.

During the Korean War the Chinese army tactic of human wave attacks against US lines of defence prompted interest in flechette projectiles in single and multiple projectile systems for small arms and antipersonnel (APERS) use. Flechette munitions include projectiles for use in the M16 rifle, CAWS (close assault weapons system), and 12 gage shotgun, as well as the 105mm M101A1/M102 howitzer, 2.75 in. FFAR (folding fin aircraft rocket), and the 70mm Hydra-70 FFAR.

Flechettes are typically designed with the intended target in mind. For example, some flechettes are designed to behave as hardened penetrators to breach harder targets, such as thin armor. Such flechettes are less effective against softer targets because they tend to pass through the target quickly with minimal damage. Other flechettes are designed to damage softer targets by fracturing or bending as they strike the target; however, they are often ineffective against harder targets because of the tendency to fracture or bend upon striking such targets.

In combat situations wherein both harder and softer targets are anticipated, flechettes for each type of target have conventionally been needed. Supplying, storing, and deploying multiple types of flechettes based upon the perceived or anticipated target may lead to logistical difficulties. Other conventional approaches to damaging both harder and softer targets have included the use of other types of penetrators, often having explosive components, which are more expensive to deploy than flechette-based weapons.

Ground-based air defense gun systems of 20 mm and larger calibers presently in service employ conventional high explosive projectiles for defeating a target. Although high explosive projectiles have good terminal effectiveness against aircraft, their inherent exterior ballistic performance is such as to result in poor hit probability in employment against high speed aircraft. High explosive projectiles contain a fuse mechanism and a high explosive filler. These components are rather voluminous and of low weight, thus adversely restricting the sectional density of the projectile. The resultant ballistic coefficient is such as to induce a high degree of velocity decay as a function of range and correspondingly long time of flight. In employment from ground-based guns against low flying, high speed aircraft, the long time of flight requires very large lead angles and superelevation angles. In the case of advanced ground-support aircraft, these angles are of such magnitudes that even with the use of sophisticated fire control systems the resultant hit probabilities are inadequate.

For ground-based gun fire to be effective, ability to hit the target is a prerequisite. To achieve high hit probability performance against fast-flying enemy aircraft, it is essential to fire projectiles having short times of flight resulting from high projectile velocity. In turn, this reduces the lead angle and superelevation angle requirement.

High velocity projectiles with short times of flight are essential for achievement of high hit probabilities regardless of the degree of sophistication of the fire control system. The desired short times of flight can be attained through the use of sabot-launched subcaliber projectiles having a high muzzle velocity. Furthermore, in order to minimize velocity loss subsequent to launch, the subcaliber projectiles should have a high sectional density, i.e., should consist of a high density material, such as a tungsten alloy for example, having a density of approximately 16 to 19 g/cm.sup.3. These features and related exterior ballistic characteristics are found in advanced discarding-sabot, armor-piercing projectiles described in that patent.

However, while providing the desired hit probabilities, the terminal effectiveness of this type of ammunition against aircraft-type targets is unsatisfactory. Armor-piercing projectiles are of limited terminal effectiveness against soft targets such as high speed aircraft in that the projectile can hit the target causing superficial damage without destroying it.

Owing to the configuration of the flechette, they are difficult to launch from a launcher having a bore. Specifically, the guiding fins of the flechette define a diameter which extends well beyond the diameter of the body of the flechette. Thus, the diameter of the launcher bore must be no smaller than that of a circle defined by the diameter of the guiding fins. Otherwise, the flechette will not fit through the bore. The diameter of the bore is generally far greater than that of the body of the flechette, and so a sabot is used to retain the flechette within the bore.

A sabot, or shoe, comprises at least two pieces of a hard material placed over the exterior of a front portion of the body of the flechette as a covering before the flechette is launched. The interior of the sabot conforms to the shape of the exterior of the body of the flechette. The exterior of the sabot generally conforms to the shape of the interior of the bore, and has a diameter greater than that of the guiding fins. The rear end of the sabot, together with the rear portion of the flechette including the guiding fins, is encased within a cartridge which serves to hold the sabot and flechette together. Additionally, the cartridge will retain therein gunpowder or other propellant with which the flechette is launched from the bore.

The front end of the sabot conventionally is configured aerodynamically, so that once the flechette/sabot assembly leaves the muzzle of the launcher, the sabot is aerodynamically peeled away from the flechette, thereby permitting the flechette to fly toward its target unfettered by the sabot. The sabot, having served its purpose, falls to the ground. A common configuration of the front end of a sabot is that of a cup, with the rim of the cup at the front end of the sabot, and the bottom of the cup toward the rear thereof. The pressure of the air on the bottom of the cup after launching peels the sabot away from the flechette.

Conventional sabots are made of a hard and inelastic material, such as, for example, fiberglass-reinforced plastic, and so generally are extremely rigid. This is considered necessary, so that the sabot will not dissipate the propulsive energy applied to its rear end during launching. A problem arises with the use of a sabot, namely retaining the sabot in place with respect to the flechette during launching. Since the force applied to the sabot/flechette assembly during launching is directed axially thereto, and much of that force is applied to the sabot, the sabot tends to slip off the flechette. To counteract such a tendency, a radial force is exerted between the flechette and the sabot, thereby holding the sabot in place. In some applications, this is accomplished by providing a slight tapering at the rear of the sabot. When the propulsive force is applied to this tapering, a portion of the (axial) propulsive force is translated into a radially inward retaining force, tending to retain the sabot in place about the flechette.

This tapering has a disadvantage, since it serves to dissipate a portion of the propulsive force. Additionally, forcing the rigid sabot against the flechette leaves a small space between the exterior of the sabot and the interior of the bore, through which a portion of the propulsive force may leak. Also, the sabot has an exterior diameter which is equal to or slightly less than that of the bore. Any difference in dimension leaves a small space between the sabot and the interior surface of the bore, through which a portion of the propulsive force may dissipate. Furthermore, the existence of the small space between the sabot and the bore means that the sabot is not retained tightly within the bore, and so has a slight freedom to separate while in the bore. Any separation of the pieces of the sabot is also extremely undesirable, since it will further permit some dissipation of the propulsive force therebetween, and will permit the sabot to release the flechette.

In some applications, a retaining member, or obturator, such as a rubber band, is affixed about the sabot, to exert a radially inward retaining force. The obturator also serves to fill the small space between the sabot and the interior of the bore, and so seals off the bore from any leaking of propulsive energy about the sabot, or between its pieces. The use of an additional retaining member is also a disadvantage, however, since it increases the cost of the assembly, and adds to the labor involved in its construction.

It is known to use a plastic sabot to surround a flechette and to have the barrel rifling only engrave the sabot, which transfers the rotation to the flechette by mechanical engagement with the fins of the flechette, instead of by a friction grip, and therefore a low coefficient of friction material can be used for the sabot with a resulting low friction loss in the barrel. One consequence of using a flechette however is that the combined weight of the sabot and flechette is very light when compared to a conventional bullet of the same diameter and length so that a special automatic gun must be used to function with the reduced impulse. A further problem with all sabot launched projectiles is that since the sabot and projectile exit from the barrel at the same velocity, the energy of each is determined by their relative mass to one another. The heavier the sabot is in relation to the projectile, the greater is the percentage of lost energy, since the sabot serves no useful purpose as a projectile. In the prior art, the body diameter (shaft) of a flechette is small in comparison to the sabot diameter, with a resulting large proportion of mass and energy in the sabot, so that the flechette gets a relatively small amount of the total energy and is therefore the least efficient of the sabot type projectiles.



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