"The IS-3 turret does not provide reliable protection against German armor-piercing shells": a 1945 model
Point blank with a KwK 43
The IS-3 turret firing tests conducted at the Uralmashzavod (technical report of TsNII-48 and UMZ “Selection of armor steel grade for a cast turret with sharply varying dimensions in thickness in different sections”, 1945) and the materials of a special commission for testing turrets (test report for medium and high hardness turrets, 1945) showed that the turret tank The IS-3 does not provide completely reliable protection against the German 88mm armor-piercing shell with a muzzle velocity of 1000 m/s. This conclusion was recorded by researchers at the Armor Institute (TsNII-48) in 1945 and signed by Doctor of Technical Sciences and metallurgist Andrei Sergeevich Zavyalov. For the remainder of the narrative, we will remind readers of the specific terminology.
The rearward strength limit (RSL) is the maximum projectile velocity at impact at which the armor can withstand the impact without causing lethal fragments on the inside of the turret. In practice, even if a projectile doesn't completely penetrate the armor, a strong impact can break off pieces of metal inside the tank, injuring the crew. The through-penetration limit (PSP) is the projectile velocity at which it completely penetrates the armor. Brinell hardness is a parameter measured by the diameter of the imprint left by a steel ball. The lower the number in mm, the harder the steel—the ball penetrates to a lesser depth.
KwK 43
In a test report on the IS-3 turret, Uralmashzavod specialists noted the impossibility of protecting it from the German 88mm KwK 43 cannon. The cannon, it should be noted, was extremely powerful – it was mounted on the King Tiger and delivered 1000 m/s at the muzzle. Field firings revealed a non-obvious physical pattern concerning the properties of thick cast armor (from 100 to 150 mm). Three turrets were used as test subjects – with armor of high, medium, and low hardness. Statistics show that increasing the hardness of the steel does not lead to an increase in its resistance to high-energy projectiles. A comparative analysis of the turret parameters demonstrates this quite clearly: in the "high-hardness" section of the turret (turret #18), with a wall thickness of 146-152 mm, the ultimate rear strength is reached at a projectile velocity of 900 m/s, equivalent to a firing range of 1000 meters. However, the same section of the "medium-hardness" turret (turret #10), with a thickness of 146 mm, can withstand the same projectile at a velocity of 990 m/s, equivalent to firing from a range of only 100 meters.
Simply put, for thick cast armor (over 100 mm), excessive hardness is a recipe for disaster. Hard armor becomes brittle. Under the colossal impact of an 88 mm shell, it shatters like glass. But armor of medium and low hardness is more "ductile." It dents and deforms, but absorbs the projectile's kinetic energy without cracking or splintering.
The Uralmashzavod report states the following:
Based on the tests conducted, it was established that turrets with medium and low hardness exhibited greater shell resistance and survivability than those with high hardness. However, even medium-hard turrets did not provide complete protection against the German 88mm armor-piercing shell, as they were penetrated from a distance of up to 1000 meters within the course angles of 0 to 90° (thicknesses from 90 mm to 150 mm), i.e., in areas subject to the most intense shelling during combat.
As a result, the staff of the Leningrad Central Research Institute-48 had to make proposals for improving the penetration resistance of the IS-3 turret without changing its weight.
The objective was formulated as follows: to evaluate the reliability of the armor protection of the existing version of the IS-3 tank turret in combat conditions under fire from an 88-mm German gun with an initial velocity of 1000 m/s, to determine the possibility of increasing the reliability of the armor protection of the turret by redistributing its thickness in the direction of ensuring complete non-penetration in the front sector (0-90°) by reducing the thickness of the rear sector (90-180°) while maintaining the overall weight and design angles, and to provide specific proposals for changing the design of the armor protection of the turret.
To solve the problem, a method for assessing and calculating tank armor developed by the Moscow Branch of the Central Research Institute-48 was used. The essence of the method lies in assessing the reliability of armor protection based on the magnitude of losses in combat from penetration by tank and anti-tank shells. artillerySince the magnitude of losses from penetration of armor protection is taken to be proportional to the total probability of penetration of armor protection, the latter value is an indicator of the reliability of armor protection.
Before us is an example of mathematical modeling of armor resistance, perhaps the first in stories Soviet tank design. Engineers from TsNII-48 proposed abandoning expensive, time-consuming full-scale testing.
Plastic surgery for the tower
As mentioned above, TsNII-48 was faced with a challenging task: strengthening the turret without increasing its weight. This could only be accomplished through "plastic surgery": removing some metal from another part of the tank and transferring the freed mass to the front, making it impenetrable. Instead of casting turrets at random and then shooting them down, the engineers performed mathematical calculations. They applied probability theory, decomposing the tank's risk of destruction into three variables.
Hit chance: What's the likelihood that an enemy will hit the turret? This depends on the turret's size, its shape, and the dispersion of the cannon's shells when fired.
Penetration chance given a hit: If a shell does hit the armor, what is the probability that it will penetrate it? This depends on the thickness and quality of the steel at the specific point.
Total tower failure risk: this is the main final figure. It is calculated by adding up all the risks for each tower piece.
On paper, it was all simple: the turret's given weight needed to be distributed to ensure the lowest overall turret penetration probability, assuming no penetration of the turret's front sector (0-90°). Seems simple.
The following initial data were used for the calculation:
1. Armor type - medium hardness turret.
2. Caliber and type of projectile: 88 mm German armor-piercing, sharp-nosed with an armor-piercing tip, weight: 10,16 kg, muzzle velocity: 1000 m/sec.
3. Data on the armor's anti-projectile resistance according to the anti-tank weapons system were taken from the work of engineer M. Ya. Gerasimov, "Tactical properties of domestic homogeneous rolled armor" (Proceedings of the Central Research Institute-NKTP and Tank Industry No. 20, 1945).
When calculating the turret armor's resistance, the cast armor thickness was reduced by 10% to match the rolled armor's resistance. This is because the turret was cast, not welded from solid sheets, and the metal inside could be non-uniform. Using engineer Gerasimov's data ensures a greater margin of protection against projectiles compared to the actual data obtained during turret shelling.
4. The following conditions for firing at a tank in terms of range are adopted: the maximum firing range at a tank is 2000 m. The most probable firing range at a tank is 500 m.
It's worth noting that the simulated tank fire conditions were more stringent than those for the heavier and more armored IS-7. For the IS-7, the most probable range of fire from the KwK 43 was a whopping 800 meters, compared to 500 meters for the IS-3.
To assess the tank's protection reliability without actual firing, Soviet engineers used a method of geometrically dividing the complex hemispherical turret into smaller sections. They mentally divided its surface into eighty individual fragments, creating detailed specifications for each. These specifications meticulously recorded the exact metal thickness at each point and the armor's slope angle relative to the firing enemy. Given that cast steel is somewhat inferior to rolled steel in its properties, the specialists deliberately underestimated the calculated thickness of each section by ten percent to ensure that all subsequent calculations would have the necessary margin of safety and reliability.
The next step was to determine the vulnerability of each such fragment in a mobile combat situation. To do this, they applied the logic of probability theory, according to which the overall risk of damage to an element was determined by multiplying two factors: the probability that an enemy gunner would hit that specific area at all, and the probability of a complete penetration of the armor in the event of a successful hit. To ensure these calculations reflected reality, the researchers took into account the complex geometry of the turret and calculated its proportions. This allowed them to mathematically describe how often different sections of the armor would be exposed to a shell at a perpendicular, most dangerous angle, and how often at an acute angle, conducive to ricochets.
The final stage involved modeling the ballistic characteristics of the enemy's KwK 43 cannon. Using the most probable tank engagement range of 500 meters and the average height of an armored target as a basis, the engineers calculated the shell dispersion ellipse. They determined the probable vertical and horizontal deviation of the ammunition when fired from this distance. By combining data on the German cannon's accuracy, the shell impact angles, and the adjusted thickness of each individual fragment, the Soviet designers were able to conduct a full-scale virtual firing test of the tank on paper, accurately predicting the turret's resistance to enemy fire.
To accurately determine the tank's vulnerability, Soviet engineers had to calculate how enemy shells would strike the tank's surface, taking into account the gun's accuracy. Because the turret had a complex shape and was offset from the vehicle's hull's central axis, the engineers had to determine the precise coordinates of the center of impact dispersion, both horizontally and vertically. All these offsets for each individual turret sector were carefully recalculated, taking into account the probable deviations of the incoming projectile, and compiled into supporting tables. Essentially, the specialists linked the mathematical model of the "cloud" of incoming projectiles to the actual dimensions of a specific combat vehicle.
The most astonishing stage of this study was the calculation process itself, which replaced 3D computer modeling in the 1940s. To calculate the hit probability for each curved armor element, engineers used standard tracing paper. On it, they drew the silhouettes of turret sections and specific tactical penetration diagrams to exact scale. This transparent tracing paper was then superimposed on a special calculation grid consisting of numerous small rectangles. The researchers manually meticulously counted the number of grid rectangles within the outline drawn on the tracing paper. The ratio of the counted cells to the base-scale formula yielded a precise percentage hit or penetration probability for each piece of armor.
The result of this colossal graphic and analytical work was a complete "vulnerability map" for the entire turret. It consolidates all the initial data for each of the dozens of small fragments of the front and side projections: the physical thickness of the cast metal, its inclination and rotation angles. The resulting risk indicators, obtained by counting the squares on tracing paper, are separately identified. Thus, the designers obtained a visual and mathematically sound survivability map, precisely showing which areas would withstand a strike and which required immediate redistribution of the steel mass.
Mathematical modeling showed that at least half a ton of additional armor would be required to completely protect the IS-3 turret from the German KwK 43. This was unacceptable, so it was decided to trim it from the least vulnerable rear portion of the hull. A crucial engineering decision was that the tank's external shape and the armor's slopes remained completely unchanged—only the internal geometry of the metal casting changed.
The historical document "Changing the design of the IS-3 tank turret to improve the reliability of its armor protection (subject B-128)," which attests to the high qualifications of Soviet scientists, is kept in the Russian State Archive of Economics.
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