Armour with two or more plates spaced a distance apart falls under the category of spaced armour. Spaced armour can be sloped or unsloped. 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 armour that is not sloped is generally designed to provide protection from explosive projectiles, which detonate before reaching the primary armour. Spaced armour is used on military vehicles such as tanks and combat bulldozers. In a less common application, it is used in some spacecraft that use Whipple shields. Against kinetic penetrators The first spaced armour was used on iron and steel warships from the mid-19th century. Between the thin outer armour of various less important parts and the thick main armour (protecting turrets, ammunition depots, boilers and turbines) were constructed storage spaces, coal or oil bunkers, and so on (Lord Nelson class). Some ships (e.g. Tirpitz, Takao, King George) had thicker outer and thinner inner layers to lower the damage caused by the penetrating round. From 1900 some cruisers and battleships were built with modern spaced armour, where the outer thin layer was intended only to damage the projectiles (e.g. Littorio class). Torpedo bulkheads also serve as a special form of spaced armour for naval vessels. Tank spaced armour has been fielded since the First World War, when it was fitted to the French Schneider CA1 and Saint-Chamond tanks. The late variants of Panzer III had frontal spaced armour: a 20 mm thick face-hardened steel layer in front of the 50 mm thick main armour. Impacted projectiles were physically damaged by the 20mm plate, so the main armour could withstand much greater hits. Due to lack of materials, German industry eventually switched to Rolled Homogeneous Armour (RHA), which is less effective and due to the slower production process, the technique was not widespread on German tanks. It is important in designing of integral spaced armour that each layer should be thick enough to cause adequate damage to the projectile or jet. So the thickness of every layer should reach roughly the half of the diameter of projectile expected to impact. Many World War II-era German tanks used armoured skirts (Schürzen) to make their thinner side-armour more resistant to anti-tank rifles. Contrary to popular belief the German Schürzen were designed against kinetic (AP, APBC, APCBC) projectiles. The effectiveness of conventional