The battle of the sword and shield raged since time immemorial. The sword got an edge once, which led to the end of the era of knights clad in armour. However, history repeats itself, and the idea of armoured cars began to appear even before the First World War. The result of this was the tank, which over time became a fixture of the battlefield. Naturally, anti-tank weapons were created as soon as tanks became known. The battle of the sword and shield continued.
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| An example of changing requirements for tank armour. The FCM 2C had the thickest armour of any tank built during the First World War, but penetration trials conducted by the Germans in 1940 showed that this armour was weak by WW2 standards. |
Armour powerful enough to resist cannon shells became a staple of all tanks only during the Second World War. It became clear in the fall of 1939 that bulletproof armour was not enough. At the same time, the idea of armour against shells appeared long before. Protection of tanks against something bigger than a bullet was obvious, but the nature of that something changed over time too, This is what we will talk about today.
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| The A7V, the first tank with shell-proof armour. 30 mm was enough to protect against light artillery, at least in theory. |
Bulletproof armour was a necessary evil. The creators of early tanks dreamt of land battleships, but their ideas had to be reined in. First of all, the tank’s most common enemy was indeed a bullet. Second, mass became crucial. Increasing the armour thickness from 10-15 mm to 30 mm resulted in a huge jump in weight. A tank with this armour would not be twice as heavy, but still too heavy. Now, think about what would happen with the Tank Mark I, which weighed 28 tons with just 10-12 mm of armour. The power to weight ratio was already poor, and this would strain it even further. Ground pressure and therefore mobility would become worse as well.
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| 3.7 cm Tak M 1918, the world's first mass produced anti-tank gun. Its penetration was about on par with infantry guns and 25 mm of armour offered enough protection. |
Nevertheless, the first tank with armour thick enough to resist cannons appeared in the spring of 1918. This tank was called A7V. With 30 mm of front armour, the German A7V could take a hit from a British 6-pounder. However, this tank’s first outing did not go so well anyway. Tank #561 Nixie was hit in the side and abandoned. The A7V was quite a poor tank. Two massive “windows” in the front and a gun with limited traverse were not great ideas. Nevertheless, it was the Germans who first used shell-proof armour.
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| The 130 ton K-Wagen with just 30 mm thick armour should not be a surprise. This was considered enough to deflect shells. |
The issue of thick armour can be seen on the French FCM 2C. The front was 45 mm thick, but the sides were only 22 mm thick, which still offered reliable protection from heavy machine guns. The resulting tank weighed over 70 tons. This was still better than the German K-Wagen, which would have weighed 130 tons with 30 mm of armour all-around. Most importantly, the First World War showed that having good mobility was key. The heavy tank died, but only for a time.
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| The Vickers Medium Tank Mk.II and its OQF 3-pdr Mk.II gun raised the requirement for shell-proof armour from 25 to 30 mm. |
Shell-proof armour, however, survived. The First World War showed that light guns are the most dangerous for tanks, particularly 37 mm guns. This included infantry guns and the 3,7 cm Tank M 1918, the first anti-tank gun. Both the German and Soviet 37 mm guns descended from Hotchkiss revolver guns. As a result, the “classical” thickness of shell-proof armour was 25 mm. Char B and Renault NC-1 tanks had armour this thick for a reason.
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| The Renault NC-27 became the first mass produced tank with all-around 30 mm thick armour. The armour was initially 25 mm thick but had to be thickened. |
This armour did not last long. The British analysed their experience in the First World War and created a 3-pounder (47 mm) gun. This gun was first introduced on the Vickers Medium Tank Mk.II. This gun could pierce 26 mm thick armour at 500 meters and 30 mm thick armour at 300 meters. As a result, requirements for armour grew in the late 1920s. Now the revised Char B, Renault NC-27, and Char D1 had 30 mm thick armour. Unlike the British, the French did not think that a tank should be a speedy tin can. For the same reason, the T-28 and T-35 had 30 mm thick armour.
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| The French were the first to mass produce armour 40 mm thick. It was supposed to protect against their 25 mm gun, but not against the German 3.7 cm Pak. |
But what about the 3,7 cm Tak, which evolved into the 3,7 cm Pak? That gun could penetrate 40 mm of armour from 200-300 meters. The state of the German army in the early 1930s did not make it into a serious opponent. In addition, their guns were a secret, as they violated the Treaty of Versailles. Therefore, 30 mm could be considered effective armour against anti-tank guns, at least for a time. The French discovered that their 25 mm gun could penetrate 30 mm of armour. The requirement for armour thickness was increased to 40 mm. This applied to new French light tanks still in development, which resulted in overloading.
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| It turned out that even 40 mm of armour was not enough to protect from the 3.7 cm Pak. |
The French eventually arrived at 60 mm thick armour. This was also for a good reason. The discussion of further development of the Char B series began in 1931-32. At that point, 40 mm was considered effective only at certain distances. Only 60 mm thick armour could be guaranteed to stop 25-37 mm shells at any range. This new armour was applied to the Char B1 bis. Its mass grew by 4.5 tons compared to the Char B1, from 27.1 to 31.5 tons. This resulted in worse reliability of the running gear, but became a nasty surprise for the Germans in May of 1940.
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| By 1938 the GABTU's expectations for armour increased with the rest of the world. 30 mm was now only enough to protect from heavy machine guns and small autocannons. 60 mm of armour was now needed to protect from shells. |
Other nations arrived at the same result. The T-100, T-111, and SMK-1 had 60 mm thick armour for a reason. The T-34 got away with 45 mm thick plates because they were presented at an angle, but practice showed that the sides could be penetrated at a range of 200-300 meters. French light tanks were vulnerable at the same range. It is no coincidence that the T-34M developed in 1941 had 60 mm thick armour. This improved the tank’s protection from light to fully fledged. The KV-1 had even thicker armour. 75 mm offered complete protection from the model 1902/30 field gun.
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| It turned out that protection from only machine guns was no longer enough. |
One could then consider 25-30 mm thick armour to be just bulletproof, but that is not the case. This thickness became the norm after the Spanish Civil War, where 20 mm Madsen guns that could penetrate 25 mm of armour from 175 meters were common. 25-30 mm thick armour was introduced to offer protection against these guns. However, Japan in the summer of 1939 and Poland in the fall of 1939 made it clear that the tank’s most common enemy is not a 20 mm auto cannon, but a 37 mm anti-tank gun. Some nations reacted to this information faster than others.
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| 45 mm was already not considered a reliable thickness by 1941. Timoshenko's requirement was now 60 mm, although 75 mm was needed to make the tank full proof against armour piercing shells. |
After all these changes, the fact that 30 and even 25 mm was once considered to be fully fledged armour against anti-tank cannons was forgotten. The 1920s and early 1930s are often lumped together into one period by armour historians, but that is not the case. Armour that was considered to offer reliable protection against cannons even in the early 30s quickly became unsatisfactory, and this should not be forgotten.