120 mm HESH vs 125 mm HE

Proxy wars during the Cold War weren't just a one way shipment of resources. For example, the USSR managed to obtain a functional Chieftain tank in Iran, allowing them to perform thorough trials of its various components. One of the more interesting ones was the evaluation of the L31A7 HESH shell. The Russian name for it was "armour piercing-high explosive", not to be confused with the Western style APHE.

Fragmentation vs Tanks

Previously, our friends at the Armoured Journal (Bronetankoviy Vestnik) looked at how to model the amount of damage an HE shell does to a tank. This article discusses a similar topic, but looks at how fragments from HE and HEAT shells behave once they hit a tank with the goal of damaging as much of the tank's external equipment as possible.

The article separates the effect of the fragments into two stages: primary (fragments of the shell fly from the explosion to the armour) and secondary (fragments bounce off the armour, spread out further across the surface the shell hit). Trials demonstrated that the secondary effect of HE and HEAT shells when fired at domestic tanks is more or less equivalent. However, the primary effect is drastically different.

Fig. 1: Spread of fragments from a HEAT shell after explosion (the dispersion ellipse is shown in a dotted line and is rotated 90 degrees.

The area damaged by the fragments is proportional to the length between the contact point between the shell and armour and the top of the cone from which the fragments spread, h0. Naturally, h0 is greater for longer shells. Experiments also revealed that a HEAT shell's h0 is about twice as much as that of an HE shell, meaning that the area in which a HEAT shell could hit and still knock out a Leopard 2's sight was 1.5 times larger than the same area for an HE shell.

Fig. 2: Damage diagram for a Leopard 2 tank hit with fragments from a HEAT shell (top) and HE shell (bottom).

1 - Gun sight.

2 - Shell. Fragments distribution is shown in a hatched area.

The fragmentation from a 125 mm BK-14M HEAT shell at impact velocity of 900 m/s and an angle of 20 degrees is shown in the following table:

Fragmentation	Angle of fragments	Average density	% of fragments that can penetrate an aluminium screen x mm thick:
			x=5	x=60
Primary	47-38	500	50	10
Primary	38-12	500	19	-
Secondary	0-7	300	33	-
Secondary	7-22	140	4	-

Fig. 4: Formation of a secondary fragmentation stream.

1 - HEAT shell

2 - Armour

3- Test screens

4% of primary fragments create a hole with a maximum height or width of 50-100 mm, 11% 10-50 mm, and the remaining fragments less than 10 mm. Secondary fragments are smaller and make holes no larger than 30 mm. The majority of the fragments are within 7 degrees of the armour surface.

Via andrei_bt.

Penetration Part 10: Cold War Subcaliber

I already posted penetration intel on Soviet post-war guns, but there were a lot of holes in that table. Let's skip a few years to another upgrade to the Soviet arsenal, the 125 mm smoothbore gun, and check our numbers again.

P.P. Isakov, Theory and Design of a Tank, 1982

Parameter	Type of shell (Gun)
	3BM11 (M-62)	3BM9 (D-81)	3BM15 (D-81)
Caliber, mm	122	125
Type of shell	Rotating	Fin stabilized
Shell mass, kg	7.4	5.67	5.9
Core mass, kg	2.8	-	0.26
Muzzle velocity, m/s	1575	1800	1785
Muzzle energy (kJ)	9200	9200	9400
Penetration at 2000 meters at an angle from normal of:
0 degrees	320	245	400
45 degrees	190	185	200
60 degrees	110	140	150

The M-62 blows its AP performance out of the water. However, you can see that this is an era of HEAT, as the American intelligence talked about a 460 mm penetration HEAT shell. The subcaliber ammunition of the 125 mm gun is no slouch, however. The mediocre performance of 3BM9 is due to the shell being made from high hardness steel. Once tungsten carbide ammunition is used, the penetration soars to 400 mm, more than enough to combat any tank of the era.

Via andrei-bt.

Beyond Armour Effects

I previously showed that 50 mm APCR did little damage to the components and crew of a Valentine tank after penetration. But how does APCR perform in general? B.M. Bakshinov, S.V. Lomov, and V.I. Timokhin decided to try it out.

"The main cause of damage when penetrating armour is a stream of fragments. The subcaliber armour piercing shell generates the most fragments, and its parameters are used to determine performance of crew protection measures. However, it is not enough to determine the damage caused by homogeneous steel subcaliber ammunition, since modern foreign subcaliber shells use casings from heavier metals (tungsten, nickel, iron, etc).

An experiment was carried out to determine behind-armour effect of the domestic equivalent. A "sieve" type target made up of two aluminium plates 3 mm each and a steel plate 8-10 mm thick was placed 0.5-1 meter behind 70-220 mm of medium hardness steel. The angle of obliquity was set to 60 degrees from normal, velocity was set to match a range of 2 kilometers. The number of fragments that penetrated the sieve was recorded, as well as their spread from the central axis.

The experiment allowed the formulation of conclusions on behind-armour effect of subcaliber shells:

1. Two groups of fragments are generated, differing vastly in penetrating power and number.
2. As excess penetration grows, the number of lethal fragments increases sharply, and then stabilizes.
3. The angle of dispersion weakly correlates with excess penetration.
4. There is a limited number of fragments capable of penetrating 30 mm of aluminium at a small spread. As excess penetration grows, their spread decreases.
5. Most fragments cannot penetrate more than 20-25 mm of aluminium."

Fig 1. Parameters of behind-armour effect of a heavy alloy subcaliber shell. Number of lethal fragments vs: a) their angle of spread, b) in proportion to excess penetration, d) angle of aluminium penetrating fragments and d) penetration of aluminium. Empty shapes are data points, dark shapes and lines are interpolations.

Fig. 2. Fragment spread with 180 mm of excess penetration. The penetration of the fragment is proportional to the length of the cone that represents its group, to scale.

A comparison is also made with effectiveness of domestic shells.

Shell	Fragments capable of penetrating 3-6 mm of aluminium		Fragments capable of penetrating at least 30 mm of aluminium
	Number	Angle of spread	Number	Angle of spread
With excess penetration of 100-200 mm
Heavy alloy	200-300	100	7	20-30
3BM9	200-300	100-120	2-3	20-30
3BM15	150-200	110	2	20
With excess penetration of 250-300 mm
Heavy alloy	300-400	100	20-25	12
3BM22	200-300	100	20	24
3BM26	200-300	120	37	32

The article also gives some details on the shell compositions. 3BM9 has an all steel case, 3BM15 has a front heavy alloy core, and 3BM22 and 3BM26 have a heavy alloy core in the rear. The paper reveals that in the latter cases, the fins of the shell and core fragment after they enter the armour, creating more lethal fragments and resulting in a larger angle of spread.

HE vs. Armour

I've briefly explored the effect of high explosive ammunition on armour (disappointing American HE and much more exciting Soviet HE of various calibers), but nothing exceptionally scientific. However, a paper by V.V. Gayun, A.V. Grishkun, O.P. Gusev, and A.V. Lisin titled Investigation of Damage Dealt by High Exposive Fragmentation Shells to Armour published in the Armoured Journal (Bronetankoviy Vestnik) gives some glimpse into how shells of various calibers act against armour. In the interest of brevity and not getting bogged down in scientific jargon, I will only translate a portion of the article and provide some figures.

Thankfully, there is plenty of practical data to go off here, and not just theory. The experiments involved 152 mm shells, whose devastating effects we have already seen, as well as Soviet 100 mm, 115 mm, 125 mm, and 130 mm shells. Of course, only knowing where you stand is of limited use if you cannot compare your progress with that of the potential enemy, so the British 120 mm gun with its HESH shells also makes and appearance. These shells are fired at armoured plates 2700 by 2500 by 170 mm, with sensors behind them to measure the impulse, which is judged by the scientists to be proportional to the damage caused inside the tank to its crew and equipment.

Since we've seen what a 152 mm HE shell can do, let's take a look at experimental data for this caliber first. Players of World of Tanks will be used to HE doing the same amount of damage to the tank no matter at what speed its hits, but in reality, this is not the case:

"An analysis of natural shots, static detonations, and calculations shows that the contribution of the explosive filler of a 100-152 mm HE shell contributes around 5-10% of the overall impulse (at an impact velocity of more than 700 m/s), and the main damage to the armour plate is dealt by the impact of fragments.
...
When increasing the impact velocity starting from 600 m/s, the impulse of a 152 mm HE shell grows considerably, and when decreasing it from 400 m/s to 250 m/s, it decreases by 2-4 times. When the impact velocity is 950 m/s, the impulse is 50 and 40 kNs for 0 degrees and 60 degrees respectively, which is 2-2.5 times greater than the 125 mm OF-19 shell.

At angles of impact close to zero, and the velocity of 250 m/s, the impulse of a 152 mm HE shell is less than that of a 100 mm HE shell with the velocity of 900 m/s. When the angle of the target is increased from 0 to 60 degrees, the impulse of shells hitting it at 850+ m/s decreases, but the impulse of shells hitting it at 250-700 m/s increases. Therefore, the 152 mm HE shell has a higher effect against armour than a 100-130 mm HE shell at impact velocities greater than 650 m/s and a comparable effect at lower velocities."

Fig 1. Experimental dependence of the impulse of 152 mm HE shells on the impact velocity (vc) at several angles of impact (α).

The ML-20. whose devastating effects on armour we've already seen. is not a very high velocity gun. Shells hitting the enemy would be flying at less than 600 m/s, giving us a pretty small range of impulse that causes such damage. How does this compare to other HE shells in the Soviet arsenal?

Fig. 2. Experimental dependence on impact angle α of the impulse from HE shells on an armoured plate.

1. 152 mm HE shell (vc = 950 m/s)

2. 130 mm HE shell (vc = 910 m/s)

3. 125 mm HE shell (vc = 780 m/s)

4. 115 mm HE shell (vc = 850 m/s)

5. 100 mm HE shell (vc = 790 m/s)

The 130 and 152 mm heavy artillery is the undisputed king here, but at very high velocities. Corps level artillery and tank guns would be unable to fire shells so fast. 100-125 mm tank guns (the D-10T and its post-war smoothbore cousins) seem like they woudn't do as much damage as we've seen the ML-20 dish out, but comparing the impulse values of these guns to the values in fig. 1 says otherwise. These guns achieve an impulse of about 20 kNs, same as the ML-20.

Now, let's see how these guns compare to British HESH. Unlike traditional HE, the explosive filler spreads itself around the plate it impacts to increase the effect. How much does this increase the effect? Here is the impulse achieved when hitting "a tank" (the specific type of tank is not mentioned).

Caliber, mm	Type	Impact velocity, m/s	Impulse, kNs
Turret
115	OF-18	680	8
125	OF-19	850	11
120	HESH	620	12.5
152	OF-29	800	15
Hull roof
115	OF-18	800	8
125	OF-19	850	9
152	OF-29	800	19
Upper front hull plate
115	OF-18	860	11
125	OF-19	850	14
120	HESH	620	12
152	OF-29	800	28

Turns out, not much better. 120 mm HESH is comparable to the slightly faster flying 125 mm HE, and it still greatly surpassed by 152 mm HE.