7.5cm KwK 42 L/70 had a greater AP performance than the 8.8cm KwK L/56 - why ?

[mkhulu]

Panther (armed with the 7.5cm KwK 42 L/70) had greater armour piercing performance than the famed 8.8cm KwK L/56 of the Tiger tank ?

From my limited knowledge of physics I would have reckoned the Tiger 88mm would have better AP performance than the Panther 75mm

Here's my reasoning

bigger calibre = greater amount of propellant  , hence greater speed resulting in more KE (Kinetic Energy) being possessed by the 88mm.

Or am I mistaken in KE being an important factor in armour penetration ?

Was the AP fuze used in the 7.5cm KwK 42 L/70 ammo better performing than the AP ammo used in the 8.8cm KwK L/56 ammo ?

Could the difference perhaps be due to the Panther being a medium tank and the Tiger a heavy tank ( thus resulting in different tactical application in the battlefield) But to counter this I could argue that the function of tanks is to kill other tanks ( with the Sturmgeschutz providing the infantry with mobile artillery support )

PS - Apologies if my knowledge of physics is simplified. Passed 2 semester courses in physics at Uni many years ago - knew enough to pass then , and since graduating I never used physics ! (Thank goodness I didn't study Mech Engineering   - they are real engineers   ) PPS - No offence intended towards Mech engineers - apologies if any is taken !

[dadlamassu]

I've no mathematical knowledge.

I would suspect that the difference is partly mathematical but also technological.  The 8.8cm L/56 (and its ammo) was an anti-aircraft gun converted to ground use while the 7.5cm L/70 (and ammo) was purpose designed as a tank killer. 

The Tiger 1E IIRC was designed as a breakthrough assault tank with heavy armour and heavy gun to take out enemy positions as well as tanks.  The Panther was designed to attack through the open the breach created by the Tigers and kill enemy tanks.  Its gun, while excellent at killing tanks, was not much use against positions as its shells had very little explosive inside.  They needed a lot of steel to withstand the stress of high velocity.

[mkhulu]

I've no mathematical knowledge.

no worries - as with my physics knew enough to pass my 2nd year of uni maths , but all forgotten  

I would suspect that the difference is partly mathematical but also technological.  The 8.8cm L/56 (and its ammo) was an anti-aircraft gun converted to ground use while the 7.5cm L/70 (and ammo) was purpose designed as a tank killer.  

The Tiger 1E IIRC was designed as a breakthrough assault tank with heavy armour and heavy gun to take out enemy positions as well as tanks.  The Panther was designed to attack through the open the breach created by the Tigers and kill enemy tanks.  Its gun, while excellent at killing tanks, was not much use against positions as its shells had very little explosive inside.  They needed a lot of steel to withstand the stress of high velocity.

the famous panzerkeil (Tigers upfront , Panthers taking wings and Panzer IV's at the base) ? I reckon from your input that the AP performance was due to the Panther and Tiger having different tactical roles (and that my suggestion that tanks are there to kill other tanks is a unwarranted simplification)

The Centurion would have been serious opposition to the German cats ( considering environment  , crew quality  , support and maintenance)
Must add more Centurion kits to my stash !

 

[andrewj]

The simple answer is the differing muzzle velocities of the two guns, whilst the MV of the 88mm is 820m/s the 75mm KwK 42/70 threw a shell at 1120m/s, even allowing for the lesser weight of the 75mm projectile, the penetrating power is much higher.

[GTX]

Ah the old Kinetic Energy trick...



where m is the mass and v is the velocity of the body. In SI units, mass is measured in kilograms, speed in metres per second, and the resulting kinetic energy is in joules.

[Mr.Creak]

The classic Milne-De Marre formula for armour uses shot diameter as one of its parameters:

WV²=kd³(t/d)ⁿ.

In other words it depends on (is divided by) diameter cubed: the greater the diameter (for a given mass and velocity) the (much) less the penetration (disregarding overmatch).

This might give more insight:
http://homepage.ntlworld.com/paul.a.allcock/Documents/mftw.htm

Ah the old Kinetic Energy trick...
where m is the mass and v is the velocity of the body. In SI units, mass is measured in kilograms, speed in metres per second, and the resulting kinetic energy is in joules.

And that energy is applied over a smaller area (with regard to 75mm vs. 88mm): hence greater penetration.

[rickshaw]

As others have alluded to, armour penetration is a function of both calibre (mass actually) and velocity.   

Now, armour and anti-armour technology is a very, very complex subject and one which neophytes may find confusing - particularly if they believe there should be some sort of sense and order in the world!

The questions, I'd ask about your original question are:

At what range?
At what angle?
Using what ammunition?
Against what sort of targets?
Where were the tests conducted?
Where they consecutive or conducted separately?
Who conducted the tests?

All important in understanding the results.

No two nationalities tended to conduct such tests the same way, this makes comparing results very difficult.

Then there is the problem of national chauvinism.  "Those Germans, they can't have a gun more powerful than our guns!"

Even the Germans themselves often got varying results from tests conducted at different times on their own and captured weapons.  When you compare them to British tests they often seem wildly disparate.    Its not until you realise tests conducted in the Middle-East are going to have very different results compared to tests conducted in Northern Europe because of the most obvious factor - temperature than you start to see the reasons why they produced different results.

Most often, in wartime, these sorts of tests are pretty quick affairs, merely to see if its true that your enemy's guns can penetrate your tanks and vice a versa.  Don't expect scientific accuracy.  This means that rarely are the hardness factors in the metal used in the armour actually tested.  The assumption being that all armour is the same.  In reality, thats not true.   The Germans in particular excelled in producing higher-hardness armours (as did, interestingly the Soviets).  This can cause shells to shatter when they hit the armour.  So a round which is perfectly adequate to destroy say, a Panzer III will prove inadequate for a Panther.

And the final thing I'm going to mention is "mythos".   Most people have for decades had it drummed into them by various sources that the Tiger I tank was a fearsome beast and that it mounted the dreaded 88mm gun.   In reality the Tiger I was seriously underpowered, the gun was a converted Flak gun and it was misemployed in most battled.    The first ones encountered - on the Eastern Front - were committed in the forests and marshes around Leningrad.   They of course did badly.   When the British first encountered them, they knocked their first out with a 6 Pdr AT Gun in Tunisia.  Nothing is invulnerable.   Don't be surprised if reality shows that the emperor has no clothes on. 

[sagallacci]

As mentioned, the Tiger I 's gun was merely a big gun, not nessisarily a powerful gun (what was the barrel length of the Flak compared to the KwK? could it have had a shorter tube/reduced velocity?) but all that was corrected with the 88/L71 in the TigerII, both longer tube and larger cartridge. I suspect the real "trick" to the 88 Flak was that it could be used for direct fire at a time when most other direct fire weapons were half it's size.
Another under-performing gun+tank was the T-34/85, in which the gun was a modest velocity field piece, more intended for the secondary role of tank as mobile direct fire artillery rather then specialized anti-tank weapon.

[Weaver]

Slightly off at a tangent, the idea that "the main role of tanks is to kill other tanks" is a post-WWII one. Historically tanks have spent far more time firing HE in support of infantry than in "dogfighting" with other tanks. The WWII gun-calibre escalation was driven by the need for good AP performance AND a decent HE load, which is why the alternative of smaller-calibre ultra-high velocity rounds was less favoured.

In that respect, you could claim that the 88mm L56 was a better gun than the 75mm L71 for a "breakthrough" tank like the Tiger because it was better at blowing up fortifications.....

[rickshaw]

True but the pendulum has again swung back to tanks being primarily an infantry support weapon with HE (or Pseudo-HE) performance being emphasised over that of AT performance.

Speaking of which, one of the problems encountered during WWII was that their high-velocity tank guns (ie 17 Pdr/77mm/76.2mm/88mm/etc) tended to have much poorer HE performance than the older, lower-velocity tank guns (ie 75mm), this was because shell design being largely a "dark art", designers tended to err on the side of strength in the shells so they could survive firing, rather than fracturing in the gun tube (or just after exiting it) on firing with the result that the shell walls of the high velocity weapons tended to be much thicker than those of the comparatively lower velocity guns and so therefore a much smaller HE filling.   As to why the lower velocity shells were thinner walled - remember most lower velocity guns were descended directly from artillery weapons whereas the higher velocity guns were usually purpose designed weapons and therefore had to have their shells expressly designed for them.  This is why the Soviets decided to utilise comparatively lower-velocity but MUCH higher capacity shells in their tanks.  The decision not to adopt the 57mm and instead the 85mm in the T-34 and the 122mm gun of the JS family being a case in point.

[rickshaw]

You also must take in to consideration the design of the penatrator.

Not quite that important until you see APDS and its successor APFSDS appear.  Up until then it was a case of "if it looks right, it usually works".  AP and its refinements APC and APCBC were really drawing on Naval experience in shell design.

One interesting problem encountered with APDS and APFSDS is dispersion caused by the way in which the sabot is discarded after leaving the muzzle.  Most early APDS rounds suffered quite badly from this and it wasn't really cured until the end of the 1950s.  Asymmetrical discards resulted in the penetrator yawing with the resultant loss of accuracy, even to the point of getting side on hits against the target.

I have several photos from the RAAC museum which I'm going to upload this evening, showing the differing effects of AT rounds fired against a fixed target.  It was rather enlightening seeing it.

[rickshaw]

As promised pictures of a demonstration target which is now mounted at the entrance to the Royal Australian Armoured Corps Museum at Puckapunyal here in Australia.





Most of the hits, as the display board indicate are from 20 Pdr rounds and are kinetic energy rounds.  There are several HEAT rounds - 20 Pdr, 106mm and ENTAC.  The one which I was particularly interested in though was the HESH 76mm round:



The large, circular hit, on the bottom edge is where the HESH round hit and exploded.  As you can see, no actual penetration occurs.  However, its what happens behind the plate which is interesting:



The large circular depression is the "scab" which the shockwave from the explosion has blown off and which has hit the plate behind it at considerably velocity.   Its about 10 inches in diameter.  The plate is about 5 inches thick.

The strike on the extreme left of the plate is a side hit from an APDS round which had a sabot separation failure.

The plates BTW are not armour - they weren't hardened.  They came from a ship's hull.  Why they chose mild steel plates I have no idea, except perhaps it was cheap and readily available.  The multiple layers allowed spaced armour to be simulated.  Even so, the HEAT rounds penetrated well over 4-6 layers (with air gaps).

If anybody wants higher resolution versions send me a PM and I'll email them to you.

[rickshaw]

Ah, it appears you're using an unusual nomenclature.  APC/APCBC rounds don't have a "penetrator", per se, as the whole shell is used.  A "penetrator" (a post-war term) is really only appropriately  used with APDS or its APCR rounds and suggests a separate sub-calibre shot.

The intelligence report is interesting.  Not accurate but interesting.  I suspect that the adoption of APCBC rounds had more to do with the problems encountered on the Eastern Front with sloped and hardened armour used by the Soviets.  The T-34 in particular was difficult for the Germans to counter (Guderian suggested that the Germans produce their own T-34s to counter it - that proposal ended up with the Panther entering production).  It was one of the reasons why the PaK 36 was provided with APCR ammunition and quickly replaced with the extemporised weapons such as the PaK 97/38 while awaiting the arrival of PaK 40s in adequate numbers.