Designing for a firefight: The importance of polyethylene in armor design

Ballistics is a lot like aviation. Both demand a great deal of attention to safety.

I have spent over thirty years in an aviation career, and one of the most important lessons in aviation safety is design. A safe design is redundant, and it removes as much room for human error as it can. That is because, despite our best efforts, people make mistakes.

Every material has an envelope

Redundant design is also required because materials have limits. Every material in an armor plate performs well inside a certain envelope and less well outside it. Many armor manufacturers say little about where those limits are. It is important to discuss them, because you cannot engineer out a vulnerability that no one is willing to name. Naming it is the first step toward building a layup around it.

Ceramic's glaring vulnerability: the second hit

Ballistic ceramic is a great example. It is very effective at stopping high-powered projectiles, but it has one glaring vulnerability: multiple-hit performance. The ceramic fractures as it defeats a round, and once the strike face is damaged or missing, a second shot in the same vicinity (usually within about three inches) can penetrate the armor. How close is too close depends on the caliber, the velocity and the design of the plate. A thicker Ultra-High-Molecular-Weight (UHMWPE) backing material can make a plate less vulnerable, especially to lower-caliber follow-up shots.

Consider an officer on patrol wearing an RF3 ceramic plate: a capable strike face over a standard UHMWPE (ultra-high-molecular-weight polyethylene) backer. The officer is hit by an AR-15 firing M193 ball, a 5.56mm round. The plate stops it. But the ceramic around that impact is now damaged and no longer has the resistance it was designed with. If a second M193 round lands near the first, the probability of a complete penetration goes up, because the backer was never thick enough to stop that round on its own. An RF3 plate, built for far more powerful threats, can be defeated on the second hit by an RF1 round.

Start from a backer that stands on its own

The answer is to make the polyethylene do more of the work. Well-performing polyethylene, in sufficient thickness, can resist some RF1 and RF2 threats independently of the ceramic. If the backer is thick enough to stop the RF1 threats by itself (7.62x39mm mild steel core, 7.62x51mm M80 and 5.56mm M193, the three rounds that define NIJ RF1), the plate starts from a position of strength: reduced vulnerability and increased redundancy. A thinner ceramic strike face on top then adds protection against RF2 and RF2+ threats.

Now replay the patrol scenario with that plate. The first M193 round damages the ceramic. A second RF1 round lands in the same spot. This time the backer is built to stop that round on its own, damaged ceramic or not. That is redundancy: when one layer is compromised, the next one can still do its job.

What redundancy does not mean

This does not mean the design works against every caliber. No plate does. What a redundant layup does is increase the probability of survival after the first hit has already happened.

Design for the second shot

In aviation we design for the failure we hope never happens. Armor deserves the same discipline. When you compare plates, ask what happens after the first hit, and choose a design that still protects you when part of it has already done its job.