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Fairly straightforward question. If you can build power armor for the purpose of enabling infantry to carry bigger weapons, heavier loads and thicker armor with more mobility (and for some reason haven't replaced human soldiers with robots), how large could this armor get before it starts being more trouble than it's worth?

I see two major issues: The first is visibility. A larger soldier is going to need a larger foxhole to hide in, and going to be easier to spot, and therefore easier to shoot at. The second issue is urban combat. A guy in armor that makes him as broad as an ordinary man is tall just ain't going to fit through most doors. Naturally, if you see more problems, feel free to point them out.

We all like our Starcraft Terran Marines or 40k Space Marines, but something tells me that they just aren't practical.

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  • $\begingroup$ What tech level are we looking at? $\endgroup$ – Ruther Rendommeleigh Jul 3 at 14:10
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    $\begingroup$ A fascinating twist on this is to consider the biological body of an organism to be the power armor for its brain! $\endgroup$ – Cort Ammon Jul 3 at 19:56
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It Depends

As with any military/weaponry question, the question is application.

The current broadly-deployed powered armour definitely has trouble fitting through doors, but it is pretty efficacious at helping soldiers carry heavy weapons into combat, and allows a soldier to put enough armour between themselves and the enemy that foxholes aren't really necessary.

So if you're willing to go big (and power constraints don't become a problem), then the only limit may be your ability to build actuators to move your power armour. In straight-up combat engagements, size may never be a liability.

If, however, you want to deploy your powered armour in situations where current infantry barely fit (breaking into houses and whatnot), then that is your limit right there. It can't be much bigger than a soldier, it can't weigh much more than they do. Powered exoskeletons with light armour, maybe.

It's actually the middle-ground powered armour that seems to have very little application. Not small enough to allow the wearer to do tasks that require them to be human-sized, not big enough to play with the big boys in terms of armament, speed, or armour. Looks cool, though!

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    $\begingroup$ So, TLDR... Go big or go home? ;) $\endgroup$ – Steve-O Jul 3 at 13:37
  • $\begingroup$ Weight may not be as big of a constraint as you might think. If the armor is heavy but has disproportionately large boots (like snowshoes), then its ground pressure could be lower than that of a normal human. $\endgroup$ – Richard Smith Jul 3 at 19:06
  • $\begingroup$ This is true, but then you run into the "unwieldy when trying to maneuver in spaces that a normal human can navigate" problem. We build environments for human-shaped/human-weight things. If you try to climb stairs with tennis-racquet-sized shoes to support 400kg of steel, motor, and batteries, it will not go well for you. $\endgroup$ – jdunlop Jul 4 at 0:19
  • $\begingroup$ +1 Part of the Australian role of the infantry is "seize and hold ground". If the armour is too big to even enter buildings proportioned for normal humans then the infantry are unable to perform their role, and buildings in urban areas are some of the most valuable ground around. $\endgroup$ – KerrAvon2055 Jul 4 at 4:58
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Don't do the same error the Germans did with their wunderwaffen:

  • if your soldiers are going to need a reinforced concrete floor to be able to go anywhere, you have basically made just really thick office workers. A sunken in the ground soldier is just an easy target and an obstacle for the rest of your army.
  • if your armor will take a lot of resources to be made, you are impoverishing your army, because you won't be able to make something else
  • if your armor needs a lot of fuel to be powered, you are hampering your army with an exceedly complex supply chain
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    $\begingroup$ Never underestimate the danger of terrain. $\endgroup$ – Joe Bloggs Jul 3 at 5:57
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At some size your power armor is arguably not 'power armor' anymore - it becomes some kind of mech or vehicle instead. I don't know exactly where you'd put that cut off - something like Fallout power armor is recognisable as a person in a suit of armor, while a Pacific Rim Jaeger is clearly a robot, but what about something like the AMP suits in Avatar?

I'd think that in a world where they can design and build large suits of power armor (to the point of being as wide as a person is tall), they'd create different models for different uses to avoid running into the issues you've raised.

  • Light-weight exo-suit style power armor that focuses on mobility. It provides limited protection in combat, but allows you to carry more weight and travel longer distances faster without tiring out. Used mainly for recon missions or moving forces to a forward base, and sometimes by support personnel working in secured locations.

  • 'Standard' power armor that provides good protection against small-arms fire and allows you to effectively use larger weapons that would normally not be used hand-held or on the move - high-calibre sniper rifles, Light Machine Guns - but not too large or unwieldy. Soldiers in this armor can work indoors and are only slightly larger than a regular person. Useful for standard operations like patrols and small-scale skirmishes, and used by standard infantry in most battles.

  • 'Assault' power armor that provides extreme protection against high calibre weaponry and explosives. Able to survive direct impacts from artillery fire, these are used by specially trained solders for assaulting fortified locations. Large, slower moving except in bursts of speed and unwieldy in enclosed locations, these suits usually require the support of other soldiers in standard armor to be most effective.

  • 'Support/Artillery' power armor that provides the best weight-bearing capabilities at the expense of protection. These suits definitely can not fit into buildings, they're used in place of jeeps or small trucks to transport artillery and large quantities of munitions - blurring the line between 'armor' and 'vehicle'. They could even have integrated artillery, like mortar devices, to provide support from long range.

By equipping soldiers with only the armor that is relevant for their missions, you can avoid the armor becoming more trouble than it's worth. Of course you have to manage maintenance and repairs, and the supply of suit power, but I'd assume that's built into the supply chains of a military that uses power armor often. Maybe the bigger suits are used to lug additional small suits and suit parts to the front line.

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  • $\begingroup$ Sort of like the original novel version of Starship Troopers. $\endgroup$ – The Daleks Jul 6 at 13:39
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How do you define 'Power Armour'? The mechs in Battle Tech are meant be varying sizes up to about 12 meters tall (plus), carrying weapons that would easily demolish multiple main battle tanks, large buildings and whole companies of 'mere' infantry at a time. Not excepting nuclear weapons. Tactics? Storm the battlefield and let someone else clean up the mess afterwards. All the time being walking buildings/targets for anyone with anti-armor weapons.

The power armor of Robert Heinlein's mobile infantry (MIs)? Probably less than 3 meters tall. One tank at a time (perhaps up to low double digits in total) and perhaps a squad of 'meat bag' infantry at once, while being able to 'jump/fly short distances. Tactics? shoot - jump - shoot multiplied by however many MI's are involved in the assault. Hard to hit but also bordering on the scientifically impossible, too much power and weaponry in too small a package to be realistic.

Heavy Gear Mechs? By and large pure anti-vehicle weapons. Perhaps 4 meters tall. Some limited anti-infantry capacity in the form of light Mg's but basically walking AT platforms with limited payload and endurance. Needing combined armed support, infantry, artillery etc to succeed in a mission. IMO right smack bang in the Goldilocks zone, not to big, not to small not to unbelievably powerful or maneuverable for their size.

The first 2 can (more or less) go it alone if they have to and win. The last? They always have to be closely integrated with other arms in order to win major battles.

So in terms of believably realistic battles I'd go with something something on the scale of the Heavy Gear Mechs. Not to big to be walking targets, not to small to carry believable heavy weapons and still be impossibly mobile at the same time.

Note; this doesn't mean you couldn't have lighter, power amplified armor for infantry use. That's being worked on now. It just means the troops equipped with it would carry a magnified load of gear gear and be slightly better protected than your average infantry soldier is now. But this (I assume) is meant to be a technologically driven scenario not a magically driven one.

If so, given currently envisaged material science and power chains etc you are probably not looking at more than a X 2 increase in speed and load without turning your soldiers into something to big to hide and hug ground easily. It would also be cheaper/easier to give a squad of soldiers with 'light' amour' a pumped up AFV to travel in to the battle than it would be than to give them all 'super' armor.

So I would suggest a combo of vehicle mounted troops and 'light' Heavy Gear style mechs with heavy support (tanks artillery etc as needed) e.g a combined arms approach.

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Tanks are used because they are relatively cheap, relatively fast and relatively strong. Relatively in comparison with their playing field, compared to infantry.

They don't play well with aircraft nor with boats nor with weapons that can propel themselves over long distances. But those three are so crazy expensive in comparison that they open a gap for tanks. Also, tanks are just there with the soldiers, the other three have to be called in first, which means waiting time, which means eternities if you're under fire.

Build a cheap wearable self propelled weapon (armed KI drone) and the tanks will die out in a matter of months.

The full armor medieval knight disappeared from Europe in less than 30 years when the British introduced their cheap armor piercing long bow and somewhere else the Crossbow came up. Both were invented just for those knights.

In your case, Space Marines have to have a size fitting into a "building" or maybe a space station really. No tank there, no aircraft, no boat. But little flying drones with KI yes why not.

Imagine an unarmored soldier, that guy certainly is easy food for the small drone. If that guy wears a shield and armor made of modern materials, you up the game: In order to shoot you, the other side has to use weapons which could pierce the space station walls. Oops. That weapon will hopefully be rare aboard a space station, so you gain back the advantage of a medieval knight.

Yes, I guess it has to be small and strong. Yes I think my paragraph above is a weak excuse. But Space Marines are cool. :-)

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    $\begingroup$ Please, the crossbow has been around since 1000 year before middle ages. $\endgroup$ – MakorDal Jul 3 at 7:00
  • $\begingroup$ The one which was strong enough to pierce armor was not. It needed a steel spring. Also, don't forget we've lost a lot of what was known during the middle ages. Romans were a lot more advanced than what came after. $\endgroup$ – Anderas Jul 3 at 7:11
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    $\begingroup$ Firearms made armour obsolete... There was "proofed" armour, but it was even heavier and more expensive than traditional knight's armour, and when a bullet impact could shatter metal and not only drive the lead bullet but fragments of armour into a wound, it became more advantageous to have less kit so that it was easier to dodge, than to carry heaps of armour that unless hideously expensive, probably wouldn't work. $\endgroup$ – Monty Wild Jul 3 at 12:54
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    $\begingroup$ @MontyWild, actually not just all bows and all crossbows with any sort of practical rate of fire had slim chance of penetrating full plate, but muskets and arquebuses did not fare much better! Armour disappeared because firearms got so cheap that equipping many more guys with just guns became more efficient. $\endgroup$ – Jan Hudec Jul 3 at 21:20
  • $\begingroup$ … and I mean not just cheap to make, but also cheap to train with. With cold weapons, the warriors had to spend years training and since only a few could be supported doing that, it made sense to give them expensive armour to give them all the edge possible. But guns are much less dependent on the user skill, so it became more efficient to just conscript some guys and send them off to battle with a couple of months training. $\endgroup$ – Jan Hudec Jul 3 at 21:52
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  1. A Problem with Big armor would be heat, as an object grows it's volume is cubic x^3 where as its surface are is squared x^2. This is why big animals like elephants have additional surface area, like the ears to cool more, it's also why they use water and mud to cool. Tanks have relatively large vents to cool their engines and these are very vulnerable (In WW2 Molotovs were used to break the engines). If you have power armor you might use batteries and electrometers, this presents it's own unique challenges for cooling. Big armor might require massive heat sinks to keep it at operational temperature, this would be a unique vulnerability and if sufficiently damaged might render the armor inspirational within minutes. On the other hand you could make this video gamy, as in no big heat sinks required, but whilst using the armor the wearer has to monitor their thermals, to ensure that their armor doesn't overheat. Firing a rifle too long with recoil reduction? The motors in the arm and shoulder get too hot and you need to take a break, meaning you can't fight back for a while. Running too fast for too long? Now you need to wait and if you need to scram in that time you can't. I think you get the Idea.

  2. Deformation. When a large round impacts close to a joint the material moved out of the way may be in the joints path and lock up the armor, In big armor the amount of material in the way may make it impossible to get this unstuck in combat, making them glass-cannons, that instead of dying become sitting ducks.

  3. Joints Joints are extremely hard to armor and real life power armor would require something like an armored skirt. This may be in the form of a literal skirt out of kevlar, to catch incoming rounds and stop them from penetrating an deforming the joint. A complex joint like the back of the knee or the inside of the elbow is almost impossible to armor properly, so at some point kevlar armor skirts are no longer powerful enough to stop incoming tank rounds.

P.S. My definition for power armor would be: as long as the wearers limbs are in the armors limbs it is power armor and not a mech. This would turn the "Robots" in Avatar into Mechs. Fallouts power armor would remain power armor under this definition.

To answer the question I'd say that everything that is intended to do more than stop high caliber rifle fire, across the entire body, & assist in movement and firing weapons or act like a weapons platform (btw great movie : Edge of tomorrow you should watch it) is probably to large. If you can't enter a building or vehicle with it you more or less loose your worth as infantry. Creating powerful weapons platform that tear through armor is very easy and cheap. This would completely invalidate large power armor (if you want additional info please write a comment and I'll add it). Smaller power armor, as I described would enable vastly more powerful infantry, where as the other (assuming near current levels of tech) would just be vastly more expensive and less capable than modern tracked or wheeled vehicles.

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  • $\begingroup$ The link in your last paragraph yields an HTTP 403 error and that’s why it’s a good practice to copy the info from your link into your answer. In that case, what movie are you talking about ? You got me curious. :-) $\endgroup$ – breversa Jul 3 at 14:55
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    $\begingroup$ @breversa fixed the link, thanks for letting me know :) $\endgroup$ – The_Moth Jul 6 at 6:58
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Let's Start with the Most Primitive Power Armor and Work Our Way Up

enter image description here

Medieval plate armor was the original iron man. Very carefully engineered, you could run in it at full speed in it, fight all day long in it, and do some gymnastics in it.

It was 1.5 millimeters thick, weighed between 15 and 22 kilograms, and could stop up to a shot from a modern 0.44 magnum.

Benefits: The basic armor, then, is protective against almost all small arms fire.

Liabilities: as recorded in multiple battles where armored knights attempted amphibious assault, if you could sink the boats before the knights made landfall, they would drown. You could not swim in the armor.

Scaling Up

enter image description here

With the aid of an exoskeleton, you could increase the thickness of the skin 10x from 1.5 millimeters to 15 millimeters. This would increase the weight of the armor from 15 - 22 kg, now it would weigh as much as an adult human 150 - 220 kg. For a sense of scale, the plate is ${6 \over {10}}^{th}$ an inch thick, barely adding to the dimensions of the wearer.

The muzzle energy of a .44 magnum varies between 1 kJ and 2kJ. Ballistics isn't as simple as muzzle energies and armor thickness, but for a ballpark estimate a suit of plate 10x times as thick will be able to withstand ten times as much energy ~ 10kJ to 20kJ.

Benefits: This armor thickness is protected against up to the M1A1's auxiliary 0.50 BMG machine gun, whose muzzle energies are around 18kJ to 20kJ. But an A-10 primary gun GAU-8 round, at 100 kJ, will still penetrate the armor.

Liabilities: you weigh as much as two adults combined. This is probably fine in most buildings, but I think shaky rope ladders in the jungle, dodgy unsafe structures, or unstable hillsides might all be perilous.

And Further...

enter image description here

An additional 10x increase in the thickness of the skin produces a layer that almost as thick as a computer is wide. At 44 inches wide, and weighing nearly one ton for a typical hard suit, you can still fit under a roof, but no longer fit through doors (although you could tear a hole in the wall to enter an exit a building).

Compared to a human profile of 36 inches, there's still not much increase in the target profile. A suit designed with as much mobility as medieval armor could still go prone, crouch behind obstacles, and otherwise take cover.

Benefits: with another 10x increase in the thickness of the protective material, the armor should be able to tolerate all small and heavy arms, even up to a direct hit from a GAU-8 (200 kJ). Like the sterotypical sci-fi soldier in super-heavy power armor, this level of protection requires anti-tank weaponry to engage it.

Liabilities: At one ton weight, some wooden structures might not be able to support the soldier. Modern buildings should be tall enough to allow passage, but pedestrian doors will be too small (but they could just break a hole in a wall allowing them to enter and exit). The soldier is about 22% larger in each dimension: width, height, depth.

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    $\begingroup$ 15mm of armour will not stop .50 caliber machinegun fire. Even basic Ball ammo will go through 19mm at 500 meters, armor pierving varieties will go through more, and more than that at closer ranges. . $\endgroup$ – David Hambling Jul 4 at 12:16
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    $\begingroup$ Yeah. I wanted to stay away from details of ballistics because so much can change by hardening a material, or layering, or other techniques. Think I caveated that the is an estimate. 15mm of what kind of material? $\endgroup$ – James McLellan Jul 4 at 12:56
  • $\begingroup$ @JamesMcLellan Just the notion that having up to around 200 kg of armor might be practical for power armor is helpful to know. Modern body armor is less than 8 kg and while it only covers the chest region, it can stop most assault rifle rounds. It would be reasonable to assume that with 25x more weight to work with, you can make armor that can withstand most traditional small arms, covers the entire body, and doesn't shatter when hit. $\endgroup$ – FlyingLemmingSoup Jul 5 at 8:53

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