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 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 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)

ii)

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, 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 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 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 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 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. Upon leaving the barrel, the rebound of material causes

the segments to separate.

iv) Centrifugal Force

The method of using centrifugal force to separate segments is extremely

powerful and does not rely on any internal or external pressure force. This method

also depends on the availabilities of a rifled launch tube. Due to projectile spin, centrifugal force acts on the sabot and sabot pieces separate from the projectile.

v) External Mechanical Force

A retarding force may be applied to a full-bore diameter sabot (base) to achieve an axial separation between it and sub projectile.

In this dissertation we have developed a mathematical model of sabot separation in different phases near muzzle by the external force.

Yang '^' has worked on Fin Stabilized Armor Piercing Discarding Sabot

(FSAPDS) projectile motion. The FSAPDS consists of the tlying projectile body with tail assembly and the three sabot components fixed on the projectile body symmetrically with the groove teeth. The sabot components gradually go around their rear touching point, turn backward and separate from the projectile under action of the air dynamic force. The sabot discard process is very complex. I'iie separation process can be studied with the help of three turning points and two transition periods (Fig. 1.6). The turning points can be defined as follows:

The first turning point is at the instant when the fixed circles of a sabot component reaches to the limit stress state, and its groo\e teeth break at the same time due to the air dynamic force. The second turning point is at the instant when circle groove teeth of a sabot component separate from those of projectile body and their mechanical interaction vanishes also due to the air dynamic force. The moment is defined as the third turning point, when the intersect point between projectile and shock wave at the head of a sabot component moves to the projectile base, and the projectile is considered as getting free from the influence region.

The first period is from the moment of the projectile leaving the gun muzzle to the first turning point. The second period is from the first turning point to the second one. The third period is between the second turning point and third one. The fourth period is free llight period of the projectile, during which the projectile gets rid of the sabot component influence. The first transition period is from the .second turning point of the first sabot component to that of the last sabot component. The second transition period is between the third turning point of the first sabot component and that of the last sabot component. The four periods and two transition periods stated above have the mutual association and influence.

Turning points are defined for each sabot separately. The first turning point for three sabots is at the same time and same position relative to the projectile body.

The second turning point is at the same position relati\e to the projectile body but is at different times, whereas the third turning point is at different time and different relative positions, which can be studied with the help of transition periods separately.

The motion of the projectile is divided in each time period called as phases of motion.

The total motion is studied in these phases separately. In these phases sabots get separated from projectile where projectile may gets unstable. These phases are called critical phases of motion.

Intermediate ballistics includes study of flow fluid in the vicinity of the muzzle, theoretical modeling of gas flow and sabot separation process. Most of the work done in this field by the researchers is experimental. A brief review of it has been presented in the next chapter. 41^ -H- /5IXXZLI

Key: A - Breech E - B e gmning of lands B - Breechlock F-Ritfedbore C - Reaction ( or combustion) G - Muzzle chamber H - Chase D - Shoulder I - Slide cvHnder

Fig.1.1- Parts of Gun

A-Internal Ballistics B- Internucliate Ballistic C- Ex1erniilB:illistics Muzdf D- Termiiiiil Ballistics Tijiget

V- r - Z^

-^v-B--

Fig. 1.2- Classification of Ballistics Fig.l.3-Fin Stabilised Armor Piercing Discarding Sabot (FSAPDS) Projectile

Fig. 1.4- Axially Separated Sabot (Basic Cup Sabot)

II Fig.1.5- Radially Separated Sabot (Basic Ring Sabot)

Murdf Firsi Tummg Second Turning Third Tummg Free Flight Penod Period Penod Trajectory

^_ P-^ > > > > l^lrrxi" ^ ^W- "^

•^ -^ ^ •X- First Period—) Second Period >Thifd Penod ^Fourth Penod

(First Transion Period) (Second Transition period )

Fig.1.6- Phases of Motion

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