Whereas fictional armors are often shaped through a kind of evolution whereby costume designers, artists and animators see each other’s costume ideas and iterate on them, armor development responds (within the limits of the physical materials available) not to other armor design, but to the demands of the human body (you need to be able to bend and move and armor needs to be of a weight a human can wear) and to the threats the armor is meant to defeat.
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Armor works largely by converting various kinds of piercing or slashing attacks into blunt trauma distributed over the widest possible part of the body. And that in turn is part of the advantage of using rigid materials in armor construction.
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A rigid material can spread out the energy of a weapon impact over a large surface; because assuming it remains rigid the entire armor component moves from the impact, contacting the body across a much larger area. The power of distributing impact energy in this way is pretty stark. A 50J impact concentrated into a very small, sharp impact zone (like the tip of a spear or an arrowhead) can easily produce lethal wounds. By contrast 200J applied across your entire chest is something you’ll certainly notice, but probably won’t cause any permanent injury. Indeed, as modern body armors show, impacts upwards of two-thousand joules (the energy delivery of many modern rifle rounds) is quite survivable if spread over enough of the body. So rigid elements (be that a breastplate or, as in modern armor, something like rigid plate inserts) can be of tremendous value precisely because they’re rigid and thus spread out the energy of impact.
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Thicker armor means more weight, which adds up fairly rapidly, while more complete protection around joints means reductions in mobility. So an armorer has to think pretty hard about the tradeoffs between mobility, weight and protection. And one of the key questions here is, quite simply, “where is an opposing blow most likely to land or be most dangerous?”
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By contrast, the threat profile of gunpowder warfare is slightly but importantly different. On the one hand it is a lot harder to armor against bullets because they arrive with much more energy. And I want to stress: much more energy. For a sword or spear swung by human arms, the upper limits6 are around 130J, though most blows will be much weaker than this. Arrows, as we’ve noted, top out around the same energy at launch but fall off somewhat in flight. By contrast, musket bullets can arrive with many hundreds of joules of energy and modern rifle rounds can deliver in the neighborhood of 2,000J of energy on impact. So armor that is trying to stop such a round has to be able to absorb a lot more energy and successfully spread it out over more of the defender’s surface.
The other factor is that, whereas melee strikes originate at the shoulders but can be rising strikes (‘uppercuts’) or falling strikes or horizontal strikes, bullets and other direct-fire weapons (this would be, for instance, equally true of directed energy weapons) fly very fast on relatively flat trajectories, which means the threat is mostly to the front of the body.
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Consequently, whereas armor against contact weapons tends to want fairly complete coverage of the torso (including the sides and the tops of the shoulders), armor against bullets (and other missile weapons) is much more concerned with covering the vertical surfaces of the torso and is willing to compromise armor on the shoulders and even leave gaps in protection, if that means achieving a favorable balance of coverage and weight.
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The first solution to the problem of how to use a rigid material to armor the body is of course to simply armor the parts of the body that don’t bend and then use some other material to protect the parts that do. Archaic Greek ‘bell’ cuirasses and later Greek and Roman muscle cuirasses take this approach, with the cuirass terminating at the hips and hanging leather strips, called pteryges, hanging down to cover the rest of the hips, groin and upper legs. But this is not exactly an ideal solution, as it sacrifices a lot of coverage.
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The earliest of these articulation solutions is scale armor, by which we mean an armor composed of a lot of small rigid scales (metal or hardened leather, typically) which are fixed to backing material (textile or leather), so that they hang down. The scales overlap, which presents a solid metal face to the enemy, but since they move independently, little mobility is lost, allowing a scale coat to extend down past the waist and even cover the legs. The weakness of the approach, however, is that the scales are only anchored to the backing material at the top; there’s not much to stop a blade or spear-tip from sliding up one scale and beneath another, thus penetrating the armor. That’s less of a concern for something like an arrow-strike (which is going to be descending at least somewhat when it arrives) but against an opponent with a sword or dagger in close combat, that is a very real weakness.
A way to solve that weakness is to connect the scales to each other rather than to the backing, so that an opponent cannot slide a weapon underneath them or flip up a scale to render the opponent vulnerable. That solution — small metal plates connected to each other, rather than a backing — we call lamellar armor and it was very common in a wide range of cultures, but it has very little purchase in modern fantasy or science fiction armor designs, I think primarily because it was not included in the Dungeons and Dragons armor system. Nevertheless, lamellar armor was quite common in a wide range of cultures: we see it in the Near East, in Europe, in China and in Japan. The rigidity of the overall armor for lamellar varies based on how the plates are connected together (which you can see quite clearly in Japanese armor, in which a single set of armor often includes both rigid surfaces and articulation both using lamellar, connected more or less rigidly). In Europe, we see a variation on this concept, the brigandine (also underused in fantasy settings) where the metal plates are riveted through each other and a textile or leather backing.
But of course the solution we’re most interested in is plate armor, where a set of armor (a ‘harness’) is composed of a set of articulating plates which both provide a rigid protection to the wearer but also articulate where the wearer needs them to bend. Now going through all of the different methods late medieval plate armor uses to allow the armor to articulate would run beyond the scope of this post, but the relevant part here is the way that plate armor articulates over the torso, broadly speaking. The key components here are the cuirass, composed of a breastplate and a backplate, which covers the upper-half of the torso; this component is generally entirely rigid over that surface because the human body doesn’t bend there much either (on account of the rib-cage).
Below the cuirass, often directly attached to it, is a component called faulds. This consists of a set of articulating ‘lames’ (horizontal strips of armor) connected via leather straps or sometimes sliding rivets so that the lames can telescope into each other to enable the user to bend at the waist or raise their legs or even sit down. Faulds usually extend over the hips (sometimes only on the front) and a bit of the upper legs but occasionally run down as far as the knees. Then in many armors, an additional pair of metal plates hang down from the faulds to cover the upper legs called tassets.
Above the cuirass, we have pauldrons or spaulders (we needn’t here get into the differences), which protect the shoulders and upper arms. These are structured with a shoulder ‘cop’ — a dome-shaped metal piece — covering the shoulders, to which were attached a series of descending lames (articulated the same way the faulds would be) to apply coverage to the upper arms. Crucially, these pieces generally attach to the cuirass (though spaulders often also attach to the upper-arm armor called the rerebrace) rather than just to the upper arms, because as you will recall protecting the top of the shoulder is really quite important. Indeed, even a casual look through ancient and medieval armor will quickly reveal that this armor tends to be the thickest on the shoulder: Early mail armor often featured a second layer of mail to cover the shoulders, for instance; for some medieval armor, a mail coif or aventail also provided a layer of protection over the mail covering the shoulder.
The key advantage of this setup is that by terminating the solid form of the cuirass at the ‘natural waist’ (where the body is thinnest) the cuirass allows the wearer to bend and rotate at the waist, while the faulds, with their telescoping design, allow the wearer to bend down at the waist, raise their legs or sit. Likewise, the segmented, articulated construction of the pauldron both protects the shoulder, but also allows the arms to be raised.
