Behind these elements will be some backing element designed to stop the degraded jet or projectile element, which may be of high hardness steel, or some composite of steel and ceramic or possibly uranium. Soviet main battle tanks from the T-64 onward utilised composite armour which often consisted of some low density filler between relatively thick steel plates or castings, for example Combination K. For example, the T-64 turret front and cheek was originally filled with aluminum, and then ceramic balls and aluminum, whilst some models of the T-72 features a glass filler called "Kvartz". The tank glacis was often a sandwich of steel and some low density filler, either textolite (a fibreglass reinforced polymer) or ceramic plates. Later T-80 and T-72 turrets contained NERA elements, similar to those discussed above. Ships Belt armour is a layer of armour-plating outside the hull (watercraft) of warships, typically on battleships, battlecruisers, cruisers and some aircraft carriers. Typically, the belt covers from the deck down someway below the waterline of the ship. If built within the hull, rather than forming the outer hull, it can be fitted at an inclined angle to improve the protection. When struck by a shell or torpedo, the belt armour is designed to prevent penetration, by either being too thick for the warhead to penetrate, or sloped to a degree that would deflect either projectile. Often, the main belt armour was supplemented with a torpedo bulkhead spaced several meters behind the main belt, designed to maintain the ship's watertight integrity even if the main belt were penetrated. The air-space between the belt and the hull also adds buoyancy. Several wartime vessels had belt armour that was thinner or shallower than was desirable, to speed production and conserve resources. Deck armour on aircraft carriers is usually at the flight deck level, but on some early carriers was at the hangar deck. (See armoured flight deck.) Aircraft Armour plating is not common on aircraft, which generally rely on their speed and manoeuvrability to avoid attacks from enemy aircraft and ground fire, rather than trying to resist impacts. Additionally, any armour capable of stopping large-calibre anti-aircraft fire or missile fragments would result in an unacceptable weight penalty. So, only the vital parts of an aircraft, such as the ejection seat and engines, are usually armoured. This is one area where titanium is used extensively as armour plating. For example, in the