Armor, slope, and special arrays
RHA, composites, ERA, and why a 30° plate is thicker than it looks.
RHA and the slope trick
Rolled homogeneous armour is the baseline: a hardness and a thickness in millimetres. Slope increases the line-of-sight thickness (thickness / cos θ) and encourages shots to ricochet. The T-34’s 45 mm at 60° behaved like much more. After APFSDS darts arrived, ricochet became rarer — a long-rod penetrator does not bounce the way a 1944 APCBC shell did.
Cast vs welded vs composite
Cast turrets (early Pattons, T-72 domes) are cheap in complex curves and can be thick. Welded turrets take modules and upgrades. Composite (Chobham / Burlington, Combination K, Dorchester, AMAP) sandwiches ceramics, metals, and air gaps to break both kinetic darts and HEAT jets. Exact recipes are classified; the public fact is that millimetres of RHA-equivalent are no longer a single number.
ERA
Explosive reactive armour is a sandwich of explosive between plates. When a HEAT jet hits, the plates fly apart and smear the jet. Kontakt-1 is first-generation (weak vs APFSDS). Kontakt-5 and Relikt claim useful effect against darts. ERA does almost nothing against a top-attack EFP that never touches the brick.
Spaced, slat, and the roof problem
Spaced plates pre-detonate HEAT. Slat / cage armor on the rear and side can crush an RPG’s piezoelectric fuze. Neither helps against a Javelin or an FPV that dives through the thin roof. The modern kill shot is often from above — which is why APS and roof kits now dominate upgrade budgets.