How Cumulative Projectiles Work

Picard

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Also posted on: How Cumulative Projectiles Work – History and War (wordpress.com)

Cumulative or HEAT warheads are the staple of infantry (man-portable) anti-tank weapons, and are also used by artillery and various unarmoured and armoured vehicles for the purposes of anti-tank combat. Their main advantage over other types of anti-tank projectiles is that their effectiveness does not depend on the projectile’s velocity. As a result, it was possible to produce an entire group of man-portable anti-tank weapons which allowed infantry significant anti-tank capability.

These weapons were far cheaper than the tanks they were used to destroy, by the factor of over a hundred. Today, a guided Javelin missile costs 175 000 USD, while a modern main battle tank can cost from 2 to 7 million USD. And they were highly effective. Once Germans introduced the Panzerfaust on the Eastern Front in second half of 1944., proportion of the Soviet tank losses rose to 30% of all the tanks used.

Cumulative projectile works by focusing the effects of the explosive fill onto a small surface. Portion of the armour at that point turns into small particles which the explosive jet carries into the tank’s insides. The hole itself is very small, a few centimeters in diameter. After passing through the armour, the jet turns into a funnel, spreading outwards at cca 15 degree angle and reaching several meters away. It kills the crew in its path, and can also cause the ammunition to detonate should it penetrate the bullet jacket.


Cumulative projectile


Effect of the cumulative projectile on armour

The cumulative effect is increased by lining the recess with 1 – 2 mm thick layer of metal, which melts and forms itself into the jet. The speed of the metal jet can reach 10 km/s. Compared to the conventional cumulative projectiles, pressure and densities of matter and energy are much higher, increasing penetrative power of the jet.


Metal jet

Effectiveness of the jet depends on the projectile’s diameter – larger projectile has greater penetrative power. Penetration itself is expressed in the values of thickness of homogenous steel armour which projectile can penetrate at 90 degree impact angle. “Zolja” with 64 mm warhead penetrates 300 mm of RHA, while “Osa” with 90 mm warhead penetrates 400 mm of RHA. Swedish AT4 with 84 mm warhead can penetrate 420 mm of RHA (originally 356 mm). But these are optimal values. In reality, projectile will never strike armour at a perfect 90 degree angle, and if vector of the threat is known, tank’s crew can take various measures (such as angling the vehicle) to further increase effectiveness of the armour. These values are much greater than those of World War II era cumulative projectiles, which generally penetrated only armour thickness approximately equal to their own diameter. Soviet 122 mm shells were somewhat better than early models, penetrating up to 150 mm of steel armour. Cumulative warheads were also used in armour-piercing rockets carried by the ground attack aircraft.

Various obstacles can disrupt the jet and thus reduce the effectiveness of armour penetration. Projectile’s effectiveness is also significantly reduced if it is forced to activate away from the primary armour layer. Various obstacles were used for such purposes, such as sand bags and tool boxes.


M4 Sherman with sand bags

Because the jet acts as a fluid, passage through different types of materials (even just metal – air – metal) causes it to deform, form vortices and breaks, which dissipate it and reduce its penetrative power. These principles were exploited in construction of both spaced and composite armours.


Types of Armour. Translation: a) composite armour – 1 main armour; 2 – separate armoured element, 3 – ceramic plates, 4 – carrying material, 5 – armoured plate, 6 – radiation protection; b) spaced armour – 7 – armoured plate, 8 – space, 9 – inner armoured plate; c) reactive armour, 10 – active armoured element, 11 – explosive, 12 – main armour

This is not to say that perfect protection can be achieved. Even if projectiles cannot penetrate the armour of the tank itself, it is still possible to render the tank combat-ineffective by destroying optics, tracks, or targeting the turret – hull joint. Sides, rear and especially top of the tank also tend to be less armoured than its front. Particularly vulnerable are armoured plates behind the wheels, as they tend to be very thin (not much above 90 mm for most tanks). For this reason, many tanks use armoured skirts to set off HEAT charges.

Majority of armoured personnel carriers, infantry fighting vehicles, and other armoured vehicles, tend to be much less armoured than main battle tanks. Most of the lighter APCs in particular are only expected to protect soldiers from infantry small arms fire as well as shell fragments up to 155 mm, though heavier variants exist – some being as armoured as actual tanks.

Major problem when using HEAT projectiles is explosive-reactive armour (ERA). This armour consists of the metal boxes filled with the explosives. Explosives are set off by the cumulative jet, and the shockwave of the explosion destabilizes the jet, thus reducing its effectiveness. Major weakness of this type of armour is its liability to be activated by the small arms fire. In order to counter this armour, modern HEAT projectiles have double or even triple warheads. The idea is that the smaller charge in front activates the explosive armour, clearing the path for the main warhead.

When used against base armour, this multiple-stage design also improves penetration. A sophisticated heavy two-stage shaped-charge warhead is capable of piercing armor of equivalent to 900mm thickness. A triple-shaped charge warhead offers 50mm more penetration.
 

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