[Here’s a handy graphic](https://upload.wikimedia.org/wikipedia/commons/d/da/Image_Explosive_Reactive_Armor.png)
General idea is that most anti tank rounds that are effective against modern armor are either shaped charge rounds or kinetic penetrators
Shaped charge rounds use explosives to take a copper cone and flip it around into a very narrow and very fast copper jet that can bore through armor (think water jet cutter but with molten metal). Kinetic penetrators are long thin rods moving at high speeds. Both of these require that they go straight and not get broken up or the stream just splatters and the rod shatters
ERA takes two steel plates and puts explosives between them. When they get hit the explosive goes off and causes the top plate to move outwards. Unless the round hits 100% dead on 90 degrees the top plate is going to move sideways through the stream/rod. This makes the stream cut a slash instead of a hole which eats up a lot of its energy and can push against the side of the rod causing it to shatter
Latest Answers