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Large Caliber Ammunition - Types of Projectiles

A projectile or shell is a missle fired from the muzzle of a gun; it is always the projectile, whether issuing from the muzzle of a Breech-Loading Rifle, using separate ammunition, or from the muzzle of a Rapid-Fire Gun, using fixed, cartridge-case ammunition. Projectiles for guns of and above seven inches in caliber are called major-caliber projectiles. For guns of six-inch caliber and smaller they are called minor-caliber projectiles. The principal function of the projectile is to carry its charge intact to the enemy's most vulnerable point, and its relative efficiency will be in a measure proportional to its carrying capacity. The first projectiles used were stones thrown from slings (afterwards lead bullets were projected in the same way), arrows from the long bow, and darts andjavelins thrown by hand. In the sieges of walled towns, in very early days, ballista, and catapults were used as a species of heavy ordnance, the former to hurl large stones, and the latter, wooden beams shod with iron and often covered with inflammable material. The projectile, as it is understood in modern times, came in with the use of gunpowder in warfare, and developed with the improvements in weapons using it. While lead answered all the purposes in small-arms, it was found too soft for battering with larger guns, and stone shot being not only too light for good flight, but also deficient in tenacity, early gave way to iron.

Projectiles can be broadly classified according to three main types: spin-stabilized, fin-stabilized, and rocket assisted (both fin- and spin-stabilized). Formal military classification is based on the intended use of the projectile and the composition of the explosive charge (i.e., antipersonnel, antitank, and incendiary). Some very significant progress in projectile design has been made in the past few years. The form of all projectiles is approximately the same, namely, that of a hollow steel cylindrical case with pointed head, having a soft metal band near the base which takes the rifling of the gun and gives the projectile the twisting motion which keeps it steady during flight.

SPIN-STABILIZED PROJECTILES Most guns in use today use spin-stabilized projectiles. Spinning a projectile promotes flight stability. Spinning is obtained by firing the projectiles through a rifled tube. The projectile engages the rifling by means of a rotating band normally made of copper. The rotating band is engaged by the lands and grooves. At a nominal muzzle velocity of 2800 feet per second, spin rates on the order of 250 revolutions per second are encountered. Spin-stabilized projectiles are full bore (flush with the bore walls) and are limited approximately to a 5:1 length-to-diameter ratio. They perform very well at relatively low trajectories (less than 45 quadrant elevation). In high trajectory applications they tend to overstabilize (maintain the angle at which they were fired) and, therefore, do not follow the trajectory satisfactorily.

FIN-STABILIZED PROJECTILES These projectiles obtain stability through the use of fins located at the aft end of the projectile. Normally, four to six fins are employed. Additional stability is obtained by imparting some spin (approximately 20 revolutions/second) to the projectile by canting the leading edge of the fins. Fin-stabilized projectiles are very often subcaliber. A sabot, wood or metal fitted around the projectile, is used to center the projectile in the bore and provide a gas seal. Such projectiles vary from 10:1 to 15:1 in length-to-diameter ratio. Fin-stabilized projectiles are advantageous because they follow the trajectory very well at high-launch angles, and they can be designed with very low drag thereby increasing range and/or terminal velocity. However, fin-stabilized projectiles are disadvantageous because the extra length of the projectile must be accommodated and the payload volume is comparatively low in relation to the projectile length.

In contrast to conventional spin-stabilized projectiles which derive their in-flight stability from the gyroscopic forces resulting from the high rate of spin, the finned projectiles are stabilized during flight by aerodynamic forces acting on the projectile. Although projectile spin does not contribute to the stabilization of finned projectiles, a low rate of roll around the longitudinal axis is desired to minimize the adverse effects of mass and configurational asymmetries which may result from material imperfections and from manufacturing tolerances.

Fin-stabilized projectiles are ideally launched from smooth bore guns which, due to the absence of rifling, do not impart a rolling motion. Such weapons are installed, for instance, on advanced battle tanks and commonly have calibers of 60 millimeters or more.

Automatic cannons having calibers ranging approximately from 12.7 to 40 millimeters have almost exclusively rifled barrels and generally fire various types of spin-stabilized projectiles, including armor-piercing projectiles. In order to improve the armor penetration of such weapons, it is desirable to develop technology permitting successful employment from rifled gun barrels of fin-stabilized armor-piercing projectiles with their inherent high degree of terminal effectiveness. In this case, successful employment means compatibility of the ammunition with the gun and feeder system, which in turn requires the necessary structural integrity to function reliably under all operating conditions specified for such weapons while at the same time providing a projectile accuracy which is equal to or better than that of spin-stabilized projectiles fired from the same weapon.

Commonly, fin-stabilized projectiles consist of a subcaliber penetrator and a fin assembly of four or more fins attached to the rear of the penetrator. The projectile assembly is symmetric to its longitudinal axis and is fired from the gun by means of a discarding sabot. Two important functions of the discarding sabot are to support and guide the subcaliber projectile along the centerline of the gun barrel during acceleration and to form a seal to contain the propellant gasses during travel in the barrel. The latter function is accomplished by the rotating band which engages the rifling grooves of the gun barrel and in doing so imparts spin to the projectile commensurate with the rifling twist of the barrel and the projectile muzzle velocity.

Fin-stabilized projectiles reflecting the current state of the art incorporate a sliding seat between the rotating band and the sabot body. The sliding seat is designed such as to reduce by approximately 70 to 90 percent the amount of spin transmitted from the rotating band, which picks up the full spin, to the sabot body. The degree of spin transmission within the seat of the rotating band is determined by sliding friction. Thus, upon exit from the muzzle of the gun the fin-stabilized projectile has a rate of spin equal to approximately 10 to 30 percent of that of a spin-stabilized projectile launched at the same muzzle velocity.

There are two problem areas encountered with this method of firing fin-stabilized projectiles from a rifled cannon. Firstly, it is difficult to control the spin reduction in the sliding seat with a degree of repeatability necessary to assure acceptable projectile accuracy over the entire range of operating conditions specified for military employment. Variations in projectile temperature from -40.degree. to +60.degree. C., changes in humidity, finite manufacturing tolerances, contamination by dust, salt and other substances entering between the rotating band and its seat, etc., influence the friction coefficient in the band seat and with it the degree of spin transmission.

Secondly, centrifugal forces acting on sabot components are very effective in initiating the instantaneous and symmetric separation of the sabot from the penetrator upon exit from the muzzle of the gun. With reduced projectile spin the centrifugal forces acting on the sabot components are reduced by the square of the spin ratio. As a result, the sabot separation is neither as rapid nor as precise as with a nonslipping rotating band and is increasingly more dependent on aerodynamic forces.

The access of aerodynamic forces to the projectile is delayed by the efflux of high velocity propellant gasses upon exit of the projectile from the muzzle of the gun. These propellant gasses envelop the projectile temporarily in a reverse flow field. Only upon entering into the ambient air, which occurs at a range of approximately 30 calibers from the muzzle, do the aerodynamic forces become fully effective in sabot separation. The magnitude of the aerodynamic forces prevailing for sabot separation is only a fraction of the centrifugal forces available when launching at full spin and therefore a considerably more fragile sabot construction is required to assure its fracture and separation. In addition, because of size limitations of ammunition of calibers up to 40 millimeters, the physical dimensions of sliding rotating bands, inclusive of their seats, are small, thus resulting in rather delicate and vulnerable components. In contrast, utilization of a nonslipping rotating band allows for the use of a stronger sabot which is advantageous when employed from high rate of fire cannons and their correspondingly high structural loads during feeding and ramming.

Fin-stabilized projectiles equipped with discarding sabots incorporating slipping rotating bands experience considerable variations in spin rate at exit from the muzzle due to deviations in the friction coefficient within the sliding seat of the band. As a result the subsequent acceleration or deceleration of the projectile spin may result in conditions where the spin rate is equal to the nutation frequency of the projectile and resonance instability will occur. The lower projectile spin rate at muzzle exit and consequent reduction in centrifugal forces acting on the sabot decrease the rapidity and symmetry of the discard of the sabot components and therewith result in increased projectile dispersion.

SABOT A sabot is a lightweight carrier used both to position a missile or subcaliber projectile inside a gun tube and to transmit energy from the propellant to the projectile. The sabot works much like a person throwing a dart, where the thrower's arm movement acts as both the propellant-driving gas and the sabot's energy-gathering pusher.

In general, guns operate with a fixed mass to be propelled out of the gun's tube. The sabot is necessary to transfer propellant energy but is a parasitic weight in terms of projectile target performance. Reducing the sabot's weight allows greater projectile velocity. The weapons thus penetrate deeper, with more lethal results. But materials used to fabricate sabots can only be as lightweight as they are strong enough to withstand great pressures and loads during gun-tube acceleration.

Three types of armor piercing projectiles are currently utilized in small caliber gun systems. One of the designs is of a conventional projectile shape and is full-bore diameter, consisting of a combination of high strength steel or high density material as a penetrator swaged or inserted into a suitable jacket or sleeve material. At the projectile base is an opening for a tracer cavity of adequate depth and diameter to provide a clear visual trace of the entire projectile trajectory. This type of full-bore projectile utilizes the high density or high strength penetrator and to some extent the jacket or sleeve material and its geometry to affect armor penetration. This type of projectile has severely limited armor penetration capability at target engagement ranges beyond several hundred meters, due to its high drag configuration.

It has been demonstrated that sub-caliber high density rod type penetrators are capable of penetrating significantly more armor than the full-bore projectiles at target ranges beyond several hundred meters. This is due to the high density rod's more efficient armor penetration geometry and the greater mass per cross sectional area of the sub-caliber rod flight projectile, which results in it losing less velocity from aerodynamic drag. To take advantage of the rod's high ballistic coefficient and to provide increased initial launch velocities, sabots were designed to encapsulate the rod penetrator during handling, storage, and gun firing, and to discard shortly after exiting the muzzle, thus allowing only the rod penetrator to continue in flight toward the target. One type of discarding sabot projectile has been demonstrated in small caliber guns to provide increased armor penetration over full-bore projectiles. This is the Armor Piercing Discarding Sabot (APDS) projectile, which utilizes a spin stabilized sub-caliber penetrating core as the flight projectile. APDS projectiles using high density rod penetrators have been developed for guns from caliber 5.56 millimeter through caliber 120 millimeter. Given aerodynamic considerations, APDS projectile designs below caliber 25 millimeter do not allow the inclusion of a tracer cavity without degrading penetrator performance. The tracer cavity in these projectiles significantly reduces the available high density rod material required for armor penetration.

It has been demonstrated that armor piercing fin stabilized discarding sabot (APFSDS) projectiles penetrate more armor at greater ranges than spin stabilized APDS projectiles, due to the longer allowable penetrator lengths that can be launched and flown to the target with accuracy and stability. APFSDS projectiles utilizing high density sub-caliber rod penetrators have been developed for both rifled barrel and smooth bore guns from caliber 25 millimeter through 140 millimeter, and these designs have permitted the incorporation of an adequate tracer cavity in the rear of the flight projectile without degradation of the rod's armor penetration performance.

Prior art delay discarding sabot projectiles has typically taken the form of a metal pusher having a forward facing recess surrounding a high density metal penetrator, both pusher and penetrator typically being right circular cylindrically shaped members. The prior art pusher typically had a pyrotechnic delay column and expulsion charge adapted to explode after the assembled pusher/penetrator has been ejected from the gun barrel so as to axially separate the penetrator from the pusher. The inherent problem with the prior art configuration was that there could, because of normal machining/manufacturing variations, be significant differences in dimensions between the outer diameter of the penetrator and the inner diameter of the aforesaid recess. The difference in dimensions vary from round to round and hence result in a substantial variation of release forces, i.e., the forces tending to hold the penetrator within the pusher. This uncontrollable variation in release force accordingly would dramatically and significantly change the separation point from one round of ammunition to another, greatly reducing the overall accuracy, i.e., failing to produce a projectile having a low dispersion factor.

Previously, the lightest weight sabots were made of aluminum. In the past, the search for lighter weight sabot materials focused on metal composites. But researchers were continually frustrated by failure-metal composites simply were too brittle. Attention then shifted toward polymer-based composites, which were being used extensively in thin structures for aerospace applications. Researchers began to consider fiber composites for complex shaped structures that needed to survive multidirectional stresses. Some engineers refer whimsically to a fiber composite as "string and glue." It consists of high-strength carbon fibers, which must be laid down and oriented to yield maximal strength and handle maximal stress. Polymer is used to glue together layers of these fibers in a process similar to that used to manufacture plywood. When layers are glued together, the grains of adjacent layers are arranged either at right angles or at some wide angle to each other. Once a piece of the material has been fabricated, it can be machined into the required form. Fairly thick pieces that can withstand high three-dimensional stress are used for sabot material.

Long rod penetrators are well known and are adapted to penetrate armor. Long rod penetrators have stabilizing fins which are either welded to the penetrator rod or threadably fixed to the penetrator rod. Such stabilizing fins are necessary to guide the penetrator in true flight to the target. In conventional long rod penetrators, if armor is thick enough, it is possible that the long rod remains unpenetrated if the fins remain at least partly locked to the penetrator or are only partly sheared from the penetrator as the penetrator moves through the armor.

It would, of course, be desirable to reduce the retardation effect of the fins which may prevent the penetrator from moving as far through the armor as possible. This problem is intensified when using stronger ferrous type materials for the guide fins and penetrator rod to accommodate new propellants which expose the assembly to higher temperatures. Thus, armor is constantly being improved in toughness, hardness, obliquity and is being constructed in multilayer fashion. All of these changes require improvements in penetrators by increasing penetrating power and range. Such improvements are accomplished by adjustment of the length to the diameter ratio of the penetrator, the use of new material such as tungsten and depleted uranium, the use of new propellants, new sabots and new stabilizing fin structures and materials, therefore, and the like. The new propellants require the penetrator to withstand higher temperature in the gun tube since the rod and the guide fins are heated to higher temperatures. Consequently, stabilizing fins which conventionally were made from aluminum alloys are now being made of ferrous alloys which have much higher strength and are capable of withstanding higher temperatures.

When using aluminum alloys, the fins tended to shear readily from the penetrator body when the fins reached the surface of the armor being penetrated and did not produce a substantial retardation force against continued movement of the penetrator rod into their armor. However, higher strength stabilizing fins do not shear until a considerably higher force is applied between the rod body and the fin so that a substantial retardation force is present as the penetrator shaft enters the armor and the fins encounter the armor surface. In other words, a portion of the energy which propels the rod into the armor will be used up by "dragging" the fins through the rod cavity in the armor or in shearing or tearing the fin from the rod. As a result, the full impact energy of the rod is not used in accomplishing its primary objective of passing through a given armor thickness.

ROCKET-ASSISTED PROJECTILES There are two main reasons for developing rocket-assisted projectiles: (1) to extend the range over standard gun systems, and (2) to allow for lighter mount and barrel design and reduce excessive muzzle flash and smoke by reducing the recoil and setback forces of standard gun systems. Since the ranges are different, the above two objectives represent opposite approaches in the development of rocket-assisted projectiles. Normally, one or the other establishes the performance of the rocket-assisted projectile under development although some compromise in the two approaches may be established by the design objectives.



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