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Sloped armour

Discussion in 'Armor and Armored Fighting Vehicles' started by BruceAlmighty, Mar 19, 2017.

  1. BruceAlmighty

    BruceAlmighty New Member

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    Hi, I'm new to this forum so forgive me if this question has already been addressed, but why is that, given everyone goes on about how sloped armour is better than vertical at stopping shells from penetrating - which seems logical - why then did/does so many tanks NOT have sloped armour? The T34 was famous for its sloped armour all around, but then why did, for example, the German Tiger have vertical? This was made after the T34. Plenty of tanks in WW2 had vertical or near vertical armour.

    I mean, it seems so obvious to me that ALL tanks should have sloped armour all around, end of story.

    Thank you.
     
  2. Otto

    Otto No More Half Measures Staff Member WW2|ORG Editor

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    Welcome to the site BruceAlmighty.

    So, a very brief answer here, as a full answer to this question would fill multiple books.

    A good thing to keep in mind when thinking of tank design and tank engineering is that everything is a trade off. Everything.

    Yes, sloped armor does a better job deflecting an incoming projectile, but this requires a much longer piece of armor to protect the same "enemy facing" area. That means much more weight and a slower vehicle as a result. By their nature, the sides can only be sloped so much and often are thinner to accomodate the extra weight of the front sloped armor.

    Another problem is the reduced inner size of the vehicle. Tanks are always cramped, and a squished diamond shape makes the problem of fitting all the necessary components, ammunition stowage, and crew inside more difficult. Less space inside the vehicle might mean that the vehicle must get a little taller to accomodate the contents. A higher profile is not a good thing on the battlefield. A long box shape is the most efficient in terms of free space and allows a lower overall vehicle profile.

    There are a number of other associated deficits with a smaller inner area. For example, crews of the T-34 sufferered a very high mortality when the armor was penetrated. Tanks with a larger interior not only give the crew a better chance of surviving a penetration, but also of escaping if necessary. Having your experienced tank crews survive and serve in a new AFV is a real asset. Sloped armor makes this much less likely, as a penetration will do more damage within a smaller fighting compartment.

    That's all for now but I think you get the idea. Sloped armor helps, but it must be done in balance with the other requirements of your AFV.
     
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  3. Takao

    Takao Ace

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    While it is true that the Tiger appeared after the T-34, the Tiger design dates back to 1938 with the VK 30.01(H) medium tank and the VK 30.06(H) heavy tank, before the design was finalized as VK 45.01(H) on May 26, 1941.

    So, as you can see, the Tiger design was finalized before the Germans met the T-34.

    Now, had the Tiger been redesigned with sloped armor, it would have been even longer before it entered production and combat.
     
  4. GunSlinger86

    GunSlinger86 Active Member

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    Wasn't the King Tiger sloped?
     
  5. Slipdigit

    Slipdigit Good Ol' Boy Staff Member Patron   WW2|ORG Editor

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    Hull armor was nearly vertical all around. Turret armor was a bit more off vertical, but not enough to have any desired effect.
     
  6. Sheldrake

    Sheldrake Member

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  7. von Poop

    von Poop Waspish WW2|ORG Editor

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    Along with Otto's pretty practical summation, there is this:

    David Fletcher - 'The Universal Tank' - p91:
    (My bold)
    I have not read the paper in question, though I looked for it a while back, but Col. Gordon-Hall was certainly an interesting character in this area leaving a trail of quite sensible papers and commentary. He was CO of the School Of Tank Technology from 1946-48 and I have him on a long-standing list of 'Tank Men' I'd meant to sort brief biogs of, but he's one I never took further.
    The point stands, though, back to that attempt at 'real world' appraisal of these statistical issues - even though I'll join the blessed Fletcher and admit confusion on the mentioned maths, the 90 degree perpendicular strike was (is) always going to be something of a rarity, so given a rough guess of average engagement angles in average topography it really is quite possible that a sloped plate (perhaps particularly at the sides) may even more often present an ideal perpendicular face to a hit than a flat.
    Swings and roundabouts... If slope was an uncertain factor, was it outweighed by internal space and lower weight? Quite possibly.
    Not really 'end of story'... but then, so little is in this area.
     
  8. harolds

    harolds Member

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    As an addendum here, I think British thinking was to keep the vertical surfaces. Their idea was that since opposing tanks are rarely exactly aligned opposing each other and are mostly at different angles, both vertically and horizontally. Thus, when you shot it out with an opposing tank the rounds would naturally be hitting at some sort of angle anyway.
     
  9. Sheldrake

    Sheldrake Member

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    This was supported by the Operational Research Section report No 12 http://lmharchive.ca/wp-content/uploads/2014/02/The-Full-Monty2.pdf P395-8 looked at 75mm Sherman casualties in Normandy. This study found that the proportion of hits against the front and sides were almost equal and concluded that there was no point improving the frontal armour at the expense of side armour. i.e. The Panther (thick sloping frontal armour and thinner side armour) was less effective than Tiger I (big square box)
     
  10. m kenny

    m kenny Member

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    A much more detailed AP 'hit' survey done in 1945 on 154 tanks gave 41% hits to the front of a tank and 55% in sides/rear/top.
     
  11. m kenny

    m kenny Member

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    HC hits gave a 28% frontal strike ratio.
     
  12. gtblackwell

    gtblackwell Well-Known Member Patron  

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    The Israelis captured their share of T54-55, T 62 and 72 tanks over the years. They did not like the pancake low turrets , preferring their M48-60s and Centurions for what some reported as having fighting room inside the tank. Thought not WW2 it re enforces what is said above. They are certainly experienced in armored warfare. The low russian turrets also limited the range of elevation compromising the hull down position. Might this translate to WW 2 tanks as well ?

    Gaines
     
  13. BruceAlmighty

    BruceAlmighty New Member

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    Thanks for all your replies everyone. Yes I'm sure there must be some advantages with vertical armour. Just that I keep reading that the T-34 was so good because it had sloped armour.

    And as for sloped adding more weight - isn't the shortest distance between two points is a straight line? Surely a slope between two corners of a stepped front or side of a tank would use less metal? And it wouldn't use up any more space inside....

    Anyway.....thanks all again!
     
  14. Christian Ankerstjerne

    Christian Ankerstjerne Member

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    Sloped armor has both advantages and disadvantages. I wrote an article about it some years ago, and would like to elaborate a bit on some of the points raised above.

    As Otto hints above, a sloped armor plate will have to be larger to cover the same vertical high. Assuming all other elements being unchanged, the weight is exactly the same for the same relative thickness. This is actually quite easy to prove:
    Code:
    Absolute thickness: T
    Relative thickness: RT
    Length: L
    Width: W
    Required height: H
    Angle: A
    
    Required absolute thickness to achive specific relative thickness: T = RT * sin(A)
    Length of sloped plate required to cover required height: L = H / sin(A)
    Volume of plate: T * L * W
    Replace values in volume formula with T and L formulas: RT * sin(A) * H / sin(A) * W
    Simplify: RT * H * W
    Or in other words, the volume is constant for a specific relative thickness and required height - the angle is irrelevant.

    Relative thickness is not the full story, however.

    Shot deflection has a positive effect, especially at extreme angles. As von Poop pointed out, different ammunition types are affected differently by sloped armor, and below a certain angle, traditional steel shots have an inherent normalizing effect. Furthermore, if the absolute thickness of the armor is less than the diameter of the shot, the stress on the armor plate will be concentrated in a narrow band around the impact area, which will increase the risk of plate failure.

    One significant advantage of sloped armor is that it's simply easier to manufacture a thin armor plate than a thick one. With the rapid increases in armor plate thicknesses during the Second World War, sloping the armor was largely a necessity to feasibly acheive the level of protection needed.
     
    Last edited: Mar 25, 2017
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  15. BruceAlmighty

    BruceAlmighty New Member

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    Thanks Christian, but where you state "As Otto hints above, a sloped armor plate will have to be larger to cover the same vertical high" that doesn't take into account the horizontal face as well. Image a set of steps - you have the tread and the riser. Less material would be needed if instead of steps you had a straight line slide from top to bottom (I wish I was able to draw a diagram here...). To me it seems simple. To me the Hetzer (and of course the T-34) is the epitome of what I'm talking about...rather than stepping the sides with vertical and horizontal they just have slopes. That way they are using less metal (plus more room inside for that matter).
     
  16. Christian Ankerstjerne

    Christian Ankerstjerne Member

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    I know it might seem counter-intuitive, but the volume really is exactly the same for any armor plate with a given relative thickness that covers a specifict height and width (again, all other things being equal).


    Consider this example: You have an armor plate covering a height and width of 100 centimeters, and a thickness of one centimeter. This armor plate has a thickness of (100 * 100 * 1) = 10 000 cubic centimeters, and a relative thickness of one centimeter.

    Now imagine that you want to have an armor plate at 45 degrees instead, but still covering a height and width of 100 centimeters, and with a relative thickness of one centimeter.

    The absolute thickness of the plate can be reduced to (1 * sin(45)) = 0.707... centimeters. The width of the plate is not changed, but the height has to be increased to cover the vertical distance of 100 centimeters. The question is how much.

    Because we chose an angle of 45 degrees, and because the original vertical plate was necessarily perpendicular to the horizontal plane, the plate, the horizontal, and the vertical plane forms a right, isosceles triangle. We know that the two catheti (the height and the distance the plate extends outwards horizontally) are 100 centimeters each. Using Pythagora's theorem, we can calculate the length of the hypotenuse (the plate) as (sqrt(100^2 + 100^2)) = 141.421...

    We can now calculate the volume as (0.707... * 100 * 141.421...) = 10 000 cubic centimeters. Exactly the same as the vertical plate.


    Alternatively, consider this thought experiment: Rather than sloping the armor plate, ignoring such silly things as practicalities, you slice it into slices that are each one molecule thick. Starting from the bottom, you then slide each slice back one molecule more than the previous one. The end result will be sloped 45 degrees (there are no steps - being one molecule thick means that there is no difference to a sloped plate), and will have both the same relative thickness (we never shortened any of the slices) and the same volume (it's the same mass) as the original.
     
  17. BruceAlmighty

    BruceAlmighty New Member

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    Hey this discussion is carrying on a bit long, but I'm happy to continue this if you are :).

    I'm a bit lost when you're talking about 'volume'....are you meaning volume of metal in the plate/s? Was that in response to my suggestion there would be "less metal" in my last sentence? In which case I do see your point, but you didn't include the volume of the horizontal plate. Having sloped armour (the hypotenuse) replaces the vertical AND the horizontal (or have I misunderstood you?).

    And I'm glad you brought good old Pythagoras in because I'd forgotten to use that!

    Ok, so all I'm saying is that the length of the hypotenuse of a right angled triangle is shorter than the combined length of the base and height. So...therefore the area of a sloped surface would be less than the combined area of the vertical and horizontal surface. Therefore, less weight (assuming same thickness).

    Get me?

    Hmm...and I'm still struggling with that thought experiment of yours.....o_O
     
  18. BruceAlmighty

    BruceAlmighty New Member

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    Ah.....reading again your earlier reply yes you have neglected to include the horizontal surface.
    Having said that I do admit that the armour of horizontal surfaces wouldn't be very thick on tanks, hence it may not be worth including.
     
  19. Christian Ankerstjerne

    Christian Ankerstjerne Member

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    I understand your objection now.

    My calculation assumes a free-standing armor plate. This is because adding additional armor plates complicates the calculation considerably, without little real effect

    Consider a cube with a side of 1 (drawing 1), a surface area of 6, and a volume of 1. Now, if we slope the upper half of one of the sides (drawing 2), and maintain the relative thickness of that upper half side, it is clear that the amount of steel used is decreased (by one-eight, i.e., 0.75; half of one side and an eight each of two other). In this scenario, however, the internal volume is also decreased (also by one-eighth, i.e., 0.125).

    To maintain the external volume, the top, front, rear, and bottom would have to be extended. In fact, to re-gain the lost one-eight of the volume, the sides would have to be extended by 0.125 in one direction, which would increase the surface area back by 0.5, resulting in a net decrease by only 0.25, or one-twentyfourth. If the front is also sloped (drawing 3), the volume would be further decreased, and the sides would have to be extended further outwards to maintain the volume.

    Whether such a change would increase or decrease the total amount of steel used depends on the thicknesses of each plate, since it is rare to have the same amount of armor all-round. As such, it is not really meaningful to make any detailed calculations, since these can always be disputed. I hypothesize that simple changes will have a minimal, if any, effect (it is mathematically possible to construct a structure with a finite internal volume and an infinite surface area, but this would be impractical for a physical object).

    Another consideration is that, in order to maintain the internal volume, the width would have to be increased, and the internal layout would be more difficult to fully utilize. Whether a potentially fairly small weight saving is worth the hassel depends a lot of the other design considerations.
     

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    Last edited: Mar 29, 2017
  20. harolds

    harolds Member

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    I've been away from my computer for a while, but remembered something I read on this topic. It seems that the Germans operating the Mk IV would turn their tanks so that the front corner of the tank faced the most likely threat. That gave the opposing gun a slanted surface to penetrate. Later in the war, as allied tank and AT guns were improved, Tiger I crews did the same thing.
     

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