Ballistic Protection Value of Sloped Armor

I am new to this forum and am posting this at the request of one on another board, and to seek the assistance of anyone out there that might be able to assist me find a lost document written by Richard M. Ogorkiewicz.

There seems to be a common misunderstanding about how sloped armor increases the defensive capability of the armor. I have seen many people comment and attribute the value of sloped armor only in its increased thickness at the horizontal. This holds true if you are dealing with a shaped charge but is grossly in error when involving AP weapons and the ballistics involved.

“...Another method is to slope the armor away from the vertical. This increases its effectiveness against high-velocity projectiles, at first in direct proportion to the increased horizontal thickness of metal which the projectile has to penetrate and then more rapidly because of the uneven stresses set up in a projectile when it strikes a plate at an angle: with a slope of about 50 degrees to the vertical the plate is approximately twice as effective against horizontal attack as a vertical plate which offers the same thickness in the horizontal direction”.
– Richard M. Ogorkiewicz “Armor”.

As I understand this, if we take a 1” plate and slope it so that the horizontal thickness is 2”, then this 1” thick plate will have approximately the same defensive value as a horizontal plate with no slope that is 4” thick.

Ogorkiewicz later graphed the ballistic defensive value of sloped armor, an article I once had, but I have lost it over the years. Presently I am ordering a number of his works in an attempt to find the graph but thought that someone on this board might be able to save me some time and expense if they have a copy they might share or could direct me to another source.

To give you an idea of the significance of this consider what I have seen as a hot topic on this board. “The T-34” - had 47mm of frontal armor slopes at 60 degrees. I remember the defensive multiplier for this value simply because I used it so much in my calculations. The multiplier was 3.6, which gives the T-34 an “effective” hull armor thickness of 169mm of armor from the front.

Historians and historical simulation designers have largely ignored these ballistic modifiers for years. Does anyone out there have some good source material on ballistic defensive value of sloped armor?

 

The effect a slope has on a ballistic is far too random to give it a set group of numbers to represent its effectiveness. If the T-34 did indeed have 169mm of effective plate, it wouldn't be holed as easily as it was. In 1943, the average life span of a T-34 upon reaching the front was 20 minutes. While, indeed, a slope would have a great effect on a shell launched from great distances, it didn't help the T-34 so much after the German amour was modernized.

On the other hand, the Sherman 76(w) Glacis (which had a slight less slant) was found to be all but impregnable to the standard 7.5cm KwK 40 L/48 armor piercing shell at standard combat ranges.

 

Oh Please!!!!!!!!

Those are some pretty far out claims.

Since the slope effect has been calculated by experts, one of whom I have quoted that I am familiar with, I would be interested to know the sources you quote when you say it is irrelevant.

As for your statements on the superior armor of the Sherman, again I would be interested in your source material.

As for the life expectancy of the T-34 in 1943, the Battle of Kursk would seem to refute your claim.

 

I am not saying sloped armor is irrelevant, I am saying that the effectivness of sloped armor is too random to put a definate numericle value on, just like how writen penetrations arn't always going to be correct in every situation. Angle of attack and material differences have to be taken into account. The same holds true for armor.

Russian view on Sherman vs. T-34
1. Sherman armor was softer than T34, so ricochets off T34 could severely injure crew while internal fragments were less frequent with Sherman (Sherman used 260 Brinell Hardness armor while T34 used 440 Brinell)
2. T34 had additives to HE rounds so they packed extra explosive punch, but this gave them a greater chance of exploding after tank was penetrated. Loza points out that T34 crew could be killed outside the tank, while they ran, due to the power of the exploding HE ammo. With Shermans this was less of a problem (47 degree glacis Shermans would have wet stowage for ammo, which decreases chance ammo explodes).
3. Sherman 75mm gun outpenetrates T34 76.2mm, and Sherman 76mm outpenetrates T34/85 85mm weapon.
4. Sherman has three man turret, T34 has two in turret, making Sherman more efficient.
5. U.S. 75mm HE rounds are very effective, U.S. 76mm HE has small HE burster and is inferior to 85mm HE. 6. 2.5" American armor at 47 degrees from vertical on M4A2 Sherman glacis is more effective than T34 45mm (high hardness plate) at 60 degrees from vertical versus 75mm and 88mm ammo.Germans found that PzKpfw IVH 75mm APCBC could not penetrate U.S. 2.5" plate/47 degrees at 1000m, but it could hole T34 glacis.

The T-34-85 in NKPA service had very limited success in Korea, & that succsess was vs 2.36 in Bazook's & 57mm AT guns initialy Ie, the NKPA 107th Tank Regt vs the ROK 7th Inf Div on the Yonchon-Seoul road. & Ie, the 107th vs Task Force Smith in July. The 2.36in bazooka performed absmyly against the T-34-85. This was rectified when US frces recieved the 3.5in Bazooka.

The one & only real T-34-85 success, vs US tanks, was vs M24 Chaffee's, from Co A, 76th Tank Bn, @ Chonjui, who lost 12 out of 14 M24 in the 2 week fighting. As the 75mm on the Chafee was incapable of defeating the T-34-85 frontaly.

Once the M4A3E8 Sherman & M26 Pershing arrived the tables turned, as both were more then a match, for the T-34-85 which earned the nicname; 'caviar can" with US tank crews, due to its vulnerability to both the 76mm & 90mm guns frontaly at all combat ranges.

The T-34-85 was an even match for the M4A3E8 as both tanks could kill each other frontaly at standard ranges. Lend lease Sherman advantages over the T-34-85 as identified by the Soviets concerning the M4A2 76mm during WW2 were:

- Turret traverse speed.
- Optics.
- Ammo stowage protection.
-*76mm ammunition was superior in penetration capability.
- Mechanichly more reliable.

*In Soviet live fire tests vs Tiger II @ Kubinka US "76 mm armor-piercing projectiles penetrated the "Tiger-B" tank's side plates at ranges 1.5 to 2 times greater then domestic 85 mm armor-piercing projectiles."

US inspections of KO'd T-34-85s in Korea found that 75% of the NKPA crews did not survive their tanks destruction compared to 18% in US tank crews. 97 T-34-85 were destroyed in tank vs tank actions, with another 18 claimed as probables by US forces vs losses of 34 US tanks of which 15 were total writeoffs.

And the Battle of Kursk was the only battle in all of 1943 I suppose?

 

Please take it somewhere else

Sir,

You cite hearsay and your opinion but offer no source material.

If that is all you have to offer, I will thank you to take it somewhere else.

 

And I suppose your information is any more valid than mine? My information comes from Soviet live fire tests against captured german equipment. Things like: http://www.battlefield.ru/library/books ... pons7.html

And books from authers who specialize in Armour of the WW2 era.

 

Welcome to the forum Greg Pitts.

I´m just beginning to grasp the concept of the defensive ballistic value of sloped armour, but I have a couple of other points I would like to add to the discussion.

You wrote that the T-34 had an “effective” hull armor thickness of 169mm of armor from the front. It also had a large hatch on its frontal armour plate, which could explain why it still could be knocked out from a distance of 1500m +. There´s also, as Danyel brings up, the issue of armour quality.

Also, would a slope of 10 to 20 degrees have any effect at all, except for the actual increased armour thickness ?

How sloped does an armour plate have to be before we can talk about a defensive ballistic value ?

And finally, wouldn´t different types of projectiles have different effects on the calculation of the defensive ballistic value of a sloped armour plate ?

 

Good points, I would say that ballistic cap AP rounds would be largely affected but the other types of rounds, like composite rigid AP rounds, would still do their job. This would explain a part of Danyel's claim that the T34's life expectancy in the field was 20 minutes, which I still mistrust.
Another reason for this point would lie in Soviet attck doctrine, as always, and the fact that there is no field where German technology was more ahead of anybody else than the anti-tank weapon.
However, if the T34's protective armour was something in the area of 169mm thick, then nothing in the German arsenal would have been able to take it out before the arrival of the 75mm L/70 and the 88mm L/71, if not from point-blank range or behind. I'm not saying that I believe the 20 minutes-claim but T34 were actuall taken out in quantities; how would you explain that?

Thanks for joining us here, Greg Pitts. The 'requester' of your first post was me.

 

Armor

Thank you Skua and Roel for your learned comments. What follows are excerpts from “Armor” by Richard M. Ogorkiewicz. For those interested in source material, it was Published by Arco Publishing Co., Inc. New York 1960, Library of Congress Catalogue Card No. 73-117345; S.B.N. 668-02334-I.

I hope these will answer some of your questions. What I have found is that the source creates more questions for me to research and find the answer to. You will find that certain ammunition types drastically affects penetration capability and why or why not these may have been available at certain times. I would still like to find a good source that shows what the ammunition type availability was for year and theater of the war, and what the standard distribution of these types were by tank and nationality.

Unfortunately, this source does not provide the graph on the ballistic protection value of sloped armor. This was my reason for beginning this post. Ogorkiewicz graphed this out in an article on Battlefield Survivability, which I have lost over the years. I will continue to attempt to find it again.

Still, the excerpts below give you some idea of some of the issues at hand on tank armament and armor and of the importance of taking ballistics into account on armor values. It is not simply a matter of effective thickness of armor or hardness. The impact angle of the shell has as great a part as the armor itself and has been a much-neglected topic for discussion.

So here we go...

On the eve of the Second World War typical guns were 37 – 47 mm with lengths of 32 to 52 calibres long with muzzle velocities of 2,000 to 2,800 fps. Armor penetration varied at 40 to 60mm at 500 yards with normal impact. Lacking was “shell-power”, which limited their ability to deal with ATG’s. Britain attempted to remedy this around 1930 with the adoption of close support tanks fitted with howitzers, representing up to 4% of the total tank strength. The attempt was unsuccessful as demonstrated in North Africa.

In 1926 to 1928, the Germans started with the 75mm L/24; the Soviets with their multi-turreted T-28’s and T-35’s. By the late thirties the Soviets caught up with the Germans by mounting larger guns of 30.5 and 41.5 calibres long on the KV and T-34 tanks. The Americans caught up in 1942 with the M4 and then the Germans regained a slight lead in 1942 with the 7.5cm L/48 gun.

An exception to the trend was Britain who continued to use smaller bore calibre guns as the primary tank weapon. It was only in 1943 that a gun similar to the American 75mm gun was adopted.

By 1943 the 75-76.2 mm gun formula of 1941 had outlived its usefulness and better guns were necessary to match new tank guns and thicker armor of opposing tanks. The German answer to this problem was the 75mm L/70 and the 88mm gun of the Tiger I. The Soviets followed suit rapidly with the 85mm gun on the KV-85 and then later on the T-34/85. In Britain and the U.S. little was done because of their doctrine that tanks were not made to fight other tanks. By 1944 this view changed dramatically and we find the 76mm gun and the 17pdr introduced. Right before the wars end, the M26 was introduced with its 90mm gun.

The Russians and Germans had already moved a step beyond this with the introduction of the 122mm gun on the Stalin and the 88 mm L/71 gun of the Tiger II. The Jagdtiger was also introduced with a 128mm gun which was the most powerful tank weapon used in WWII.

With the 120mm gun and velocities on the order of 3,000 fps the development of conventional tank guns and ammunition had just about reached the practical limit. The aim had been to increase greater armor penetration by achieving sufficient kinetic energy at the target by a combination of projectile weight and velocity, kinetic energy being one half the product of projectile mass and the square of its velocity.

Higher muzzle velocities necessitate disproportionate increases in gun barrel length, chamber pressure and weight of propellant charge, while barrel life decreases rapidly as muzzle velocity increases. Once this limit was reached with any given gun the only way to increase armor penetration was to either increase the gun calibre or depart from orthodox designs, particularly ammunition.

The introduction of APCBC ammunition (ballistic cap) came into use in 1942. A year earlier the Germans had introduced APCR or HVAP ammo. This was used with some success in 1941 but the disadvantage was its cost and its rapid loss of velocity with range. Still, the Soviets started use of this ammo type in 1942. In 1944 the Americans also introduced this ammo on their 76 and 90mm guns.

Earlier than this, work was done on tapered, or cone bores, originated in Germany by Gerlich. In 1941 the Germans introduced the first ATG gun of this type with a 28mm gun capable of firing a tungsten core shot at 4,600 fps! Two other guns, 42/30 and 75/55mm were also introduced but because of the shortage of tungsten the use of the ATG was limited and never applied to tanks. The only tank to use the cone bore was the British Tetrarch light tank of 1944.

The most practical ammunition was APDS ammo, which discarded its envelope or jacket after leaving the muzzle thereby reducing air resistance of the armor-piercing sub-calibre core in proportion to its cross-sectional area. The result is that the velocity and penetration of APDS falls off less rapidly with range than APCR or HVAP.

The first use of shaped charge shells from a tank was around 1942 with the German Pz IV and Sturmgeschutz. This principle was applied to anti-tank rifle grenades such as the British #68 grenade, the British PIAT anti-tank spigot of 1943 and the German panzerfaust of 1944. This weapon could penetrate as much as 200mm of armor at normal impact.

As the tank is essentially a weapon carrier, logic dictates that it should be designed around its armament. This logic was not always followed and some tanks were mobile pillboxes with armor taking precedence over armament. While greater protection may allow a tank to approach its enemy more closely, the greater its firepower, the less it needs to rely on its protection. In general however, the amount of protective armor carried by tanks stems from requirements not directly related to their armament.

A common aim was to have sufficient armor to be immune to a similar hostile tank or ATG at 500 to 1,000 yards. At the outbreak of WWII frontal armor was common at 20-30mm on a light-medium tank and 60-75mm on the French and British medium tanks. By the end of the war frontal thickness on medium tanks exceeded 100mm and reached 250mm on the German Jagdtiger. The American T28 carried a record thickness of 304mm.

This great armor imposed severe handicaps however as armor weighs .283 lb/c.in; a square foot even of half an inch (12.7mm) plate weighs 20 lbs while the 250mm frontal plate armor of the Jagdtiger weighed 400 lbs per square foot.

One method of making the most of armor used is to distribute it unevenly according to the probable direction of attack. The front of the tank being the most probable direction from which to be attacked was given the best protection. Another method is to slope the armor away from the vertical. This increases its effectiveness against high-velocity projectiles, at first in direct proportion to the increased horizontal thickness of metal which the projectile has to penetrate and then more rapidly because of the uneven stresses set up in a projectile when it strikes a plate at an angle. With a slope of about 50 degrees to the vertical the plate is approximately twice as effective against horizontal attack as a vertical plate, which offers the same thickness in the horizontal direction.

Sloped armor came into prominence in WWII, particularly with the appearance of the Soviet T-34 with it’s one-piece sloping glacis plate. Hitherto, the accepted method had been to bolt or rivet armor plates to a frame, which was heavier and less efficient. Both the Germans and Soviets had adopted welded construction in the thirties but the former existed on American tanks down to the M3 of 1941 and the British Cromwell of 1943. The French had by then successfully evolved the technique of making large castings and with the D-1 of 1931 introduced the first tank with a cast turret.

Cast armor, while slightly inferior ballistically to rolled armor, gave particular scope for improving resistance to penetration by suitable shapes. Castings were applied with success on the Stalin tanks of 1944 and the British Valiant in the form of hull fronts with double curvatures, which means that frontal attack against them was at large composite angles. The design was improved on still with the introduction of the later Stalin tanks and the American T-42, M-48, and M-103 tanks. The production of such large hull castings can tax industrial resources and the trend was to limit large castings to turrets.

Spaced plates were also employed to give some protection to the suspension components. Against high-velocity shot spaced armor has proved less effective than a single plate of the same thickness. The use of a second, relatively thin external armor plate has become more attractive since the introduction of shaped-charge ammunition. This allows the shaped charge to be as far away from the primary armor when the shaped charge is triggered, reducing its penetration ability. The Germans were the first to do this, fitting their Pz IV tanks with thin mild plates or wire mesh around the turret and the sides of the tank.

If the deployment of shaped-charge ammunition makes the use of spaced armor more desirable, which is relatively inefficient against other forms of attack, it becomes unlikely on the other hand to have much effect on the quality of armor plate.

In fact, mild steel is as effective against shape-charge ammunition as highly alloyed armor plate. Effective protections against high-velocity projectiles demands high quality steels that are both tough and ductile. These two qualities are to some extent opposed to each other and armor that has been hardened right through to achieve the desired toughness is apt to lose its ductility and crack under impact. As a consequence, armor with a Brinell hardness of 400-500 is commonly used as the best all-round solution but at times is hardened to about 700 Brinell on the surface. This type of face-hardening is intended to cause shot break-up, but piercing caps fitted to armor piercing shot spall away the hard layer and enable the uncapped shot to attack the softer armor underneath. As a result, protection is largely obtained by the thickness of the armor and its arrangement or slope.

More another time ...

 

Wow, an amazing post there, thank you for the many quotes. I believe that if you have thorougly studied this man's articles you should be able to invent both the perfect AP shell and the perfect protection against it; something in this area must have been the man's occupation?

 

I really do not recall anyone on these forums ever stating that the only value of sloped armor was the increase of thickness to verticle.

 

OK Danyel,

You do realize that if you spent as much time helping research the issue as you are trashing my posts, we would have probably already found the article, the answer, and the truth.

How about a truce? I really need yours and others help on this issue.

- Greg

 

i read that huge post by greg just now and didn't understand a word of it. i'd like to learn about different types of armour and shells and how they work..i really don't know anything about it, if someone could post a link to a website that might help me, i'd greatly appreciate it.

 

Yeah, I agree Dayve, this is a little to in-depth for me.

 

I also don't recall trashing anyones post in this thread.

 

you will find the same info posted by him, the thing you need to do is to understand all the terms so you can get the meaning of the post, sound hard but is not, happy hunting

 

WW II BALLISTICS: Armor and Gunnery, by L. Bird and R. Livingston

Does anyone know where a copy can be obtained?

:smok:

 

if you go to google.com and search for the name of the book youll more than likely find a few copies on E-bay or somethin

 

The Summation of this debate

Hello, I have read the many opinions and statements of the number calculations. Here is what I get out of all this.

The armour thichness is important and the sloped armour, even against AP rounds, is also a very big factor.

I site the survivability rate of the front of the first Tiger and it's rather thick front plate.
The front slope of the T-34 at 60 degrees is also a fine example of a tech and or design edge.

Putting both of them together in one design like in the King Tiger or the Panther are great examples of combining both.

However I also include the quality of the armour casting, one entire piece vs. welded pieces, where in the first King Tigers to enter the war had many welded joints crack and or out right fail near the welded plates.
This to me signifies that the armour thickness and the slope combined with the quality of the steel plating are too closely related to exsist without one another.

Now with all this said todays standards are effected by velocity of the round and the density of new materials being invented, to the point of too exotic for me to understand, will definitly change everything on how to calculate armour plating survivability.

 

Found this site while looking for ballistic info:

http://www.warships1.com/index_nathan/M79apdoc.htm

It has a penetration program who the author claims to be accurate from a rifle bullet to a Sturm Tiger using ammo within the limits of the program. The program can be downloaded for free but it is not simple to use. It is a DOS based program (I hate DOS) and so far I have not been able to get it to print out properly.

Perhaps some of you PC guru's can do better.

:smok: