Hard body armour: A possible compromise

I wrote enough about the folly of hard body armour in conventional warfare in a pre-powered-exoskeleton world. It's simply too heavy - the users are slowed down, exhausted and the necessary endurance, speed and agility is lost.

Radical positions are rarely correct, and also rarely followed. There's usually a majority preference for compromise, and even more so when intuition/feelings are biased against the radical proposal. Thus let's look at what might be a sensible compromise about body armour.

The current typical HE munitions still allow for a sensible role for about 2-3 kg heavy torso-protecting 'flak vests', which provide but the lowest level of fragmentation protection to stop the weakest about 80% or so fragments (more where the impact angle is a good one). Such vests are not "bulletproof" except against some weak pocket pistols with lead bullets.

Example distribution for an old HE shell (no preformed fragments, no internal serrations to control the fragmentation pattern):
0.1-1.0 g: 77% of fragments
1.0-10.0 g: 21% of fragments
10.0 -1 40.0 g: 1% of fragments.

To stop all but the heaviest 1% of fragments would require a NIJ level IIIa vest (the highest grade that's still soft body armour) that weigh several kilograms more, and even then it would only protect at a considerable distance to the explosion (way outside of the lethal blast radius).
Still, that's one possibility for a compromise between the lightest flak vests and the 15 kg nonsense. Such NIJ level II vests are actually quite common, especially with pouches for the heavy level IV plate inserts.

Another compromise could be the citadel armour principle. Citadel armour was introduce in battleship and battlecruiser design prior to the First World War. It was impossible to harden the whole ship, so the ship designers limited the shell-proofing to the citadel which usually reached from the forwardmost main artillery turret barbette to the rearmost one. This included protection for the boilers and steam machines/turbines as well as for the munition magazines. Everything else got at most thin armour plates meant to mitigate the effect of shell explosions inside, not to keep shells out.

Battleship Yamato armour layout - the citadel is visible
A benefit with citadel armour / all-or-nothing-armour was that armour piercing shells that could still penetrate this citadel armour at too short or too long distances (the 'immunity zone' was a few thousands metres wide) would do little damage in the non-hardened compartments because their explosives load was small compared to SAP or HE shells.

Let's look; where are our human equivalents to munition magazines?

I'm no medical doctor, but to my knowledge the only parts below the lungs and heard that lead to quick death if perforated are the main blood vessels; abdominal aorta, iliac, fermora. A hit in the lower spine would also be bad, for it almost guarantees paralysis of the legs and other discomfort.
Meanwhile, perforations of the lower torso's intestines can lead to death when perforated, but a 500 ml saline solution should in most cases buy enough time that CASEVAC/MEDEVAC and surgery would save the patient AFAIK. This is AFAIK different with the heart and aorta. Lung hits are really bad as well, and lungs are big. The upper spine is also useful.

So I suppose we could define the area above the diaphragm as the citadel area, and the front is much more relevant than the back. That's about half of the area of a 10" x 12" plate.

example of a 10" by 12" plate carrier
Neither is a good representation of relevant bullets, especially if one thinks of indoors combat (urban combat) where machineguns aren't that prominent, while assault rifles are. AP bullets are not as commonly used in assault rifles, but not terribly expensive steel cores are present in many Kalashnikov cartridges (7.62x39 and 5.45x39). I suppose the 7N22 bullet (5.45x39 mm, penetrates about 10 mm RHA at 100 m*) is about the most penetrating bullet that one should care about in regard to body armour until powered exoskeletons become practical and affordable. This threat is in between NIJ levels III and IV.

The "defense" industry has attempted to cover that gap with unofficial "level III+" and "level III++", but did not set its threshold there. Instead, they focused on common 5.56 mm bullets. Still existing III++ plates give an idea of the weight of a citadel armour plate; its weight would be in the 2-3 kg range including protection against excessive blunt trauma and with little multi-hit resistance.

Well, would such a little armour plate be worth its weight? I suppose there may be niches where that's a "yes" (mostly indoors), but even 2 kg can make a huge difference when you have to clear many buildings with many stairs. You'll only get the weight down and be agile if you muster the self-discipline to shave off weight on every opportunity.

There's probably but a weak case for combining both a flak vest and such a single chest plate. The only common fragments indoors are from AP mines, 30 and 40 mm grenade launcher HE/HEDP grenades**, HE/HEDP rifle grenades**, defensive hand grenades and offensive hand grenades with fragmentation sleeve added.
I don't have experiment test results on hand, but I suppose that blast, bullets and effects from large calibre hits from outdoors are the main problems indoors, not those fragments that a vest would stop at 2-4 m distance.

So there may be a case for a spare plate carrier with a single anti-7N22 chest plate (about 500 square cm) that could be worn instead of a nearly identical weight flak vest for indoors missions. This could be grabbed together with other equipment for combat in settlements (fighting in built-up areas / OHK) such as crowbars, collapsible lightweight ladders, demolition equipment for going through walls et cetera. This stuff would/should be stored in the infantry squad's transport vehicle (2 ton 4x4 or APC).


*: Afaik penetration may actually be less than that at very short distance because of vibration issues, but I'm not sure about this.
**: Shot into the building from outside, thus typically hitting the back. Nose-fused HE is then much more dangerous than base-fused HEDP.


  1. Focusing on weight exclusively tends to ignore heat load.

    The USMC seems to be leading in systems integration. So, how does all the armor & LBE fit together?


    1. Elsewhere I argued against chest rigs, so my position is that "LBE" should be little more than a padded belt.
      And the USMC is leading only in marketing.