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If you have a <1mm thick layer of vacuum inside an armour plate, be it for vehicles or body armour, wouldn't it stop explosive shockwaves from penetrating through the armour? This is, of course, presuming the explosion is not powerful enough to crumple the plate and either force it to make contact with the back plate, or simply rupture it and ruin the vacuum.

Strength of Steel

Flexure Strength of 2mm of mild steel: 381,870,000N/m^2. According to one answer, that is almost four thousand times the force of 1 atmosphere of pressure, and over twice the force of 1kg of TnT at 1m distance. Unless there's some weird mistake with that source, I don't understand why people are insisting, without calculations or sources, that vacuum is impossible to contain without a huge amount of steel, when glass jars can do it.

I have added the hard science tag, in the hopes someone will actually demonstrate whether several mm of steel can't withstand a vacuum, despite glass jars being able to.

However, please note I never limited the material containing the vacuum to steel... I just mentioned it as an example.

Back to the Question

I was thinking of arming soldiers with this sort of vacuum plate, due to the prevalence of explosive-based weapons in the setting.

Notably, shockwaves can still travel through the joints of the armour, since you can't have it totally free floating:

Red is for the explosive force, black is the armour plates with the white vacuum inside of it, blue is the part which joins the two plates, and orange is the padding focused on that area.

However, that should reduce the effectiveness of the explosion, if it is forced to go through narrow chokepoints? You can also focus padding on these areas to dissipate it.

The joint areas might be at the shoulders and hips, arm and leg joints, protecting the lungs, heart, and other organs from direct shockwaves. Helmet a bit like an astronaut or deep sea diver, with a vacuum gap between the visor's glass panels. Neck will have to make do with a vacuum gorget and plenty of padding, unless you want to make it rigid like with tournament armour.

So, would this work? I don't mind some magic to make it work, but I'm interested to know why this hasn't been tried.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Aug 17 at 17:39
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    $\begingroup$ I’m voting to close this question because it belongs on the physics stack $\endgroup$
    – John
    Aug 17 at 17:40
  • $\begingroup$ @John It wasn't intended to belong there at all. I didn't expect it to come to an argument that steel can't withstand a vacuum, but a glass jar can. $\endgroup$
    – Nail
    Aug 17 at 17:47
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    $\begingroup$ Steel can survive it, if you make it the right shape I personally have built several large (big enough to fit a 5 gallon bucket inside) vacuum chambers. in this case shape matters far more than material. but the combination of thinness and shape you want will not work and this is a physics or engineering question not a worldbuilding one. $\endgroup$
    – John
    Aug 17 at 18:13
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    $\begingroup$ @John Would've saved us a lot of effort if you started from that. Your reference to 12mm tanks and such, and your other comment about it needing heavier steel, and that point about the pascals made it seem you were arguing steel wasn't capable of it. It lead to me finding out neat statistics about steel, though. Thanks for the info and sources and helping me to learn, John. $\endgroup$
    – Nail
    Aug 17 at 19:15

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I am going to bring up some considerations that I have seen lacking or poorly explained so far. The long and short of this is that this armor is impractical in addition to not being viable.

For Infantry Armor?

We can rule this out for infantry armor simply because soldiers generally want (and must) interact with their environment. Unless they are fully encased in armor, the blast will still affect some part of their body. That some part is likely their eyes, ears, or hands!

Sound infamously doesn't travel through vacuum, so you would be asking soldiers to give up a valuable sense in favor of blast protection. While various armors in history have dampened soldiers sense of sound, this also brought up issues with the ability to hear orders, detect enemies, and detect battlefield conditions. This armor is simply a no-go from a sensory perspective, even if it did work otherwise.

In theory, if a individual had armor like this encapsulating their whole body, they could have sounds "piped in" from the outside, but this seems impractical for arming every foot soldier. Now, if this were a sci-fi world and every soldier needs a space suit, the environment may act to deaden or defeats blasts anyways.

For Vehicles?

We are going to detour into two subjects here: adding forces (and stresses) as well as force concentration.

This armor, by virtue of the cavity on the inside, already has forces conspiring to make it collapse. Projectiles and blasts from the outside would team up with these forces, making the armor more easily overcome than if it were filled with air. For an easily accessible analogue, consider the humble soda can. When empty, it is easy to crush and dent. When full, it resists denting and crushing significantly more. This armor is much like an empty soda can.

Let's talk concentrating forces! It is impractical (and likely impossible) to have armor floating within armor. Supports and joints are needed. These supports and joints will act as force concentrators. When the blast hits the out plate, assuming it is rigid enough, that force will be transmitted to the supports. The force which was once covering a large area is now in a smaller one! This is a recipe for making little holes in the backside where the supports were.

Both of these principles are acting against making this viable for vehicles and infantry. Yes, a vacuum would prevent a blast from propagating (which is why we cannot hear the sun while on earth...). But the force concentration and the influence of atmosphere means this armor is less effective than simply two plates with an internal 1 mm gap.

What Makes It Better?

Aerogel or foam in the cavity would help in two ways. Firstly, it can help resist the atmosphere's pressure by transmitting some forces. Secondly, the various aerogel/foam/air interfaces would act to disperse the blast wave. This is more manipulating the speed of the blast wave in different materials (and reflection/refraction) to spread the blast over an longer time. Thirdly, these could act to help disperse shrapnel (assuming the interior material is strong enough). One sees something like this in debris shielding for spacecraft.

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    $\begingroup$ Well, many US soldiers already have headphones as hearing protection and comms. So it's not really implausible. USMC were testing out some sound-cancelling ones in 2017, but those have been around for decades. A woman also recentl broke the record for running in a bomb suit, making a ten minute mile. However, you make a fair point that while vacuum prevents explosions propagating, that offers no special advantage. I thought it might in relation to vibrations having to travel further, but now I'm not so sure. Thanks for the help and the good explanation, Pipper. $\endgroup$
    – Nail
    Aug 17 at 17:56
  • $\begingroup$ For an important bit of context, though, according to a source Flexure Strength of 2mm of mild steel is 381,870,000N/m^2, almost 4,000 times the force of 1 atmosphere. So the empty can example gives the wrong impression, since supposedly it'd make a 0.025% difference. Apparently, design can make a massive difference to this, though, as discussed in other comments. And even so, I'm now doubtful of vacuum plate. Just wanted to share that trivia since it seems really interesting. $\endgroup$
    – Nail
    Aug 17 at 18:04
  • $\begingroup$ @nail yes, flexural strength is very design dependent. That's why you frequently do not see it in "pure" material property databases and catalogues. In any case, the armor can take 1 atm less pressure from a blast wave due to that vacuum. I am unsure (off the top of my head) the pressure differentials you get from things like grenades and bombs, but I have seen a tennis ball go through plywood with less than 1 atm $\endgroup$
    – PipperChip
    Aug 17 at 18:57
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    $\begingroup$ @nail check this out: response.restoration.noaa.gov/oil-and-chemical-spills/… $\endgroup$
    – PipperChip
    Aug 17 at 19:07
  • $\begingroup$ Someone claimed 1kg of dynamite had just over half the pressure I listed at 1m. True that its flexure is specific, though. Good source example. Thanks again for the help. $\endgroup$
    – Nail
    Aug 17 at 19:25
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So, would this work? [...] I'm interested to know why this hasn't been tried.

Making vacuum in an environment with largely uneven dimension is difficult, and your case looks exactly like this situation: 1 mm thick and spread all around a larger object.

That apart, I don't think one wants to fully reflect back the pulse produced by the explosion, because that itself would give a big jolt. One wants to dampen and disperse the pulse in such a way that it is degraded to thermal noise without producing damage. That's better achieved with other medium than vacuum.

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    $\begingroup$ This doesn't really answer the question. What system of dispersing the pulse are you thinking of? Some sort of padding? If so, that can easily be layered over and/or under the vacuum plate. Layering some of it over the joints might be a good idea. $\endgroup$
    – Nail
    Aug 17 at 9:56
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    $\begingroup$ The question is "would this work" not "what alternative would work better" -- right? $\endgroup$
    – Zeiss Ikon
    Aug 17 at 11:09
  • $\begingroup$ @ZeissIkon the underlying question is would it work better than the same steel as a flat sheet with no vacuum. to which the answer is no at any weight wearable by a human. $\endgroup$
    – John
    Aug 17 at 16:31
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No

Just because there is a vacuum does not mean that the pressure wave would stop propagating. Instead, as the pressure wave hit the armor, the pressure differential would push the armor towards the body.

With frequent supports, the armor would funnel the blast pressure through the pads. This would not be good because, even not concentrated, the blast pressure can do significant damage, so when confined to the supports and the padding beneath them, it is likely that the explosion would do even more damage.

With infrequent padding, the armor would bend so that the layers that contain the vacuum touch, and more or less the same pressure would be exerted on the skin and body that way (the bending of the armor might do something to protect the soldier, but that would cause an increase to the pressure on the pads, causing the pressure to be higher on them).

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    – L.Dutch
    Aug 17 at 17:02
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Let's have numbers!

Air has mass. The tens of kilometres of air above us pressing down creates, by definition, an average pressure of 1 atmosphere at sea level. You don't notice this pressure pushing down on you because almost everything you interact with is at the same pressure, but 1 atmosphere is 101,325 N/m^2. Put another way, a 1 square metre section of armour with a vacuum layer in the middle has over 10 metric tons of weight pressing on it - unless it is ridiculously thick armour - too thick to wear - it is going to flex by 1 mm easily.

The practical upshot is that the vacuum gap will disappear just by the armour existing. It gets even worse if an explosion does occur, since the overpressure caused by an explosion is significantly more than atmospheric pressure. A single kilogram of dynamite can cause overpressure of 1200 kPa (1,200,000 N/m^2) at 1 metre and 280 kPa (280,000 N/m^2) at 2 m. Even if the armour could sustain a 1 mm vacuum gap under normal circumstances - which it couldn't - it would have even less chance of sustaining the gap under the very conditions where it is critical that it works.

The other problem is that even if a literally magic ability is used to maintain the vacuum gap, "nature abhors a vacuum". In order for there to be a vacuum within each section of armour, each individual section must be sealed, which means that there are bits of material joining the outer and inner plates. This material around the outside of each plate becomes a weakness, as all the energy will be transmitted through it.

Finally, overpressure from explosions comes in second place vs shrapnel on the kill boards. Soldiers will want their armour to be as strong as possible to resist incoming shrapnel, not stressed-to-breaking-point by trying to maintain an internal vacuum. (This is also why it's a bad idea to use magic to create a vacuum around all your soldiers and put them in spacesuits - shrapnel is really bad news in vacuum.) The whole principle around some grenades with pre-fragmented casings that create really small individual pieces of shrapnel - such as the Australian F1 - is that they have a very constrained deadly radius and are relatively "safe" to soldiers once the shrapnel has travelled even 20 metres or so.

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  • $\begingroup$ Thank you very much for the fine, detailed answer. This has still left questions in my mind, though. Steel has a flexural strength of 381,870,000N/m^2 for 2mm of mild steel: iopscience.iop.org/article/10.1088/1755-1315/445/1/012044/pdf So isn't that 3,800 times the force of 1 bar, and over twice the force of an entire kilogram of dynamite at one meter? $\endgroup$
    – Nail
    Aug 17 at 15:12
  • $\begingroup$ @Nail, you're reading it wrong. First, that "381.87 N/mm^2" is the peak stress the test specimen survived, not the load needed to start bending it, and second, the "per square millimeter" is cross-section area, not surface area. In order to get your desired 318,870,000 N of strength, you'd need to make your armor at least a meter thick. $\endgroup$
    – Mark
    Aug 18 at 1:20
  • $\begingroup$ @Mark Are you sure about that? They actually gave higher numbers for the other strengths, so it surely has to be peak bending strength, rather than break or yield stength. Here: "The peak force obtained for mild steel was 2495.43, 4643.33 and 6215.67 N for 1, 1.5 and 2 mm respectively. The elongations of mild steel were 48.88, 54.77 and 56.11 mm for 1, 1.5, and 2 mm respectively." By N, they seem to mean N/mm^2, based off another passage in the abstract: "For stainless steel tensile strength analysis, the peak forces obtained are 9486.66, 9558.00 and 9522.33 N for 1, 1.5 and [cont] $\endgroup$
    – Nail
    Aug 18 at 1:51
  • $\begingroup$ @Mark [cont] 2 mm respectively. The deformation of the material occurred at similar loads having stresses of 535.20, 583.51 and 486.88 N/mm2 for the thicknesses investigated." Clearly, ~500 N/mm^2 is completely different from ~900 N/m, so they must mean the same unit in each case. And sine they mention 2mm and such, with increasing strength, I figure it has to be thickness? Making it longer/wider should make it weaker against flexure testing? $\endgroup$
    – Nail
    Aug 18 at 1:55
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    $\begingroup$ @Nail, N is never a unit of pressure. It's always a unit of force. In order to switch between pressure and force, you need to know the cross-section area of interest. $\endgroup$
    – Mark
    Aug 18 at 2:35
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A thin vacuum layer would not be possible.

Your idea for a small inside layer of low pressure (vacuum) would be akin to a submarine. Submarines are built to withstand enormous water pressure from the outside while still keeping a normal livable pressure inside. The way a submarine is structurally build means it won't instant collapse when there is a very small hole, but it will dent in and be destroyed if a hole is big enough.

submarines can withstand the pressure difference by their bulky and rigid design. As said by others: just to make the idea work of a vacuum layer you would have to make something strong, bulky and rigid. This makes it unviable for infantry. One wrong move of a suit could compromise the structural integrity leading to an instant failure of the intended purpose of such a vacuum layer.

The closest you could come in my opinion with a different absorbing layer would be water. But that also has many downsides (leakages for example).

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  • $\begingroup$ Hello, I see you're also a new contributor. Not sure about water for bomb armour. Water normally enhances shockwaves. Human bodies are also mostly water, so it may propagate freely to the wearer. Maybe if you had a hard layer beneath it, in a sandwich? But then, I don't think water makes particularly good padding, since it's (nearly) incompressible? $\endgroup$
    – Nail
    Aug 17 at 18:10
  • $\begingroup$ Water transmits shockwaves quite well; water in contact with a human transmits shockwaves extremely well (humans are ugly bags of mostly water, so there's little impedance mismatch). $\endgroup$
    – Mark
    Aug 18 at 1:22
  • $\begingroup$ my assumption was that have water sandwich'd between two hard layers would mean the shockwave would be more evenly distributed to the next layer. But at the same time :') I probably have been completely wrong about the water part of my answer. $\endgroup$ Aug 18 at 7:47

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