Friday 20 January 2023


As usual, I did an AMA on the Ask Historians subreddit to promote my newest book. This was the most popular one yet with over 1000 upvotes and over 20 top level questions answered. Check it out!


  1. Good stuff.

    I still would like to know more about how the Soviets calculated theoretical armor protection. It seems in cases (and this makes sense) they use one set of criteria for their guns versus enemy armor (the "certified penetration" 80 % probability values) where what's important is that you have a high degree of certainty that said gun will work against specified enemy tanks. However, when it comes to how well your own armor is protected against enemy AT weapons, you use some different criteria--for instance, the "partial penetration" table you published recently showing weapon performance where the armor is compromised but not penetrated. If your armor meets a criteria like "partial penetration" against a gun you can have a high degree of confidence that it will offer good protection against said gun.

    As an example--you mention in the Youtube video, I believe, how the IS-2's armor was originally designed to resist the 88 mm Kwk36 on the Tiger 1, as said gun (by Soviet metrics) penetrated 120 mm amour and the driver's plate was 120 mm at a 30 degree slope. However, that 120 mm metric was a 'certified penetration' value--80 % of penetration. It would have been cold comfort to an IS-2 driver to learn that said plate only lowered the chances of penetration to, say, 40 %, for good reason. So was some other value used?

    Similarly, there's the problem of rounded mantlets. In some instances (like the proposed upgrade from 100 mm to 130 mm for the IS-2 front turret) we are told (Baryatinskiy) that this upgrade should offer protection against the 75 mm Kwk42 on the Panther. However, a different document you have posted:

    shows the 130 mm thick turret front would be vulnerable out to 1100 meters from the same gun.

    How to explain this discrepancy? My strong suspicion is that the first calculation that the 130 mm rounded IS-2 turret should survive a hit from the German 75 mm Kwk42 is based upon an average slope factor calculated for the IS-2's rounded front turret face. The second document saying it won't is a very conservative estimate that assumes a round is hitting that turret face orthogonally or nearly-so (a very small chance). My reason for thinking this is that:

    a) the 1100 m vulnerability figure is close to the 90 degree Soviet figures for certified penetration of the Kwk42 (actually, it's closest to the 500 meter figure, so some other metric than "certified penetration" is likely being used). So it makes sense the writer of that report is assuming also a direct, orthogonal, hit, despite the fact that in practice such a hit is quite unlikely.

    b) Indeed, in real life, the rounded IS-2 turret even on the model 1943 resisted with a turret front of only 100 mm resisted the Panther's 75 mm Kwk42 rather well, at 1000 meters, as here:

    The 130 mm upgrade thus may have made it close to invulnerable.

  2. Continued...

    So--obviously, when proposing these changes, Soviet tank designers had to be calculating armor resistance some way in order to make these proposals. But I've seen no details. I have taken scale drawings of IS-2s, and loaded them on online protractor websites to calculate something like a 50 % criterion for angle of impact--in other words, at what angle does half the shots impact at a smaller angle than that, and half at a greater angle. I come up with a value of about 40 degrees; though the error in my measurement could mean it's 35 degrees (I don't think it's less than that). I've seen one paper that used 45 degrees as the mean angle of impact with the T-34/85's rounded turret face, but I think that's too much.

    I don't think you can just plug in one value for every tank; both T-34/85 and IS-1/IS-2 turrets seem to be elliptical in shape, tapering to a narrow flatter area. This makes sense, as by doing this you both reduce the vulnerable flatter areas, making them harder to target and hit, and can limit the use of the thick armor on the turret to the flattest area, making it harder to penetrate, thus keeping down the overall weight of the tank. However, some rounded mantlets are barely rounded at all (say, the Sherman Jumbo) so any mean angle of impact would be low.