Chapter 1 Chapter 1

CHAPTER 1 CHAPTER 1 INTRODUCTION The "Ballistics" is derived tVoni the Latin word "baiiista" which is an ancient machine designed to hurl a javelin. The word baiiista owes its origin to a Greek word "Ballein" meaning 'to throw' '", The stone hurled by prehistoric man was the first example of ballistics. The ability of throwing farther, with more power led to devices such as slings and spears. Thus ballistics can be defined as scientific study of motion of a projectile in some medium: either in space or in atmosphere or in liquid. Motion of gun projectile is studied in Gun Ballistics. The gun can be viewed as a mechanical device in which heat, liberated by a burning propellant, is converted into the useful kinetic energy of projectile. Its function is to propel projectiles toward specified targets. When a round is fired propellant starts burning, ga.ses start coming out which develop pressure inside the gun. Projectile gains velocity and starts moving inside the barrel. It leaves the gun from the muzzle end (Fig. 1.1). Then projectile moves in air till it hits and damages the target. Gun ballistics is studied in following classes: ' i) Internal Ballistics ii) External Ballistics iii) Intermediate Ballistics iv) Terminal Ballistics (Fig. 1.2). i) Internal Ballistics Internal ballistics is the scientific study of the operating processes within the gun from the moment burning of the propellant is initiated. Change in the pressure and gain in the projectile velocity are the oiilcomes o\' these processes due to the chemical energy generated hy burning the propellant. ii) External Ballistics External ballistics deals with characteristics that influence the motion of the projectile as it moves along its trajectory. Trajectory is the flight path of projectile as it flies from muzzle of weapon to the point of impact. There are number of factors which affect the motion of a projectile; some are associated with the projectile itself; like its mass, calibre, shape, axial spin rate and others with the atmosphere through which the projectile moves; like density, wind velocity, air resistance. iii) Intermediate Ballistics It is the study of the transition from internal to external ballistics, which occurs in the vicinity of gun muzzle. Motion of the projectile is affected due to precursor flow as well as propellant gas flow Held. Separation of sabot parts in ca.se of subcalibre projectiles also takes place during this part of motion. Different types of muzzle brakes are developed to reduce/bifurcate the flow of gases and to reduce the blast/flash observed. iv) Terminal ballistics Terminal ballistics includes the study of the effects of impact of projectile on a target and the projectile itself. The conditions under which projectile impacts against target vary widely depending on strike velocity, strike angle, type of projectile and target. The damaged a target is affective in two ways due to kinetic energy and due to explosive effect, accordingly the projectiles are called as kinetic energy projectile and chemical energy projectile. In kinetic energy projectiles the main effect is due to impact and penetration. A chemical energy shot contains some amount of high explosive. On impact high explosive detonates and its shock energy is utilized to damage the target in three dit't'erent ways. i) It concentrates at a single point as a jet. ii) High explosive spreads over the target surface and then detonates. iii) High explosive is detonated in the vicinity of the structure and it blasts a structure '"'. The kinetic energy shot damages the target like tanks using its high kinetic energy. The shot is strong and small, whose outer contour may be harden to over come the impact loads of a strong target material. The material of the target melts on impact of the projectile to almost liquid form due to high linear momentum. The projectile moves inside the target, till velocity reduces, the state of the target material changes to plastic. Here the material shears past the projectile and comes out in petal forms and also mo\es axially forward. In this state is the major adxance of the projectile in the target. Finally as the target state changes to an elastic mode, the projectile ceases to move further. As the de\elopment of strong armors is becoming a reality, the projectile is allowed to yield, shatter and consumed but its length is large and defeat of the target continues. It retiuires the projectile to be thin and long to have more penetration at a target. A fundamental necessity for obtaining the optimal effect of a projectile is that it reaches the target with maximum accurac}. It requires the projectile to be stable. The stability of the projectile can be achiexed by either giving spin to the projectile or by attaching fins at the rare end of the projectile, 'fhe requirement of spin for stability is very large for projectiles with large length to calibre (1/d) ratio which is practically not possible. That is where tins stability is used. There are three common types of projectiles: i) Bore-calibre projectiles These projectiles are of the same diameter as the bore of the gun. Their sides are directly in contact with bore. Most of the large bore-calibre projectiles are fitted with driving band(s). to allow location, obturation and spin of the projectiles within the barrel. ii) Shot The term shot is applied to loosely supported lead shot or a number of flechettes positioned ahead of a gas sealing wad. Its use is limited to shotguns and warhead fillings. iii) Sabot Projectiles A thin projectile has more penetration when tired from a higher calibre gun due to more muzzle velocity. This is a sub-calibre projectile which requires support to fit in the gun during launching phase. These supports, sabots are discarded once the projectile leaves the muzzle. This is known as Fin Stabilised Armor Piercing Discarding Sabot (FSAPDS) round (Fig.1.3), FSAPDS ammunition is a highly effective anti armor ammunition which is being used for almost all ad\ anced tanks. It consists of a very dense metal projected at an extremely high velocity whose armor piercing power is due to its high kinetic energy. The de\elopment of this ammunition started with under calibrated shell which is made up of dense metal (,APDS round). The ultimate step was the making of a particularly dense metal bar or rod, streamlined and stabilised by tail fins. The performance in terms of penetration is measured in terms of a parameter mv" / d^ kinetic energy per unit cross-section area. Higher value of this parameter gives more penetration. This thin projectile is guided within the gun barrel by sabots that are Jettisoned at the muzzle of the gun ' ". The 'Sabot" is a French word for wooden shoe. A military use of sabot is inherited from; its description as "a piece of soft metal formerly attached to a projectile, to take the groves of ritling". hi modern term, a sabot is a device conforming on one surface to a gun bore and on the other surface to a projectile. The sabot guides the projectile during its motion inside the barrel and provides obturation. After the exit from the muzzle end, within a few meters (2m to 5m) of travel these are separated and only core body (projectile) moves ahead. These projectiles are called "Discarding sabot projectile"' '. Types of Sabot Projectile Taking into consideration, the mechanism of separation, the sabot projectiles can be divided in to two groups: (a) Axially separated sabot projectile (b) Radially separated sabot projectile. a) Axially Separated Sabot These are cup sabots. They are either attached by push fitting or screw fitting. The basic cup design is shown in (F-ig.1.4). Cup sabots are attached to spin stabilized projectiles as well as fin or flared aft body projectile. It is e\ident that finned projeclilcs will ha\e only pushed rilled sabots and design will be such that Tins do not obstruct the sliding of sabot. b) Radially Separated Sabot These are the ring sabots. 3-4 in number placed radially around the penetrator (Fig. 1.5). Mostly these are used in finned projectiles where l/d ratio is large which cannot be stabilized through spin alone. The stabilising fins fixed at the rear end of the rod constitute a parasitic load during the acceleration phase and is needed at that location only after exit from the launch tube. Such consideration led to the idea to incorporate the fin assembly to the sabot so that during acceleration it is used as a structural element and after launch to have it slide to rear where it can serve as stabilizing element, discarded when the projectile is free in air. Sabot separation takes place with the help of one of the five types of separating forces. i) Propellant (las Force In this case sabot is split in two or more pieces with a hole or cavity machined in the base at the interface of sabot pieces and core body. This cavity is filled with the high-pressure propellant gas while the projectile is accelerated in the gun. Upon leaving the launch tube, due to this high internal pressure forces these sabot pieces may apart away from the projectile. ii) Aerodynamic Forces These forces acting on the hollow front section of sabot pieces, force the pieces radially outward away from the penetrator. This method requires both a high air density and lightweight sabot segments (relative to projeetile weight) to obtain good separation. iii) Internal Elastic force In such case the segmented sabot is made slightly oversize and compressed when loaded into launch tube.

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