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Assume that roughly at 1200 AD, Europe discovers a new metal (or alloy). By terms of various strengths and hardness it's comparable to high grade steel, however instead of steel's density of ~8 g/cm³, this material has a density less than even aluminum at ~2 g/cm³. The ease/difficulty to work it is roughly comparable to steel. By terms of rarity it is comfortably more costly to acquire than steel is to create, but not nearly enough so to make it considered a precious metal.

While this is not the focus of the question, weapons would also have access to this metal.

This armor crafting techniques have the next couple hundred years or so to mature, and we reach roughly 1450 AD.

Would plate armor maintain the same form given this new material? Practically, what might this new armor change for the knight on the battlefield?

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    $\begingroup$ Have you considered leaving the heavy metal to the heavy cavalry and using the light metal for light cavalry? $\endgroup$ – Kys May 23 '17 at 19:36
  • $\begingroup$ I believe that this question has already been covered. $\endgroup$ – ShadoCat May 23 '17 at 19:43
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    $\begingroup$ Out of curiosity do you have a metal in mind? I ask because it sounds like you're describing a world with a naturally occurring HEA such as CrMnFeCoNi. $\endgroup$ – Myrdden Wyllt May 23 '17 at 20:24
  • $\begingroup$ @MyrddenWyllt No real-life metal in mind, actually. $\endgroup$ – Nex Terren May 23 '17 at 20:27
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    $\begingroup$ @Shadur Price depends on offer and demand. They are demanded because they're pretty, but they are valuable because they are rare and so, there's not a lot of offer. An exception would be diamonds, which are pretty common, but scarcity is forced by a de facto monopoly by the De Beers company. $\endgroup$ – Rekesoft May 24 '17 at 11:16
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I'm seeing other answers here which assume no change in armour design, I would argue that that is unlikely to be the case.

Disclaimer: I am not an expert in metal working or medieval armour, though I do have an interest in the area.

One answer argues that lighter metal does not help against blunt weaponry, but this is not necessarily the case. There are several ways armour designs can be changed to help absorb the impact of a heavy blow, but can only be implemented to a certain degree in traditional smithing because of the added weight. Areas of overlapping, but not touching, plates are one example. Each plate takes a certain amount of energy to move, and so consequently decelerates the incoming weapon.

If you're metal only weighs a quarter as much as steel, than you could have three layers of armour. These layers would be most effective if the hung slightly away from rather, or we interspersed with padding.

-The innermost is heavily fluted (the lines you see on Maximilian plate) holding the middle layer away from it (and strengthening the metal), and is the heaviest, this layer covers important areas such as the torso and head, as well as a fauld/tasset structure.

-The middle layer is lighter, a kind of maille with well placed scales or plates upon which small spokes are mounted, which sits on the fluting over the torso, but also covers the limbs.

-A third and outermost layer resembles the classic "knight in shining" armour, sitting atop fluting on the helm and the spokes on the middle layer elsewhere on the body.

There are a couple bonuses to this type of layered design. Assuming each of the three layers masses similar to traditional Maximilian style plate, it is lighter. Plate maille with padded armour underneath was very warm, but you could place staggered ventilation holes in each layer without compromising your protection. It would be considerably stronger than traditional armour, as it offers three times as much material between your meat and your opponents weapon.

It would not make the knight immune to blunt force trauma but it would certainly help, and other design choices could be made to improve the armour further. It would however be in essence three suites of armour, and made from a more expensive metal, so likely be very uncommon due to the price-tag.

As for any concerns of speed or agility, it likely wouldn't change an enormous amount, knights were trained to move in their​ armour and it's weight was not really much of a hinderence to them.

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    $\begingroup$ Puttingn 3x more material between the "meat" and the opponents weapon also means putting more limitations on how freely you can move your limbs. For example, with just a t-shirt on, I can fold my arm straight across my chest. With a thick snow-jacket on (equivilant thickness of plate, (actually less)), I end up with perhaps a 10-15 degree angle at maximum "folding". With 3 times that thickness on my chest area? I wouldn't be able to bring my arm across anymore! My upper arm would be stopped by the material, and my movement is now severely impaired. Note: This applies to all joints. $\endgroup$ – Aify May 24 '17 at 18:08
  • $\begingroup$ @Aify I've some basic training with a bastard sword, and I can't say I was ever asked to fold my arms across my chest.. your also assuming uniform thickness in the armour, which could be thinner around joints, though I'd be able to better address your comment if I knew what angle you were measuring at 10°-15°. It is worth keeping in mind though that plate armour was only a few millimeters thick, and layering and spacing it as suggested would still likely be under an inch in the thickest areas, such as the middle of the chest. $\endgroup$ – Myrdden Wyllt May 24 '17 at 18:23
  • $\begingroup$ @Aify if you have more experience with medieval armour than I, I would be more likely to defer to you, but the snow jacket thickness comment makes me doubt that somewhat. $\endgroup$ – Myrdden Wyllt May 24 '17 at 18:24
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    $\begingroup$ For me, it's less experience with classic medieval armor but my experience with building my own armors for combat training, cosplay, and costumes from which I make the point. I don't have any specific training in medieval combat styles, but I have semi-extensive training in the Eastern arts, in which many movements require the arms to be placed close to the body - something that thickened armor only ever prevents. When I said snowjacket = thickness of armor, I had included all the padding and space between the user and the armor itself as part of the thickness. $\endgroup$ – Aify May 24 '17 at 22:22
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You'd think that a lighter material just as strong as steel would make better armor, as there are two possible ways to go about it; the same design and thickness but lighter weight resulting in increased battle mobility, or a thicker design which keeps the total weight but has more material since it's so much lighter.

You thought wrong.

Knights in full plate armor typically had to worry about 3 things:

  • Artillery (giant rocks from catapults, trebuchets and ballista ammunition, etc)
  • Heavy weapons (Maces, war hammers, etc)
  • Weapons slipping into the cracks of their armor (Rapiers, Daggers, possibly lucky arrows). Note that well developed steel plate (eg: french plate) was designed with various angles and slopes, rendering the user pretty much invulnerable to arrows in general. Also note that this can't really be defended against.

Two of the above kill the knight due to mostly concussive injury. Even if you go with the armor thickening route, you'll still die from the impact (or at least be seriously injured). Thickening the armor will also result in decreased mobility in terms of joint areas (or more exposed joint areas to allow the same mobility). This is bad.

In fact, you would fare worse off against heavier steel weapons in general if you went with the lighter weight route. Consider that if every knight had the same material type armor, each knight would get a similar speed increase; the knight can now hit harder with a heavier weapon compared to your lightweight thin armor - crushing your skull in the process.

Certain bladed weapons might be made lighter - allowing for faster manipulation, which also works against plate armor (see slipping into cracks point above).

All in all - It would change nothing for the knights wearing the armor on the battlefield.

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    $\begingroup$ Your answer was exactly what I was going to say +1. The weight wasn't just a disadvantage, it was an advantage! $\endgroup$ – Erin Thursby May 23 '17 at 19:04
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    $\begingroup$ @Christoph I'm saying that the knight with the heavier armor against the knight with the lighter armor has an advantage against the weapons that worked best against plate armor - it's just physics. Imagine a car crashing into another car - both are made of similar strength materials, right? It's likely that at a high speed, both cars will be totaled. However, imagine what happens when a heavy truck slams into a smaller, lighter car - still the same strength material, same external forces involved, except the heavier car is barely damaged while the small one is totaled. 1/2 $\endgroup$ – Aify May 24 '17 at 8:22
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    $\begingroup$ 2/2 It just so happens that when the knight has a heavy steel mace - against the lighter armor, damage is amplified. Since damage is a result of forcing the head/body into the armor at high speeds (blunt force trauma), and taking into consideration that the impact force is going to be the same in both cases, the only variables that change are the weight. When object A hits object B, the lighter object B is relative to object A, the farther it flies. (Assuming no aerodynamic calculations). $\endgroup$ – Aify May 24 '17 at 8:30
  • $\begingroup$ The differential advantage to reducing the weight of a rondel dagger (the most likely weapon to slip between plates) is probably negligible. Knives and daggers already move at nearly the speed of empty-handed fighting, which is why knife fights in real life are so terrifying. Mostly it worked like grappling: You need to place the point at or near the gap in the armor, then forcefully thrust it in; you may have to work past secondary layers; at the neck, it'd be the gorget for instance. To get the target to hold still for that, you'd need to establish control over them for long enough. $\endgroup$ – tex May 24 '17 at 13:15
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    $\begingroup$ Exhaustion was a factor that was pretty significant too. $\endgroup$ – enderland May 24 '17 at 18:03
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Weight is not an issue

The Wikipedia page for Plate Armor says the following:

A complete suit of plate armour made from well-tempered steel would weigh around 15–25 kg (33-55 pounds). The wearer remained highly agile and could jump, run and otherwise move freely as the weight of the armour was spread evenly throughout the body.

...until it is

One of the reasons people stopped running around in plate armor was because it wasn't bulletproof. One of the reasons they didn't try to make bulletproof armor is that it'd be too heavy. I'd imagine that with superlight metal, armorers could make bulletproof (and crossbow-bolt-proof) cuirasses and helmets, which could keep the fully armored knight relevant for a few more centuries, if not all the way into modern times.

The main tradeoff I see, though, is that you're still going to need to increase the armor's volume; that's going to make it hard to increase armor to the limbs, especially near the joints.

And if you have a few more centuries...

With more time comes more available armor to buy/inherit, and thus more availability of hard-to-produce items. I'd think that aside from better personal armor, you'd start seeing things like bulletproof horse armor and bulletproof Pavises. This should have drastic effects on how battles are fought in the gunpowder era. Cannons should still be effective, as will artillery, so the age of the knight should still end in the early 20th century, but until then I'd like to imagine a battlefield where a musket stands on equal ground with a warhammer.

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    $\begingroup$ I'm picturing Marines in plate armor, with slightly modified guns (larger trigger guards, etc.) so they can be used by them, and I just realized I'm thinking of space marines. I like this answer. $\endgroup$ – Nic Hartley May 24 '17 at 18:21
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I don't think much would change. If I'm understanding this right, the volume of metal being used isn't changing, we're just saying that volume weighs less.

You need to look for places where less weight is better - and there weren't a whole lot of those in medieval warfare. I think the biggest candidate would be boots + legguards. Reducing their weight 75% (or even 25-50% if you use an alloy of this and steel) would make it much easier to march with said equipment on. Making all the armor out of this would make marching even easier, but it would have a lot less impact than the items on your feet/legs (consider how much easier it is to walk with a child on your back vs. clinging to your leg). This may also apply to gauntlets (at least the finger portions).

In terms of weapons, if your sword weighs 1/4 as much and moves 2x as fast, you've still only got 1/2 the momentum you used to (if my maths are right). This reduction makes it harder to injure your opponent and easier for him/her to deflect your blows. Probably not what you want (but may be worth the trade-off for a stiletto or rapier where you already can't compete).

For overall weapon+armor applications, you'd probably see people experiment and wind up wanting to make this into an alloy with steel. Probably something like 10-20% new metal. There would likely be a point where the speed/agility gains outweigh the resilience/impact losses

I would anticipate seeing this in auxiliary uses much more. Look for cases where weight might be an issue and more strength is desirable. This new material is only 3-5x as heavy as wood instead of 10-20x so you could see people replacing (or reinforcing) wooden products with this.

You could look at places we use aluminum and other light metals today as candidates for similar things in the past. Camping gear, cookware and storage vessels would be ones I'd pick in particular. Cookware would potentially be a big one, if you can cut down on 75% of the weight you can bring a bit more and increase the quality of food. A 55 gallon drum weighs around 20-25kg, with this metal that would drop to 5-8 kg making this a much more viable storage solution. This might make really good wire rope as well, or be used in place of chain where less weight is beneficial.

If you know one, I'd consult an engineer on the full ramifications of reducing mass while changing nothing else.

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  • $\begingroup$ I had to upvote for the child-weight example alone, if nothing else. So true... $\endgroup$ – Angew May 24 '17 at 8:45
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    $\begingroup$ Your maths are half right. Momentum is mass by velocity but when dealing with impact weapons, it is the kinetic energy that counts, and that is mass by velocity squared. Double the mass and you double the energy, but double the speed and you quadruple the energy. Consider the damage caused by big man (100Kg) walking slowly into you (1m/s) compared to a small cannon ball (1Kg) hitting you at 100m/s, the momentum is the same, the kinetic energy is not. $\endgroup$ – Paul Smith May 24 '17 at 11:14
  • $\begingroup$ This isn't necessarily a valid comparison, as the energy of the cannonball is focused on a much smaller area. If 1kg of cannonball was spread over your whole body at 100m/s, it would deal a lot less damage. $\endgroup$ – Puppy May 24 '17 at 20:20
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@Aify has done a marvelous job of making salient points, for example that concussive force had an er...impact... but let me expand a little.

The biggest thing perhaps is this: better steel won't just go into your armor.

There's no way that they'd develop this and not incorporate it into weapons. What this means is if you are picturing bladed weapons simply snapping when coming in contact with your armor--it won't. Bodkin arrows, at close range, could already penetrate armor. Load it up with the new steel and see what happens. Possibly it would need to be weighted for extra force, but edge it with the the newer stuff.

And there's also no way that they wouldn't make use of weaponry with greater density, in order to bop the knight about. So the cheap steel might actually be BETTER against it BECAUSE of the weight. Or make bladed weapons more nimble using the lighter steel, to get in through the cracks.

The actual thickness and strength was secondary to good design as far as armor was concerned. For instance, designing a surface so that it was more likely to BOUNCE rather than take the full force of any given thing--or so that the kinetic energy is redistributed more fully.

There's also something else you might want to consider. It's hard to design armor for every situation. Some armors did better against arrows, but didn't fare as well against, say, a heavy mace. Even today, it's difficult to get armor that's proof against everything, and we do need joints to move, so that's always a weakness.

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    $\begingroup$ Why edge a bodkin head with the new allow? The penetration power of the arrow depends on its inertia and its cross-section; since the launching force depends on the bow, both an iron-headed arrow and a alloy-tipped one will have the same momentum (mass x velocity). The alloy-tipped arrow will accelerate and decelerate more swiftly. $\endgroup$ – Daniel Holz May 23 '17 at 22:01
  • $\begingroup$ @DanielHolz For the cutting part of the the broadhead, I would edge it with the new stuff, so the edge has more of a chance to get through. Wouldn't make it the whole thing, because weight is actually needed for effectiveness. $\endgroup$ – Erin Thursby May 24 '17 at 2:31
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    $\begingroup$ @Daniel Holz Are you sure that it's the momentum that's important? From comparisons of modern ammo, I see that it's mostly the kinetic energy that's being compared. It grows linearly with mass, but quadratically with speed, which is why smaller bullets have gained popularity in the last decades. $\endgroup$ – AndrejaKo May 24 '17 at 9:59
  • $\begingroup$ @AndrejaKo I don't understand; the kinetic energy is the work needed to accelerate the projectile to a given speed, but an archer will always do the same amount of work: the alloy-headed arrow have a higher velocity due to its lower mass, and vice versa for the iron-tipped one. Oh, I was taking the impulse into consideration: the more massive arrow will decelerate more slowly, and penetrate further into the target's armor. $\endgroup$ – Daniel Holz May 24 '17 at 23:06
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Couple things you should consider.

  1. Rarity: If your super metal is x percent more expensive it will have a corresponding reduction in commonality. Basically the concept here is that in the medieval era, armor was not issued to soldiers (nobles) they each purchased and maintained their own. If the armor is more expensive correspondingly fewer will have it.

  2. Functional changes: Most importantly the armor will be thinner and lighter. The main goal of plate is obviously the protection of the wearer but the counter point to that is that they were slow and would tire easily.

The density difference between your material and steel (which lets be honest, medieval steel was not of comparable quality to what we have today) will allow for a whole range of changes to medieval war/combat.

  • With lighter armor knights could function far better unmounted. While there was certainly a prestige aspect to riding into battle it was also practical, walking with plate armor on is exhausting.

  • Speed. Faster troops give a commander many more options particularly in scenarios where mounts are not practical, sieges and forested areas as examples.

  • Agility. Obviously related to speed, agility was severely limited when fully armored. This limited weapon/fighting style options, but with increased mobility you could do all sorts of fun things with staves, dual blades etc etc etc.

As far as appearance goes it probably would not change overly much. Plate armor's value is in the fact that it covers the body literally from head to toe. There are no unprotected body parts. I can see the armor changing in small ways though.

  • More plates. More, smaller plates, particularly around the joints would allow the wearer to make use of the lightness and would be necessary for the increased agility mentioned above.

  • Less padding. Lighter armor would require less padding underneath. This further improves agility and also keeps the wearer cooler meaning they wouldn't tire quite as fast.

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