Blocking a shaped charge with your body

[ericthered]

We just had a PC in an expendable body jump in front of a shaped charge (hurrah for robots!).



Its going to penetrate, of course, but I'm not sure how much of it will attack the player in the non-expendable body behind the sacrificial dodger.



I resolved this as turning the charge into a explosion that has no divisor and is divided by 3* distance in yards, but I'm not sure if this is correct.



How would you handle this sort of damage?

[Kromm]

If the distance is "in the same hex" or "one hex apart," the jet and slug of a shaped charge will be cohesive enough to remain a deadly penetrator. At such close distances, work out the cover DR of the target, subtract just 1/10 of this, and have the remaining damage affect the next target with the (10) intact. Treat damage type as piercing, not crushing.

At two or more hexes, cohesion will break up rapidly, but the effect will still be directional – not a classic concussion wave. It would be closer to a projectile with drop-off at 1/2D range. I'd do the above but drop the (10) and give the "projectile" a 1/2D of 2.5×(dice of original damage) and a Max of 5×(dice of original damage).

In all cases, if you allow a Dodge roll vs. bullets, you should really allow one vs. this as well. It's narrow and highly directional.

For a concrete example:

A projectile that does 6d×6(10) cr ex hits a robot with DR 20 (front and back) and HP 20. It rolls a dead-average 126 points of damage.

The robot takes 126 - 20/10 = 124 points.

Per p. B408, the robot's cover DR is found by adding together the target's DR on both sides and 1/2 HP for a machine; that's 20 + 20 + 10 = 50. It counts as just DR 5 vs. a shaped charge, so 126 - 5 = 121 points passes through.

If someone is in the robot's hex or the hex directly behind, this is a 121-point piercing projectile with armor divisor (10). They can try to Dodge for zero damage.

From two yards out to 2.5×6×6 = 90 yards, it's a 121-point piercing projectile with no armor divisor. They can try to Dodge for zero damage.

From 91 to 5×6×6 = 180 yards, it's merely a 60-point piercing projectile with no armor divisor. They can try to Dodge for zero damage.

Beyond 180 yards, it's too broken up to matter.

In all cases, if you use optional blowthrough rules to limit damage, these would apply to both the robot and any target behind it, at any range.

This has nothing to do with any ordinary linked explosive damage. If that 6d×6(10) cr ex were from an AT4 that also has a linked 7d×2 cr ex explosion, that explosion would function normally, affecting the robot at full damage and the person behind the robot at 1/(3×distance in yards) damage. If the robot is bigger than the person and qualifies as cover, full cover DR 50 would apply to this blast before division for distance; since average damage is 49 points, the robot is likely to completely absorb the blast damage.

But in general, being in the path of a shaped charge is really bad. Explosively forged penetrators were developed based on this observation. Those behave as pi++ instead of as pi, and retain a divisor of (2) out to a 1/2D range on the order of 200 yards.

[ericthered]

Ok, that mostly makes sense...


But then how does that interact with the side skirts on tanks? I know this was a common armor upgrade in WWII, and I don't think the space gained is more than a yard?


Thanks for the quick reply!

[Kromm]

I forgot to mention one thing:

Real-world shaped-charge projectiles rarely initiate against soft targets. It could conceivably be better to be shot by one and have it not explode than to not be shot by one but have a hard-target friend in front of you set it off.

Being shot by one would be a lot like being shot by a huge bullet. I'm not sure we have Fourth Edition rules kicking around for that, but Third Edition said:

The target also must be rigid enough to activate the fuse. Roll 3d against the DR+3 of the target. If the roll is equal to or less than the target's DR+3, the warhead explodes. Otherwise, the warhead does not explode. It does 1/6 base damage as crushing damage, with no armor divisor.

So a fleshy person with no DR would only activate one on a roll of 3. That 6d×6(10) cr ex + linked 7d×2 cr ex HEAT round would act more like a 6d crushing bullet. In Fourth Edition, we'd probably call it a 6d(0.5) pi++ bullet, so it would be crummier at piercing armor but make a bigger hole. Still, it would be nowhere near as bad as having the thing go boom.

[Kromm]

Originally Posted by ericthered

Ok, that mostly makes sense...

But then how does that interact with the side skirts on tanks? I know this was a common armor upgrade in WWII, and I don't think the space gained is more than a yard?

It might be fairer to say the (10) goes away immediately, if such measures were effective . . . I'm not familiar with them. But that would still leave a deadly jet and slug, just not a great penetrator. For a person with modest or no DR, "121(10)" and "121" aren't very different; for a tank with side DR in the 100-200 range (GURPS High-Tech, p. 238), they are.

[Phoenix_Dragon]

Shaped charge warheads are extremely dependent on proper stand-off distance. Detonate them early (Or late!), and they lose a huge amount of penetrating power.

In High-Tech, armor like side skirts or cage armor is called Spaced Armor and multiplies DR by 1.5 against shaped charges. If you wanted to do it as an Armor Divisor mod, it might work to just have it drop AD by a step.

That's over a very short distance. Less than a yard, often less than a foot. Drop-off of penetration, from what I have found, is close enough to linear to use that as a guide. Exact distances will depend on the warhead diameter, but a reasonable approximation might be dropping AD by one level per yard from the point of detonation.

[Jack Sawyer]

Shaped charges also don't like different materials (composite armor) but many kinds of complex armor (like NERA or the armor used on the M1 Abrams) also are more dynamic and disrupt the jet (which is very prone to disruption compared to a large, heavy APFSDS slug.)

One must also consider that not all shaped charges are designed in quite the same way. There are anti-fortification shaped charge munitions like the M908 HE-OR-T which is itself a modification of the M830 HEAT-MP (a round that incorporates HE-frag properties into a shaped charge, similar to HEDP munitions) The AT4 also has Anti bunker or anti structure munitions which involve shaped charge made with a shallower cone - this costs some penetration, but it makes a wider hole which can be useful for the intended purpose of the round. Obviously cone geometry (and precision) have a huge role in terminal effects (quality of the liner has an impact on how many 'charge diametrs' of penetration a HEAT round might achieve.

Which itself brings up the fact that the 'caliber' of the charge (diameter) also matters. HEAT rounds or shaped charges range from tank size (120mm or more) down to 25-40mm grenades (which get fewer 'charge diameters' than tank rounds because it's harder to manufacture smaller liners with the same complexity I've read.) a Grenade launcher might only penetrate a couple of inches, as opposed to the hundreds of inches you might expect for a anti-tank HEAT round. Caliber/size of the munition can also have an impact on 'behind armor effects' (lethality to living beings or vulnerable fiddly bits inside a vehicle.)

What the liner is made of also makes a difference. Most shaped charges are copper or tantalum (the latter being costlier but more effective) but you can make shaped charge liners from lots of materials and with different effects. Aluminum for example reduces penetration (it's not as good as copper or tantalum) but it adds an incendiary effect to the shaped charge. Others include DU or Tungsten, or even gold has been tested.

Also fun fact about shaped charges from here

Originally Posted by Globalsecurity

The shaped charge jet tip reaches 10 kms-l some 40 µs after detonation, giving a cone tip acceleration of about 25 million g. At this acceleration the tip would reach the speed of light, were this possible, in around 1.5 seconds. But of course, it reaches a terminal velocity after only 40 millionths of a second. It is difficult to think of any other terrestrial event as fast as a shaped charge jet tip. The jet tail has a velocity of 2-5 kms-l and so the jet stretches out to a length of about 8 cone diameters (CDs) before particulation occurs. The stretching occurs at a high strain rate, requiring the cone material to have excellent dynamic ductility at temperatures up to about 450°C. On reaching a target, the pressure developed between the jet tip and the forming crater can be as high as 10 Mbar (10 million atmospheres), several times the highest pressure predicted in the Earth's core.

Shaped charge liners for various reasons are very complex to form and thus easily disrupted (which is why it's easy to design complex and compsoite armor against them compared to APFSD rounds) but if the jet could be sustained longer before breakup it would keep accelerating (higher tip velocity, further penetration.) It's something that lends credence to the idea of Thanix from Mass Effect being credible (it's at least better than the interpretation they went with in the games....) Or the Earthlight weapon (which Thanix itself drew parallels to, as well as to the DARPA Magneto-hydrodynamic explosive munition.

[Pursuivant]

Originally Posted by ericthered

But then how does that interact with the side skirts on tanks? I know this was a common armor upgrade in WWII, and I don't think the space gained is more than a yard?

WW2-era spaced armor "schürzen," like used on late marks of the German Panzer IV, were actually designed to defeat anti-tank rifle shots aimed at bogies, return rollers, and other parts of the tracks. They also had a slight benefit against conventional AP rounds since they could cause the round to slightly deflect or tumble before it hit the main armor.

They were not designed to defeat HEAT rounds.

I'll defer to Kromm on rules questions, but there is the possibility that spaced armor on a WW2-era armored vehicle would only partially eliminate the armor divisor for an AP or HEAT round.

[tanksoldier]

.If the distance is "in the same hex" or "one hex apart," the jet and slug of a shaped charge will be cohesive enough to remain a deadly penetrator.

Respectfully disagree.

The explosion will still be significant but a stand off of even six inches will disrupt many shaped charges. The stand off armor on Strykers is about a foot from the hull and is effective against current RPG style shaped charge warheads.

Having the warhead and plasma stream hit one relatively solid object, penetrate that, travel across a measurable space then impact another... the stream won’t be coherent at that point.

[Dalillama]

Originally Posted by tanksoldier

Respectfully disagree.

The explosion will still be significant but a stand off of even six inches will disrupt many shaped charges. The stand off armor on Strykers is about a foot from the hull and is effective against current RPG style shaped charge warheads.

Having the warhead and plasma stream hit one relatively solid object, penetrate that, travel across a measurable space then impact another... the stream won’t be coherent at that point.

It will still be a blast of superheated metal though, which is probably going to be pretty fatal for a meatperson in the way.