Spaced armour is a way to make vehicles more durable against specialty APS shells and possibly HE. There are a few ways to create spaced armour and each have their pros and cons.
Air Gaps[ | ]
Shaped charges (also called HEAT, APS and missiles only) and squash heads (also called HESH, APS only) are made to bypass strong armour by creating fragments on the inside of the vehicle. While the mechanics differ slightly between HEAT and HESH, defending against both is achieved through similar armour setups. The basic principle is to create a gap in the armour. This will cause both HEAT and HESH to spawn their fragments. If there is more armour behind the gap, then that armour will get damaged by the fragments which is a much better outcome than the fragments damaging ammo, AI, engines, and other internals directly.
These gaps can be created in two ways. You can simply leave a 1-meter gap or use one of two kinds of blocks: structural blocks that do not fill the space they occupy, or non-structural blocks. For non-structural blocks, the ideal blocks are applique panels. While applique does not benefit from added AC from armour layering, it does offer good protection for its cost. ERA armour blocks also work as they, in theory, destroy themselves and the shell/fragments hitting them but are single use until repaired.
The other alternative is using structural blocks with airgaps in their model. This is somewhat difficult to explain, but it does allow to protect a vehicle against the specialty shells while using the space that would otherwise need to be empty. Slopes, for example, will work and add a small amount of AC via stacking although less than a full block would. Poles laid parallel to the armour also work and have considerably more HP than slopes.
Finally, in the case of HESH, the last layer of your external armour (as illustrated in the picture below), has special importance as it has a stronger impact on the final AP of the fragments that will spawn. Wood will have the most reduction in the fragments' AP. The lower the armour value of the blocks, the lower the AP.
Here are some potential armour setups, the heavy armour on the left represents the outside layer where fire is coming from whereas the metal and applique represent the internal layer protecting your fragile components:
Empty Space[ | ]
A lot of Onyx Watch designs use this approach. The key idea here is to use empty space as a form of protection by having your exterior walls be far from important internal components. This is especially good against high explosives (HE) as shells and missiles will hit the outer layer first and the damage simply should not reach anything important or be severely weakened by the time it reaches the next wall. This does tend to create very large vehicles with poor drag profiles, making it a bigger and slower target.
This also works against HEAT and HESH, as the space between both layers of armour allows fragments to spread out over a very wide area. Spreading the damage over a larger area reduces the chance the internal armour will be broken and hit fragile internal components.