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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.
2.2 Mortar bomb
All maneuver units require indirect fire to win. Mortars provide indirect fires that are organizationally responsive to the ground maneuver commander. Of particular value of these weapon systems is their capability of curved firing at different ranges which is an inestimable advantage during military actions in cross-country terrain. Military history has proved that mortar had the effectiveness in the war. Their rapid, high-angle, plunging fires are invaluable against dug-in enemy troops and targets in defilade, which are not vulnerable to attack by direct fires . They play a unique and vital role on the AirLand Battlefield.
Mortars allow the maneuver commander to killing indirect fires on the enemy, independent of whether he has been allocated supporting artillery. Heavy forces use carrier-mounted mortars to allow the mortar platoon to move cross-country at speeds compatible with the battalion task force. Light forces use wheeled vehicles or hand carry mortars into firing positions. Some companies have light mortars that can be manpacked across all terrain. All mortar sections and platoons exist to provide immediate, organizationally responsive fires that can be used to meet the rapid changes in the tactical situation on the AirLand Battlefield.
Mortars have existed for hundreds of years, first seeing use in siege warfare. However, the early incarnations of these weapons such as the Pumhart von Steyr were large and heavy, and could not be easily transported . 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.
In 1940 the development of mortar rounds comprising projectiles of various purposes was started under the leadership of Alexandr I. Zverev, a prominent designer and a winner of three State Prizes. The mortar bombs were designed to be fired from smoothbore mortars of all callibers adopted for service with the Army.
Wide use of mortar weapon systems is attributed to the combination of design simplicity, ease of mortar handling and mortar bombs high efficiency. For 50mm, 82mm, 107mm, 120mm and 160mm smoothbore mortars FSUE "SRPE "Bazalt" has developed HE, HE/frag, illuminating, incendiary, smoke and practice (training) mortar rounds.
The main goal of mortar systems development in the post-war period was to enhance ammo operation reliability and crew safety alongwith improving of technical characteristics. New types of mortar bombs were developed to be used with mortars of all calibers, these bombs were fitted with strengthened stabilizers, their bodies were manufactured of steel and steel cast iron. The precision of manufacturing bomb basic components was improved. These measures eliminated the possibility of short falling and mortar bombs destructing and provided for twofold fire accuracy improvement.
240mm M-240 superpowerful mortar firing a HE steel mortar bomb weighing approx 140kg and capable of destructing heavy log-and-earth, brick and concrete structures and buildings was fielded in 1950. So far, in terms of fire power this system has no equivalents in the world.
The smoothbore mortars featuring improved ballistics characteristics (120mm 2B11 towed mortar, 82mm 2B14 mortar and 82mm 2B9 automatic mortar) were adopted for service in the late 70s-early 80s and so far have no foreign counterparts. For the above mentioned mortars FSUE "SRPE "Bazalt" developed in a very short period of time two crucially new series of mortar bombs featuring improved efficiency and encreased firing range, among them there were bombs fitted with proximity fuzes. The family of bombs developed for the 2B14 and 2B9 mortars comprises fragmentary, illuminating and smoke rounds. The family of bombs developed for the 120mm mortars comprises HE/frag, illuminating, incendiary and burning bombs.
Nowadays the development mortar rounds follow feature below :
increased firing range owing to the use of long-range propelling charges
improved HE/fragmentation effect due to design parameters optimization, use of projectile steel, high-strength cast iron and powerful high explosives
application of red phosphorus based compositions
equipping rounds with proximity fuzes
introduction of new illuminating and incendiary compositions.
Figure 2.0: type of mortar bomb
2.3 Types of mortar
Basically there are no precise types of mortars. Even though, these mortar bombs can be group into its function or purpose. The three primary types of mortar are as follows:
High-explosive rounds are used to suppress or kill enemy dismounted infantry, mortars, and other supporting weapons, and to interdict the movement of men, vehicles and supplies in the enemy's forward area. Bursting WP rounds are often mixed with high-explosive rounds to enhance their suppressive and destructive effects.
Obscuration rounds are used to conceal friendly forces as forces maneuver or assault, and to blind enemy supporting weapons. Obscuration can be used to isolate a portion of the enemy force while it is destroyed piecemeal. Some mortar rounds use bursting WP to achieve this obscuration; others employ more efficient technology. Bursting WP is also used to mark targets for engagement by other weapons, usually aircraft, and for signaling.
Illumination rounds are used to reveal the location of enemy forces hidden by darkness. They allow the commander to confirm or deny the presence of the enemy without revealing the location of friendly direct-fire weapons. Illumination fires are often coordinated with HE fires to both expose the enemy and to kill or suppress him.
Even these mortar had been group into three main group which are high explosive, obscuration, and illumination but these mortar more easily recognize if they are group into more specific subgroup which are;
2.3.1 Light Mortars
The 60-mm mortar, M224, provides air assault, airborne, ranger, and light infantry rifle companies with an effective, efficient, and flexible weapon. The inherent limitations of a light mortar (short-range and small-explosive charge) can be minimized by careful planning and a thorough knowledge of its capabilities. The M224 can be employed in several different configurations. The lightest weighs about 18 pounds; the heaviest weighs about 45 pounds. Each round weighs about 4 pounds.
The 60-mm mortar round cannot penetrate most rooftops, even with a delay setting. Small explosive rounds are effective, however, in suppressing snipers on rooftops and preventing roofs from being used by enemy observers. The 60-mm WP round is not normally a good screening round due to its small area of coverage. In urban combat, however, the tendency of smoke to linger and the small areas to be screened make it more effective. During the battle for Hue in South Vietnam, 60-mm WP rounds were used to create small, short-term, smoke screens to conceal movement across open areas such as parks, plazas, and bridges. Fragments from 60-mm HE rounds landing as close as 10 feet away cannot penetrate a single sandbag layer or a single-layer brick wall. The effect of a 60-mm mortar HE round that achieves a direct hit on a bunker or fighting position is equivalent to 1 or 2 pounds of TNT. Normally, the blast will not collapse a properly constructed bunker but can cause structural damage. The 60-mm mortar will not normally crater a hard-surfaced road.
Figure 2.4: soldiers using one of the light mortar.
Figure 2.5: One of the light mortar, The M224 60mm Mortar
2.3.2 Medium Mortars
The 81-mm mortars, M29A1 and M252, are the current US medium mortars. The M252 is replacing the M29A1, but both will remain in the Army inventory for several years. Medium mortars offer a compromise between the light and heavy mortars. Their range and explosive power is greater than the M224, yet they are still light enough to be man-packed over long distances. The M29A1 weighs about 98 pounds. The M252 is slightly lighter, about 93 pounds. Both can be broken down into several smaller loads for easier carrying. Rounds for these mortars weigh about 15 pounds each.
The 81-mm mortar has much the same effect against urban targets as the 60-mm mortar. It has a slightly greater lethal area and its smoke rounds (WP and RP) are more effective. A direct hit is equivalent to about 2 pounds of TNT. The 81-mm round cannot significantly crater a hard-surfaced road. With a delay setting, the 81-mm round can penetrate the roofs of light buildings.
Figure 2.6: One of the medium mortar, The M252 81mm Mortar
2.3.3 Heavy Mortars
The 107-mm mortar, M30, and the 120-mm mortar, M120, are the current US heavy mortars. The M120 is replacing the M30, but both will remain in the US inventory for several years. The M30 is a rifled mortar, stabilizing its projectile by spinning it rapidly. The M120, like all other US mortars, fires fin-stabilized ammunition from a smooth bore. Although heavy mortars require trucks or tracked mortar carriers to move them, they are still much lighter than field artillery pieces. They outrange light and medium mortars, and their explosive power is much greater. The M30 weighs about 675 pounds. The M120 is much lighter at about 320 pounds. Rounds for the 107-mm mortar weigh about 28 pounds. Those for the 120-mm mortar weigh almost 33 pounds each.
The 120-mm mortar is large enough to have a major effect on common urban targets. It can penetrate deep into a building, causing extensive damage because of its explosive power. A minimum of 18 inches of packed earth or sand is needed to stop the fragments from a 120-mm HE round impacting 10 feet away. The effect of a direct hit from a 120-mm round is equivalent to almost 10 pounds of TNT, which can crush fortifications built with commonly available materials. The 120-mm mortar round can create a large but shallow crater in a road surface, but it is not deep or steep-sided enough to block vehicular movement. However, craters could be deep enough to damage or destroy storm drain systems, water and gas pipes, and electrical or phone cables.
Figure 2.7: One of the heavy mortar, The M120 120mm Mortar
2.4 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 2.8: A typical mortar bomb
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)
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.
AREA FOR FILLINGS
Figure 2.7: Cut-section of the casing
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.
220.127.116.11 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)
18.104.22.168 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)
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.
22.214.171.124 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.
126.96.36.199 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)
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.
2.5 Aerodynamic Drag.
In a vacuum, for a projectile in flight the only force acting to the projectile is that due to gravitational acceleration is the weight of the projectile. The horizontal component of the launch velocity remains unchanged while the vertical component is reduced by the gravitational acceleration . In a real fluid there will be an additional force opposing the forward motion of the projectile caused by the resistance of the fluid . This is what we call drag. In general terms, this force is function of the forward speed of the projectile. For the projectile which is flying with a very high speed the drag force can affect or modifies the trajectory of the projectiles. During the rising part of the trajectory, drag acts an addition force to the gravitational acceleration. The vertical component of the launch velocity is eroded more quickly. The horizontal component also reduced by this drag.
The drag force act on an object that moving through a fluid medium essentially contributed by two different effects of drag force which are skin friction and pressure drag.
A is the frontal area
is the density of the air
is the speed of the bomb relative to the air
Figure 2.11: Forces and moment during flight (Arrow Tech)
2.5.1 Skin friction
Skin friction is produce by the viscosity of the fluid . Viscosity is a measure of the resistance to shearing motion on the surface or the projectile. This happens when an object is in motion through fluid, the molecules immediately adjacent to the surface of the object and stick firmly to the surface. Molecules which are little further from the surface will move parallel to the surface. The molecules that stick on the surface of the body will form a thin layer also known as the boundary layer which shearing is taking place. The resolution in the flight direction of the force generated by these shear stresses acting on local surface of the body is skin friction.
2.5.2 Pressure Drag
Even viscosity in air is small it can still make a substantial contribution to the drag of a projectile because of its effect on the drag force. In general the average static pressure at the front of the moving body is greater than the average static pressure at the rear body due to the viscosity . This imbalance pressure produce force acting toward the rear of the body against the motion of the body. This is called pressure drag.
At subsonic speeds, smooth shape with rounded noses and gently tapering rear section with pointed bases are required in order to minimize the drag force. If sharp corner is represent elsewhere on the object the boundary layer may separate from the surface. This phenomenon will form a region of fluid behind the body which is called wake. In this wake the static pressure is lower than it would otherwise be and therefore the drag is increased. This form or drag usually called ââ‚¬Å“base dragââ‚¬Â. The drag force generally can be calculated as;
2.6 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.
2.7 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.
2.8 Stability of the Bomb
Mortar bomb obtain stability through the use of fins located at the 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.
2.9 Vortex Shedding
An unsteady flow also known as vortex shedding usually that takes place in special flow velocities. In this flow, at the back of the body vortices are created and detach periodically from either side of the body. The inertia effects are negligible and the flow pattern is very similar to that for ideal flow and the pressure recovery being nearly complete at very small values of Re. With increasing of Re will cause the fixed eddies to elongate and then begin to oscillate. The fixed eddies then breaks away from the cylinder depending upon the free stream turbulence level. The breaking away occurs from one and then the other side of the cylinder, the eddies being washed away by the main stream. This process is intensified by a further increase of Re, whereby the shedding of eddies from alternate sides of the cylinder is continuous, thus forming in the wake two discrete rows of vortices. This is known as vortex street or von KÃÂ¡rmÃÂ¡n vortex street . A vortex shedding will only be observed at range of Reynolds numbers (Re), at the value of about 90. The Reynolds number is a ratio of inertial to viscous forces in the flow of a fluid. This Re can be defined as:
d = the diameter of the cylinder (or some other suitable measure of width of non-circular bodies) about which the fluid is flowing
V = the steady velocity of the flow upstream of the cylinder
= the kinematic viscosity of the fluid.
The range of Re values will vary with the size and shape of the body from which the eddies are being shed, as well as with the kinematic viscosity of the fluid. Over a large Re range (47<Re<107 for circular cylinders) eddies are shed continuously from each side of the body, forming rows of vortices in its wake. The alternation leads to the core of a vortex in one row being opposite the point midway between two vortex cores in the other row, giving rise to the distinctive pattern shown in the picture. Ultimately, the energy of the vortices is consumed by viscosity as they move further down stream, and the regular pattern disappears.