Friday, 29 November 2013

Shell Shape

WWII era artillery shells are all look more or less the same to the untrained eye. However, much ink was spilled in the discovery of what shape was best. Documents in CAMD RF 81-12042-85 tell a small part of that story. The document is almost entirely text, so I will not include scans. They can be found here.

"In all nations, without exception, armour piercing shells for anti-tank and field artillery are produced with sharp tips. Our shells (45 mm and 76 mm) are made only with blunt tips. Comparing their effect on tank armour shows an advantage of sharp armour piercing shells over blunt armour piercing shells. 
Blunt tipped shells migrated to land artillery from naval artillery, because a blunt shell penetrates heterogeneous (for example, cemented) armour, hardened on the surface and softer in the back. This armour is commonly used on battleships.
Homogeneous armour, armour with uniform hardness, resists blunt shells much better than sharp shells. On tanks of all countries, including Germany, armour is homogeneous, which means that using sharp tipped shells will be more effective than blunt tipped shells. Sharp tipped shells also have an advantage when fired at an angle, which happens most often. A blunt nosed shell that hits armour at an angle ricochets. A sharp nosed shell, when hitting the armour, bites into it, and penetrates. The comparative effects of sharp and blunt 45 mm armour piercing shells are shown on the table below."

The performance of the sharp tipped shell is the upper row. The performance of the blunt tipped shell is the bottom row. The table shows distances at which the shells penetrate, in meters, at the specified angle. The blunt nosed shell performs more poorly against 40 mm of homogeneous armour every time, and does not penetrate at all when the armour is sloped 45 degrees.

"Trials showed that sharp-tipped 37 mm shells work better than blunt tipped 45 mm shells, at higher muzzle velocities.
There are two reasons why we do not need to worry about the armour piercing properties of our shells. One is that our 45 and 76 mm guns are very powerful. The other is that German tanks are weakly armoured (40-50 mm in the front, 30 mm on the sides), and German armour is of poor quality.
These two factors somewhat mitigate the deficiency of blunt nosed shells.
Experience shows that the Germans keep increasing the thickness of their armour (in part, the front section of all German tanks has additional screens), and the mass of German heavy (PzIV) and medium (PzIII) tanks keeps growing. We must be ready to penetrate thicker armoured German tanks. This can be accomplished by means of using sharp-tipped shells. The penetration of sharp tipped shells may also be increased by using any of well known methods (variable hardening, cementing, welding on a ballistic cap, etc). 
I think that it is imperative to immediately begin production of sharp-tipped armour piercing shells for anti-tank artillery.
June 27th, 1942
V. Malyshev"

Malyshev was right, German tanks kept getting more and more armour. However, GAU KA responded to his letter a few days later:

"The influence of tip shape on penetration was studied at GAU KA as soon as armour piercing shells were introduced to land artillery. 
In 1935-36, ANIOP, under orders from the GAU KA, performed a study on this topic (NIR #25, 1936), which led to the following conclusions:
"All armour piercing shells must be produced with a blunted part about 0.7 calibers long, as shells with this blunting need less speed to penetrate an armour plate than shells with a sharp tip under most conditions." (part 8 in NIR-25, 1936).
Further along, as armour quality improved, and as necessity to produce improved armour piercing shells increased, trial shells frequently were developed in two variants: sharp tipped and blunt tipped, and were tested on homogeneous and heterogeneous armour plates.
During the last pre-war years (1939-1941), development and testing of improved armour piercing shells was performed with cemented armour, over 30 mm thick, with resistance coefficient K=2500-2700. The reason for this was that the Izhor factory sent GAU KA the sturdiest armour plates, the cemented ones. Additionally, the so called "destructiveness" of the tip of the shell was higher for cemented plates than homogeneous ones. As such, tests against cemented armour were considered more rigorous when it came to examining shell quality. New types of shells were usually also tested against homogeneous armour.
We did not have precise information on armour plates of our enemies, so GAU KA had to make sure its engineers produced shells that are effective against both homogeneous and cemented armour, as ordering shells that work only against cemented or only against homogeneous armour would have been incorrect.
The main characteristics of a shell hull are the following: speed of penetration, hull sturdiness, and lack of ricochet.
NII-48 trials (see report #1757ss dated November 19th, 1940), ANIOP trials (see report #0264ss-40), and trials of the Main NKSP Directorate demonstrated that sharp tipped shells work better against homogeneous armour at 0 degrees, and require less speed than blunt tipped shells, but work worse against cemented armour.
When shooting at a plate at an angle away from normal, as the angle increases, performance of the sharp tipped shell decreases compared to the blunt tipped shell, against both cemented and homogeneous plates. 
  1. The blunt nosed shell can survive an impact against armour better.  Usually, the sharp tipped shell penetrates the armour 30-40 mm away from the place of impact at a 30-45 degree angle, which means the chance of ricochet is higher.
  2. The rigidity of the sharp shell is worse when shooting at an angle, compared to a blunt shell.
The above examples of theoretical and experimental works fully back the decision to adopt blunt-tipped shells. Study of captured armour piercing shells and their comparative trials showed that:
  1. All captured shells without ballistic caps are sharp tipped. If there is a ballistic cap, the tip is dulled.
  2. The front part of the shell is very hard (up to 62-65 Rc), which is achieved by:
    1. use of instrumental steels
    2. welded on tip
    3. very fine grinding, and even polishing, of the shell tip
Domestic armour piercing shells are made from more ductile 35 HGS grade steel, and, according to work of NII-24, are not improved by the above methods.
Sharp tipped armour piercing shells are only superior when manufactured from instrumental steel. At this time, the transition to instrumental steel, requiring special thermal treatment for the required high hardness and polishing of the shell tip or a welded on shell tip, would be difficult, and production of large amounts of these shells is not possible.
In order to evaluate the penetration of sharp-tipped armour piercing shells made from instrumental steel using our calibers, GAU KA gave an order to produce these shells at the beginning of 1941, but they were still not manufactured."

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