HEAT and Armor Overmatch

[serfCharlemagne]

Hi guys, I just found out about armor overmatch, which is basically the phenomenon where the effects of sloped armor tend to be negated as the diameter of the shell becomes greater than the actual thickness of the armor plate. Much more info about it can be found in the following two posts:
https://ruhrpottpatriot.tumblr.com/post ... what-do-we
https://ruhrpottpatriot.tumblr.com/post ... d-why-some

Now, my question is is there a similar phenomenon as overmatch for HEAT warheads, or is it simply penetration value of the copper jet vs. line of sight thickness of the armor (only considering RHA, not composite armor, ERA, etc.)?

[Mobius]

No. In WWII only the spin of the shell and the deflection mattered. Actually, the earliest HEAT shells had steel liners (and maybe aluminum) and they were hemispherical in shape so that mattered.

[T. A. Gardner]

It's not just diameter, but weight as well. The mass of a shell can simply overcome the strength of the steel in the plate even if it can't penetrate it. Instead, the round might well "bounce off" but shatter the plate and spray the interior of the vehicle with hot shrapnel anyway.

[serfCharlemagne]

@Gardener right, the best "simple" explanation of it I heard was over on the World of Warships reddit page, where someone said it's like when a bullet ricochets off a wall, it still leaves a dent. If the "wall" (i.e. armor) is thin enough, it'll still fail even if the round should have ricocheted. The reason I wasn't too concerned about weight was that I figured minus specialist APCR shells, most shells would have been made of the same or similar materials.

@Mobius, the spin of the shell mattered? How is that?

[Mobius]

@Mobius, the spin of the shell mattered? How is that?

Probably the molten jet gets dispersed by centrifugal force. Also, it seems there is always some yaw in the projectile so that could change its focus.
Some early tests of these projectiles were done in a static setup where the shell was placed against the armor and detonated. They over estimated the real penetration.

Modern armies came up with ways to overcome the rotation problems. The French came up with a HEAT shell that had roller bearings in its driving bands so it wouldn't rotate much. Then the smooth bore guns allowed non-rotating fin stabilized shells to be used.

[Peasant]

You are quite right, Mobius.

THE EFFECT OF ROTATION UPON SHAPED CHARGE JETS 2-121.
The Deterioration Process.
The development cf a triple-flash X-ray system for studying jets from large rotated charges has clarified the details cf the deterioration cf the jet. The sequence cf events as the rotational frequency increases is shown in figures 2-41 through 2-44, which show the effects of increasing rotation upon the jet from a 105-mm copper liner. The deterioration process can be broken down into the following distinct steps.

a. The jet, which is normally continuous when unrotated, begins to break up into separate pieces along its length.
b. As the rotational frequency increases, the cross section of the jet starts to deviate more and more from a uniform circular shape and shows evidence of deformation into a ribbonlike structure.
c. There is finally a definite bifurcation or separation cf the jet into two essentially parallel jets, with each jet broken into separate pieces. When the bifurcation first appears, the two portions of the bifurcated jet generally seem to lie in a plane of bifurcation.
d. Increasing rotational frequency causes the plane of bifurcation to be distorted into a helical surface.

The bifurcation in the jet appears tobe associated with a critical frequency that depends on the caliber. Thus, bifurcations have not been seen in jets from 105-mm charges rotated at 15 rps, whereas all jets from 105-mm charges rotated at 45 rps show bifurcation, as domostjets from 105-mm charges rotated at 30 rps. The incidence of bifurcation is clearly associated with the steepening portion cf the penetration fall-off curves (see figure 2-45).

Finally, the plateau region associated with the highest spin frequencies indicatesthat the later modifications of the bifurcation process contribute very little to further reduction in penetration. It was originally conjectured that the original bifurcation was perhaps followed by bifurcation of each of the new portions of the jet. This has not been ruled out, but the observations on the target plate upon which this was based can also be explained by the distortion of the plane of bifurcation into a helical surface.

If you were able to read all that without doing off, congratulations, you can read the rest of this "sciency stuff" here: https://apps.dtic.mil/dtic/tr/fulltext/u2/389304.pdf

[serfCharlemagne]

That's a great source Peasant, thanks all for the responses!

Does anyone know what rps typical WWII shells spun at? Also, did bazooka rockets, panzerfausts, etc. spin in flight?

[Mobius]

Sometimes I get this backwards but here goes.
The rifling of the 7,5 cm KwK 40 is 1/24 to 1/18.
[Corrected]
At 1/18 is 0.075mx18=1.35m.
At 450 m/s for HL/A gives 450/1.35 = 333.33 rps.

[Yoozername]

Sometimes I get this backwards but here goes.
The rifling of the 7,5 cm KwK 40 is 1/24 to 1/18.
At 1/18 is 0.75mx18=13.5m.
At 450 m/s for HL/A gives 450/13.5 = 33.33 rps.

It is one turn in 18 calibers. So, it would be 75mm*18= 1350mm, or 1.35 rev/m

So, the math is actually 450 M/s/1.35 rev/m or 333.33 rps, or ~20K RPM

While most of the energy is in the translational velocity, a lot is also in the rotational velocity.

A top of my head example is a 155mm M107 arty shell goes 175 rps. The bigger the caliber, the lower the rotational spin. Rifle bullets are much faster then.

Most WWII light AT weapons like bazooka, panzerfaust, etc, did not impart spin. The post-war RPG weapon does impart a slight spin. The increase in accuracy being more important than any slight loss in penetration.

The actual penetration appears to vary over range with better penetration at longer ranges.

[Yoozername]

Perhaps a bit off-topic, but interesting data on stand-off...

[Mobius]

Oops, right 7.5 cm = 0.075m not 0.75m.

[critical mass]

Hi guys, I just found out about armor overmatch, which is basically the phenomenon where the effects of sloped armor tend to be negated as the diameter of the shell becomes greater than the actual thickness of the armor plate. Much more info about it can be found in the following two posts:
https://ruhrpottpatriot.tumblr.com/post ... what-do-we
https://ruhrpottpatriot.tumblr.com/post ... d-why-some

Now, my question is is there a similar phenomenon as overmatch for HEAT warheads, or is it simply penetration value of the copper jet vs. line of sight thickness of the armor (only considering RHA, not composite armor, ERA, etc.)?

The short answer is no. The penetration mechanics are different. Solid penetrators attempt to perforate by means of attacking the strength limit of the plate, causing a displacement of armor (pushing it away from the nose), by pure shock effect (shattering the armor plate), or by attacking its shear limit (driving out a plug / disc).

HEAT shaped charge attack is well within the range of fluid dynamic effects. Neither the attacker nor The armor act solid or plastic , but work as liquids.

[Yoozername]

No. In WWII only the spin of the shell and the deflection mattered. Actually, the earliest HEAT shells had steel liners (and maybe aluminum) and they were hemispherical in shape so that mattered.

I believe the Germans did use zinc in some PF30k. Perhaps to increase it's penetration? But most German hollow-charge ammunition was soft steel. This liner is certainly a casting.
Edit: Actually, HL/C ammo is also said to use zinc too! I revisited the thread where they discuss this.

[Mobius]

The reason I mentioned steel liners is that the first model of bazookas had steel liners. They were replaced by copper liners giving a 30% increase in penetration.

[Yoozername]

I don't know if the Soviets used anything but soft metal in their liners.

An interesting thing about copper, as opposed to other commonly used metals, is that the stand-off can be shorter.