Steve Jackson Games - Site Navigation
Home General Info Follow Us Search Illuminator Store Forums What's New Other Games Ogre GURPS Munchkin Our Games: Home

Go Back   Steve Jackson Games Forums > Roleplaying > GURPS

Reply
 
Thread Tools Display Modes
Old 10-05-2017, 06:28 AM   #1
Erling
 
Join Date: Sep 2014
Default 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?
__________________
When in deadly danger,
When beset by doubt,
Run in little circles,
Wave your arms and shout.
Erling is offline   Reply With Quote
Old 10-05-2017, 07:43 AM   #2
Varyon
 
Join Date: Jun 2013
Default 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.
__________________
GURPS Overhaul
Varyon is offline   Reply With Quote
Old 10-05-2017, 12:36 PM   #3
Ulzgoroth
 
Join Date: Jul 2008
Default Re: Flexible ballistics armor: right through or totally stopped

Quote:
Originally Posted by Varyon View Post
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.
This is a puzzling statement, since obviously there's only going to be a limited range where that's true...

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 over-estimate damage by as much as:
2/3 * R + 1 - sqrt(2R+1)
Which is, for instance, about 24 damage at R=50.
__________________
I don't know any 3e, so there is no chance that I am talking about 3e rules by accident.
Ulzgoroth is offline   Reply With Quote
Old 10-05-2017, 10:07 PM   #4
Varyon
 
Join Date: Jun 2013
Default 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.
__________________
GURPS Overhaul
Varyon is offline   Reply With Quote
Old 10-05-2017, 11:09 PM   #5
Ulzgoroth
 
Join Date: Jul 2008
Default Re: Flexible ballistics armor: right through or totally stopped

Quote:
Originally Posted by Varyon View Post
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.
I think 51 damage hitting DR 50 and doing 35 damage instead of 10 is seriously ungood, personally.
__________________
I don't know any 3e, so there is no chance that I am talking about 3e rules by accident.
Ulzgoroth is offline   Reply With Quote
Old 10-06-2017, 06:03 PM   #6
Varyon
 
Join Date: Jun 2013
Default Re: Flexible ballistics armor: right through or totally stopped

Quote:
Originally Posted by Ulzgoroth View Post
I think 51 damage hitting DR 50 and doing 35 damage instead of 10 is seriously ungood, personally.
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 human-scale combat.
__________________
GURPS Overhaul
Varyon is offline   Reply With Quote
Old 10-06-2017, 06:28 PM   #7
Anthony
 
Join Date: Feb 2005
Location: Berkeley, CA
Default 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.
__________________
My GURPS site and Blog.
Anthony is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Fnords are Off
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -6. The time now is 06:22 AM.


Powered by vBulletin® Version 3.8.9
Copyright ©2000 - 2017, vBulletin Solutions, Inc.