Increasing the protection offered by a helmet has been an age-old quest. From edged weapons to arrows, fragmentation to bullets, it's always been a type of arms race to provide the next best solution to protect our heads.
In World War One, Germany offered one of the first up-armor or applique solutions to their common M17 helmet. This front plate was applied via ventilation lugs on the side of the helmet and a leather strap going around the back. This provided increased protection, especially in trench warfare where the user's head was generally the first target presented. You can still buy them today here, but it will cost you!
Moving on to more modern-day solutions, you will see that the materials have changed slightly, but the general concept of adding extra material or an applique is largely the same. Velocity Systems has created what they call a SLAAP plate that attaches to modern day high cut helmets to provide the user with increased ballistic protection. This plate is designed to stop 7.62 X 39 caliber, 123 gr. FMG with mild steel core penetrator at 2,400 FPS. All while keeping backface deformation under 25.4 mm (1 in).
Photo From http://soldiersystems.net/2012/09/27/ach-slaap-velocity-systems/
Another concept that we have worked on here at Hard Head Veterans in conjunction with Revolution Armor is to not only defeat 7.62 X 39, but also 7.62 X 51 and 5.56 at full rifle velocities utilizing next-generation metals. The problem with these is they are much heavier than ceramic or fiber up-armor options. They do however stop the mentioned threats above, at full rifle velocities and with minimal backface deformation. The other issue with these next-generation metals is their extreme hardness. They are very difficult to form into curvatures that will fit a ballistic helmet and still maintain their ballistic qualities. Designs like these would only be best suited for static type helmet use, such as in a gun turret, or for a very short period of time while mobile.
UPDATE: After a ton of R&D and working with Diamond Age Armor, we are excited to be offering our up-armor solution for our ATE ballistic helmets. Not only is it lightweight (right at a 1 lb), but it defeats the most common rifle rounds out there! Made from silcon carbides it allows us to achieve incredible protection ratings with a thin and lightweight solution.
A common question we hear is "even if you stop the round wouldn't the impact turn you into a vegetable or break your neck?" The short answer is no, not at all. As long as the BTD/BFD is within acceptable standards you will suffer a light concussion worst case. In terms of your neck snapping, not even close. Does the recoil of the rifle against your shoulder rip your arm off? Modern ballistic helmets are designed specifically to combat these threats! If you would like to learn more about this, we have a rather in-depth blog about it here!
Another question you may be asking, is what about the face? As with everything in helmet armor solutions, there seem to be trade-offs and with face protection, it's no different. You have to trade off the cost of weight and vision, for protection and coverage. On the market currently, there are a number of options that address this issue. From the common face shield, you see riot police wearing that may offer blunt to ballistic protection. To full mandibles and cheek protection like you would see on helmets of ancient times. 3M captured the most recent U.S. Army ballistic helmet award for their IHPS helmet that provides all of these things.
As more and more technology is improved, created, and applied. Up-Armor helmet options will as well. Perhaps, in the near future, we won't even need to wear helmets, but just activate our star trek type force shield for protection.