Summary of the Drone-Delivered Fragment Threat

I just wrote a short piece for Body Armor News:

Characterizing the Drone-Era FPV Fragment Threat

You should read the whole thing, but, for convenience and emphasis — and because I know that some engineers are in the habit of checking this blog from time to time — the key points are below.

– Fragments from conventional artillery/mortars and from aimed FPVs and other drones are very different.

– Typically, conventional shell fragments travel quite far before impact, decelerate rapidly in air, and if you’re unlucky enough to be struck by any fragments, it’ll be few or even just one of them.

– The statistically average shell fragment is something like 14 grains at 1,000 feet per second.

– The drone-carried fragment threat is the opposite in practically every respect: detonations are close, preformed fragmenting elements maintain their velocity fairly well — not that it matters, given the distances involved — and you’ll never get just one, and rarely only two. You’re likely to be struck by anywhere from six to seventy near-simultaneous fragments.

– There is no statistically average drone-carried fragment; there are three distinct buckets: light and fast, light and slow, and heavy. Different munitions produce different fragment types. The statistically average fragment in the light and slow category is 4–6 grains at ~2,300 fps. (!) Less than half the weight, but more than twice as fast as the average artillery fragment. And that’s just the light+slow bucket. The absolute median is about 6.8 grains at 4,200 feet per second. (!!!)

– Conventional standard-issue soft armor only stops roughly 1–15% of drone-carried fragments. Yeah, not the other way around: in some cases, ~99% of fragments are going to penetrate. Though current soft armor works well against conventional fragmenting munitions, it’s ineffective against the drone-delivered threat, which has become the dominant threat in Ukraine. Modern soft armor was built to artillery-era assumptions and needs revision.

– The absolute minimum soft armor 17gr FSP V50 should be 2,060 fps. 2,130 fps would be somewhat better. Full reasoning is in the BodyArmorNews post. Anything with a V50 under 2,060 fps is legacy-era armor that might be useful in certain circumstances, but is underpowered in the drone age. Current standard-issue soft body armor is ~1,800 fps.

– The fragment sleeves that drones carry may contain tool steel elements which are much harder than the standardized FSPs used in lab testing: ball bearings are hardened to 60–66 HRC, whereas the FSP is hardened to just 30 HRC. If hardened steel frag is a prominent threat — something which intelligence and military medical teams can easily determine — the above V50 recommendation may need upward revision to roughly 2,840 fps. See, e.g., Martinez-Guo, Zherui; Martinez-Morales, Stephenie; and Chen, Weinong. (2019). “Projectile strength effects on the ballistic impact response of soft armor targets.” Textile Research Journal. 90. 282–293. 10.1177/0040517519862882.

– Coverage is at least as important as V50. The priority should be broad torso coverage, followed by the neck and upper yoke, pelvis and junctional region, and deltoids. All of those areas should reach roughly the 2,060–2,130 ft/s class.

– The largest unresolved variable is median body-to-burst distance. Military medical, intelligence and ordnance teams should recover fragments and record mass, geometry, material characteristics such as hardness, and coarse standoff category. That dataset would improve armor requirements dramatically — far more than further modeling based on assumed distances.