ACE3/docs/wiki/framework/frag-framework.md

layout	title	description	group	parent	order	mod	version
wiki	Fragmentation Framework	Explains how to set-up fragmentation system for explosives using ACE3 fragmentation system.	framework	wiki	7	ace	major minor patch 3 0 0

1. Overview

The fragmentation system in ACE3 is a significant improvement over the fragmentation system in ACE2. Previously the system relied on fuzzy math from the values of indirectHit and indirectHitRange in CfgAmmo to calculate roughly the velocity and range of fragmentation. This had some serious drawbacks, especially in the case of smaller explosives such as hand grenades and 40mm grenades where lethality was lower than desired.

In ACE3 the system has moved away from what "feels" right to actual explosive engineering equations, primarily the Gurney equations. This allows us to get close to the actual fragmentation velocities that would be produced by an explosive configuration similar to type of ammo we are simulating.

The system for the end-developer is easy to use, and only requires minimal research into the type of ammo being represented.

2. Config Values

class CfgAmmo {
    class MyGrenade {
        ace_frag_enabled = 1;  // Deprecated
        ace_frag_fragCount = 1200; // Approximate number of fragments the explosive makes - information below
        ace_frag_metal = 210;  // Amount of metal being fragmented (grams) - information below
        ace_frag_charge = 185;  // Amount of explosive filler (grams) - information below
        ace_frag_gurney_c = 2843;  // Gurney velocity constant for explosive type - information below
        ace_frag_gurney_k = 3/5;  // Gurney shape factor - information below
        ace_frag_classes[] = {"ACE_frag_large"};  // Type of fragments - information below
        ace_frag_skip = 0;  // (Optional) Skip fragmentation for this ammo type (0-disabled, 1-enabled) - information below
        ace_frag_force = 1;  // (Optional) Force fragmentation system (0-disabled, 1-enabled) - information below
    };
};

2.1 Fragment count

ace_frag_fragCount

The maximum number of fragments that the real munition would create. Frag count affects the chance of being hit by a fragment and the maximum range in which a fragment may hit a unit. Frag count may be found online, inferred from similar munitions, or derived based on desired hit distance. For a desired distance, the frag count ($$N_{frag}) is given by,

N_{frag} = 4\pi\cdot P_{hit,min}\cdot distance_{max}^2 = 0.02\pi\cdot distance_{max}^2

where $$P_{hit,min} is 0.5%, the minimum chance to hit that is considered when generating fragments. Therefore, calculating the frag count for a frag grenade with a wounding radius of 15 meters, calculating for a distance of 80 meters may be desired instead.

Dimensionless value, count of number of fragments.

2.2 Metal amount

ace_frag_metal

The amount of metal being fragmented. Generally taken as the entire weight of the warhead, though in some cases you might want to only include the fragmentation jacket or body.

As long as the units match ace_frag_charge, the total mass of fragmenting metal given in any unit of mass (i.e., both use kg, g, or lbs).

2.3 Explosives filler amount

ace_frag_charge

The mass of explosive filler in the warhead. This may include any detonation/ignition charges, but usually such charges are relatively small.

As long as the units match ace_frag_metal, the total mass of explosive filler given in any unit of mass (i.e., both use kg, g, or lbs).

2.4 Gurney velocity constant

ace_frag_gurney_c

The Gurney constant for explosive force. You can find a list of common explosive types below. If you can not find it here, or want more accurate numbers, just google the type of explosive and Gurney constant and you can find substantial information. This is not the detonation velocity of the explosive, do not confuse them†!

Type	Speed
Composition B	2700 m/s
Composition C-3	2680 m/s
Cyclotol 75/25	2790 m/s
HMX	2800 m/s
LX-14	2970 m/s
Octol 75/25	2800 m/s
PBX 9404	2900 m/s
PBX 9502	2377 m/s
Pentolite	2750 m/s
PETN	2930 m/s
RDX	2830 m/s
Tetryl	2500 m/s
TNT	2440 m/s
Tritonal	2320 m/s

†A rule of thumb from literature is that the Gurney constant is given as 0.338 times the detonation velocity.

2.5 Gurney shape factor

ace_frag_gurney_k

The shape factor for the explosive configuration. You should choose it based on the general configuration of explosives/metal in the warhead. Most grenades for example are a sphere. Artillery and aircraft bombs are a cylinder. Mines generally a flat plate. Below is a list of the three common shapes and their factors.

Shape	Factor
Sphere	3/5
Cylinder	1/2
Plate	3/5

There are other configurations but these are the most common. If you are interested in others check out the wikipedia link given above. Most of these will not correctly function in ACE3 though due to additional variables for the equation.

2.6 Fragments type

ace_frag_classes[]

There are different types of fragmentation fragments to choose from, and they can be defined in this config value.

Type
ACE_frag_tiny
ACE_frag_tiny_HD
ACE_frag_small
ACE_frag_small_HD
ACE_frag_medium
ACE_frag_medium_HD
ACE_frag_large
ACE_frag_large_HD
ACE_frag_huge
ACE_frag_huge_HD

Tinier fragments do less damage, and generally correlate to lower mass fragments. The _HD variants are all higher drag versions. Higher drag version are useful for fragments that are irregular or would not fly very far. Experimentation here is important.

2.7 Ignore fragmentation

ace_frag_skip

When 1, the ammunition type will not produce fragments. ace_frag_skip does not stop submunitions of the ammo type from producing fragments. ace_frag_skip may be helpful for ammunition types that might cause high network load or for explosives that do not produce fragments. Experimentation under network conditions may be required. ace_frag_skip takes a higher priority than ace_frag_force.

2.8 Force fragmentation

ace_frag_force

When 1, the ammunition type will fragment, ignoring internal hit value-based qualifications. ace_frag_force takes a lower priority than ace_frag_skip.