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The design of the mortar bomb

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It is important to do studies on existing information regarding the design of the mortar bomb. This will help in giving a critical review about the subject in this study which is the ammunition for 81mm mortar. This chapter will discuss about the parts of the mortar bomb, types of 81mm mortar ammunition, ballistic of mortar, fragmentation of the bomb, aerodynamic forces and moment acting, bomb stability and software used for simulation. From the research, all information will be the guideline in developing this study. This chapter will also increase the understanding of this study in order to ensure success at the end of the second semester.

2.1.1 Background of Mortar

Mortars started to be developed when tactical trench lines came into use in the World War I. The objective was to bring casualty into the enemy trenches. The early idea and complex design was the German mine launcher, Minenwerfer but the archetype of a mortar was the British Stokes design in 1915 which was a simple tube with a fixed firing pin at the bottom end, where a bomb was dropped and ignited to launch the bomb out from the barrel to the target. Basically, mortar is a stumpy tube designed to fire a projectile at an angle higher than 45 degrees but lower than 85 degrees so that it falls on the enemy territory.

Figure 2.1 (a): Minenwerfer (www.landships.freeservers.com)

Figure 2.1 (b): Stokes Mortar (www.landships.freeservers.com)

2.1.2 Types of Mortar

There are no precise definitions in categorising the mortar. Therefore it is helpful to group them as light, medium or heavy.

2.1.2.1 Light Mortars

Mortars of approximately 50 to 70mm size of calibre which are laid by hand meaning they have no baseplate or bipod and have very simple sighting systems. They are generally carried at platoon level.

Figure 2.2: Light mortar (www.flamesofwar.com)

2.1.2.2 Medium Mortars

All other conventional man portable mortars, with calibres sizing up to approximately 110mm. They are usually pooled in specialist support sections at company or battalion level. They have base plate, bipods, and sophisticated sighting system.

Figure 2.3: Medium mortar (www.gosfordhobbies.com.au)

2.1.2.3 Heavy Mortars

Mortars which are too heavy to be carried and which are therefore vehicle mounted or towed, although it should be noted that light and medium mortars are frequently vehicle mounted for tactical even though they may be man portable.

Figure 2.4: Heavy mortar (www.missing-linx.com)

2.1.3 Mortar Ammunition

It is the mortar bomb, a streamlined metal shell having stabilising vanes at the tail which is normally filled with explosives. The mortar bomb gained its thrust through the burning of an amount propelling charge placed in the tube. The size of mortar bomb varies depending on the inner diameter of the mortar. Mortar ammunition can be categorised depending on their fillings and purposed as:

i. High Explosive (HE) is use for fragmentation and blast. It causes troop casualties and damage to light material.

ii. Red Phosphorus (RP), White Phosphorus (WP) smoke. It is used to screen, signal, and act as an incendiary.

iii. Illumination. Used to illuminate, signal, and mark.

iv. Training Practice (TP). Training items are completely inert. Practice items may or may not contain explosive sections such as propellant charges or spotting charges.

2.2 Mortar Bomb Parts

The construction of a mortar bomb is normally consists of fuze, casing with obturation baffles, cartridge and fin. Every part mentioned has different purpose on the bomb.

Figure 2.5: A typical mortar bomb

2.2.1 Fuze

The purpose of a fuze is to initiate a projectile when it strikes a target or at an appropriate point in its flight. It cannot be accidentally initiated in storage, transportation, or in the weapon when it is fired. Fuze used on mortar bomb is the nose fuze type, a simple percussion fuzes which function when the nose of the shell is crushed on impact with the target. This type of fuze is normally fitted to High explosives (HE) and white phosphorus smoke ammunition. Those used with HE shells often incorporate an optional delay setting which allows the projectile to penetrate the target before functioning.

Figure 2.6: Projectiles with nose fuze (www.globalsecurity.org)

2.2.2 Casing

The casing carries fillings which determine the purpose of the ammunition. For HE fillings, it is designed to provide maximum fragmentation during explosion when detonated by the fuze. The material used in governing the casing is normally forged steel and cast iron.

Figure 2.7: Cut-section of the casing

2.2.3 Obturation

The diameter of a mortar bomb must be less than that of the tube from which it is to be fired or otherwise it could not be loaded. For the bomb to drop straight to the bottom of the barrel without being supported on a cushion of air there must be a gap between the outer wall of the bomb and the inner wall of the tube. This gap is known as windage. Windage allows expanding propellant gases to flow past the bomb and vent into the atmosphere and thus lower the thrust of the bomb when it is launched. Obturation provides a close down to this gap.

2.2.3.1 Obturating Baffles

To prevent the excessive loss of gas on firing is to machine series of baffles around the widest part of the casing. The baffles create turbulence in the windage gap between the bomb and the internal surface of the barrel, and thus prevent the gases from flowing freely upwards.

Figure 2.8: Obturating baffles system (Cranfield Institute of Technology)

2.2.3.2 Obturating Ring

One of the most significant advances in modern mortar bomb design was the invention of the plastic obturating ring, an expanding split ring sitting in a single groove in the bomb casing. This system provides excellent obturation.

Figure 2.9: Obturating ring system (Cranfield Institute of Technology)

2.2.4 Cartridge

Cartridge carries propellants. Upon firing, a pin strikes the primer at the base of the cartridge and ignites the propellant powder, which burns rapidly and generates expanding gases. The gases are forced down the length of the barrel, pushing the projectile in front of them and eventually out of the barrel.

2.2.4.1 Primary Cartridge

The primary cartridge carries the initiating system and the first increment of the propelling charge. It fits into the central channel in the spigot of the tail section. When the propellant in the primary cartridge is ignited, the cartridge ruptures at point corresponding to the holes in the tail spigot. The flames which come from the tail spigot then ignite the augmenting cartridges, which are fitted around the tail of the bomb.

2.2.4.2 Augmenting Cartridge

Most mortar bombs have augmenting cartridges which are ignited by the primary cartridge and which provide the full charge for achieving maximum range. For firing at shorter range, increments can be removed quickly and discarded.

Figure 2.10: Primary and augmenting cartridge (Royal Ordnance)

2.2.5 Fin

Fin provides stability to the projectile. Attached fin projectile does not need some sort of rifling bore to be launched since it does not require spinning in order to gain stability in flight.

2.3 Ballistic of Mortar

Ballistic is characteristic for the motion of objects moving under their own momentum and the force of gravity. Mortars operate at low pressure compared to guns. It is possible to increase the pressure generated in the bore on firing but this requires a stronger, and heavier barrel and a bigger baseplate. Such solutions are possible for vehicle-mounted or towed equipments, but not for manportable mortars.

All the work done by the expanding propellant gases in accelerating the bomb to its maximum velocity is achieved in the short distance travelled in the bore by the widest part of the bomb, which carries the obturating ring or baffles. After this part of the bomb has emerged from the muzzle the expanding gases continue to accelerate through the increasing gap into the atmosphere. In a typical mortar the distance travelled in the bore by the obturating part of the bomb is less then one meter. Any increase in this distance would produce a higher muzzle velocity and thus increased range, but this would be at the expense of portability.

The muzzle velocity of typical 81-mm mortar bomb fired at maximum charged is around 300 m/s and this produces a maximum range in the region of 5000-6000 m. The tactical need for the infantry to engaged targets beyond this range is not so great as to outweigh the advantages of current weapon systems, with their portability, flexibility and speed into and out of action.

Most mortar fire bomb at subsonic velocities and this avoids the ballistic complication of the transonic and supersonic zones. It is called subsonic if all the speeds considered are less than the speed of sound, transonic if speeds both below and above the speed of sound are present, supersonic when the flow speed is greater than the speed of sound. In the past the transonic zone presented a barrier through which mortar bomb could not fly without becoming catastrophically unstable, but this was largely the consequence of crude manufacture and assembly which resulted in asymmetric and inherently unstable ammunition. Modern mortar bomb are manufactured to close tolerance and they are thus more stable in flight an can be fired at supersonic velocities if greater ranges are required. Tampella long-barrelled 81-mm, 120-mm and 160-mm mortars fire bombs at muzzle velocities of up to 400 m/s.

2.4 Fragmentation

The act of fragments scattering after the bomb is detonate. Fragmentation performance is controlled by fragment mass, fragment velocity and payload.

2.4.1 Fragment Mass

Factors governing fragment mass are:

i. material properties of the casing

ii. thickness of casing wall

iii. quantity of explosives

iv. detonation velocity of explosives

The material of the casing must be neither excessively ductile nor excessively brittle.

2.4.2 Fragment Velocity

Factors governing fragment velocity are:

i. Quantity of explosive inside casing

ii. Energy of the explosives

iii. Density of casing material

To calculate fragment velocity, Gurney Formula is used:

V = (2E) . [ (C/M) (1+C/2M) ]

Where:

V is the fragment velocity

E is the Gurney explosives constant

C is the mass of explosives per unit length

M mass of casing per unit length

Variations in the parameters would lead to a combination of fragment size and velocity which could be optimised for particular applications. In the case of mortar casing, the constraints imposed on the shape by aerodynamic considerations and on both shape and material choice by structural considerations will mitigate against an ideal fragmentation performance.

2.4.3 Payload

It is usually desirable to carry the maximum high explosives payload to the target. Such considerations can therefore have a substantial effect on the design of extended range projectiles solutions may include using an extended length of ogive to reduce drag or use a sub-calibre round or to use base bleed. These solutions compromise the payload carrying capacity.

2.5 Aerodynamic Forces and Moment Acting On the Bomb

The aerodynamic forces and moments which have measurable effect on a finned type projectile are the drag force, lift force, and pitching moment. Once the projectile leaves the muzzle, its trajectory is determined by many forces. Primarily, gravity exerts a constant pull on the body and acts through the centre of gravity which is determined by the distribution of weight throughout the body. Gravity always produces a uniform vertical acceleration of about 9.8 m/s2.

Figure 2.11: Forces and moment during flight (Arrow Tech)

2.5.1 Centre of Gravity

An unspin projectile must have its centre of gravity well forward so that it travels nose first. This governs the shape of the typical mortar bomb, which is wide at the nose and tapers toward the tail. The tail assembly must be as light as possible, and in modern designs this is achieved by making of lightweight aluminium alloy. If the bomb body is roughly cylindrical, as in a bomb used as a carrier for an ejecting payload such as smoke canisters or bomblets, the centre of gravity can be moved forward in relation to the overall length of the complete bomb by fitting a long tail boom.

2.5.2 Centre of Pressure

The centre of pressure is the point at which wind forces exert no turning moment, and in any unspun projectile this point must be behind the centre of gravity. The lift generated by the fins of a mortar bomb provides a force the move the centre of pressure towards the rear, behind the centre of gravity. This generates a restoring moment that rotates the projectile through its centre of gravity towards the direction of its trajectory, thus progressively reducing yaw.

2.5.3 Drag Force

Drag force opposes the forward velocity of the bomb. Drag forces act at the centre of pressure which is a function of the body's shape and are in the opposite direction as the motion of the bomb. There are three types of drag force that apply, which are:

i. Skin drag- friction on the outer surface as it moves through the air

ii. Shape drag- caused by low pressure behind the body due to the flow of air around its shape.

iii. Wave drag - a loss of energy that is put into acoustic waves as the body passes through the air. Particularly strong near the speed of sound in air.

Drag coefficient is mainly dependent on the shape of the bomb. In addition to this shape-related coefficient, the aerodynamic drag also depends on the frontal area of bomb, the air density, and the square of the relative air speed. The relationship between drag and these factors can be expressed by:

Drag =

Where:

A is the frontal area

is the density of the air

is the speed of the bomb relative to the air

2.6 Stability of the Bomb

Mortar bomb obtain stability through the use of fins located at the aft end of the bomb. Normally, six, eight, ten or twelve fins are employed. Additional stability is obtained by imparting some spin to the bomb by canting the leading edge of the fins. Fin-stabilized projectiles are very often sub-calibre. A sabot, wood or metal fitted around the projectile, is used to centre 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. For projectiles fired without spin or only with a small spin the stabilising influences must be created by aerodynamic forces. For the bomb to be stable, the center of pressure location is required to be behind the center of gravity location when measured from nose.

2.7 PRODAS

Simulation software is very important in order to simulate data and to see the behaviour of the projectile. Utilisation of simulation software reduces the cost and the probability of failure for this study. In this study, simulation is the main method determining the projectile behaviour in term of ballistic theory generally, external ballistic theory specifically.

PRODAS is produce by the Aero Tech, an Engineering Consulting business with a focus on the defence industry. This software is focuses in advance weapon design with the standard world integrated weapon design tool. Simulation tools provided by PRODAS are:

i. Modelling - Build a model from a drawing or even a picture.

ii. Aerodynamics - Compare aerodynamic coefficients from multiple aero estimators.

iii. Launch Dynamics - Interior ballistics, balloting and jump.

iv. Trajectories - Fly 4DOF, 6DOF and Body Fixed and Guided Trajectories.

v. Terminal Effects - Estimate penetration of KE projectiles and lethality of fragmenting or shaped charge warheads.

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