The sense of sloped armor in tank design.

[TheFlemishDuck]

When people would explain you what they perceive to be the advantage of sloped armor, they would say that putting armor under an angle makes it thicker for a projectile going in a strait line. So they would argue that for example 50 mm of armor that is hit under a 45° angle would translate to 70mm of effective armor. (50mm/(cosinus 45))

When considering that from the perspective of armor design however, it must be noted that a metal plate that is mounted at a 45° angle will also need to be longer to cover the same height. Indeed if you would replace a 0.5m plate that is upright with one that is 45° slanted then the slanted plate of armor would need to be 0.7m long (0.5m/(cosinus 45))

Which basicly means that it kinda shouldn't matter wether you used sloped armor or not from the perspective of the mass of the tank, and afaik the mass of the tank is pretty much the most crucial design propperty. The same goes for the metal plate above in that a 50mm thick plate angled at 45 degrees will have the same mass as a 70mm plate that is upright atleast in terms of height again.

Worse in fact to use sloped armor, because one must remain in the situation where that angling is effective. If the enemy can hit youre tank so that the armor is not at an angle anymore (which could be for example due to terrain) then youre basicly weaker than a boxy 70mm armored tank that could actually get its armor in a better position due to terrain.

So ... is sloped armor actually useless?

 

[Gerle]

If the projectile trajectory is the same, the chance of a ricochet would be different if it hits a piece of sloped armor compared to if it hits a vertical piece.

 

[TheFlemishDuck]

If the projectile trajectory is the same, the chance of a ricochet would be different if it hits a piece of sloped armor compared to if it hits a vertical piece.

Sure, but would a 50mm plate sloped at 45° be better than 70mm positioned vertically?

 

[Gerle]

I believe that depends on the type of projectile, and the type of armor, so many variables. For WW2 era gear, sloped armor might be better. Today, with ceramic armor, long rod penetrators, etc, it seems to me there must be other variables involved. Looking at modern vehicles, some of them have near vertical surfaces, so there must be other concerns that are more important than the sloping of the armor.

 

[Xeorm]

I've never heard of that being an advantage to sloped armor. I've always seen it as being an advantage with ricochets and glancing blows.

 

[Antediluvian Monster]

If you look at penetration tables of '40s and '50s tank guns in works like Hunnicutt, almost all projectiles will penetrate less of angled armour plate than simple geometry indicates. So that 60 degree slope (from vertical) as in T-34 doesn't double the effective thickness but can actually more than double it. Of course there are other variables, but as long as the diameter of the offending projectile is no more than twice the thickness of the plate it should be a pretty reasonable expectation in era of steel projectiles and steel armour.

 

[Henry IX]

Sloped armour is more effective that flat armour per gram - the effect of deflection is significant. However, sloped armour has a significant drawback - it makes the internal space of the tank smaller. Hence why the German tanks, such as the PzIV, had one more crew member than the similar sized Soviet T-34. This extra crew member makes the tank significantly more combat effective as it has a faster fire rate and much quicker target acquisition time (a critical factor in combat). This is part (but only part) of the reason why the otherwise inferior German tanks of 1941-42 massively outperformed their Soviet counterparts.

 

[TheFlemishDuck]

I've never heard of that being an advantage to sloped armor. I've always seen it as being an advantage with ricochets and glancing blows.

I see that reoccur a lot, but often mention is also made that sloped armor will produce more ricochets. Seeing the comments here i think most of those people should leave the narrative of "increased effective armor" behind them and simply make the argument on bounces as that seems to be the only real advantage. But to give an example of people making that error:

This person could leave behind the arguments "increases armor thickness" and "saves weight". Ricochets is the only gain arguably. But you can see that he made the 100mm/(cosinus 45) calculation to come up with the 141mm of effective armor.

If you look at penetration tables of '40s and '50s tank guns in works like Hunnicutt, almost all projectiles will penetrate less of angled armour plate than simple geometry indicates.

Yes i guess penetration tables should be the way to look at it, they do seem to show that sloped armor is simply significantly more effecient and that would be something for designers to go by. By the tables a 50mm piece of armor sloped at 45° does indeed perform better than a vertical piece of armor of 70mm and this effect increases as the velocity of the projectile goes up too. I guess you could try to come up with several theory's for why sloped armor produces that result but eitherway the tables just show how it is, now to just convince some youtubers that they are making a few errors in their narratives as that illustration of "extra thickness trough angle" is quite coman when people try to explain the advantage of sloped armor.

 

[bz249]

Basically sloped armor is better against small caliber high velocity munition (the typical anti-tank stuff at the beginning of WW2) but worse against high caliber low velocity munition (because no matter how you angle it a large, thin sheet is never going to have the roboustness of the small thick one) than a vertical plate of equivalent thickness. There is also a minimal weight saving (the relatively thin top armor is saved) but at a sacrifice of internal volume.

 

[Denkt]

Pretty sure was already known in the medieval era that sloped armor protected better against projectiles than flat armor, but at that time this knowledge was used in Construction of defensive structures.

Today Im not sure armor is used much for protection anymore because modern projectiles use concentrated heat beams which simply melt the armor but can be countered by explosions which deflects the heat beam so it don't go through the armor.

WW2 projectiles on other hand had to physically go through the tank so the same principle that have been used against cannons probably worked well.

 

[Kovax]

Today Im not sure armor is used much for protection anymore because modern projectiles use concentrated heat beams which simply melt the armor but can be countered by explosions which deflects the heat beam so it don't go through the armor.

Depends on the weapon. The M1 Abrams uses depleted uranium penetrators, rather than shaped charge explosive shells. Most of the anti-tank weapons on the battlefield, however, DO rely on projecting a jet of intense heat at a point on the armor. Still, if the explosive shell doesn't land flat enough with respect to the target's surface, the explosive force will not be properly shaped or directed. It's difficult to get the explosive warhead to "stick" to an angled target long enough for the jet to form and penetrate the target (a mere fraction of a second, but that's an eternity for something moving at several hundred m/s or faster), rather than bounce off or tumble on impact.

The use of ceramic composites, layered armor, and other techniques to deflect or break up the jet of super-heated metal all degrade the effectiveness of shaped charge weapons, but it's a case where the presence of the weapons forces the use of more expensive technologies to defeat it, and the presence of the defensive measures drives progress on alternative anti-tank weapon designs to bypass or ignore the extra defenses (such as attacking the thinner top armor of the tank). The mere existence of armored vehicles in your army forces all enemy units to carry anti-tank weapons, whether they ever actually encounter them or not. Without any armor, there's no defense against small arms fire, and no point in having a "fighting vehicle" at all, other than for transportation.

 

[kaoss]

Also, thinner plate is probably cheaper to manufacture. Specially since amrour plate is often carburized and heat-treated.

 

[Henry IX]

Also, thinner plate is probably cheaper to manufacture. Specially since amrour plate is often carburized and heat-treated.

That is an excellent point. Annealing the armour plate is correctly is harder the thicker the plate is, as the outside will tend to cool much faster than the inside, and modern tank armour tends to aim for a high level of consistency throughout the material, as this decreases the chance of cracking or shattering. Points of different crystal structure in the steel are weaknesses. Thick plate requires specialised furnaces for annealing it correctly, which is a bottleneck in production and an expensive piece of equipment.

Welding armour also gets harder the thicker the plate you are trying to weld as heating the whole area of the join to the correct temperature for a strong weld, all at the same time, is very difficult. Again this slows production and increases the costs of production.

For this reason the think plates of tank armour are often cast as a single piece, but this requires very complex and expensive equipment to produce, again driving up the price and production time of the vehicle.

Even without the benefits of ricochets the cost and time savings of sloped armour on a mass produced tank could be considerable.

 

[pithorr]

Why not just to use few thinner plates together?

 

[Antediluvian Monster]

Why not just to use few thinner plates together?

Generally, multiple thinner plates are weaker together than a single thick one.

That doesn't mean they didn't do it, of course. In American Civil War the monitors were armoured with stacked 1-inch plates for some kind of reasons of production. But they knew this was bad for performance even back then.

 

[Kovax]

Most warships are armored with multiple layers of overlapped plates, rather than a single layer of massively thick blocks. Current tank armor designs often stack layers of totally different materials, rather than one homogenous layer of hardened metal. There's a tradeoff between manufacturability and absolute protectiveness, as well as compromises to cover different forms of attack (solid penetrators versus shaped charge devices), so different situations may call for different solutions.

 

[Antediluvian Monster]

Most warships are armored with multiple layers of overlapped plates, rather than a single layer of massively thick blocks.

Not entirely clear what you mean here? Are you talking about the thin structural plate of non-armour steel that the massively thick belt armour plate was attached on? Or the protected decks of vessels of the incremental scheme where they were generally a layer of splinter protection backing the belt-plate?

I would generalize the opposite way: since rise of the armoured warships, massively (though possibly variably) thick single plate was preferred. This applied to vertical armour since HMS Warrior, and with rise of the all-or-nothing protection scheme it also applied to horizontal armour. Before the era of all-or-nothing, engagement ranges were expected to be short enough that no projectile could have physically reached deep into the ships innards (i.e. engines, magazines) without impacting some form of belt armour, owning to the shallow angle of descent. Hence the decks were just extra thick structural plating mainly intended to contain damage, rather than prevent it.

Current tank armor designs often stack layers of totally different materials, rather than one homogenous layer of hardened metal.

Yes, this is why I used the word "generally". In course of 20th century militaries did start experimenting with both layered/laminated armour and spaced armour with the intent of providing greater protection than the sum of it's parts (when the previous standard was less protection). Experiments with spaced armour were already ongoing in 1930s and Italian and German ships of the period came to incorporate it. For example, the Littorio class battleships featured an experimental belt scheme where there was 70mm plate outboard, followed by ~250mm gap, followed by the the actual 280mm face-hardened belt plate, with the intent that the outboard plate and the gap would be sufficient to strip the armour piercing cap (intended to defeat face-hardened armour) off striking projectile. The actual effectiveness of these schemes is debated.

I think laminated armour first came up around '50s when Soviet and US researchers started looking for ways to defeat HEAT projectiles.

 

[Kovax]

I've seen WWII vintage film footage of welders layering armor plates over the underlying layer, and carefully filling in the gaps between them where they meet. You can't easily fill in a welded seam 200+ mm deep, even with tapered plates, so it was done in a couple of layers. The overlaps also avoided a single weak weld line between the plates, which you would have if it were a single plate from inside to outside. You obviously can't cast a single-piece beltline of that size.

 

[Antediluvian Monster]

Some basic information of US naval armour, from USN manuals: https://eugeneleeslover.com/ARMOR-CHAPTER-XII-A.html

In short, forged slab of armour steel that is bolted on.

I'm not entirely sure, but I suspect welding would be a poor method for this kind steel, essentially a superalloy of it's period. I have understood that even the high strength Ducol manganese steel British used for hulls was poor for welding. Not entirely sure what you saw (welding of extra thickness HT steel skin plating?), a link would be helpful.

(Edit) That being said I did forget about one prominent kind of early laminated armour: the compound armour. This was a steel plate forge-joined with wrought iron backplate, essentially providing the effect somewhat akin to face-hardened armour that trended for a while around 1880s until replaced with actual face-hardened armour.

 

[Antediluvian Monster]

Found a post by Bill Jurens (salvage expert and publishing author on technical naval history) on the belt plate attachment on NavWeaps forum. If you enable PMs @Kovax I can link it to you (or at least, I can't see the option to PM to you in usual place). Or, I suppose, you can just find it by Google.

A very complete description of the armor plate arrangement for the South Dakota class battleships was published in Warship International No. 43-3 and No. 43-4 a few years ago. Iprepared the drawings for that. Although differing in details, the U.S. practices described in that paper were quite similar to the procedures used in other navies as well.

Most side armor plates were in the vicinty of 10 feet x 30 feet in size, arranged horizontally or vertically, weighing in at about 150,000 lbs apiece. The larger deck armor plates were typically about 25 feet x 12 feet in size, typically weighing in at about 70,000 lbs each, most were somewhat smaller. Usually the 'short' dimension was held constant and the length was allowed to vary as required. Plates were typically held together with keys of various shapes. Many navies used removable keys, basically square rods inserted between the sides of plates that were grooved along the edges. Others machined the grooves and tongues right into the plates, so they were much like 'tongue and groove' plywood. Usually the plates were pre-assembled at the shipyard to ensure proper fit, adjusted slightly to fit the actual situation at hand, and hoisted into place afterwards. Often, the joints between the plates were seam-welded on one or both sides in order to ensure water-tightness.

A heavy supporting structure made up of 'backing' plates was usually used to hold the side armor in place, with large bolts passing into tapped holes in the backing plates themselves. As the backing plates could be held to a higher flatness tolerance than the plates themselves, it was quite common to leave a small gap between the plate back and the backing plate itself which was typically filled with a concrete like substance, or wood. Sometimes an 'armor shelf' was providd at the bottom of the plate, which would take much of the load off the bolts, and sometimes allowed omitting the backing plates entirely. Deck plates were bolted into place in somewhat the same way, although the backing plates were sometimes omitted and the armor bolted right to the frames.

Plates continued to be bolted in place until about the middle of World War II, when it became more common to use welded joints instead. In the USN, the Montana class would have made extensive use of welded armor joints.

Contrary to popular belief, it was really quite easy to attach structure to the outside of face-hardened plates, as notches could be left at the plate boundaries allowing access to the softer back portions of the plate via various brackets, etc. Further, it was quite easy, and common, to leave 'soft spots' at strategic locations in the face-hardened surface, which -- at least in that small area -- retained the qualities of homogeneous armor all the way through.

Hope this answers (most of) your questions...

Bill Jurens