WP Effects on an Armored Target

[Raellus]

If you play in my PbP, be forewarned, there are SPOILERS ahead.



I need some advice on how to resolve a recent development in my PbP campaign. I just rolled a natural 1 for a 120mm mortar strike on a landing craft converted into an armored monitor (styled on the ones the U.S. Brown Water Navy used on the major river systems of South Vietnam). The hit location is the hull and the round type is WP. Since the terminal trajectory of the mortar round would have been straight down, I've interpreted this to mean the the mortar round struck the top deck between the monitor's main gun (mounted over what used to the be the troop compartment) and the superstructure/bridge. Besides blinding the bridge crew for a while, I'm not sure what kind of damage this would have done to the vessel itself. The area the round struck isn't really armored, it's just roofed over with thin sheet metal.

Would the round have punched through before exploding?
Would it have exploded on contact?
In the case of the latter (which I assume to be the likely result), would the explosion have ruptured the roof? I assume it would, to some degree.
Could WP fragments burn through thin sheet metal?
Could WP fragments set dried paint on fire?

I'm just not sure how much damage the impact and explosion of the WP bomb would do. If it was a HE round that hit, I think I'd have an easier time of it. I've read that 120mm HE mortar bombs are at least as destructive as 105-123mm tube artillery HE shells. My impression of WP artillery rounds is that they don't so much explode as they burst. This may seem like a spurious distinction, but I think it would make somewhat of a difference in terms of the resulting blast/penetration effects. I want to handle this realistically. Would the monitor be knocked out of commission by a 120mm WP hit, or could it shrug it off the damage caused by the impact/bursting? Either way, the chemical smoke and incindiary effects of the WP will have to be dealt with but I feel confident that I can handle those fairly realistically.

Thanks in advance for your help.

[pmulcahy11b]

The only thing I can add to this is a story from a Cobra pilot in Vietnam. A PT-76 was attacking friendly troops, but all he had was a Minigun in a chin turret and rocket pods full of HE rounds. He know the HE rounds he had weren't going to penetrate a PT-76's armor, not even from the top, but he thought he'd fire anyway, hoping to distract the PT-76 (and probably out of frustration too).

He emptied his pods, and the PT-76 stopped. No smoke, no fire, no gunfire from the PT-76, but it stopped dead.

Turns out that the HE rounds couldn't penetrate the PT-76, but the concussion from the hammering of the rounds killed the crew.

A WP round or bomb might be able to do something like that, maybe?

[schnickelfritz]

I've read of a similar encounter in France in 1944...M4 Sherman encounters a German 15.0cm SFH18 crew with a HE shell loaded...15.0cm fires, hits the glacis of the Sherman, and with it not being sealed, the cuncussion killed the crew stone dead. The author was a repair section commander for the US 3AD, so I took his word on it.

With that said, it is my impression of WP rounds that it is a bursting charge, not a true explosive payload, with the aim to scatter the WP material for maximum effect.

I would think that charge on a 4.2" or 120mm mortar round to be on the scale of a standard grenade, maybe two, three tops.

It depends on the thickness of the sheetmetal and condition of the corrugated material here in particular. It's been a while since I was at my Grandparents and saw the stuff up close.

I would assume you would have some penetration and concussive effects on the crew. It depends on how sadistic (if the vessel is friendly) or generous (if an enemy vessel and you wish to reward the gunner) you want to be.

I would probably put in enough havoc to drive the vessel off (if enemy) or require the players to stop, come along shore and get sand or something to snuff out the WP if they don't have something like that aboard.

I know corrugated siding isn't very thick...maybe .125" or so, but if taken off something common (like a barn or equipment shed) and you have a good supply, it could be layered and/or spaced, increasing the protection.

You also get benefits from the shape, what you put underneath it (if anything), and any reinforcing structure/support structure.

If I was trying to roof over a USN LCM or a barge, that's what I'd do...probably run a layer of metal. a layer of timber...4-6" thick, then a layer of sheemetal. If you are getting the materials from a farm or farms, there should be plenty of both to do an LCM.

Yes, that probably means getting materials from a couple of good sized equipment sheds (big enough to store tractors and combines) and a barn or two.

I hope that helps-

Dave

[schnickelfritz]

Additionally, from what I read of the US 4.2" Mortar in WW2, they found that the HE payload was actually greater than that of the standard US 105mm Howitzer HE round of the era. It wasn't as big as a 155mm HE round of the era, but not by much. The troops were really impressed by the wallop, as were the Germans on the receiving end of the Soviet 12.0cm...enough that they captured all they could, type standardized it, copied it verbatim and issued it in the official TO&E.

What you gain in mobility and punch you give back in range.

Dave

[Raellus]

Thanks for the input so far. I neglected to mention that the PCs have also just scored an 82mm HE direct-fire hit (from a Vasilek automortar) on the monitor's superstructure. I wasn't sure how to handle that result either because the HE armor penetration number was a -2, meaning no penetration against even light armor. From what you guys have posted, it sounds like this would at least temporarily incapacitate any crew behind the detonation site. Combine this with the smoke from the WP and it's likely that the monitor will likely be motoring out of control for at least a minute or two.

Additional input is still welcome.

[Benjamin]

Some other affects to take into account relate to the ad hoc design of the target vehicle. Being hit by two rather large rounds could easily damage any weapon mounts, especially field installed turrets. Spreading WP could get through any viewports and almost certainly ignite anything stowed on the exterior of the craft (camo tarps, personal effects, extra fuel, etc.).

Also, the added armor and weapons were probably mounted using nuts and bolts. Violent shaking has a tendency to sheer bolts off and send the nuts flying about the interior. Small bits of metal can be blown off the interior as well and act like shrapnel. The real original USS Monitor had both of these problems even though its armor was never breached.

A purpose made armored riverine craft could shake these hits off, but a kitbashed vessel constructed post-Twilight would be hurting.

Benjamin

[Apache6]

The hit from the WP round is going to set anything flamable aflame. WP generates a great deal of thick smoke and consumes the oxygen in immediete area, at the least, it will cause great confusion for the crew. I'd assume that an improvised monitor would likely have ammunition, fuel and other items that would catch fire. Secondary explosions from fuel and ammo may destroy the vehicle after several minutes.

The 82mm HE wll not penetrate any significant amount of armor but in a confined area, aboard a boat for example, the fragments and explosive concussion are going to be severe.

[Raellus]

Thanks again, guys. I've resolved the combat turn based on your input.

The 82mm HE round stunned the bridge crew with concussion, badly wounding one of them with flying bolts or spall. They'll all likely be deafened for a few minutes at least.

The WP smoke is furthering adding to the chaos on board, impeding the sight and breathing of anyone near an open vision or ventilation port. Externally stowed flamables are now burning (a nice touch I may have otherwise omitted).

Using the scatter results, I rolled to see which way the monitor would drift while its bridge crew was incapacitated by concussion and smoke and the results were hard to port, directly towards the riverbank. I'll have to figure out how long it will motor towards a sure grounding before the crew regains consciousness and control of the vessel.

We'll see what the PCs do next.

[pmulcahy11b]

Originally Posted by Raellus

Thanks for the input so far. I neglected to mention that the PCs have also just scored an 82mm HE direct-fire hit (from a Vasilek automortar) on the monitor's superstructure. I wasn't sure how to handle that result either because the HE armor penetration number was a -2, meaning no penetration against even light armor. From what you guys have posted, it sounds like this would at least temporarily incapacitate any crew behind the detonation site. Combine this with the smoke from the WP and it's likely that the monitor will likely be motoring out of control for at least a minute or two.

Additional input is still welcome.

Which version are you using? In 2/2.2, an HE penetration of -2 would actually be read as 2D6-2 penetration (2D6 being base penetration for anything, unless the PEN is "Nil.")

[Raellus]

Originally Posted by pmulcahy11b

Which version are you using? In 2/2.2, an HE penetration of -2 would actually be read as 2D6-2 penetration (2D6 being base penetration for anything, unless the PEN is "Nil.")

Thanks, Paul. I am using v2.2. One of my players was sneaking around here and read my post. He PM'ed me to inform me of the applicable rules covering explosive effects and armor penetration. I factored that in and the result was one wounded bridge crewman- in the narrative it was flying bolts and spall jolted/propelled by the exploding HE round that got him.

[Haztech5]

As a HazMat Tech I was involved with an incident where a Highway Patrol trooper had found some, and not really understanding that it has a nasty tendency to auto ignite, stored it in the local sheriff's office evidence locker. The officer over the evidence locker retired and a new one took over and "discovered" it freaking out. He knew just enough about hazmat to make the situation... I'll just say... interesting.
We took the package out and using some det cord blew it up. There was an intensely bright flash and pieces of burning phosphorus flew all over starting a bunch of small fires. The smoke was thick and nasty, very acrid. The heat generated was quite intense as well.
So in the case of the boat I'd say very little damage from the bursting charge, but lots of acrid smoke and fire. It would easily ignite any nearby flammables.

[kota1342000]

Ah very cool. A tech for the PD or Fire side of the house?

[The Rifleman]

Interesting post. Something to consider is the weight of the 120mm mortar round and the velocity that it gains while falling. Here's my experience in Iraq: We had mortar rounds used as IEDs all the time. They blew straight up in the air as they were in a hole in the ground that tampered the explosion completely around them. Sometimes even when close to the trucks, nothing really happened. No one took them seriously. Then, one day, we took mortar rounds from the tubes. They were landing all around us, the concussion was unbelievable. One of them landed next to a humvee and flipped it over. Craters were everywhere.

A mortar round landing on just thin sheet metal is going to go right through it. There is just too much weight, travelling too fast to stop it. Also, you've got to have really thick metal, or armor, to stop the effects of fire. For example, an M1 is designed to take a fire in the engine compartment. But you can google car fires and you'll see that thin metal in cars goes up VERY quickly.

[Tegyrius]

Is there such a thing as a prox fuse on a 120mm WP round or are they always impact fused?

(I'm one of Rae's players, too, but neither of my PCs has the knowledge necessary to affect this particular event.)

- C.

[Haztech5]

I'm from the fire side of the house. The team I'm on has both PD and FD members.

[NanbanJim]

I forget the source, it may have been a US Army FM on close assault of armoured vehicles... but the Germans in WWII had discovered that if you manage to get 2-3 infantry smoke grenades on an armoured vehicle, you will cause asphyxiation within a very short amount of time... as in, the crew needs to bail without delay to have a chance to survive.

Granted, you've doubtless moved past this in the campaign, but anyone also looking here may give great consideration to the effects of WP's smoke in a confined space!!