IIam going back to my space physics degree here (15 yes ago) ... Using a structure with 2 gapped layers is generally a good way of protecting something. The reason being that the projectile tends to have enough velocity such that the first layer causes the projectile to become unstable and break in-between the two layers making smaller projectiles that cannot penetrate the subsequent layers. The problem is that they air-gap needs to be of quite a size making it unsuitable for tanks. The gap size is generally due to the size/velocity of the projectile. Spacecraft are subject to impacts from small particles (<grammes) travelling at km/s. Tanks are hit by larger projectiles at sub km/s. I.e. the tank would be unfeasibly large in order to use this type of armour.
As for sloped armour one of the things to remember is that a KE penetrator (hehe) will impact with a relatively small point. The transfer of all that energy onto a single point with small surface area makes the armour act in a non-solid way. It begins to act more like porridge as the weapon passes through. As such the sloped armour is less likely to deflect a round as it's like firing a nerd dart into porridge. The armour does offer additional thickness though. And then there is all of the layering which is not that well known.
If anyone is ever near Swindon/Oxford, my university has a bunch of tanks and some test fires at plates of armour. Open invitation to come and climb in/around our tanks!
