Category: Weapons 101

Intro

Time to talk about the basics of Tank Armor. After all, we all wanna know the basics when we are diving into tank designs in upcoming videos. Note that this video is limited in scope and mostly deals with developments from the interwar period up to the 1980ies. Anyway, let’s get started with armor materials.

Armor Materials

The usual material for armor was and is steel, but there are different techniques of producing steel and also other materials. Let’s take a look.

Rolled Homogeneous Steel Armor

Rolled Homogeneous Steel Armor was for quite some time the standard steel armor for tanks. Rolled steel means that the hot steel was rolled through one or several pairs of rolls during the production. It can be easily produced in large quantities, but can only be bent to limited degree. Usually it is used for armor plates, Germany in World War 2 used for the most part rolled Armor, thus their tank hulls and turrets have great boxy features. In contrast the cast steel turret for the Sherman had round features. (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 1)

Now, a few words about terminology, rolled steel plates are usually welded together, hence the term welded armor is usually use instead of rolled armor. Although, this can be a bit misleading since cast armor is also welded together unless the part is completely cast. Thus, cast turret or hull implies that large parts of the element are made from cast steel.

Cast Homogeneous Steel Armor

Now, the other main method for producing tank armor is steel casting. In this case the liquid hot metal is poured into a mold. This has the main advantage, because the armor can be molded into various shapes easily, allowing for curved areas and specific thicknesses. (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 1-3)
Initially, this technique was rather rate, but it was already used in World War 1 for several versions of the French Renault FT tank’s turret. (Ogorkiewicz, Richard M.: Technology of Tanks, p. 359)
In World War 2, the British, Soviets and US used various cast turrets, but it isn’t so straight forward, e.g., the Churchill Mark III had a welded turret, whereas the Mark IV had a cast turret and for certain variations of the T-34 there exist both welded and cast turrets. As you can see, it can get quite complicated, even up to this day certain tanks have some variants with cast and welded turrets, like the T-90.

But back to World War 2, in general, although the Allies used more cast turrets than the Germans as the war progressed. After the Second World War, cast turrets became almost universal for main battle tank turrets. Since the 1950s it is also common to cast complete hulls. Nevertheless, as mentioned before even current tank models use also welded elements. (Ogorkiewicz, Richard M.: Technology of Tanks, p. 359)
Chemically, rolled and cast armor are almost the same. The main advantage of cast armor is that It can be molded into almost any shape.(Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 1-3)
Now, let’s look at the advantages and disadvantages. The disadvantages of cast armor is that heat treatment and other refining techniques are complicated or not possible, thus it is not as though and shock-resistant as rolled armor.

A Manual from the US Army Materiel command from 1963 states:
“In general, rolled armor is about 15% better in resistance to shock and penetration than cast armor. However, this advantage is offset to some extent by the varying angles of obliquity and irregular shapes possible in castings. These variations in shape considerably decrease the penetrating ability of certain types of projectiles.” (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 1-3)
Note that I don’t know if this value is also correct for World War 2 steel nor current steel.

Cast armor although reduced the number of welded joints, especially considering turrets or hulls that are made out of one piece. (Ogorkiewicz, Richard M.: Technology of Tanks, p. 359)

Face-Hardened Homogeneous Steel Armor / High Hardness Armor

One way to improve the hardness of armor was to process the surface of the armor, this armor is called face-hardened homogenous Steel Armor. In this process, called carburizing, the armor is heated in a furnace for a considerable amount of time. Usually rolled armor plates were used for this. The advantage is it increases the hardness, thus increasing the chance that projectiles shatter on impact, but increased hardness also increases the brittleness. Additionally, the welding of such armor plates could often lead to cracking during the welding or afterwards. (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 3) Thus, early example of face hardened armor before World War 2 were usually bolted or riveted, which wasn’t ideal. Furthermore, the process is quite expensive and not suited for mass production. During the 1960s the problem of cracking could be overcome and high hardness armor was used on light armored vehicles mostly. Only in 1980s the technology was suitable to produce dual hardness steel thick enough for main battle tanks. (Ogorkiewicz, Richard M.: Technology of Tanks, p. 359-361)

Nonferrous Armor Materials

There were also various non-iron-based armors (nonferrous), like titanium, aluminum, magnesium alloys, nylon, fiberglass and others. (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 1-4)

Aluminum Armor

Probably one of the most notable non iron amored vehicles is the Armored Personal Carrier M113, which has aluminum armor and is also one of the most produced armored vehicles outside of the Soviet Union. Also other aluminum armored vehicles like the M114, M 108 and M109 were built. Although aluminum is lighter, for the same amount of protection about the 3 times the thickness is needed compared to rolled Steel. There are various advantages and disadvantages for aluminum. (Ogorkiewicz, Richard M.: Technology of Tanks, p. 367-368)

As pointed out by the author Ogorkiewicz:
“In addition to the savings in weight, aluminum armour is also easier to machine and the greater thickness of its plates makes it possible to use stepped joints, which provide a partial interlock between plates and require therefore less welding. All this has helped to reduce the cost of producing vehicles with aluminum armour but its cost per ton has been significantly higher than that of RHA [rolled homogenous armor].” (Ogorkiewicz, Richard M.: Technology of Tanks, p. 368)
There are various armored vehicles that use aluminum and/or aluminum alloys to a large degree, like the M551 Sheridan, the British Alvis Scorpion, the French AMX-10 and also the M2 Bradley Infantry Fighting Vehicle. (Ogorkiewicz, Richard M.: Technology of Tanks, p. 368-369) Now, the Bradley also has composite armor, so let’s take a look at it.

Composite Armor

The wide adoption of shaped or hollow charges like the Panzerfaust, RPG and HEAT shells, allowed the penetration of thick monolithic steel armor quite easily, this lead to the development of composite armor. (Ogorkiewicz, Richard M.: Technology of Tanks, p. 369-370)

[NOTE: shaped and hollow charges are used interchangeably here]

To spare you and me some complicated math here, basically hollow charges are not too much affected by the density of the material, thus certain lower density material provide better protection for their mass in comparison to steel, hence the term for this is also called mass effectiveness, which almost sounds like a really cheesy title for a computer game. The problem is that the resulting thickness usually makes those materials impractical to protect against shaped charges. Furthermore, they are also quite useless against regular anti-tank ammo or to use the technical term kinetic energy projectiles. (Ogorkiewicz, Richard M.: Technology of Tanks, p. 370)
Yet, the combination of low and high density material, can provide effective armor protection. The US started to develop composite armor at the end of the Second World War, there were firing tests with Shermans. Later on different version of composite armor were developed for the M48 and M60 Patton, but didn’t see mass production due to cost and difficulty in production. (Ogorkiewicz, Richard M.: Technology of Tanks, p. 370-371)
Yet, the British developed the so called “Chobham Armor”, which was also used by the US and Germany in their designs since the early 1970s.
“Since then almost all new battle tanks have been built with some form of composite or multi-layered armour instead of monolithic steel armour”. (Ogorkiewicz, Richard M.: Technology of Tanks, p. 371)
There are various materials like glass, ceramic and aluminum oxide, that offer greater protection against shaped charges than their density might suggest. Yet, those materials often have their disadvantages. The most effective approach is to use multi-layered armor consisting of steel and said materials. The effectiveness can also be improved by spacing those layer, although this makes the armor more bulky.(Ogorkiewicz, Richard M.: Technology of Tanks, p. 371-373)

Explosive Reactive Armor

Another protection against shaped charges was explosive reactive armor. It was developed in the 1970s and was first used by the Israelis in their operations in 1982 in Lebanon with British Centurions and US M60A1’s. A few years later the Soviet T-64 and other Soviet tanks were also equipped with reactive armor. (Ogorkiewicz, Richard M.: Technology of Tanks, p. 374-375)
Now, to properly explain reactive armor, we need some basic understanding of shaped charges. To put it very simple, a shaped or hollow charge creates a kinetic effect that punches through armor, reactive armor solves this problem by exploding. Of course it is a bit more complicated than that, reactive armor is basically a hollow brick consisting of an explosive charge between two metal plates. Now, if the brick is penetrated by a shaped charge, the explosives go off and brick expands towards the shaped charge. There are two effects that reduce the effectiveness of the shaped charge, first its velocity and angle is changed and second the expansion of the plates requires the molten jet to go through more space.
Of course reactive armor must be designed resistant enough to be unaffected by artillery fragments and small arms fire. Also it can be a potential hazard to unbuttoned crew and nearby supporting infantry. (Ogorkiewicz, Richard M.: Technology of Tanks, p. 374-375; Cooney, Patrick J.: Armor, January-February 1988, p. 7; Yap, Chun Hong Kelvin: The Impact of Armor on the Design, Utilization and Survivability of Ground Vehicles, p. 68-70)

Physical Properties

Now, before we look at the ballistic properties, let’s take a look at the physical properties, because those are determining the ballistic ones. And the most important physical properties are:
“(a) Hardness: the ability of the armor to resist indentation.
(b) Toughness: the ability of the armor to absorb energy before fracturing.
(c) Soundness: the absence of local flaws, cavities, or weaknesses in the armor. Unsoundness is not so often found in rolled armor as in cast armor, because of the mechanical working which has been done during the hot-rolling process.” (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 1-7)

Note that a high hardness, which is measured by the Brinell Hardness Number (BHN), usually makes armor quite brittle and easier to break, thus reducing the toughness rating. Thus, increasing one value can also lead to the reduction of another value, hence the proper balance is more important than one local maximum.
Ballistic Properties / Armor Characteristics
So, let’s move on to the basic ballistic properties that are most important for tank armor.
“The necessary ballistic properties which are required of armor consist of resistance to penetration, resistance to shock, and resistance to spalling.” (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 1-6)

Resistance to Penetration

Resistance to penetration is quite simple, it is the ability of the armor to resist the partial or complete penetration, which is called perforation by the way, through the armor plate. (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 1-7)

Resistance to Shock

Next is resistance to shock, which means the ability of the armor to absorb energy without cracking or rupturing. Note that resistance to shock is referring to energy, thus it includes both projectiles as also explosion. Also atmospheric condition can change this property, low temperature makes most materials more brittle and thus more likely to crack. Something you should consider, especially if you want to invade Russia, Canada or Finland. (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 1-7)

Resistance to Spalling

Finally, resistance to spalling, which is the property of armor resisting to partial cracking, flaking and breaking away of smaller elements, especially on the opposite side of the penetration. Usually, spalling results in an expanding hole from the entry to the exit of the armor plate. (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 1-7)

Or to put it another way, resistance to spalling is the property of your armor plates preventing themselves from transforming into a shotgun blast that turns your crew into Swiss cheese.

Penetration vs. Perforation

Now, while reading I encountered a very interesting distinction, it seems that most of us use the term penetration not quite precisely. To quote:

“The term penetration is reserved for the entry of a missile into the armor without passing through it. The term perforation implies the passage of the missile completely through the armor.” (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 11)

Now, if one thinks in more biological terms this actually makes quite a lot of sense. But, in case you wanna go full Penetration-Perforation-Nazi, here is a list of subreddits that will really enjoy your comments:
/r/tankporn
/r/worldoftanks
/r/warthunder
/r/destroyedtanks

Whereas the word “enjoy” is used rather loosely here.

Surface Design and Features

The overall Surface design of tank armor should be focused on providing appropriate protection in relation to the expected direction of attack, e.g., strong frontal armor and weaker rear armor. Furthermore, the tank should have an overall convex surface and as a short reminder, this is what concave looks like. Now imagine some shot ricochets here with the convex shape the projectile will fly always away from the shape, but with the concave shape it can hit the shape after bouncing off. (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 4)

Shot Traps

In context with armor design convex is reached by the absence of reentrant angles. These so called “shot traps” would often occur between the turret and the hull. What makes them so dangerous is that the deflected projectiles could strike weak spots in the armor that were usually hard to hit, like the top of the hull. Probably the best known shot trap of World War 2 is the early Panther. As you can see here a shot that bounces from the gun mantlet will deflect into the upper side of the hull, which is weakly armored. This was the reason, why the gun mantlet was changed. As you can see here, the lower Panther is a later variant. Here, the same shot will not be directed towards the hull if it ricochets. Reentrant angles are also relevant when attack by high-explosive shells, because they will also redirect the explosive blasts and fragments into lesser protected areas. (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 4)

Regularity

Yet, another aspect that is less obvious is that the surface should be as regular as possible. Basically, every irregularity that breaks the uniformity of the armor will restrict the uniform absorption of energy and as a result could damage the armor. (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 4)

Thus, “A flat, smooth wall of constant thickness offers the best resistance to severe attack, principally because the shock of impact can be uniformly absorbed over the entire area.” (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 4)

Sloped Armor

Now, probably one of the best known armor features is sloped armor, which was one of the features the Russian T-34 is well known for. Sloped Armor is basically armor that is not angled at 90 degree. Sloped Armor increases the effectiveness of armor in two ways, first it increases the distance the projectile has to perforate. In this case, an armor of the thickness of 1.2 has an effective armor thickness of about 1.7 if it is angled at 45 degree. And Secondly, due to the angle deflections and also shattering of projectiles becomes more likely.
Note that sloping usually doesn’t reduce the effectiveness of shaped charges. (Ogorkiewicz, Richard M.: Technology of Tanks, p. 363)

Spaced Armor & Armor Skirts (Schürzen)

Another way to improve armor rating is by using spaced armor, one of the first tanks that was fitted with spaced armor was a late Panzer III in 1942. After the Second World War spaced armor was not used commonly until the 1960s. Yet, sometimes spaced Armor is not so obvious than in World War, e.g., the Leopard 2A5 uses spaced armor at the frontal part of the turret. Probably the best known use of spaced armor are the German “Schürzen” or armor skirts in World War 2. (Ogorkiewicz, Richard M.: Technology of Tanks, p. 363-365)
These were originally introduced to protect the sides of German armored vehicles against Soviet anti-tank rifles that fired conventional Kinect rounds. Why do I mention that? Because there is a on ongoing myth out there that the skirts were introduced to protect against shaped charges, yet at the time of the introduction of the armor skirts in 1943 shaped charges weren’t common on the battlefield yet.(Spielberger, Walter: Sturmgeschütze, S. 92-93)
Skirts were not common the first decades after the Second World War, but were reintroduced with the British Centurion and other tanks in the 1960s and 70s. Although this time in order to protect against shaped charges. (Ogorkiewicz, Richard M.: Technology of Tanks, p. 365)

Slat, Cage, Chain and Bar Armor

There are also other forms of spaced armor, namely slat, cage or bar armor, which was also used in World War 2 with wire meshes instead of metal plates for the skirts. It usually consists of steel bars that are located at a certain distance to the main armor of the vehicle. After World War 2, slat armor saw a reintroduction in the 1960s and recently it is used by Israeli and US troops in the Middle East to protect against shaped charges. Also, since it is relatively easy to produce, vehicles used in the current conflicts in Iraq and Syria are equipped with all kinds of slat and chain armor. You might check out the galleries that the blog “Tank and Armored Fighting Vehicles News” put up, as always the link is in the description.(Ogorkiewicz, Richard M.: Technology of Tanks, p. 365; https://tankandafvnews.com/2016/01/18/armored-oddities-of-syriairaq/)

Feasibility, Cost & Strategic Resources

As a final remark, one important aspect that we need to consider then it comes to armor is the feasibility in terms of industry, cost and resources, which is probably very well expressed with this remark from 1963:
“The alloys of certain light metals show future promise for use as aircraft armor where the importance of weight saved would offset the disadvantages of substituting a more expensive, strategically critical material in place of steel.” (Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics, Cp. 10, p. 1-4)

Summary

To summarize, steel was and is a common material for armoring tanks, once it was used almost exclusively. It has a high density and is quite easily to produce in large quantities. The introduction of shaped charges although allowed to penetrate even very thick steel plates easily. To counter shaped charges various measures were introduce like spaced, composite and explosive reactive armor. Thus, nowadays a tank is usually armored with a multiple layers of different materials and/or additional armors like spaced and reactive armor.
Although steel was the main material for main battle tanks for light armored vehicles aluminum alloy armor is not uncommon since the 1960s.

Armor design is a complex topic, because many factors affect each other, for instance the key physical properties of tank armor are hardness, toughness and soundness, whereas increased hardness usually decreases toughness.

Furthermore, certain materials and techniques are quite expensive, thus armor design is not only influenced by military aspects, but also by feasibility in terms of the industrial capabilities and resources of the producing country.

Sources

Headquarters, US Army Materiel Command: Elements of Armament Engineering Part Two Ballistics.

Ogorkiewicz, Richard M.: Technology of Tanks, Jane, Volume 1-3.

Yap, Chun Hong Kelvin: The Impact of Armor on the Design, Utilization and Survivability of Ground Vehicles: The History of Armor Development and Use

Cooney, Patrick J.: Armor, The Professional Development Bullentin of the Armor Branch PB 17-88-1, January-February 1988.

Spielberger, Walter: Sturmgeschütze

Tank and AFV News – Armored Oddities of Syria/Iraq

https://en.wikipedia.org/wiki/Rolling_(metalworking)

https://en.wikipedia.org/wiki/Penetration_(weaponry)

https://en.wikipedia.org/wiki/Casting_(metalworking)

https://en.wikipedia.org/wiki/Reactive_armour

https://en.wikipedia.org/wiki/Slat_armor