impacting projectile (RPG, ATGM) to destroy them. Steel slats placed at a specified distance have a 50–60% chance of breaking an RPG-7's warhead so no cumulative beam can be formed. It also provides some protection against grenades. In response to increasingly effective HEAT, HESH and APFSDS warheads, integral spaced armour was reintroduced in the 1960s on the German Leopard 1 and later the Merkava. Spaces between plates increase the distance a projectile must travel to reach the interior of a vehicle. Sometimes the interior surfaces of these cavities are sloped, presenting angles to the anticipated path of the shaped charge's jet or kinetic penetrator to further dissipate their power. The two (or more) layered spaced RHA armour were highly effective against early steel and tungsten APFSDS munitions, because the rod was severely damaged by penetrating the first layer and was thus ineffective on the inner armour. Therefore, a much thinner total steel thickness and weight was enough against a specific projectile. For example, a given weight of armour can be distributed in two layers 15 cm (6 in) thick instead of a single 30 cm (12 in) layer, giving much better protection against HESH and APDS munitions, but their effect on shaped charges was limited. So military researchers tried to increase the efficiency of spaced armour by changing the used materials and increasing the number of layers, from the early sixties. Composite spaced armour Multilayer spaced armour, which also use special materials, are a transition to composite armour, most of the latter are also partially spaced armour. In the case of the Leopard 1A3 and later variants, the outer layer of spaced armour was hardened steel and the space was filled by elastomer, thus the effectiveness of the shattering effect of the outer layer against APFSDS was outstanding, and the protection against early HEAT warheads was increased, too. The BDD add-on armour of T-55 and T-62 series based on the same effect, but it had multiple layers within elastomer, therefore it roughly doubled the frontal protection of these tanks against APDS and HEAT weapons, and made the areas of add-on immune to HESH rounds. In T-64 and early T-72 (up to T-72M1) and T-80 (to mid T-80A) used stakloplastika (a special military-grade dense glass-fiber reinforced pressured plastic) as filling in the frontal upper glacis spaced armour. This plastic was effective in lowering the concentration of the jet of shaped charges