81mm Mortar Effectiveness

[rexford]

The mortar is often described as one of the primary casualty makers of WW II, and TM9-1907 suggests that mortar effectiveness may be underestimated if one goes on the basis of shell size alone.

Comparing the 1 hit-per-10 square feet blast area drawings for an 81mm mortar (M56 shell) and direct fire 155mm M107 HE shells after ground contact detonation, the mortar covers more ground with lethal fragments.

When a Sherman 75mm HE round detonates 30' in the air, the 81mm mortar round covers about 3 times the area with lethal fragments (1 hit-in-10 square feet), which seems remarkable.

While the above results seem surprising, consideration of blast direction suggests that the drawings may make sense when contact angle is considered.

When 155mm HE rounds quick fuze detonate on direct fire attempts or Sherman shells detonate in the air, the majority of the fragments appear to be thrown out at an angle that is close to being perpendicular to the shell sides.

Pieces that are driven down or up (the majority) from the 155mm ground blast would not pose much of a threat to low level soldiers or equipment, and much of the explosive fragment effect would be lost. On the Sherman air burst, fragments heading upwards or to the sides would be lost against targets close to being under the round.

Since a mortar round is coming down almost vertically (about 70° from horizon), fragments that move perpendicular from the sides would form a roughly circular pattern of lethality close to ground level. The percentage of mortar fragments that may potentially strike low level targets in the open would be higher than a 155mm HE direct fire hit or a Sherman 75mm ricochet air burst, making the mortar round more efficient.

This is not to say that an 81mm mortar round is the best under all circumstances, since the advantage would be maximum against un-entrenched targets in the open. Against dug-in infantry or bunkers, one would probably want to use ricochet 75mm HE attempts or 105mm direct fire HE.

What the above suggests is that HE rounds should probably have different ratings against different target types. Against an infantry attack across a field, the mortar with its high rate of fire and nearly circular fragment pattern might be significantly more effective than an SU 152 or 105mm Sherman (the Su 152 HE round is roughly the same as U.S. 155mm in terms of total and HE filler weight).

[jrcar]

I haven't seen this bought up before, and it is something that always confused me. In my Army officer training I was always told that weight for weight a mortar shell is more effective, and in fact danger area wise a 81mm morter is more effective than a 105 (but the 105 has a significant range advantage).

The reason for this is due to the high pressures that a 105 has to withstand in comparison to a 81. Because of these the walls of the shell are thicker and so tend to break up into larger shrapnel sizes (so there are smaller numbers of pieces), whereas a 81 has thinner walls and the pieces are smaller, and more are created.

Mortars also form circular blast patterns, whereas gun and howitzer blast patterns are oval in nature, with most of the danger area behind the point opf impact of the round.

Cheers

Rob

[Redwolf]

Well, specifics are not known, but CMBO is supposed to have a more complex model than just "blast". Behind the scenes the engine is supposed to work with different values for bpure last, shrapnel effect and possibly others, and the value displayed to the user shall be a convinince summary only.

Tests show that the CMBO 150mm infantry gun can drive infantry out of foxholes easier than the Hummel, although the latter has more blast value displayed. The difference could be due to precision (when shooting at foxhole in even ground lower velocity is better), however I watched the fall of the shells and couldn't find any noticable precision difference, while the difference in effect is very noticable. Hence I assume that CMBO in fact has a more complex model.

[lcm1947]

Very interesting posts guys. Thanks for the info even though I didn't ask the question. It really makes the game so much more interesting when you understand stuff like this.

[rexford]

Originally posted by jrcar:

The reason for this is due to the high pressures that a 105 has to withstand in comparison to a 81. Because of these the walls of the shell are thicker and so tend to break up into larger shrapnel sizes (so there are smaller numbers of pieces), whereas a 81 has thinner walls and the pieces are smaller, and more are created."

Good point. TM9-1907 indicates that the initial fragment velocity of 81mm mortar rounds is 6,180 fps while 75mm and 105mm HE reach 3120 and 3500.

[rexford]

Paul Lakowski noted on Russian Battlefield site that 155mm round dropping at 70 degree angle from horizon has 10 times the effective blast area of the same round at 10 degrees, due to sensitivity of fragment spread to impact angle.

[jrcar]

I thought we were also told that the smaller fragments travel further than the large fragments, I glad you confired that!

And a small fragment still causes a battle casualty.

During my intelligence officers course we also covered identifing arty and mortars by the pattern they made, crude ASCII art to follow:

P= point of impact

X= danger area

Mortar Gun How

X X

XXX XPX XXX

XXXXX XXXXX XXPXX

XXXPXXX XXXXX XXXXX

XXXXX XXX XXX

XXX X X

X

Effectively the mortar is evenly spread, the guns blast is behind the point of impact, and the Howitzer is between the two. The lower the angle of impact the less effect.

cheers

Rob

[jrcar]

H'm well that didn't work, I'll have to do some pictures... but later

Cheers

Rob

[ May 03, 2002, 11:39 PM: Message edited by: jrcar ]

[Londoner]

Great post rexford, agreed, IMO onboard mortars in general seem to be undermodeled.

[Michael Emrys]

And the same for off-board mortars as well. The discrepancy may not be huge, but it seems to be there.

Michael

[TSword]

Would be nice to see in CMBB, a 81 mm Mortar shell being more then a handgrenade as it is now.

[rexford]

British report WO 291/491 presents comparative dispersion data for British 3" and German 8cm mortar rounds. Ground dispersion of German 8cm was about 2/3 of the 3" figure.

Based on the data, German 8cm mortar would range onto targets faster than British 3" and drop a higher percentage of effective rounds.

More data from report is posted on John Salt site for weapons effectiveness.