Quote of the Day
The whole point of being a citizen soldier is that you cannot wait until you are no longer a soldier.
— Gary Gallagher
When I was a boy, most of the fathers in my neighborhood had served in WW2. One of these fathers, Alvin Weese, was an Army veteran who was very specific about his WW2 service by saying that he had "served under Patton" and you could clearly see his pride in having been a soldier in Patton's 3rd Army.
As a boy, Alvin's comments about "Old Blood and Guts" got me curious about the American use of armor during WW2. Most of what I read or heard was quite disparaging (example) about the M4 Sherman Tank (Figure 1).
The comments I heard about the M4 can be summarized as:
- caught fire too easily
- inadequate armor
- inadequate main gun
- the M4 should have been replaced earlier by the M26 Pershing.
This weekend, I saw a Youtube video by a gentleman, with the handle "The Chieftain", who works for wargaming.net as a researcher and he had a completely different take on the M4 Sherman than I had heard before. While he addressed each of the concerns that I listed above, in this post I will limit my focus to his statement that the sloped frontal armor on the M4 Sherman was actually comparable to the unsloped frontal armor of a Tiger I. Specifically, he states that the Sherman had an equivalent frontal armor thickness of 3.6 inches compared to that of a Tiger I's 4.4 inches (see the video below, about 34 minutes in).
Since the Sherman is listed as having 2 inches of frontal armor, I thought it would be interesting to examine his statement more closely to understand the reasoning behind the 3.6 inch statement. In response to an excellent response from a reader of my blog, I will also look at how the armor was constructed and how a Sherman's armor had a much more difficult attack to resist than the Tiger I did.
We can thank the gaming community for bringing so many of these facts about WW2 weapons into light.
Video That Motivated This Post
Figure 2 shows the Youtube video that got me thinking. The lecturer does an exceptional job describing the complex managerial context of US armored forces during WW2.
Figure 2: Good Lecture on American Armor During WW2.
- Cast Homogeneous Armor
- As the name states, cast armor is formed in a mold. As such, it allows for high-rates of production. Unfortunately, cast armor provides less protection than an equivalent thickness of rolled armor.
- Rolled Homogeneous Armor
- As the name states, rolled armor is formed through a rolling process. This process generally provides protection superior to that of an equivalent thickness of cast armor.
- Armor Overmatch
- This is a complex topic, but as the diameter of a shell nears the thickness of the armor, the armor provides less protection than you would predict based on its thickness. I have not been able to find a definitive description of overmatch, but it appears to be related to shock. There are many online discussions on how to model this effect (example). The Sherman, with 2 inches (50 mm) of frontal armor, was overmatched against 75 mm and 88 mm armor.
- Line of Sight Thickness (τLOS)
- The horizontal thickness of a tank's armor, which increases as the armor is sloped (see Figure 2).
- Normal Thickness (τN)
- The thickness of a tank's armor normal to its surface.
- Slope (θ)
- The angle of the armor plate measured relative to vertical. Note that some tanks specify this angle measured from horizontal – you have to check.
Armor Evaluation Criteria
Evaluating armor protection is complex process. Years ago, I spent some time reading articles on how battleship armor was designed (see the excellent work by Okun). I now see that designing tank armor is just as difficult as designing battleship armor.
I will limit my discussion of Sherman versus Tiger I armor to four topics:
- Armor thickness
The two main reasons for sloping armor are two (1) increase its effective thickness, and (2) increase the likelihood of causing incoming rounds to glance off. The Chieftain mentioned armor thickness during his discussion of "Myth 8: Sherman Was a Death Trap". An assumption of this discussion is that armor can be compared strictly on a thickness basis. Like all interesting engineering questions, the answer is "it depends." In that case of tank armor, it depends on the projectile you are trying to defend against.
- Armor Quality
There are numerous ways to build armor. The Sherman was initially built using Cast Homogenous Armor (CHA) and later transitioned to the superior Rolled Homogeneous Armor (RHA). Tiger I used RHA. Evaluating the relative merits of these CHA versus RHA is difficult, but RHA provides superior protection for a given thickness of armor.
- Threat Faced
A Sherman tank's armor had to fend off attack from a high-velocity 88 mm gun, while the Tiger I had to resist attack from a low-velocity 75 mm. For a Sherman to provide levels of crew protection comparable to what a Tiger I provided, the Sherman would have needed much more armor.
- Theater Conditions
As I read the various articles about the Sherman, you see that its characteristics were more suited to some battlefields than others.
Frontal Armor Thickness
The Chieftain said that Sherman's frontal armor is usually listed as 2 inches thick, while the frontal armor of a Tiger I (Figure 3) is usually listed a 100 mm (4 inches) thick.
While the Tiger I's armor is not sloped, we can see that the M4 Sherman's frontal armor is sloped at 56° (as shown in Figure 1). According to The Chieftain, this means that the M4 Sherman's effective armor thickness is really 3.6 inches relative to a horizontal strike and is roughly comparable to the frontal armor on a Tiger I.
Figure 4 shows how The Chieftain got his answer of 3.6 inches of effective armor thickness. The key formula here is .
Unfortunately, the M4 Sherman did not use sloped armor for its sides, but at least the frontal armor was sloped, thus making more effective use of the 2 inch frontal armor plate. While I am very familiar with the sloped armor on the T34 and Panther tanks, I had never thought about the M4 Sherman's armor being sloped.
The Sherman's sloped armor had significant advantages when facing opponents armed with 50 mm or 57 mm main guns (e.g. PzKpfw III). However, these advantages vanished when faced with opponents armed with 75 mm or 88 mm main guns (e.g. Panther and Tiger I) because of armor overmatch, which occurs when the shell diameter is greater than the armor thickness. When armor is overmatched, the slope plays minimal role. For a good description of how overmatch affects the level of armor protection, see Appendix A.
The fact that the bulk of Sherman production used CHA rather than RHA like the Tiger I meant that an inch of Sherman armor was less capable than an inch of Tiger I armor. The exact difference is difficult to estimate – some folks claim that CHA could be penetrated 500 meters further away than RHA.
You really need to evaluate the Sherman's protection relative to the threat it faced. A Tiger I's 88 mm main gun could penetrate a Sherman from ranges beyond typical visual ranges, while a Sherman could not penetrate a Tiger I's frontal armor even at close range. Thus, the level of crew protection in a Sherman is not really comparable to that of a Tiger I.
The Sherman appears to have performed well in the Pacific Theater where Japanese tanks were few and were relatively light. It also performed well in Africa, where it mainly dealt with PzKpfw IIIs and PzKpfw IVs. It received good grades in the Italian campaign where the mountainous terrain force the Sherman into more of a mobile artillery role. However, the Normandy campaign did not play to the Sherman's strengths of reliability and mobility. In the Normandy Campaign, the Sherman faced a well-led opponent who out-gunned and out-armored it. This meant that the Sherman could only depend on its remaining strength – its vast numbers.
The Chieftain brought up a number of good points about American armor during WW2, but I do not agree with his point that the Sherman protection might not have been as bad as most people say – I do think its armor protection was grossly inadequate. However, I enjoyed his video, and I will be checking out his Youtube channel for more interesting morsels as time goes on.
Appendix A: Quote on Overmatch
I found the following quote on overmatch that gave the best description I have seen on how armor overmatch is modeled. It still does not explain the physics behind the phenomena.
Behind the decision to retain the the 88 mm KwK 36 L/56 as the main gun of the Tiger I, instead of the Rheinmetall 75 mm KwK 42 L/70, was the fact that at that time armor penetration was mainly a function of thickness to diameter (T/d) ratio. During World War II, the Armor Piercing (AP) round relied on its own weight (and a 88 mm KwK 36 L/56 gun APCBC shell weighed 10.2 Kilograms, as opposed by an 75 mm KwK 42 L/70 gun APCBC shell, which weighed 6.8 Kilograms) to penetrate the enemy's armor. Theoretically, the higher the muzzle velocity, the more penetration any kind of AP round would have, all other variables remaining constant. In real World War Two tank combat, however, other important variables intervened, such as the thickness to diameter (T/d) coefficient, which means that the bigger the diameter of any given round relative to the thickness of the armor it is going to strike, the better the probability of achieving a penetration. Furthermore, if the diameter of the armor piercing round overmatches the thickness of the armor plate, the protection given by the inclination of the armor plate diminishes proportionally to the increase in the overmatch of the armor piercing round diameter or, in other words, to the increase in this T/d overmatch. So, when a Tiger hit a T-34, the 88 mm diameter of the Tiger's round overmatched the 45 mm glacis plate of the T-34 by so much that it made no difference that the Russian tank's glacis was inclined at an angle of 60 degrees from vertical.