penetration limits and definition

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Peasant
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Re: penetration limits and definition

Post by Peasant » 23 Apr 2019 20:50

Mobius wrote:
23 Apr 2019 19:44
--snip--
Good catch. Honestly, I have no idea. A 6pdr AP(not APCBC) would need to have a muzzle velocity of about 3025fps to retain 2700fps at 338m(370yrds). Other ranges associated to the Ballistic limits are off as well. The charts we have are for the "Shot DD/L/13381 (weight 6pdr.4oz.13drm.)". I know there were several Mk.'s of the 6pdr AP shot, maybe it's a different one? Or an experimental one with a windshield? Everywhere in the report it just refers to the projectiles as AP/APC or APDS shot so no luck there.

Edit: at least there is no reason to assume the critical velocities themselves are wrong.

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Re: penetration limits and definition

Post by Peasant » 29 Aug 2019 17:46

It appears that most ballistic limits computed at 0°and 30° against rolled steel armor have SE of about 40-50 fps. This might help normalize US data against that of other nations.

Image

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Re: penetration limits and definition

Post by Peasant » 01 Dec 2019 17:49

critical mass wrote:
07 Aug 2017 13:41
Providing that the data of successful penetration and unsuccessful penetration are distributed normally without skewness, than an harmonization might be attempted on the US/British 50% probability criterion at least on 30° impact condition:

USSR: 50% limit=(IP@30°+CP@30°)/2
Brit: as given
US: as given
german: 50% limit=(G(d)+(G(d)/0.92))/2
I recently saw somebody on wot forums using this formula to calculate the "actual" performance of the german guns. But after reflecting on it a bit, it has occurred to me that this is actually not the correct way of determining the 50% point of G(d).
I have edited the diagram taken from this thread to illustrate my point:
Image

Unless the probability distributions of G(d) and G(s) are fully overlapping with each other, averaging them does not bring us to the 50% point of either of them.

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Re: penetration limits and definition

Post by Mobius » 02 Dec 2019 15:25

Mobius wrote:
23 Apr 2019 02:21
I believe that was early war British penetration.

The March 1944 British Tank School Definition of CV.
The Critical Velocity is an average velocity at which a 50% projectile success rate occurs. A projectile success being an impact velocity at which at least 20% of the projectile (or the less subjective "significant proportion of the projectile nose") passes completely through the plate. A projectile "failure" is any impact velocity which does not meet the definition of projectile "success".

At least this is what I copied.
I copied this from Tank net a long time ago.
USA Naval Ballistic Limit Method
CP = More than 50% of the mass of the projectile passes completely through the armor.
PP = Some but 50% or less of the projectile makes it through the armor.

The USA method for determination of BL(N) would be to employ an “up and down” firing method until 3 complete penetrations (CP) and 3 partial penetrations (PP) are obtained within a velocity spread no greater than 100-fps (in some documents 120-fps).
Or; If the zone of mixed PP and CP results is greater than 100-fps, “fire additional rounds to obtain a minimum of 3 CP and 3PP in the mixed zone.
Or; If no mixed zone is obtained, average the velocities of the lowest CP and the highest PP.
Or; If the mixed zone is less than 100-fps, average the velocities of the 3 lowest CPs and three highest PPs.
Or; If there are an uneven number of PPs and CPs within the mixed zone, than one of two weighted averages is employed to determine the limit velocity. The first weight average applies if the number of PPs in the mixed zone exceeds the number of CPs in the mixed zone:

EQUATION 1
BL = VA + [((Np – Nc)/(Np + Nc)) x (Vhp – VA)

VA = Average of all velocities in the mixed zone
Np = the number of PPs in the mixed zone
Nc = the number of CPs in the mixed zone
Vhp = the velocity of the highest PP

The second weighted average applies if the number of CPs in the mixed zone exceeds the number of PPs in the mixed zone:

EQUATION 2
BL = VA - [((Nc – Np)/(Nc + Np)) x (VA – Vlc)

VA = Average of all velocities in the mixed zone
Np = the number of PPs in the mixed zone
Nc = the number of CPs in the mixed zone
Vlc = the velocity of the lowest CP

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Re: penetration limits and definition

Post by critical mass » 05 Dec 2019 14:31

Peasant wrote:
01 Dec 2019 17:49
critical mass wrote:
07 Aug 2017 13:41
Providing that the data of successful penetration and unsuccessful penetration are distributed normally without skewness, than an harmonization might be attempted on the US/British 50% probability criterion at least on 30° impact condition:

USSR: 50% limit=(IP@30°+CP@30°)/2
Brit: as given
US: as given
german: 50% limit=(G(d)+(G(d)/0.92))/2
I recently saw somebody on wot forums using this formula to calculate the "actual" performance of the german guns. But after reflecting on it a bit, it has occurred to me that this is actually not the correct way of determining the 50% point of G(d).
I have edited the diagram taken from this thread to illustrate my point:
Image

Unless the probability distributions of G(d) and G(s) are fully overlapping with each other, averaging them does not bring us to the 50% point of either of them.
The red marked G(D) does not represent the actual average of 50% G(D) but instead the modus of the ultimate distribution. The actual average of G(D) is where the ultimate and penultimate curves cross over due to the two datapoints in the middle, dragging the whole average down a little. Averaging may not be a solution if a discrete result are required, it does provide, however, a reasonably close approximation for the BL (Army), which is also used by the US and british for penetration definition of Army AP.
The BL(Army) required min. a hole or crack through from one side to the other. Passage of the projectile is not required here.

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Re: penetration limits and definition

Post by Peasant » 28 Jul 2020 08:17

Shower thought: if 5% of the mass of a 25kg shell (25 * 0,05 = 1,25kg) passes behind the armour, its not considered a defeat, while if the same amount of mass enters from a hit of a 1,43 kg shell (1,25/1,43 = 87% of its mass) it is considered a penetration. Logic?

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Re: penetration limits and definition

Post by critical mass » 29 Jul 2020 10:47

I think the soviet mass % count is somwhat related to the tendencies of their projectiles to not survive penetration intactly. A fairly complete break up will eventually cause some fragments never making it past the plate. It´s also highly variable, depending on when the break up occurs during penetration. Initial break up (shatter) can cause events where not a single piece of the projectile passes the plate, a very late break up during exiting the plate can mean that 100% sans driving bands, cap and nose covers enters the space behind the plate.

Break up is sometimes bad for penetration (not at high obliquity, though), but its always bad for detonation of the projectile (causing duds and low order bursts).

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Re: penetration limits and definition

Post by Peasant » 10 Dec 2020 16:18

Assuming all other variables being equal, the mechanism of interaction between an intact shell and the back of the plate near it's ballistic limit remains unaffected by the overall thickness of the plate (at least at normal obliquity). Which is to say, the energy it takes for a shell to move from just cracking the back of the bulge to completely passing through is constant for any thickness under this model.
Mathematically it can be written like this:

Image

where C is a semi-empirical constant indicating energy difference between the two limits.

Therefore we should be seeing the relationship between Navy BL and Army BL as is shown in Chart n.1
As we can see, there is no simple multiplier to convert between these two, as the difference changes from 250fps at 1500fps BL(N) to just half of that for 2875fps BL(N).
On the other hand, as the absolute thickness of the plate grows, it is usual for it to become heat treated to a lower hardness. This has a side effect of making its material more ductile and able to stretch more before breaking, and so the back of the plate can absorb more energy between cracking and letting the whole projectile through, increasing the energy difference between the two limits. But its unlikely that this phenomenon completely negates the reduction in BL difference at different thicknesses/striking velocities.
Therefore that helpful rule used by german engineers to convert between G(d) and G(s) is but an approximation good enough only for a limited set of conditions (german APC projectile attacking german RHA at 30°).
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Peasant
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Re: penetration limits and definition

Post by Peasant » 19 Mar 2021 16:59

For the AP projectiles with tangent ogive nose penetrating MQ RHA at low obliquity under condition their nose remains intact, the C constant is approximately equal to 835 when velocities are expressed in fps.

For example, the penetration chart for 75mm M61 APC from Terminal Ballistics Vol.3 gives the Navy BL for this projectile against 3,22in./0° as 1870 fps(500m.). Using this formula, the estimated Army BL for this target would be about 1673fps(1100m.)

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Re: penetration limits and definition

Post by Peasant » 12 Apr 2021 15:00

For a usual 6 round US ballistic limit, if we assume that all 6 velocities are distributed uniformly inside the 150fps interval, at fixed 30fps intervals, the standard deviation(SD) for the (assumed)normal distribution would be 50fps (correct me if I'm wrong). From the table of normalized standard variable, the critical velocity for 80% of shots succeeding would be 0,84 SD above the 50% limit, or +34fps.

Using DeMarre formula, this translates into +2,4% rated penetration for 2000fps striking velocity for the 50% criteria compared to soviet 80% criteria.

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Re: penetration limits and definition

Post by Peasant » 26 Apr 2021 11:45

There is some confusion about interpretation of ballistic trial reports from either nation, so I hope this table will help:
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Re: penetration limits and definition

Post by critical mass » 29 Apr 2021 08:05

The british were somewhat distinct in using the term "penetration" for a hole or crack through the armor and the term "perforation" for a complete projectile passage through the plate. That was not the case for the US use of terminology, where term "penetration" is frequently employed in relation to the US (Navy) BL, while the US (Army) BL refers to the Holing limit and does not use the term "penetration" here.

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Re: penetration limits and definition

Post by Peasant » 24 Jul 2021 14:25

The projectile mentioned is .50cal AP.
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