On American Armour

After my articles on Soviet and German armour, I think it’s only fair to sail across the Atlantic and turn our eyes at the manufacturing juggernaut that was the United States. A lot of people know that the US was good at casting armour, and that their armour was softer than that of their European counterparts, but that’s about it. While this is true, it’s far from the whole story.

To begin, let’s go far far back to the 1920s. There are no Shermans, Tigers, or T-34s. The greatest war mankind has ever known has just ended, and everyone is busy developing the latest and greatest killing machine. Huge and slow monsters did not seem interesting to the US Army, and development was focused on vehicles that were quick and agile, but paid for that agility with thickness of armour. For instance, here is an attempt to produce 6-12 mm armour plates that protect against .30 and .50 caliber bullets in 1922.

Maximum hardness!? This isn’t the American armour we know and love! Don’t worry, they have two decades to get there. In the meantime, let’s see how well thin armour like this turns out.

Well that’s not good. To be fair, the unrolled plates didn’t crack, but were covered in free cementite, which was what the metallurgists were trying to avoid. Ok, so the early armour wasn’t of amazing quality. How well did it resist attack? A document about corrugated armour conveniently mentions the ballistic limits for flat armour defined by specification AXS54 Rev. 2 in a table. We see that the 12.7 mm plate can be penetrated by a .30 caliber bullet at a speed of 2250 f/s and a 14 mm plate can be penetrated by a .50 cal bullet at 1930 f/s. Both of those bullets were very capable of achieving those speeds, so early American vehicles were still vulnerable to small arms fire. I suppose it’s worth mentioning that Soviet vehicles with similar armour thicknesses were not vulnerable to .30 caliber bullets, but that was a few years later.

All right, so the armour can be penetrated by bullets. Does it still spall? The Americans invented a pretty sweet process for taking pictures of rapidly moving things, which indicated that yes, yes it did.

Oh dear. Let’s fast forward to 1938, a year before the world erupts into total war once more. American armour has improved to this point. It got a little softer (400 BNH ideal hardness) and a little thicker, now expecting protection from proper 37 mm guns.

1700 m/s is the ballistic limit of an inch of armour against a 37 mm gun. This gives protection from 37 mm AP from about 850 meters. To put this value into perspective, English reports indicate that the American 37 mm guns penetrate 30 mm of German armour from 1600 yards (1463 meters). This is very high quality armour. Also note that the armour no longer spalls when hit with .50 cal bullets. These values are satisfactory against the current German army, but they won’t be limited to PzIs, PzIIs, and PzIIIs with 37 mm guns forever.

The Americans needed more tanks and needed them quickly. How do you get more tanks quickly? Casting!

119 cast armour plates from 9.5 mm to 76 mm were tested in 1940 in order to figure out how well this sort of thing can be done. The report only mentions that “some” plates passed the specifications, but fails to mention how many. Another report, however, does mention some pretty serious issues with casting.

Yikes, that’s not a good thing to hear when you’re about to participate in a huge war. This is where the Americans rapidly started leaning towards softer armour.

Watch as the hardness falls from 400 BNH to only 255! Also, note the superiority of Canadian manufactured armour. Take that, Americans!

Let’s look at slightly thicker armour. A document on welding provides a table of ballistic limits of 1.5-2 inch plates with and without weld repairs. Not only is the ballistic limit of a plate with a weld not that much less than an undamaged plate (remember all those popping German seams?), it’s only decreased when the shell strikes within an inch of the seam. This is pretty damn good. Furthermore, the ballistic limit of a 2 inch plate is 1512-1640 f/s, which corresponds to 500-900 meters (at least according to Soviet tables for the M3 gun). Recall that both Soviet and German data indicates that the Tiger can be penetrated in the side from the outer limit of that range. The quality of American armour far surpasses the quality of German armour, even though the plate is overmatched in the American case. While this assertion is only backed by theory, battlefield performance seems to agree.

Damage to vehicles consists mainly of broken tracks, tank fires, broken turret rings and damaged suspension system. Apparently armour plate quality superior to that of Germans.

While the cast turret rings seem to still be a bit of a problem, the armour shows itself splendidly.

No observed complete penetration to front sloping plate, front tank doors, nor gun shields. Final drive housing struck by what is believed to be 6-pounder armor piercing projectile was dented with no effect on operation.  One penetration reported by armor piercing 6-pounder on edge of door next to T member, numerous penetrations of side plates and back plates with no effect on operation of vehicles.

And this isn’t even the Sherman we’re talking about, but the meek M3 Stuart. 

My communist benefactors are looking over my shoulder, so I have to say something negative about American armour now. Uh, let’s see here…

Ha! The tanks are falling apart in winter! How cold did that winter get, anyway?

Oh, pretty damn cold, huh. Sadists can read some of the detailed descriptions of the damage in extremely cold temperatures (1, 2, 3, 4, 5, 6), but for those of you that don’t have time, the damage is very similar to that of German vehicles at room temperatures: cracking, spalling, fragments falling off.

So there you have it. Both cast and soft armour came from necessity instead of some kind of inherent superiority, but the Americans pulled it off. The quality of armour sent to Europe and the Pacific was largely excellent, and definitely superior to the armour of the vehicles it was fighting against.

32 thoughts on “On American Armour

  1. YAY FIRST! So if severe temperature broke tanks as much as it sounds like it did, were tanks conditioned for their native climates? EG tanks made in the north behave different from tanks made in the south?

      • I read somewhere the SS Titanic sank because the plates were brittle in the freezing waters: a crack just an inch wide ran the length of the ship upon impact with the iceberg. It wasn’t the iceberg gouging a furrow.

        • The plates were less ductile at cold and colder temperature and also had a slightly higher sulfur content then usual, iirc. That being said, at room temperature the metal was some of the best available at the time.

        • It’s often believed that it wasn’t the steel plates that cracked/shattered, but the rivets. The iceburg overstressed the rivets and caused them to sheer, which opened up seams in the steel plates.

    • Depends, for instance, the USS Enterprise was armored only by Bull Halsey’s brass balls, but it held together pretty well under Kamikaze attack.

    • Althought this is more of a deisgn failing than through defective materials, several liberty ships broke apart in freezing seas due to brittle fracture thanks to the welded construction creating various stress concentrations throughout the vessels.

      • Also, remember that welding on that large a scale was also very, very new. Liberty Ships were some of the largest all-welded objects ever produced up until that time. Especially using the techniques that were used to speed up the production.

        The solution was very American. Gigantic steel belts to hold it all together.

    • The armor in the belt and turret faces of the Iowa-class ships is generally considered by metallurgists to be some of the highest quality steel ever produced by mankind, ever.

      US naval steel has always been of very good quality. And armor layouts, from the Nevada-class onward, were generally superior to anything else in the world. The US’s modified “All-or-nothing” concept worked well.

      • The Iowa class had very mediocre armor layout. They had to reduce the weight so that it could keep up with the carriers. I know a fellow that has made a life long study of the Iowa class. It is his belief that if it 1 v 1 a Yamato it would have suffered badly. The Montana class was designed to correct this.

        • I agree. Look at how the Yamato easily dismantled mere destroyers and destroyer escorts which doesn’t have any meaningful armor at all. Oh wait.

          They had to switch to explosive rounds because the armor is too thin to trigger detonation in the AP shells, they just made holes along the side of the DDs and DEs (and in some cases even comes out through the other side LOL). And as far as can be confirmed it was the cruisers and torpedoes from the destroyers that eventually sunk the “tin cans”, rather than the biggest battleship afloat.

        • Iowa’s protection was far from “mediocre”. It wasn’t designed to be protected against it’s own armament. It was designed to be protected against the 16″/45 Mk6. It’s actual effective armor layout (Which was very similar to the South Dakota) was one of the best in the world. Especially over vitals. Remember, Iowa had a ‘modified’ All-Or-Nothing armor scheme. Protection was prioritized over vitals. Like the turret faces. And the turret faces are just ridiculous.

          1v1 Yamato, Iowa never would’ve even gotten hit. It had the speed advantage, it had the effective range advantage. Iowa would’ve dictated the fight up one side and down the other. Yamato would’ve been helpless. Then you factor in Yamato’s inferior armor quality and the fact that the 2,700lb Superheavy AP was just as good as Yamato’s 18.1″ AP and, well…

          Montana corrected nothing. Montana was simply designed to be able to be protected against it’s own armament. And it probably would’ve failed at this. Because the 16″/50 Mk7 with Superheavy AP is awesome.

  2. Huh. Would love to see The Cheiftain cover this. Not because I don’t believe you, but because I want to see what else he could dig up.

  3. The key was the US made things so damned uniform that you could rely on them being good or bad everytime which is what you want.

    Thank god we stole the hardness tech from the T54 and made things like the T95 medium with both hardness and superior craftsmanship.

  4. “Huge and slow monsters did not seem interesting to the US Army”

    Lets see here… US Heavy Tanks.

    British Mark IV(Training in the UK and used in France in combat at least one made its way to APG.),
    British MarK V(Training in the UK and used in France in combat. Post war one was in use at APG in 1929) 301st Tank Battalion,
    British Mark V*(Used in France in combat.) 302nd Heavy Tank Battalion,
    British Mark VIII “Liberty/International”(I need not say anything about it.)
    Then there is the Steam Tank.

    And then after that, yes you have to wait until the T1 Heavy Tank. But the Army was always interested in Heavy Tanks the problem was more budgets and, at times, practical reasons which is why the M6 really never got on the ground in Europe.

    • Actually if you’ll notice the overall speeds of the tanks, they increase as the first war went on. The U.S. never dd like slow paced wars, and the desire to make mobility a priority over protection goes back as far as the first tanks we ever made. They COULD have made the liberty tank more armored, but they didn’t because they wanted it to be quicker across the battlefield.

  5. Considering that some of the people in the 1940s producing cast armor for US tanks had never produced armor and considering that each company fiddled with the armor to suit their own facilities and considering that the companies only had to meet the end results(specs of the armor given to them) they did it in several different ways with different mixes of alloys and different heating and cooling.

    The rest,

    I can’t even finish reading it after I hit this “so early American vehicles were still vulnerable to small arms fire.” The research is horrible. Truly.

    Random armor plate test = American Tank Armor test ergo, early American vehicles were still vulnerable to small arms fire…. This mind you when the ONLY countries that fielded engough armor to stop a .50 cal Browning was the French in their 1917/18 FCM 1A and the Germans with maybe their K-Tank.

    Tests with much better armor from September 1942 has the .50 cal AP M2 going through 27-28mm of 368-387 BHN Homo Ni-Cr armor plate.

  6. You can not judge a country “armor quality level” on that few reports.

    Engineering tank armor is far more complicated than suggested here. A good example are the thinner german side plates. Those had high hardness, but not to make them crack easier, but to increase the effektive armor against uncapped shells. Uncapped shells which should nominally penetrate the thin armor failed because the shell shattered on the hard surface of the plate.

    • I already covered German armour. Their hard armour was only good against undermatching shells. Too bad the armour you’re talking about is overmatched by even the smallest Allied anti-tank guns.