Spaced armour

Armour with two or more plates spaced a distance apart, called spaced armour, when sloped 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 reduces the protection offered by the armour, and detonates explosive projectiles 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 early-WWII German tanks had spaced armour in the form of armoured skirts, to make their thinner side armour more effective against anti-tank fire.

The principle of spaced armour protects against high explosive anti-tank (HEAT) projectiles which create a focussed 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 molten metal is focussed well before the main armour, becoming relatively ineffective. Factory-made and improvised stand-off armour was introduced in the Second World War to defend against the new Bazooka, Panzerfaust, and other HEAT weapons.

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 15cm (6in) thick instead of a single 30cm (12in) layer, giving much better protection against shaped charges.

Today light armoured vehicles mount panels of metal rods, known as slat armour or cage armor, and some main battle tanks carry rubber skirts to protect their relatively fragile suspension and front belly armour.

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.