Estimating AFV Protection Levels

(Created 1 May 2016)
(Updated 19 May 2016)

In today's strange post-cold war world, where weapons manufacturers like to flack their wares and are curiously coy on just how good they are...it's hard for the beginner to figure out what statistics to assign to AFVs for protection, particularly if the system is still in first line operational service (M1A2 SEPv2 Abrams) and thus very, very classified.

There are a few approaches you can take to figuring out protection levels:

The Easy Way:

Sometimes, weapons manufacturers might say: “Frontal Protection against NATO STANAG Level IV, all around protection against NATO STANAG Level 2” (or “Frontal protection against 7.62 Ball”) in their brochures, which makes it a bit easier to figure out protection levels, as that's all standardized (see below).

Steel RHA at 0 degrees

Areal Density
(pounds/ft2)

Thickness
(mm)

Protection Level

15.31 psf

9.5mm

Protection from 5.56mm M193 Ball / 7.62x39 Ball and 155mm Fragments at 100m
(NATO STANAG 4569 Threat 1)

20.42 psf

12.7mm

Protection from 7.62x39 AP and 155mm Fragments at 80m
(NATO STANAG 4569 Threat 2)

30.62 psf

19mm

Protection from 7.62x51 AP and 155mm Fragments at 60m
(NATO STANAG 4569 Threat 3)

71.46 psf

44.45mm

Protection from 14.5x114 B32 AP
(NATO STANAG 4569 Threat 4)

142.9 psf

88.9mm

Protection from NATO 25mm Autocannons with APDS-T
(NATO STANAG 4569 Threat 5)
(GUESSTIMATE)

245 psf

152.4mm

Protection from NATO 30mm Autocannons with APFSDS
(NATO STANAG 4569 Threat 6)
(GUESSTIMATE)

The mock statement I gave you earlier: “Frontal Protection against NATO STANAG Level IV, all around protection against NATO STANAG Level 2” -- would equate to roughly 4 cm frontal protection and 1 cm side and rear protection against KE threats.

You would have to figure out HEAT protection on your own as manufacturers are much more coy about that (with the exception of low-level anti-tank threats such as the RPG-7).

The Moderately Hard Way:

In a 1968 article in ARMOR magazine by Richard M. Ogorkiewicz titled Thoughts on...FUTURE TANK DESIGNS, there was a simple equation for estimating frontal armor protection for a given weight for a conventional tank.

With some modifications, it's somewhat decent for guesstimating protection levels given a certain tank mass and making some assumptions about the tank design's efficiency.

T = ( E * W ) * A

Where:

T = Horizontal thickness of armor in millimeters.

E = Efficiency of hull design.

W = Mass of tank in Metric Tonnes

A = Aspect Correction Factor (1.0 for frontal armor, 0.65 for side armor)

To compute “E”, you have to have the tank mass in metric tons; plus detailed armor schematics/angles, in order to find a correct “E” number for that design ethos. In general, tanks with straight sides will have lower E numbers, while tanks with excellent sloping will have higher E numbers. Additionally, higher “E” numbers also mean the tank is significantly more cramped, e.g. Soviet post-war tanks.

Known “E” Numbers:

0.425 = WW1 Mark IV Tank
1.4 = WW2 British TOG II Tank
2 = WW2 Tiger I (use 0.75 A ratio for side armor)
2.8 = WW2 Matilda II (use 1.0 A ratio all around)
2.75 to 3 = US WWII tanks
3 = WW2 Tiger II
4 = T-54 Hull
5.5 = T-54 Turret and IS-7 Hull/Turret

The Really Hard Way:

Sometimes, the manufacturer just isn't very open with specifications. For these cases, you'll have to figure it out based off known public statements such as:

Which are likely to be very accurate, as you have to have correct public statements in order to ship them around the world to various arms expos – or for everyday operations – Abrams tanks require bridges classed as MLC 70 (as in 70 short tons).

If the vehicle is new enough, or from a secrecy intensive state (China or North Korea), you can calculate vehicle dimensions based off high enough resolution photographs and scaling the dimensions from the sizes of known dimensions in the photograph – for example, the heads of vehicle crewmen in the photograph, etc.

Calculating weight is a lot trickier, but you can figure out weight by calculating the track area in contact with the ground and then using that to calculate forward using assumptions about “acceptable” ground pressure from other vehicles the nation uses.

Once you have a good handle on the dimensions and weights, you can start playing with math to see what the best armor protection the vehicle can mathematically support will be.

This is where a good set of 3-view drawings (Side, Front, Top) is invaluable, as it makes your task significantly easier.


M113 Dimensioned Drawing

From the above dimensioned image, the following specifications shake out:

Front Hull: 36.49 ft2
Left Side Hull : 64.17 ft2
Right Side Hull : 64.17 ft2
Rear Hull: 42.26 ft2
Hull Top: 101 ft2
Hull Bottom: 125.16 ft2
Total Surface Area: 431.25 ft2

NOTE: If you need to calculate sloped surface areas (such as for the M113), use the following calculator used for simple shed roofs
(http://final-analysis.com/calculators/roof_area.htm)

If you assume that the M113A1 protects against NATO STANAG 4569 Threat 3 all around (same plate thickness for ease of manufacture) with an areal density of 30.62 psf, then the total mass of the M113's armor scheme (in Steel) is about 5.98 metric tons and the protection level is:

Frontal: 3 cm (2 cm @ 45 degree LOS)
Sides: 2 cm
Rear: 2 cm
Roof/Floor: 2 cm

Calculating Other Accessories

Given that the M113A1 weighs about 10,900 kg in combat weight; we're missing some stuff here.

Adding all this up together, it ends up being 12.245 metric tons, quite bit heavier than the M113A1. But wait, wasn't the M113 made out of Aluminum?

MATERIALS OTHER THAN STEEL RHA

One of the big "technology goals" for the US Army's Future Combat System (FCS) was to get 12.7x99 M2 AP protection level mass down from 60~ psf (Steel) to 10 psf so that FCS could be fielded successfully and meet it's goals.

This of course, didn't happen – the US Army managed to get a successful protection system on 25~ psf in the mid-1990s with the Composite Armored Vehicle (CAV) Advanced Technology Demonstration (ATD), which was refined down to 20~ psf, but it wasn't enough for FCS. It did however, provide a useful baseline of data on other armor materials for our purposes.

Armor Capabilities Compared
(12.7x99mm M2 AP Protection)

References:
ARL-RP-8 October 2000 –
Performance Metrics for Composite Integral Armor (2.5 MB PDF) specifically THIS image.

Material

Areal Density
(PSF)

%
change
over
Steel RHA
Areal Density

Armor Thickness

%
change
over
Steel RHA
Thickness

RHA Steel

Aprox 60~ psf

1.0
(Baseline)

2.74” (69.6mm)

1.0
(Baseline)

Aluminum

Approx 55~ psf

0.92

4.72” (120mm)

1.72

Titanium

Approx 45~ psf

0.75

3.01” (76.45mm)

1.10

Aluminum-Ceramic

40~ psf

0.67

3.4” (86.36mm)

1.24

CAV-ATD
Composite Ceramics

Approx 25~ psf

0.42

1.77” (44.96mm)

0.65

Baseline 20 psf
CAV Solution

Approx 20~ psf

0.33

1.545” (39.24mm)

0.56

Baseline 20 psf
Aluminum-Foam Solution

Approx 20~ psf

0.33

2.05” (52mm)

0.75

NOTES: This doesn't cover every detail – for example, the much thicker Aluminum hull on FMC's T113 prototype counter-intuitively made it more spacious inside than the T117 steel hull prototype (also from FMC), because the thicker aluminum hull eliminated the need for internal stiffeners, opening up more internal volume.

The various Ceramic/Composite Armor systems are composed of the following systems:

  • Baseline 20 psf CAV Solution: (0.05” cover plate, 0.7” Al2O3, 0.125” Rubber, 0.67” S-2 Glass V/E)

  • Baseline 20 psf Aluminum-Foam Solution: (0.05” cover plate, 0.7” Al2O3, 0.75” Al-Foam, 0.55” S-2

Re-calculating the armor mass for Aluminum (30.62 psf x 0.92 = 28.17 psf for STANAG Level III Aluminum), reduces the armor mass from 5.98 metric tons to 5.5 metric tons, reducing total mass to 11.765 t; still a bit too heavy.

What about the Hull and Floor?

What if we reduced the roof and floor thickness to only resist STANAG 4569 Threat 2 (7.62x39 AP)?

Given that the hull top and bottom area is 226.16 ft2; going from Threat 3 (28.17 psf Al) to Threat 2 (18.78 psf Al), saves 2,123 pounds (962.9 kg), bringing mass down to 10.8t, and passing the “SANITYcheck.

So why do all this math?

Well, sometimes, it's the only way to actually figure out general order of magnitude protection levels and to see whether manufacturer/fanboy claims for protection are valid or not.

Also, it gives you an idea of what you could expect protection wise if you hypothesized fantasies like:

for things like imagining tank development if the Cold War had continued past 1989.