Monday, 1 January 2018

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.

Analysis of the L31A7 shell was evaluated against homogenous and composite armour. Effectiveness of the armour piercing function was evaluated through the frequency of the oscillations that were caused after detonation using sensors mounted on tank armour. Shells were also fired against 160 mm thick homogeneous armour at 60 degrees to measure the spalling effect.

The impact against a tank's armour was judged to be similar to the effect of 115 and 125 mm HE shells: 15% less effective than the impact of a 125 mm HE shell, but 5-10% more effective than the impact of a 115 mm HE shell. The impact against homogeneous armour was judged to be similar to the effect of 100, 115, and 125 mm HE shells. 

Acceleration of the tank's upper front plate at certain frequencies on impact from HESH and HE shells.
1. 120 mm HESH at 645 m/s 2. 125 mm HE at 850 m/s 3. 115 mm HE at 780 m/s 4. Effect of 125 mm HE against the upper front hull armour of a Chieftain tank (calculated).

Acceleration of the tank's turret at certain frequencies on impact from HESH and HE shells at angles of 23-30 degrees.
1. 120 mm HESH at 645 m/s 2. 125 mm HE at 850 m/s 3. 115 mm HE at 780 m/s


Performance of HE and HESH rounds against homogeneous armour, where
J = impact acceleration
K = J/J1, where J1 is the acceleration for 120 mm HESH
Ammunition
Impact velocity (m/s)
J (m/s²)
Impact frequency (Hz)
K
120 mm HESH
646
2300
95
1.0


670
50

125 mm HE
850
2800
95
1.2


780
50

115 mm HE
780
2400
95
1.0


700
50

100 mm HE
890
2050
85
0.9


580
50


Results of effectiveness of the 120 mm HESH shell based on spalling established that 150 mm homogeneous armour at 60 degrees is the maximum where spalling occurs. Damage to the armour consists of a 10 mm dent, 2.2 calibers in diameter.

Plate thickness (mm)
Plate hardness (HRC)
Mass (kg)
Impact velocity (m/s)
Damage
Spall velocity (m/s)
Spall fragment mass
Direction of spall fragments from normal
160
3.65-3.7
17.8
645.8
No spalling. 30 mm deep bump, 240 mm, 130 mm crack. 350x260 mm ellipsoid dent on the front (280x260x10 mm formed by HE)
-
-
-
150
3.6-3.65
17.2
653.2
250x285 mm spall. Maximum spall depth: 30 mm. 390x270 mm dent on the front (290x270x10 mm from HE)
75
-
30°
140
3.55
17.16

200x190 mm spall, 20-25 mm thick. 380x260 mm dent on the front (290x260x10 mm dent from HE)
36
5.3
43°

Conclusion:
  1. The beyond armour effects of the 120 mm HESH shell from the Chieftain Mk.5P on composite armour of a tank is approximately equivalent to that of the 125 mm HE shell.
  2. When striking a 150 mm thick homogeneous armour plate at 60 degrees from normal, the 120 mm HESH shell from the Chieftain Mk.5P causes spalling that can damage personnel and equipment inside the tank.

11 comments:

  1. Very interesting Peter. One of the reasons British Army stuck to rifled guns in tanks was the capability of HESH ammunition against different targets, however, it did become obsolete as composite armour became available.

    Happy new year, and all the best for 2018. I read your blog every day.

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  2. Huh...what's even the point of HESH then? :/

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    Replies
    1. It's a pretty good all-around demolition tool I understand, and among the better ones for knocking out bunkers and the like.

      ...which isn't really a very good argument for insisting your MBT main gun needs to shoot it but eh, #justbritishthings. Then again at the time the Chieftain and its 120 mm rifle were introduced the *rest* of the West was still using 105 mm so...

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    2. From what I know, regular HE rounds with delay fuses would be far more flexible than HESH.

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  3. "Shells were also fired against 160 mm thick homogeneous armour at 60 degrees to measure the spalling effect."
    This is not clear to what it is 'also' to. Are all the tests on 160mm at 60°?
    I agree that tests don't show any advantage to HESH over HE.

    Plus what is the 'X' axis label?

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    Replies
    1. Also refers to other tests on composite armour. The x axis is frequency in Hz.

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  4. Rc hardness of this test armor steel appears to be fairly low compared to previous soviet practices, and falls in the approx. 270-290BHN range. This is what would be expected from period 42-SM homogenious grade armor steel (replicating typical german ww2 tank armor BHN ranges, btw, and introduced in T54 obr. 1953).
    This steel is much less brittle and suspect to catastrophic plate failure when exposed to shock effects than were the high hardness types used previously on T34, T44 and IS tanks.

    Notice that HESH does produce spall on 140mm & 150mm RHA plates, where plain HE does only produce dents. HESH isn´t much affected by obliquity effects.

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    Replies
    1. That is a dent from the HE filler of the HESH round, not an HE shell. The table does not include test results from HE shells.

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    2. I don't quite understand what you mean. In the fourth table impact velocities are given. Do you mean this is only for the 120mm HESH round and the HE that it is being compared to is static HE filler from the same size shell?

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    3. The original says that it's "damage caused by the detonation of the flattened high explosive" in the sentence that discussed HESH impact, so it seems to me that this is the damage from the HESH detonation, but it's unclear where the bigger dent comes from. Maybe they were measuring the total dent and the part of the dent that is 10 mm deep separately?

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  5. The HESH round did produce a wider dent than the HE rounds plus it seems to be the only one that produced any spall.

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