Building a Better Boxtank?

The following video came out a few weeks ago, and upon seeing it several of my friends (including a German) accused me of writing the script for it. Upon challenging this claim they pointed out that British tanks tend to be a bit boxy and heavily riveted, and as I like British tanks its a pretty easy conjecture to make.

Then we began to talk about the merits of sloped vs vertical armour (I know, exciting right...). As I've just read a presentation to a large gathering of armour officers about this very subject I figured it'd be good to make an article on it. So what follows is a non-mathematician trying to explain a horribly complex subject (and we all saw how well that went in the nuclear bomb article).

It is a commonly held belief that sloped armour is better than vertical armour, this isn't strictly true (some claim it isn't even slightly true). Equally it's often claimed that the Russians invented sloped armour on the T-34. First of all let's address the last point and consider what the claim is saying, that every engineer between the ancient Greeks (I'm using Greeks, as Pythagoras' measurements of triangles are the one most used in later on in this article), and the Russian designer of the T-34 had forgotten or were never taught the maths that make up geometry. Yet tanks before and after the T-34 continued to be made with flat plates... why? Maybe it's because flat armour isn't as bad as many claim, if not superior. Consider this, if sloped armour is so inherently superior why do modern tanks use fairly shallow angles on their front plates, and some like the Leopard 2, use none?

Its a Tiger!

Like all things in armour design the slope/vertical choice is a compromise. If you assume that the best idea is to wrap the armour as tightly as you can around components, and thus use the minimum armour for protecting these parts, then with the engine bay, transmission and fuel you quickly get a large rectangle. Which is great as it allows you to fasten the suspension onto the side plates, and already you pretty much have a layout that resembles most tanks ever built. On top of this, literally you have to consider the turret ring. Sloping the upper hull sides means the size of the turret ring you can fit is smaller, and with a smaller turret ring you get a smaller gun. You can't just increase the width of the tank, as most tanks have a restriction on their width. In the Second World War the restriction was limited by the railway gauge that would be used for transporting the tank.

The Sherman is narrower than the T-34, yet the Sherman could carry the bigger gun.

But what of the armour itself? An example given in the paper I mentioned is a 100mm vertical plate vs a German KwK 42 L/70 75mm, the gun most famously mounted on the Panther, at 2000 yards. At that range the KwK 42 could penetrate 104mm, and so beat the 100mm vertical plate by 4mm.
But what if we slope it?
Well to cover the same area, at say 30 degrees, the plate now weighs more. Before you all grab for calculators scratching your head or reach for the comment button, remember the missing part is the roof and thus normally much thinner and lighter, and is a different calculation and balancing act for the designer. We're just talking about the ability to protect fire from the flanks. Most people when working this out have the “roof” of the triangle the same thickness as the sides for simplicity. However even the newest student of armour design can spot that the idea of having a 100mm thick roof is a bad idea. (Note: I'd actually be interested in seeing a comparison of weights and thickness that includes the difference in thicknesses of side and roof plates)
So using the same weight of armour means you could get, 80mm of armour at 30 degrees. The same gun at the same range as used before has a penetration of 89mm. So you're actually worse off as the gun has beaten you by over twice the margin of the vertical plate.

But one thing we've failed to take into account is ballistics. All the numbers listed so far are for a shell approaching on a dead flat trajectory. As the range increases the arc the projectile needs to take to impact on the target also increases. This has the effect that the strike gradually moves closer to a 90 degree angle on sloped armour. At the same time on vertical armour the angle is getting steeper, actually increasing the thickness of the plate. A similar effect could be achieved by the tank driving cross country, which would have dips and be uneven.

Yeah, now go down the slope and the angle decreases, and add in a ballistic curve to the shot.

Also to add to the mix is the type of projectile being shot at you. A pointed projectile is best for shooting at a vertical plate, whereas a blunt nosed projectile does best against sloped surfaces. The latter is because the corner is the tip of the impact. So knowing your enemy is going for sloping armour in a big way, start firing blunt projectiles at him. During the Second World War British rounds were designed to be fired against tanks with 30 degree sloped armour..

So in summary, a vertical plate will always give its designed level of protection, and may actually give more. It's also technically (possibly?) lighter. With that in mind why are modern tanks not universally square? I honestly have no idea, it is reported that sloped armour is actually harder to spot, and blends into the background better than a square tank, and that right angles show up really well to radar. There's also a question of crew morale, as sloped armour is seen as better. So if you ever see another “design your own tank” competition, give the poor humble vertical plate another chance!


Image Credits:
www.brhoward.com, www.fprado.com and globeatwar.com