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Armour with two or more plates spaced a distance apart is called spaced armour. When sloped it reduces the penetrating power of bullets and solid shot as after penetrating each plate they tend to tumble, deflect, deform, or disintegrate; when not sloped it increases the protection offered by the armour because explosive projectiles detonate on it before they reach the inner plates. It has been in use since the First World War, where it was used on the Schneider CA1 and St Chamond tanks. Many WWII German tanks had spaced armour in the form of armoured skirts, Schürzen, to make their thinner side armour more effective against anti-tank fire. Its introduction was to counter Soviet anti-tank units using conventional kinetic penetrator type rounds (anti-tank rifles), not the bazooka, panzerfaust, and other HEAT weapons as commonly thought.
The principle of spaced armour protects against high explosive anti-tank (HEAT) projectiles which create a focused jet of plasticised metal, very effective at the focus point, but much less so beyond there. Relatively thin armour plates or even metal mesh, much lighter than fully protective armour, can be attached as side skirts or turret skirts on tanks and other armoured vehicles. This light armour detonates the warhead prematurely so that the jet of metal is focused well before the main armour, becoming relatively ineffective.
In response to increasingly effective HEAT warheads, integral spaced armour was reintroduced in the 1960s on the German Leopard 1. There are hollow spaces inside this type of armour, increasing the length of travel from the exterior of the vehicle to the interior for a given weight of armour, to reduce the shaped charge's penetrating power. Sometimes the interior surfaces of these hollow cavities are sloped, presenting angles to the anticipated path of the shaped charge's jet in order to further dissipate its power. For example, a given weight of armour can be distributed in 2 layers 15 cm (6 in) thick instead of a single 30 cm (12 in) layer, giving much better protection against shaped charges.
Today some main battle tanks carry rubber skirts to protect their relatively fragile suspension and front belly armour.
Whereas the material of normal armour is always a compromise between hardness and ductility, spaced armour can also comprise plates with different material properties to increase its effectiveness against kinetic energy penetrators. An example for this construction is the armour of the Leopard 2 tank, which provides a slanted first armour stage (disturber), a specially hardened second stage (disrupter) and a softer third stage with high ductility (absorber). The disturber is to ideally deflect or at least manipulate the direction of incoming kinetic energy penetrators, which are then shattered and fragmented when hitting the disrupter. The absorber stage finally erodes and contains spalls and fragments.
The Whipple shield uses the principle of spaced armour to protect spacecraft from the impacts of very fast micrometeoroids. The impact with the first wall melts or breaks up the incoming particle, causing fragments to be spread over a wider area when striking the subsequent walls.