Spaced Armour

Definition

Spaced armour plates are just regular armour plates, but instead of being a structural part of the tank’s armour, they are “removable”, or at least they would be (in real life).

Generally, spaced armour plates are plates such as sideskirts and stowage boxes. Gun mantlets are spaced armour, but generally are far thicker pieces of spaced armour than the usual sideskirts, and thus they behave more like regular armour would.

Spaced armour is much more effective and beneficial to the tank when it is not close to the base armour. The worst type of spaced armour are very thin plates (3-5mm) that are stuck right on top of base armour.

Why is this important?

Since most spaced armour plates are so thin, many of them allow the double overmatch mechanic to occur. This mechanic increases a shell’s normalization angle, which increases their effective penetration. It can mean that in some cases, the spaced armour is more unhelpful than it is helpful.

112’s side armour is an example of this “unhelpful spaced armour”.

The thin flat bar, highlighted grey, is a 30mm spaced plate, directly on top of its 80mm base side armour.

It causes this 225mm penetration AP shell from a 122mm gun to normalize at 28.46 degrees, instead of the usual 5 degrees.

There are loads of examples of “unhelpful spaced armour” in game, and its caused by spaced armour being either very thin, or being right on top of the base armour.

Generally you don’t want very thin spaced armour (less than 15mm), and you don’t want spaced armour that hugs your tank’s armour (ie: the distance between the armour plates is very short), as it causes a huge degree of normalization, and it can negate the auto-ricochet angle, like shown with the 112 in the image above.

AP and APCR vs spaced armour:

AP and APCR behave the same way when they hit a spaced armour plate.

1: Armour angle and thickness are calculated when the shell hits the spaced armour

2: The shell loses penetration when going through the spaced armour (If your penetration is 175mm and you hit a flat 25mm spaced armour plate, your shell will continue to the main armour, but only have 175-25=150mm penetration.)

3: Double overmatch is calculated (is shell caliber 2x larger than armour plate?). If the double overmatch occurs, then the shell’s new normalization is calculated.

4: If the shell hits base armour behind the spaced armour, its new lower penetration is calculated with the new angle of normalization from double overmatch (only if a double overmatch actually occurred).

Note:
Once hitting and penetrating spaced armour, the shell will only continue flying for 10 times the caliber (Example: 75mm shells will continue for 0.75 meters).
This part is not so relevant, because usually spaced armour is not that far from base armour. This only matters when you shoot at an angled spaced armour plate, in this case the shell may have to fly further to hit the base armour.

HEAT vs spaced armour:

Contrary to what players think, HEAT ammo can deal damage through spaced armour, its just far less effective. There are images below to demonstrate this.

1: Effective thickness is calculated when the shell hits the spaced armour (no normalization for HEAT ammo)

2: The HEAT shell detonates on the spaced armour plate, the chemical jet continues through the armour (this is how HEAT shells work, they penetrate by essentially pushing a chemical jet which goes through the armour. The metal shell itself is just a delivery case.)

3: If there was enough penetration to go through the spaced armour plate, the penetration of the chemical jet decreases according to the effective thickness of the spaced plate (so 175mm HEAT hitting 25mm flat spaced armour will become 150mm of penetration)

4: After successfully going through the spaced armour, the chemical jet loses 5% of penetration for every 10mm (0.01m) of distance travelled. If the distance to the base armour is too far, or the penetration is too low when it hits the armour, no damage is dealt.

HEAT should only ever be fired at spaced armour if its either very thin, or the HEAT is very high penetration.

The spaced armour needs to be as close to a perpendicular angle as possible, otherwise it really has no chance to penetrate. This is shown in the image below.

Note that these shells are 270mm of HEAT penetration, but could not penetrate 194mm of effective armour, just because it was spaced.
The tank had to be angled even further, decreasing effective armour to 165mm, at which point the 270mm HEAT could go through.

HE/HEP/HESH vs spaced armour:

HE-type shells only deal full damage when they penetrate enemy base armour. Spaced armour will always cause a HE-type shell to detonate before it can hit the base armour, so HE-type shells will never deal full damage through spaced armour.

1: When HE-type shells hit spaced armour, they immediately detonate.

2: Damage is mostly absorbed by the spaced armour plate. In this case, higher distance or thickness will absorb more of the damage.

3: Damage is dealt in a spherical radius around the point of detonation.

Note that HE-type shells will deal more damage if they detonate close to the base armour, and if the spaced armour isn’t very thick.
So spaced armour that is thin but is distanced from the base armour will absorb HE damage very well; and thick spaced armour, even if close to base armour, will also absorb HE damage very well.

Also note that how HE damage is calculated. The shell explodes and the damage (representing shell fragmenting) goes out in a sphere from the center of the explosion.
Higher damage is dealt if the center of the shell explosion is close to the armour plate is damaging. Higher damage is also dealt if the base armour plate that gets damaged is very thin.

Any spaced armour, no matter how thin or how close to base armour it is, will stop HE from penetrating the tank and dealing full damage.

What is the most effective type?

Basically what you want in spaced armour is:
Good distance from the base armour if its thin
Good thickness if its close to the base armour

The below examples demonstrate the different types of effective spaced armour.

15mm spaced armour, great distance
The E 75 TS and Keiler both have large 15mm spaced armour plates covering their side armour.
Their sides are both quite hard to penetrate without the required amount of penetration.
In this case, even though the thickness of the plate is same as the WZ-114 (next example) and the armour angling is worse, its the distance of the spaced armour to the base armour that makes it so effective.

15mm spaced armour, little distance
WZ-114 has very well angled upper-side armour, it is covered by a 15mm spaced armour plate. This area is actually penetrated fairly reliably by AP and APCR, even though it shows up as red. (240mm AP vs 305mm armour, should not penetrate, but it does)
In this case, its the distance of the spaced armour to the base armour that is ruining the effectiveness, and the lower thickness of the spaced armour allowing the double overmatch normalization to have a larger affect.

30mm spaced armour, little distance
IS-3 also has very well angled upper-side armour, it is covered by a 30mm spaced armour plate. You will find this area nearly impossible to penetrate unless shooting at it from a close distance or perpendicular angle.
In this case, the thickness of the spaced armour is stopping shell’s double overmatch normalization from being extremely high, and its taking off a larger amount of penetration from the shell.

Why the big difference between the IS-3 and WZ-114? Their angling is so similar and its only a 15mm difference.
If you calculate the formula of double overmatch:
Shooting the IS-3’s side gives a 122mm AP shell 28.46 degrees of normalization.
Shooting the WZ-114’s side gives a 122mm AP shell 56.93 degrees of normalization

At this angle, the armour is 305mm, the armour shows up as red, and the gun shooting it has 240mm AP penetration.

Yet the shell is able to penetrate quite reliably. That 15mm spaced armour makes a huge difference to the normalization.