Greetings! This week we will continue discussing the subject you voted for the most: Ballistic and Ammunition system overhaul. In the last developer blog (#46) we explained the various shells that cannons and tank guns will use. Today we will discuss armour types and their ability to resist penetration.
It is important to note that the same ballistics calculations are used for both small arms and large projectiles. This includes information such as: Shell Type, Caliber, Speed, Trajectory, Penetration, and Damage. Likewise, the armour calculations include Armour Slope (or angle) and Thickness. In the future, Armour Type (RHA, FHA, Cast, High Hardness) and Armour Quality (flaws) may be included as well. (Depending on the results of internal testing.)
Now, the game may use the familiar UI colors to indicate if a shell will successfully penetrate armour (Green = Yes, Yellow = Maybe and Red = No), but we have implemented major changes to the calculations and formulas happening behind the scenes. To help demonstrate results of these changes, our game mechanic engineer has created a web based armour resistance calculator specifically for GoH. We will cover this in more detail at the end of this dev blog.
The Slope Effect
All armour plates have a specific amount of physical thickness, often measured in millimeters. For example, the front plate of a Soviet T-34 medium tank is 45mm thick. The armour plate is also sloped at angle of 60° from the vertical. Because of the tilted armor, the relative thickness increases and projectiles must go through more steel to penetrate. In the case of the T-34, relative thickness is 90mm. 45mm / Cosine (60°) = 90mm.
While the equation is mathematically correct, it does not account for the effect that sloped armour has on various shell types. This effect often increases resistance above the relative armour thickness.
The “slope effect” is defined as: the multiplier that converts an angled impact penetration to an equivalent resistance at 0°. To paraphrase, the slope effect is a number (usually greater than 1) that is multiplied by the physical thickness, to give the real penetration resistance as if the armour plate had no slope. Thickness x Slope Effect = Armour Resistance @ 0°. Since all of the penetration data we use for guns is at 0°, we also want armour effectiveness data at 0°. This may be difficult to understand so we will use the T-34 example again.
Lets use an example of the T-34 front plate versus a German 75mm APCBC shell. After running slope effect formula we get a value of 2.7. If we multiply this by the armour plate thickness we get a equivalent resistance of ~122mm at 0°. 45mm x 2.7 = ~122mm. Meaning, the T-34 front plate that is 45mm thick and with a 60° slope resists penetration from the 75mm APCBC shell like a 122mm thick armour plate would with a 0° slope.
This is assuming that the T-34 plate is rolled homogeneous armour (RHA), is perfect quality (no flaws) and has the Brinell Hardness Number (BHN) of 240. We will discuss these armour types and factors next.
*BHN 240 is the hardness of the U.S. Test plate used in ballistic testing in the 1940’s. Penetration data has also been converted to how it would perform against standard (RHA) US Test plate, allowing direct comparisons between nations. (Each nation had its own methods for calculating penetration data and it is important that they be converted to a common standard.)
There were several types of armour that World War II Vehicles could be made from. Each nation had its own methods of manufacturing that affected how well the armour resisted penetration.
Rolled Homogeneous Armour (RHA)
(RHA) is cast steel that is hot rolled to a desired thickness. The rolling creates structural changes that help disperse stress loads throughout the plate. It was produced by all nations and is the most common armour type. In game, this is the default armour type and all penetration equations are set with RHA in mind.
Cast Armour is the most basic form of steel armour. Firing test results from several nations indicate that cast armour is less resistant than rolled armor due to beneficial structural changes that take place when armour is rolled.
Face Hardened Armour (FHA)
FHA has a thin outer layer of 450-650 BHN hardness on top of a standard rolled plate. FHA will be the most difficult to implement in game since there is no equation to easily account for its effects against various shell types. (It seems to vary greatly from each shell size and type.) For this reason it is not included in the armour calculator.
High Hardness Rolled Homogeneous Armour
It is best to consider High Hardness armour as a “Type” of armour for our purposes. Armour with a hardness below 375 BHN is expected to have reasonable impact resistance, allowing it to withstand multiple hits without cracking or spalling. As hardness increases above 375 BHN, it becomes brittle and loses penetration resistance when the diameter of projectile (caliber) is equal or greater than the physical thickness of the armour plate. High Hardness is a common feature for Soviet vehicles except for the KV-1 and KV-2.
If we apply the High Hardness multiplier to the previous T-34 example, it would reduces the armour effectives from ~122mm to ~94mm against the German 75mm APCBC shell. (Assuming a BHN value of 450.) This matches German field reports describing the ranges at which T-34s could be defeated by 75mm guns.
As the war progressed, important materials for armour production were rationed which would affect armour quality and penetration resistance. Also unskilled labor and poor quality control contributed to armour quality in certain situations. An example would be, American quality control permitted flawed armor in many tanks prior to October 1943, including Sherman front plates. It is possible the British underwent major quality control improvements around the same time. Many German Panther and Tiger II tanks suffered from medium to large flaws in their armour (especially Panther Glacis) which contributed to lower than expected penetration resistance.
Again, testing still needs to be done to determine which or if any of the armor types and factors will be in the finished game.
Armor Effectiveness Calculator
As mentioned before, our game mechanic engineer has created a web based calculator with all the formulas for these armour mechanics built in. You will notice that the default values are from the T-34 example. Feel free to change the various inputs to see the armour resistance change against the various shell types.
While there are many sources available on the topic, none are quite as comprehensive as “World War II Ballistics: Armor and Gunnery” by Lorring Rexford Bird and Robert D. Livingston. The full equations for slope, armour and flaw multipliers can be found here, and the converted penetration data. Any formation that cannot be found in the WWII Ballistics book, have been collected from various websites including but not limited to: panzerworld.com , battlefield.ru , wwiivehicles.com , gva.freeweb.hu … etc.
We believe that the redesign of ballistics and armour will make our game unique in the RTS market. We also aim to make the whole system second nature for our players. While we pursue historic authenticity, we do not want it to sacrifice fun and competitive gameplay. No one shall be left behind!