would have a muzzle energy of 5.6 MJ, but heavier projectiles can reach energies of 9 MJ – the equivalent, explained Keçeli, of “two kilograms of TNT concentrated on an area with the cross-section of a ping-pong ball.” It is enough energy to theoretically lift a corvette-class ship off the ground. The key is to understand the very small area that the round impacts, which can be less than 2.5cm across. APFSDS is a nightmare for armour designers, according to Keçeli, and a death sentence for crew of a heavy AFV if successful penetration is achieved. As always, the behind-armour effects depend upon the type of armour, as well as the type of projectile – some made from from advanced alloys, like tungsten carbide, others from depleted uranium (DU). The raison d’être of these metals is their density: older projectiles, such as the Soviet 3BVM-3, were made of steel, which has an average density of 7.8g/cm3, so penetration is somewhat modest. Tungsten or DU rounds, however, can have densities up to 17g/cm3, dramatically increasing energy potential. One behind-armour effect against monolithic steel armours is a phenomenon known as plugging, whereby “localised shear bands can not hold the armour matrix together and the armour part in front of the projectile slides through localised melt pools, like it is sliding on a rail,” Keçeli explained. As the projectile penetrates, it begins to create massive stresses within the armour, cracks appear, and a plug of armour travels with the projectile until it exits the armour and enters the crewed area. Once through the armour, “the plug and penetrator residue most likely ricochet off of surfaces or simply carry on penetrating various equipment inside the vehicle,” Keçeli said. In the absence a plug, it is the penetrator itself that creates the behind-armour effects. According to Keçeli, the after-effects of large calibre penetration are significantly greater than with smaller munitions. Large calibre munitions feature enormous energy, transferred to the armour and vehicle, creating a fragmentation cone (known as spalling) from the rear face of the armour, which hits the crew at supersonic speeds. A second-order effect is the penetrator itself, which can hit crew and other equipment and, in some cases, generating enormous friction In 2006, the Israeli Defence Force (IDF) deployed to Lebanon as part of Operation Change of Direction, taking a number of its Merkava main battle tanks with it. During the operation, the IDF deployed its armoured forces in small isolated packets, providing Hizbullah the opportunity to engage the tanks at range with anti-tank guided missiles (ATGM). A total of 50 Merkavas were hit with a variety of missiles: 21 were penetrated, but only ten of the incidents resulted in IDF casualties. The conflict is an example of the critical role that behind-armour effects play. It is not sufficient to simply penetrate a vehicle’s armour: that penetrator must be able to inflict enough damage behind the armour to kill or wound the crew, or disable the vehicle. There are, of course, many ways in which a vehicle can cater for behind -armour effects - but it is worth examining how those effects can vary, depending on the type of weapon used. “Regardless of design, it all comes down to a kinetic force acting on a physical body. You have to move enough material out of your trajectory so that you can reach the personnel or the equipment inside an armoured body to render them non-operational,” Alper Keçeli, a materials scientist and armour designer based in Turkey told MilTech. “In principle, penetrators are no different to a rock that is thrown; the bigger the rock, the higher the damage - the faster the rock, the higher the damage,” he said. However, he added the simplicity of this problem should not disguise the magnitude of the damage that can be delivered. Large Calibre APFSDS Large calibre (76-125 mm) Armour Piercing Fin Stabilised Discarding Sabot (APFSDS) rounds, also known as long-rod penetrators, are the primary anti-tank round used by most tank forces. “In most cases, a penetrator is a projectile made out of ferrous or non-ferrous alloys,” Keçeli explained. Multiple factors decide the efficacy of APFSDS munitions: length to diameter ratio, material, velocity, projectile yaw, and body design. If any of these are impacted or suboptimal, the penetration of an APFSDS round can be reduced. The mechanics behind this type of round, however, are brutal. A 5kg penetrator fired at 1,500 m/s A Research Associate with the Royal United Services Institute in London, Sam Cranny-Evans has a legacy of expertise in armoured warfare and the underlying technologies. Samuel Cranny-Evans Behind-Armour Effects Feature MT 3/2022 · 35 An M1A1 fires a kinetic energy round on a training range. The APFSDS is one of the greatest challenges for armour designers. (Photo: L/CPL Casey Jones via Wikimedia Commons)
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