Medium caliber HE blast values in CMBB

[The Green Rascal]

Hi, I'm hoping that one of the experts around here can help me out regarding 50mm and 75mm HE in CMBB.

A 75mm Pak40 has a blast value of 45. A 50mm Pak38 has a blast value of 14. It seems to me that there is a huge difference in performance, and I have seen squads taking 50mm hits for very little damage.

If someone could explain why the 75mm is/was almost four times as powerful according to the numbers it would be very much appreciated. Was 50mm HE pretty useless in real life as well?

Thanks in advance and spot the non-Grog

[Redwolf]

Keep in mind that we are talking about three dimensions here.

50mm to 75mm are only a factor of 0.666. But in practice a shell with 2/3rds the diameter also has 2/3rds the length and the diameter goes into the surface witha power of two.

So 0.666 ^ 3 = 0.3

Blast 45 * 0.3 = blast 13.3

But I guess this is a simplification. The smaller shell can as well be more powerful in reality.

[Michael Emrys]

Redwolf is on the right track. As a rough generalization, the effectiveness of an HE shell as depicted in CM terms would be proportional to the quantity of fragment producing metal in the casing and the quantity of HE filler, especially the latter. These in turn are proportional to the weight of the shell and that to its volume which, as redwolf stated, is more or less proportional to the cube of the caliber.

Michael

[Moon]

I am far from being an expert on this stuff, but one thing I do know is that the blast value assigned in CM has fairly little to do with just the "mm", and much more so with the actual amount of explosives contained in a round. Now obviously a bigger round usually has more of the latter, but not always, and certainly not always in a straight relation mm to mm. IIRC the manual even gives an example of two 75mm rounds having different explosive fillers and hence different blast values.

BTW, it is not necessarily correct to say "a 75mm Pak40 has a blast value of 45". CM does track various types of ammo used throughout the war, so the blast value for ammo typically used in, say, '45 might be dramatically different than for the same 75mm ammo used earlier on.

Maybe this helps for the time being until one of the real grogs comes along.

Martin

[tar]

Also complicating matters is the problem that the effect of increased explosives is also not linear. With regard to causing casualties, the effect is sublinear. I'm not sure exactly what the relationship is (Rexford, John Salt?), but having twice as much TNT in the shell does not cause twice as many casualties -- but rather less than twice as many.

The principle behind it is that overkill in the immediate vicinity of the impact is wasted. One person can only become a single casualty, no matter how much explosive power is used to blow him up.

[Redwolf]

Well, it's the fragments again, more explosive must have something to drive, and that is suitable fragments.

In reality, the smaller blast value of the smaller HE shells really means you have to aim more precise to do harm to unarmored targets.

At least in CM you are always hitting the ground and the distribution's surface raises quadratically with the average error.

So you only have to increase your precision by the root of the different blast values to get the same effect.

Or in other words, while the the 50mm shell only has 1/3rd the blast, but only have a disadvantage of 1/2 when it comes to aiming to get the same effect.

Confused yet

[The Green Rascal]

Thanks for the replies so far. Very informative and only slightly confusing

In my naivety I had figured a shell of 2/3rds the size of another should be doing around 2/3rds of the damage

After lots of CMBB'ing I couldn't figure an apparent large disparity of shell lethality for a seemingly modest disparity of caliber, and was (probably ignorantly) wondering if the 50mm HE at blast value 14 could be undermodelled. However I had no idea that ammo effectiveness varied depending on time period as well, that's great detail.

[JasonC]

"a shell of 2/3rds the size of another"

A 50mm round weighs 5-6 lbs. 75mm rounds weigh 15-18 lbs. 37mm rounds weigh all of 2 lbs. 105 rounds weigh 33 lbs, 155s weigh about 100 lbs, 81mm mortar rounds weigh 7 lbs.

There are other factors involved, but the basic intutition that a bigger shell will do more is correct. You just don't know how much bigger some of the higher caliber shells are than the low caliber ones.

Caliber only measures bore diameter. It doesn't say anything about length, weight, how much of that weight is shell casing and how much interior HE "payload", etc.

[Mr. Tittles]

The artillery modeling in CM is pretty simple. I can't imagine that the end effects of artillery is ground breaking either.

Casualty effects of guns can be traced to angle of descent, velocity, shell casing thickness, HE payload, distribution errors of aiming. Further factors are fuzing, ricochet, etc.

Direct fire, with high velocity weapons, reduces errors but has its own uniqueness.

[ November 12, 2003, 12:09 AM: Message edited by: Mr. Tittles ]

[John D Salt]

Originally posted by tar:

Also complicating matters is the problem that the effect of increased explosives is also not linear. With regard to causing casualties, the effect is sublinear. I'm not sure exactly what the relationship is (Rexford, John Salt?), but having twice as much TNT in the shell does not cause twice as many casualties -- but rather less than twice as many.

[snips]

I'm afraid my copy of "Applied operations research: Examples from defence assessment" (R W Shephard, D A Hartley, P J Haysman, L Thorpe & M R Bathe, Plenum Press, 1988) is in South Wales at the moment, but it contains an exercise to find the optimum charge weight for an uncontrolled-fragmentation HE shell. Because of the amount of steel required to make an artillery shell strong enough to withstand the stresses of firing, the proportion of HE is always below the optimum for conventional field artillery.

The rule of thumb that effectiveness of HE shell is proportionate to weight is a good one. The best table I have yet been able to find rating the effectiveness of specific shells is the following, extracted from PRO document WO 291/955, "Attack on Wesel", which gives the "25-pdr equivalent" of a number of typical American and British shells and bombs.

<pre>

American shell Total wt (lbs) Filling (lbs) Equiv wt (lbs)

75mm M3 shell HE M48 14.6 1.7 25

3" or 76mm shell HE M4281 12.9 0.9 20

90mm shell HE M71 23.4 2.7 30

105mm how shell HE M1 33.0 4.9 40

155mm how M1 shell HE M107 95 15 70

155mm gun shell HE Mk III 95 15 70

8" how shell HE M106 200 37 110

8" gun shell HE M103 240 21 85

240mm how shell HE Mk IIIA1 345 49 130

British shell Total wt (lbs) Filling (lbs) Equiv wt (lbs)

20mm Oerlikon or Hispano 0.25 0.02 2

2 pr Bofors 2 0.125 5

6 pr 6.2 0.4 10

17 pr 13.4 1.1 20

18 pr 18 1.1 20

25 pr 25 1.75 25

95mm tank how 25 3.1 35

3.7" inf how 19.5 1.75 25

3.7" HAA 28 2.0 25

4.5" 55 3.8 35

5.5" 80 lb 80 12 65

5.5" 100 lb 100 10 65

7.2" how 201 28 100

9.2" how Mk XVIA 290 44 125

Bombs Total wt (lbs) Filling (lbs) Equiv wt (lbs)

20 lb F 20 3 30

204 Frag M41 (US) 20 2.7 30

23 lb frag (US) 23 nom. 2.7 30

100 lb GP (US) 100 nom. 54 250

260 lb M81 (US) 260 nom. 35 200

250 lb MC or US GP (avg) 250 nom. 100 400

500 lb MC or US GP (avg) 500 nom. 240 600

1000 lb MC or US GP (avg) 1000 nom. 500 800

</pre>

On a cursory inspection, the larger shells and bombs generally seem to do less well than a linear relation to weight would imply.

All the best,

John.

[ November 05, 2003, 06:17 AM: Message edited by: John D Salt ]

[Andreas]

Originally posted by John D Salt:

I'm afraid my copy of "Applied operations research: Examples from defence assessment" (R W Shephard, D A Hartley, P J Haysman, L Thorpe & M R Bathe, Plenum Press, 1988) is in South Wales at the moment.

And you are not? If you are in London, give me a shout.

[Mr. Tittles]

American shell

Total wt (lbs) Filling (lbs) Equiv wt (lbs)

75mm M3 shell HE M48 14.6 1.7 25

76mm shell HE M4281 12.9 0.9 20

If I read this correctly, the sherman 75mm is about the same as a 25 #r?

The much maligned 76mm sherman shell is about 4/5 of a 25#r?

Both shells have roughly the same unfilled weight? the 76mm would have to be smaller in length but thicker in shell casing?

[ November 05, 2003, 10:15 AM: Message edited by: Mr. Tittles ]

[Redwolf]

Originally posted by The Green Rascal:

However I had no idea that ammo effectiveness varied depending on time period as well, that's great detail.

Well, you were right. There is no shell changing blast value in CMBB.

[Mr. Tittles]

Would the following be true?

1. As shells get bigger, they rely upon HE blast more than fragmentation to have a target effect?

2. As HE blast increases, fragmentation size decreases? Smaller fragmenst scrub velocity quickly?

3. HE blast falls off quickly, like an inverse cube?

4. Fragmentation falls off like an inverse square?

5. Shells made of harder material can be made thinner?

[John D Salt]

Originally posted by Mr. Tittles:

[snips]If I read this correctly, the sherman 75mm is about the same as a 25 #r?

The much maligned 76mm sherman shell is about 4/5 of a 25#r?

Yes: At least, so thought British OR staff in WW2, and I have yet to hear anyone express a more definitive opinion on the matter.

What's this "25hashr" business? It's better than 25-lber, granted, but "25 pr" or "25 pdr" are the right ways to write it.

PRO doc WO 291/741, "Comparison of the performance of 75mm and 76mm tank gun ammunition", gives the "vulnerable areas" of their HE shell, against men in the open, as 2200 sqare feet for the 76mm and 2900 square feet for the 75mm.

WO 291/113, "Lethal effect of artillery fire", reports a trial fired at the School of Artillery in 1943, which indicated a "vulnerable area" of 2500 square feet for 75mm M48 ricochet fire, as against 2300 square feet for 25-pdr fuzed superquick (Fuze 118, cap off).

WO 291/128, "A theory of fragmentation", indicates the variation in "vulnerable area" for the 25 pdr HE shell depending on its filling, as follows:

<pre>

Filling Vulnerable area (sq ft)

TNT 3000

Amatol 50/50 2650

Amatol 60/40 2000

Ford steel casing,Amatol 60/40 3000

</pre>

Originally posted by Mr. Tittles:

Both shells have roughly the same unfilled weight? the 76mm would have to be smaller in length but thicker in shell casing?

That would be a feasible explanation for the observed performance, certainly. One might expect the 76mm round to be thicker-walled to withstand higher firing stresses in a higher-velocity gun.

All the best,

John.

[John D Salt]

Originally posted by Mr. Tittles:

Would the following be true?

1. As shells get bigger, they rely upon HE blast more than fragmentation to have a target effect?

Probably yes, but for artillery shells of a size one is likely to meet on the battlefield blast effects are to all intents and purposes negligible (British WW2 OR studies negelected it, and still produced theoretical results in good agreement with practice). Humans are quite blast-resistant, and anyone close enough to be harmed by the blast is going to have a much bigger problem with the lumps of fast-moving steel.

Originally posted by Mr. Tittles:

2. As HE blast increases, fragmentation size decreases? Smaller fragmenst scrub velocity quickly?

I don't know. Other things being equal, I would expect higher brisance to result in a smaller mean fragment weight, and higher fragment velocity, but that's mere supposition on my part. The fragment velocity also depends on the ratio of filling to case material.

Originally posted by Mr. Tittles:

3. HE blast falls off quickly, like an inverse cube?

4. Fragmentation falls off like an inverse square?

I believe so, yes, at least to a first-order approximation.

Originally posted by Mr. Tittles:

5. Shells made of harder material can be made thinner?

"Stronger" rather than "harder", I'd say. Brittleness is presumbaly a bad thing in a shell when it comes to resisting acceleration up the bore, but a good thing for fragmentation.

The link

http://www.milparade.ru/security/24/131.htm

may be of interest, showing that much of this stuff is still the subject of active research.

All the best,

John.

[Redwolf]

This is also a tradeoff.

If you purpose is to blow up bunkers then you want blast and don't care much about fragments (respectivly your fragments come from the material you destroy). If you fire at infantry in the open or in trenches (but not in woods) you care fore fragments only and not about the blast at all.

[MikeyD]

Mr Tittles writes:

"Direct fire, with high velocity weapons, reduces errors but usually decreases many of the above."

This line reminds me of a practice I came across in an old u.S. M48 MBT Tank Commander's Guide involving 'skipping' HE shells with a time delay fuse in order to achieve an air burst over a soft target. (I believe an old post mentioned the Germans using this same technique in WWII).

High velocity direct fire weapons simply don't have an optimal dispersion pattern against dug-in targets, and you apparently have to jump through a lot of hoops (trick shots, timed fuses, etc.) to to achieve anything even remotely as effective as a proximity fuse air-burst artillery shell, for instance.

[ November 05, 2003, 11:26 AM: Message edited by: MikeyD ]

[JasonC]

First on John's numbers - if you examine the chart I think you will find the "25 pdr equivalents" measure is tracking not weight, but the square root of the ratio of filler to that in the 25 pdr shell. Thus the US 75 with essentially the same filler weight is rated as 25 pdr equivalent. Shells with 1.1 lbs of filler get 1.1/1.75 ^ .5 = .79 x 25 = ~20 lbs rating. It tracks for all of the rest, pretty closely, until you get to the largest aircraft bombs, where it remains within a factor of 2.

CM blast values, on the other hand, do not reflect filler weight differences. Any correction for those differences is second order and minor; the basic determinant of CM blast is simply shell weight. Thus 7 lb 81mm mortar rounds have half the blast of 75mm gun HE, when the filler weight is essentially the same for the two rounds. In CMBO, the 4.5 inch gun, which had very low HE filler weight to shell weight, and in fact had no more filler than a 105, still has much higher blast than a 105, tracking its higher shell weight.

This simplification tends to favor the low filler and low quality filler shells, and the heavier tube artillery shells, while penalizing the high filler and high quality filler shells, and mortar bombs. Mortar bombs have more HE to weight because their low velocity allows much thinner casings (the same is true of aircraft bombs).

US shells tended to have higher HE loads to weight, particular the US 75, 105, and 155. And US shell, to a lesser extend also UK, had higher quality filler - much more likely to be pure TNT when most German filler by mid to late war was 60/40 amatol (due to nitrate shortages in Germany). The quite common Russian 120mm mortar is also quite strong looked at in pure HE filler terms.

As for the relative importance of splinters to blast, testing vs. exposed targets always shows splinters as the essential element. But WW I experience was that shrapnel rounds (with less HE filler, and balls carried inside the round like shotgun pellets) was singularly ineffective against men with any cover.

This was unexpected. On examination two things were found. One was that the size a splinter needed to be was much smaller than expected. Even quite small ones were lethal or could cause disabling wounds, if moving fast enough. There was little point in bringing along sizable "bullets" when tiny fragments of the casing served adequately as secondary projectile. More HE drove fragments at higher velocities and was more than sufficient compensation for no pre-made shrapnel ball, against men in the open.

The other finding was that blast is what mattered against men in cover. Men actually in the open are so vulnerable to arty fire that increased efficiency against them is quite secondary to increased overall lethality. Those truly in the open get whacked regardless. But cover has enourmous effects on arty effectiveness. Even lying prone reduces lethality not by a few tens of percent, but by a full factor of 10.

The men it is hard to hurt are those protected by earth from the direct path of fragments. Earth stops fragments very effectively. But it transmits blast more effectively than air does. The universal experience of WW I was that arty effectiveness depended on HE delivered, and the obvious reason is that the fragment component of the threat is typically "saturated", while continued additional increments of effect remain available via increased blast.

This fits with the notion of calculating 25 pdr equivalents by square root of HE filler weight, as shown in the British planning table. (It was, incidentally, a planning table based on such analysis, not an independent measurement of actual results seen).

The reason for the square root is simply to account for more numerous smaller shells having a smaller "closest distance" than the same weight in fewer, larger shells. That is, the impact of any one shell is assumed to track its HE filler weight. But 4 x 1.7 lb filler will also put the nearest shell half the distance to be expected from 1 x 6.8 filler.

It is possible this estimate, which the Brits used for planning, is overly kind to many-small vs. few-large barrages. Because practical experience against dug in troops was that the heavier the caliber used, the greater the expected effect. The better coverage of many-small might be as useful as the Brit analysis thought against men in the open, I suppose.

Note also that the Brits were using quite a small round for their standard div arty, in HE filler terms, compared to other powers. The Russian 76 and the Brit 25 pdr are both quite limited in HE load, compared to a German 105 even corrected for its 60/40 amatol, let alone compared to a US 105 (14% HE by weight) with 100% TNT.

[Mr. Tittles]

Heres an interesting passage from a German FO...

(13) On October 13, the Canadians were pounding us in preparations for their attack. Our observation house (10) was shot into rubble, leaving only the chimney. However, we stuck it out in the cellar. At dusk it was my turn to go up on watch. With a field telephone and binoculars, I climbed up the chimney and saw what seemed to be several officers looking over this bunker with binoculars and having maps before them. I called (whispered) for a single high velocity artillery round. (A straight shot that gives nobody time to duck). It was right on target and I saw a steel helmet flying like a Frisbee. A few minutes later a van came and men ran towards the bunker. It was getting dark and I couldn’t tell who they were, but I assumed they were medics and thus I refrained from further shelling of the area. - In the book 'Semper Paratus, The History of the Royal Hamilton Light Infantry', page 278, the author writes "On Friday the 13th, the Black Watch of the 5th Brigade went in, east of Woensdrecht, against the center of the isthmus. Joe Pigott watching through his binoculars from the RHLI positions near Hoogerheide, saw them cut to pieces by machine-gun fire (all four of their company commanders were killed) and their attack, too, failed." - I believe that my one 105-mm howitzer round killed these four officers.

Notice that the velocity of the 105mm shell, giving no warning, led to its effectiveness This is similar to direct fire from many weapons.

http://members.shaw.ca/calgaryhighlanders/knolle.htm

[ November 05, 2003, 12:52 PM: Message edited by: Mr. Tittles ]

[Mr. Tittles]

I will have to dig it up but I remember two studies that showed that men in trenches, with light overhead cover/small dugouts into the walls of trenches are fairly well protected against 81mm/60mm mortar fire. The main danger is 81mm delay fuze mortar fire which can necessitate a hefty roof. The steep angle of descent and lack of ricochet burys the HE and makes it much more effective.

Even larger weapons, 120mm mortar and 105mm arty need fairly close hits to defeat a defensive system like this. Proofing against these direct hits from these weapons requires major construction.

[Mr. Tittles]

I believe every nation went to some amatol mixture as the war progrossed. The sherman 75mm may have started out with all TNT but went to a mixture of TNT and some amatol also. It was just too good a way to extend explosives for a small setback in bang probably.

[JasonC]

105mm howitzer rounds are subsonic. You can hear them coming, though not be a lot. Not so for the German 105mm cannon, which equipped 1 battery in each mobile division, used primarily for long range counter-battery work. Likewise the 88 shell was supersonic.

Mortar rounds are quite slow, but typically can't be heard descending. Sometimes the "pop" of the mortar firing can be heard if nearby - it takes several seconds after that for the rounds to land.

These factors are probably overrated, however. They seem large to the men under fire because they would like to think there is something they can do to effectively avoid arty fire. Objectively speaking it is mostly a matter of chance.

The shell just lands somewhere too close for anything anyone might do about it, or it doesn't. If it is accurate it doesn't matter if you duck. If it isn't, ducking may help your chances vs. the shrapnel, but probably has little overall effect.

[JasonC]

On amatol, 80/20 gives about 90 of the effectiveness of TNT, while 60/40 gives about 80 of that effectiveness. If the point is to stretch nitrates into overall explosive power that is worth it. If the point is to pack explosive power into each shell it is not. The US used 100% TNT for most arty rounds throughout the war. Some common rounds went to 80/20 amatol. The Germans used 60/40 as a matter of course.

This combines with filler weight differences to make for quite a difference in the explosive power of otherwise similar rounds. Both the US and the German 105 rounds weigh 33 lbs. CM gives them the same blast rating. But in reality the German round was something like 10% 60/40 amatol while the US was 14% TNT. Meaning 4.6 vs. 2.65 lbs TNT equivalent, or more than 50% higher for the US round.

If the CM blast rating of the US 105 were up to a third higher than the German, it would be perfectly realistic. Similarly, a US 81mm mortar round could easily have as high a blast rating as a German 75mm tank round, instead of the half it has in CM. But CM blast ratings contain only minor corrections for shell characteristics besides overall weight.

[Mr. Tittles]

http://users.skynet.be/jeeper/page57.html