10052017, 06:28 AM  #1 
Join Date: Sep 2014

Flexible ballistics armor: right through or totally stopped
Mr. Cole suggests a special rule for flexible armor in his Armor as Dice: if a bullet outmatches a kevlar vest by even just a little, the vest provides very little protection and barely slows the bullet down.
How would I model that with standard GURPS damage rules? Perhaps like that: "If basic damage from piercing attack exceeds DR of flexible armor after applying armor divisors (if any), protection is further reduced: divide leftover DR by 4, round down". Or maybe there's a better way?
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10052017, 07:43 AM  #2 
Join Date: Jun 2013

Re: Flexible ballistics armor: right through or totally stopped
Flexible ballistic armor in GURPS functionally gets a multiplier against cutting and piercing damage  removing this multiplier to determine injury may be appropriate. Interestingly, from what I understand of realistic armor behavior, roughly dividing by 3 any time armor is penetrated (removing the multiplier would be on top of this) is fairly realistic. This is because realistic armor penetration would follow a difference of squares approach  that is, you square damage to get rough kinetic energy and square DR to get a rough value of how armor resists KE (which is linear with the square of thickness, rather than linear with thickness as in GURPS DR), take the difference, then take the square root of that to determine actual penetrating damage. As luck would have it, dividing DR by 3 actually gets you very close to the same result, with much less math.
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10052017, 12:36 PM  #3  
Join Date: Jul 2008

Re: Flexible ballistics armor: right through or totally stopped
Quote:
Letting R be the DR value and R+x be the damage value, the indicated relation is: ε < (R + x)^2  R^2  (R + x R/3)^2 < ε ε < 4/9 * R^2 + 2/3 * x * R < ε ε < 2/3 * R * (x  2/3 * R) < ε So, basically, it's accurate while the damage is close to 5/3s of the DR. Perhaps you don't care about the error increasing without bound on the high end (on the theory that at that point the target is dead either way). But on the low end it can overestimate damage by as much as: 2/3 * R + 1  sqrt(2R+1) Which is, for instance, about 24 damage at R=50.
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10052017, 10:07 PM  #4 
Join Date: Jun 2013

Re: Flexible ballistics armor: right through or totally stopped
It fails with attacks that barely penetrate extremely high DR. As such attacks would still cause pretty severe wounds using difference of squares, and as such errors have a fairly small (relative to the full damage) range where they occur, I'm alright with overestimating them if it means only having to do simple arithmetic at the table.
As for the error on the far end, you are correct that I'm fine with it largely because, at that point, the character is almost certainly dead anyway.
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10052017, 11:09 PM  #5  
Join Date: Jul 2008

Re: Flexible ballistics armor: right through or totally stopped
Quote:
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I don't know any 3e, so there is no chance that I am talking about 3e rules by accident. 

10062017, 06:03 PM  #6 
Join Date: Jun 2013

Re: Flexible ballistics armor: right through or totally stopped
I agree, but when given the choice between a system I can do the math for fairly quickly in my head and one I need to pull out a calculator and plug in a decent amount of data for, failure in edge cases becomes more acceptable for the ease of use on the first. With a computer at hand, however, I'd absolutely do difference of squares, and just have a simple spreadsheet setup to do the calculations for me. Note also that using the damage caps from HT can significantly reduce the problem  that 35 damage is likely instead only 13 or 14 (for a strong target) damage for determining injury, with the rest just making bleeding worse... which in a setting where you'd expect to have characters wearing DR 50, isn't much of an issue (thanks to hypercoagulin). The failure is more severe when dealing with vehicles, of course, but this is meant more for humanscale combat.
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GURPS Overhaul 
10062017, 06:28 PM  #7 
Join Date: Feb 2005
Location: Berkeley, CA

Re: Flexible ballistics armor: right through or totally stopped
Note that a difference of squares model isn't really accurate anyway; the amount of energy that is required to breach armor may not be the same as the amount of energy absorbed by successfully breaching armor. If you were to make a table of projectile energy vs energy absorbed, the difference of squares model implies that it would rise up to the point of penetration and the turn flat, but what it's typically going to do is rise up to the point of marginal penetration, have a bump of some sort, likely dip back down below the initial peak, and then gradually start rising again.
All this is why armor models have never been very concerned with the effectiveness of armor at reducing damage that penetrates, they're far more concerned with what is stopped outright. 
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