Vulnerability of IS-2 Turret Front/Mantlet/Cupola

[rexford]

The subject topic has been discussed quite a bit recently, so I did a ballistic analysis for the probability of hitting the turret front/mantlet, and what the resulting impact angle would be.

I assumed that 75L48 APCBC is being fired, with a 65% chance of putting a round within a 2m x 2m box. Important results follow:

A.

6% of hits land on turret front/mantlet/cupola that is curved in one direction with impact angle of 56 to 90 deg.

6% land with 42 to 56 deg. angle

6% strike at 30 to 42 deg.

6% land at 19 to 30 deg.

6% land at 10 to 19 deg.

6% land at angle below 10 deg.

Curved in one direction means that the turret front areas that have vertical and side slopes are assumed to be too resistant for 75L48. This is the edge of the turret front where it starts to curve towards the turret side.

B.

About 36% of the hits land on the turret front/mantlet and cupola areas that are curved in one direction ("potentially vulnerable"). Turret sides and top that are seen from front view are not vulnerable for analysis purposes.

Although the "potentially vulnerable" turret area/cupola is only 18% of total turret/hull area, about 36% of hits land in the "potentially vulnerable" area. This is due to way that shots are distributed. Most of the scatter is vertical and the mantlet/turret front is close to the aim point where a high percentage of shots land, and the narrow width of hittable turret front is a very insignificant factor.

C.

If cupola and turret front/mantlet are 100mm thick, and range is such that 75L48 needs a hit within 30 deg. impact to have a chance to penetrate, 18% of the hits would hit armor with resistance equal to or less than penetration when target armor is facing directly at firing weapon.

Boring technickel Deetales follow:

D.

The turret front/cupola area equals 17.2 square feet.

Hull front area is 37.5 square feet (area below hull bottom is assumed to be blocked by earth banks)

However, potential penetrating hits are limited to cupola and a 3.2' width of turret front which includes the mantlet (eliminates turret front where it is curved in two directions). Turret front/cupola area that might be penetrated ("potentially vulnerable") is 8.6 square feet.

Turret front/mantlet and cupola that might be penetrated is about 18% of total area.

E.

65% of shots hit tank when aim is at intersection of turret and hull (common German practice that simplifies estimates). 29% of shots hit turret/mantlet/cupola, 36% hit hull front.

F.

30% of hits strike turret front/mantlet that is curved in vertical plane ("potentially vulnerable"), and 6% hit cupola. This is close to one-third factor that CMBB supposedly uses for chance that turret will be hit.

[ October 31, 2002, 01:20 PM: Message edited by: rexford ]

[Andrew H.]

Thanks Rexford, that's helpful as always. Let me summarize your post to make sure that I understand it, though.

(1) WRT the IS-2, BFC's abstractions [as we understand them] for determining whether the hull or turret is hit work relatively well.

(2) If an AFV sporting a 75L/48 does hit the turret of an IS-2, there is approximately an 18% chance of penetration, assuming an approximately straight-on shot from a range where the 75L/48 can penetrate 100 mm. (Out to about 750 meters, IIRC).

[Larsen]

Originally posted by rexford:

E.

65% of shots hit tank when aim is at intersection of turret and hull (common German practice that simplifies estimates). 29% of shots hit turret/mantlet/cupola, 36% hit hull front.

F.

30% of hits strike turret front/mantlet that is curved in vertical plane ("potentially vulnerable"), and 6% hit cupola. This is close to one-third factor that CMBB supposedly uses for chance that turret will be hit.

I'm not sure I understand. There is a 65% chance to hit the target. Right? Now, out of those 65% 29% hit the turret. Out of those 29% only 18% will penetrate. If I understood correctly that means that out of 65% only about 5.2% will get a turret penetration. Right? Or am I missing something?

[Mark Gallear]

Not claim this is gospel but I was messing about with one of he scenarios to check a mod. It was a large number of Tiger IIs attacking a town wiith one or more JSIIs. I attacked the JS2 from very short range and drove a King II right up to it and noticed numerous hits from the 88mm on the front but all bounced off. The JSII managed to kill at least 2 Tigers IIs.

The impression I got was that the 88mmL71, (a better gun then any German 75mm) could not pentate the fromt of JS2, and my understanding from battle accounts was that it could.

[rexford]

Using shot distribution tables for the case where 75L48 APCBC where gun has 65% chance of hitting a 2m x 2m target, and rolling dice against a 1/72 scale model, following results obtained against IS-2 front which is aimed directly at firing gun:

59% hit probability (274 hits in 464 shot rolls)

17% of hits strike inpenetratable turret side or top armor

57% land on hull

20% hit turret front/mantlet/cupola with impact angle under 43 deg.

16% land on turret front/mantlet/cupola with angle of 30 deg or less

6% strike turret front/mantlet/cupola with impact angle 43 deg or greater

Above figures slightly different from calculations due to use of PST 1/72 scale model for dice rolls and measured shot location relative to aim point, which differs from average figures used in computations.

So 16% of 75L48 hits that can penetrate 100mm of armor with an impact angle of 30 deg will defeat turret front/mantlet/cupola armor.

Since hit probability is about 60% in above case, this means that roughly 10% of the 75L48 shots will penetrate IS-2 front when gun has 60% hit probability and penetration allows shell to defeat 100mm x 30 deg slope effect x 0.90 quality multiplier (about 113mm penetration for 75L48 at 0 deg).

[rexford]

Originally posted by Mark Gallear:

Not claim this is gospel but I was messing about with one of he scenarios to check a mod. It was a large number of Tiger IIs attacking a town wiith one or more JSIIs. I attacked the JS2 from very short range and drove a King II right up to it and noticed numerous hits from the 88mm on the front but all bounced off. The JSII managed to kill at least 2 Tigers IIs.

The impression I got was that the 88mmL71, (a better gun then any German 75mm) could not pentate the fromt of JS2, and my understanding from battle accounts was that it could.

88L71 penetrated IS-2 Model 1944 turret front/mantlet/cupola and lower hull nose (127mm at 30 deg). Sounds like bad rolls.

[Andrew H.]

Mark: that's a fun scenario; you should play the whole thing. The 88 L/71 will usually penetrate the front of the IS-2 at close range, but sometimes it doesn't. I don't know whether the Tiger II's gun can always penetrate the glacis of the IS-2 from straight on (120mm @ 60 degrees), but I know it's close enough that if there is enough obliquity in the shot the shell won't penetrate.

[Lt Bull]

Redford,

I have been thinking of undertaking a similar analyisis of modelling projectile distribution at a target.

To come up with any probabilty for hitting a particular target, you must assume a probability distribution for the "scatter" of the rounds. I would expect, and you have asssumed, that the scatter would not be the same in the vertical plane as it is in the horizontal plane.

What did you assume for the relative scatter of shots in the vertical plane as compared to the horizontal plane? Your general assumption that that a 75L48 APCBC is being fired, with a 65% chance of putting a round within a 2m x 2m box does not refer to this key bit of information. Where did that assumption come form and at what range does that data apply?

Lt Bull

[ October 31, 2002, 09:29 PM: Message edited by: Lt Bull ]

[rexford]

Originally posted by Lt Bull:

Redford,

I have been thinking of undertaking a similar analyisis of modelling projectile distribution at a target.

To come up with any probabilty for hitting a particular target, you must assume a probability distribution for the "scatter" of the rounds. I would expect, and you have asssumed, that the scatter would not be the same in the vertical plane as it is in the horizontal plane.

What did you assume for the relative scatter of shots in the vertical plane as compared to the horizontal plane? Your general assumption that that a 75L48 APCBC is being fired, with a 65% chance of putting a round within a 2m x 2m box does not refer to this key bit of information. Where did that assumption come form and at what range does that data apply?

Lt Bull

We have German ballistic tables for vertical and lateral scatter based on constant aim. Then one cranks into the scatter spread due to range estimation errors.

For the 75L48 against IS-2 front, I assumed 65% chance that vertical scatter will fall within 1m of aim point, and 90% chance that lateral scatter will fall within 1m of aim point. Overall probability of landing shot with 2m x 2m is actually 59%.

For our wargames we estimated scatter probabilities for first, second, third and following shots based on ability to put shots within the 2m x 2m box. Separate probabilities for vertical and lateral scatter, since range estimation errors make vertical scatter much larger.

If you e-mail me directly and my scanner works I'll send you some of our tables. One table has shot scatter from aim point as a function of scatter probability within 2m x 2m box and dice roll with two decimal dice (1 through 0 on each dice).

Interesting thing is that assuming 25% average range estimation error on the first shot and test based lateral scatter grossly overestimates hit probability at close and medium ranges. According to the ballistic model, being 25% off on range at 200m should never miss, however, actual combat results show misses do occur, particularly when gunners are scared, tired or inexperienced. A book I read had a Jagdpanther 88L71 miss a stationary Sherman at close range, the math says this would never happen but a "straight out of gunnery school" novice missed.

The bottom line is that people act like people on the battlefield and do not follow ideal statistical trends, which is something BTS has been saying for a long time.

P.S. In the Unforgiven, when Clint Eastwood faces off against a room full of "bad guys" at the end, most of them hold their guns out and shoot without taking careful aim. S.L.A. Marshall did studies where about 25% of infantry fired their weapons. Getting people to accurately fire is alot different from what some wargames assume.

[Lt Bull]

OK. Will do.

I have been interested in modelling ballistics mathematics and statistical accuracy since woking on a computer model of it for a university project.

Lt Bull

[rexford]

The width of Russian tank mantlets is generally narrow, but so is the lateral dispersion of certain German ammunition:

88L56 APCBC

500m

68% of lateral scatter within 0.11m of aim point

1000m

68% within 0.16m of aim point

1500m

68% within 0.23m of aim point

----------------------------------------

75L48 APCBC

500m

68% within 0.17m of aim point

1000m

68% within 0.40m of aim point

1500m

68% within 0.68m of aim point

------------------------------------

88L71 APCBC

500m

68% within 0.12m of aim point

1000m

68% within 0.21m of aim point

1500m

68% within 0.38m of aim point

----------------------------------------

Above figures based on scatter of shots when aim is fixed, and they come from German WWII documents. Same documents suggest that above figures be doubled for combat results where gun sights may be slightly out of adjustment, gunner is under pressure and does not aim at center of target mass, target center of mass is indistinct, etc.

IS-2 turret mantlet and turret front that is curved in one plane to directly facing gun is about 1m wide, or 0.5m to left or right. Call this width the "vulnerable area".

At 1000m, Tiger E round that has vertical trajectory landing on IS-2 "vulnerable area" will have almost all of the rounds (88%) scatter laterally within 0.5m of aim point and hit that area (with doubled scatter).

At 1000m, 75L48 round with vertical trajectory falling on IS-2 "vulnerable area" have 47% of shots scatter laterally within 0.5m of aim and land on area (with doubled dispersion).

Guns like 88L56 not only have good penetration, but possess exceptional scatter characteristics which make a large difference on the battlefield and result in added effectiveness against turret fronts with narrow vulnerable areas.

British firing tests with Tiger 88L56 show gun puts shots within a narrow band (when Tiger aims at an ATG or the turret/hull intersection, it has good chance of hitting it).

[ November 02, 2002, 07:11 PM: Message edited by: rexford ]

[Andreas]

Bump - very interesting