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. 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 or kinetic penetrator to further dissipate their power. The two (or more) layered spaced RHA armors 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 armor. 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 2 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.