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Armor 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 projectiles tend to tumble, deflect, deform, or disintegrate; spaced armor that is not sloped is generally designed to provide protection from explosive projectiles, which detonate before reaching the primary armor.
Against kinetic penetrators
Spaced armour has been fielded since the First World War, in use on the Schneider CA1 and Saint-Chamond tanks. Many World War II-era German tanks utilized armoured skirts (Schürzen) to make their thinner side armour more resistant to anti-tank rifle fire. In contrast to most modern spaced armour, Schürzen was not designed to counter HEAT weapons, such as the American Bazooka or the British PIAT.
Modern spaced armor is designed to protect against High Explosive Anti Tank projectiles. HEAT-type warheads use a focused jet of super heated copper to penetrate armour; however, in order to be effective, HEAT warheads must detonate at a specific distance from the target's primary armour to ensure maximum penetration. Relatively thin armour plates or even slats or 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, causing the projectile to detonate too early. This early detonation greatly reduces the penetration of HEAT ammunition.
However, the use of add-on spaced armour skirts can sometimes have the opposite effect and increase the penetration of some shaped charge warheads. Due to constraints in the length of projectiles, some designs intentionally detonate closer than the optimum distance. In such cases, the skirting effectively increases the distance between the armor and the target, and the warhead detonates closer to its optimum stand-off.
In response to increasingly effective HEAT warheads, integral spaced armour was reintroduced in the 1960s on the German Leopard 1. Hollow spaces between plates allow increase the distance a projectile must travel to reach the interior of a vehicle. 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.
Some modern main battle tanks carry rubber skirts to protect their relatively fragile suspension and lower glacis.
Whereas normal armour must compromise between hardness and ductility, spaced armour can be constructed from plates with differing material properties to increase effectiveness against kinetic energy penetrators. The Leopard 2 utilizes a slanted first armour stage (disturber), a specially hardened second stage (disrupter) and a softer, high ductility third stage (absorber). The disturber is designed to either entirely deflect or manipulate the direction of incoming kinetic energy penetrators. If penetration does occur, the projectile is then shattered and fragmented when striking the disrupter. Assumig the first two stages work properly, the absorber stage captures spalling 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.
- Thomas, Steven, Why were Schürzen introduced in WW2?[self-published source]
- Hughes, Matthew (2000), The Panther Tank, Staplehurst: Spellmount, p. 30, ISBN 978-1-86227-072-5
- WILEY-VCH Verlag GmbH, D-69451 Weinheim (1999) - Propellants, Explosives, Pyrotechnics 24 - Effectiveness Factors for Explosive Reactive Armour Systems - page 71