At the beginning of war against Croatia, Yugoslav Army had available almost 3.000 pieces of armoured vehicles. Large portion of these vehicles were quickly lost or disabled. Due to very limited anti-armour capabilities of Croatian military, especially early in the war, most of these were destroyed by cumulative (HEAT) projectiles. Cumulative rounds themselves have many advantages. Unlike subcaliber projectiles, their effectiveness does not depend on the impact velocity. This led to development of a wide range of portable antitank weapons. Price of such a weapon can be >450 times less than price of a tank. They are also highly effective: once Germans introduced Panzerfaust to the Eastern front, Soviet losses in terms of completely destroyed tanks reached 30% of all the tanks used.
Cumulative / HEAT projectiles work by focusing the stream of hot gasses onto a small surface area. The shell is designed as a conical cavity with a copper lining, behind which there is an explosive charge. A stream of hot gasses and particles created by the lining penetrates the armor plate at hypersonic speeds. A portion of armour at the point of penetration turns into small particles that are carried into the tank’s insides. The opening created by the stream is only a few centimeters across. After passing through the armour, stream turns into a funnel, with typically 15 degree expansion zone and a range of several meters. Crew in the path of the stream is killed, and stored munitions can activate if the stream penetrates the projectile or the shell. Effectiveness of a HEAT projectile is directly related to its calibre (diameter) – greater the diameter, greater the effectiveness. As such, 60 mm “zolja” has a penetration of 300 mm RHA (RHA – equivalent of a homogenous steel armour), while 90 mm “Osa” penetrates 400 mm RHA. Armour of T-55 tank is only up to 100 mm, meaning that these weapons easily penetrate it. However, cumulative projectiles never strike the armour at 90* angle, meaning that effective penetration is less (that is, effective armour of the tank is more than the nominal values). More modern HEAT projectiles can penetrate armour thicknesses up to 700% as thick as the projectile diameter.
For maximum effectiveness, HEAT projectile has to be touching the armour when it detonates. This can be prevented in several ways, such as slat (cage) armour and spaced armour, which detonates the projectile away from the main armour, causing focus of the stream to be lost. Tool boxes are also an effective obstacles. Israelis also utilized bricks and sandbags to enhance basic armour. Composite armours are also effective against HEAT threats. Since gas stream acts as a fluid, when passing different materials it gets misshapen, forming vortices and breaks, which reduce its penetrative power. By using such techniques, modern tanks achieve effective armour thicknesses against HEAT of up to 2400 mm (for basic B-84, HEAT equivalent thickness of the front hull plate is 600 mm). Even so, tanks remain vulnerable to hits to turret ring / turret-hull junction, side and rear armour (depending on armour distribution and effective thickness), top armour and drive gear. Particularly vulnerable is side armour of wheel / drive area, which is typically not thicker than 100 mm. For this reason, side skirts are often mounted with ERA or composite armour blocks.
Another important factor is armour angle. In reality the projectile never strikes the armour at the perfect 90* angle, which results in reduced penetration (increased effective thickness of the armour). Some tanks also have highly angled armour, particularly at the front, but the effectiveness of this measure is somewhat reduced by necessitating thinner armour due to greater area being protected (a 100 mm plate has effective thickness of 200 mm at 30* and of 292 mm at 20* angle).
Armoured personnel carriers and infantry fighting vehicles typically have armour much thinner than that of a contemporary main battle tanks. Typically, armour of such vehicles is expected to provide protection against small-arms fire only, as well as shrapnel from projectiles up to 155 mm. Frontal armour is expected to provide protection against light antimateriel weapons, such as sniper rifles up to 20 mm in caliber. They are however easily penetrated by any dedicated antitank weapon. This is especially problematic because such vehicles carry infantry inside. Exception are few APCs that are actually built on the tank chassis, such as Israeli Namer APC which has armour protection thicker than that of Merkava MBT. Despite typically lesser armoured protection, APCs and IFVs have greater firepower against infantry when compared to the MBT, especially at small distances. Alongside vehicle-mounted weapons, which aside from machine guns can include small-calibre automatic cannons and/or grenade launchers, APCs and IFVs may have gun ports for the infantry. Consequently, infantry carriers are much more dangerour opponent to infantry at small distances due to far smaller dead zones.
For defense against HEAT projectiles, tanks and other armoured vehicles can employ ERA (explosive-reactive armour). This consists of explosive tiles, which activate upon impact, destroying the warhead and destabilizing the jet stream before it reaches the main armour. However, such armour can be activated by small-arms fire as well as by tandem warheads, whereas smaller warhead mounted on the nose activates the explosive armour whereas larger aft warhead attacks the primary armour of the tank. Even more modern missiles can employ triple warhead, whereas first warhead penetrates the spaced armour, second penetrates the ERA and the third warhead penetrates the main armour. Some countries, such as Russia and Israel, have started deploying active defenses aimed at destroying the projectile before it even reaches the tank in the first place.
Main disadvantage of HEAT projectiles is that they have larger diameter than subcaliber projectiles, and thus shorter effective range due to significantly reduced accuracy. When used in guns with rifled barrels, projectile rotation disperses the charge jet due to the centrifugal force, reducing penetration due to reduced jet density. More modern HEAT projectiles however can have a counter-spinning jet stream, which cancels out the projectile spin, resulting in a non-spinning jet. This is done by specifically designed copper liners. Non-spinning cumulative projectiles can be fired from a rifled barrel, by using rotating plastic girdle or a rotating body..
While plumage stabilization allows the full penetrative power of the jet to be preserved, it also causes problems of its own. In unitary shots, feathers must be placed into the sleeve, taking up propellant space. Plumage can also fail at high initial velocities of the projectile. For optimum operation it should exceed diameter of the projectile, which requires it to be foldable, which is not suitable for implements with a muzzle brake, as its opening is prevented by the powder gases. When using caliber plumage, space restraints mean that streamlined shape of the projectile head has to be abandoned, causing a drop in speed. Last possibility is usage of a sub-caliber projectile, which results in reduced penetration. Plumage stabilized projectiles also tend to have lesser velocity and thus lesser likelihood of hitting.
A variant of the shaped charge concept is the explosively formed penetrator (EFP). This variant uses the interaction of detonation waves to deform a dish or a plate of metal into slug shaped projectile. Projectile, which has low length-to-diameter ratio, is then propelled towards the target at two kilometers per second. Its impact thus causes wide but shallow hole. More modern variants however can produce rods / stretched slugs, which have far greater penetration. Other variants are multi-slugs, which are more effective against lightly armoured or area targets, and finned projectiles which have better accuracy. These projectiles have relatively low penetration, and are thus restricted to usage against more lightly armoured top surfaces of MBTs, as well as usage against less heavily armoured vehicles.
Due to reduced effectiveness of cumulative (HEAT) rounds against heavily armoured targets, modern HEAT rounds are often multipurpose (HEDP). These types have the warhead surrounded by the conventional fragmentation casing, allowing it greater effectiveness in blast and fragmentation role against unarmoured targets while still retaining armour penetration capability. Another example of such rounds is HESH, which is effective against tank armour and also against buildings. Important advantage of HEDP rounds is that it is no longer necessary to store two or more different types of HE rounds, which is an important concern due to modern tanks having limited ammunition space (Leopard 2 – 42 rounds, Challenger 2 – 49 rounds, Leclerc – 41 round, M1A2 – 42 rounds, Merkava IV – 48 rounds). Multipurpose HEAT projectiles can also be used against helicopters if equipped with proximity fuze / proximity switch, as well as against bunkers.
Increases in tank armour have made man-portable HEAT missiles larger, heavier and thus less useful. This, along with advances in active protection systems, may swing the balance back towards the tanks and against infantry unless new solutions are found.
HEAT – High Explosive Anti Tank
HEDP – High Explosive Dual Purpose
HESH – High Explosive Squash Head
RHA – Rolled Homogenous Armour
ERA – Explosive Reactive Armour
EFP – Explosively Formed Penetrator
MBT – Main Battle Tank
IFV – Infantry Fighting Vehicle
APC – Armoured Personnel Carrier