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There are plenty of high tech gases that humans have weaponised to give advantages not only in wars, but in close combat (not really a gas, I know) and riot control, just to name a few uses.

All of the weaponised gas (and gas-like) compounds that I know of are primarily based on chemical reactions, or, as a better way to put it, the attributes of the gas itself. They are used to blind, harm, or kill, among other peculiar methods of weaponisation.

But what about using air pressure? Would it be possible to raise the pressure in a house, so that the air, trying to escape, blasts out the windows? If so, how would this be done? If not, how might something equivalent be accomplished?

There are no restrictions on technology, but size needs to be kept minimal. This should be just as useful as a frag grenade, in the sense that it is deadly but small enough to be handheld. I get a funny feeling that someone will come up with something that demands FTL technology. So yes, FTL is allowed.

I was thinking something along the lines of a small canister that has the capability to compress air (a lot of it), when it is activated. A soldier could crack it out, take some readily available air, make it into a solid (pretty far fetched) and then release when ready. The only problem being... It wouldn't work. So...

How might local air pressure be weaponised?

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    $\begingroup$ I'm not willing to do the research and math for this, but I think a viable solution is available in a hand grenade sized block of frozen helium, wrapped in just enough thermite to sublimate the entire mass into an exponentially larger volume of gaseous helium. For this, helium is better than the smaller hydrogen because it is non-combustable. ...and for once, my answer does not involve FTL! $\endgroup$ – Henry Taylor Feb 7 '15 at 6:20
  • $\begingroup$ @HenryTaylor ok. so... how do i use that with FTL? heh heh... pretty good idea, though. $\endgroup$ – blaizor Feb 7 '15 at 7:06
  • $\begingroup$ FTL affects a whole load of things, especially in future settings, because it will affect if the design of your weapon may need to then also adapt to be able to deal with aliens and alien worlds. So yeah, general rule of thumb, from experience, is that if you do not mention FTL, then there is no FTL. $\endgroup$ – grimmsdottir Feb 7 '15 at 10:02
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    $\begingroup$ You have exactly described a concussion grenade. They use an explosive charge to generate large amounts of gas very rapidly, building up pressure in the form of a shock wave to cause damage to an area. $\endgroup$ – 2012rcampion Feb 7 '15 at 15:04
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Such weapons exist and are called thermobaric bombs. They do rely on chemical reactions to trigger the overpressure blast though. Oh, and it doesn't blast out just the windows, the walls too... Sorry about that.

A thermobaric explosive works by having the initial blast spread fuel in the targeted area, and then a secondary charge ignites the cloud. This achieves a longer burn and much higher temperatures than a regular plastic explosive. In doing do the high temperatures end up creating a devastating overpressure wave that does most of the damage. This is particularly effective in closed areas such as caves, bunkers or buildings. How does this work? Well, the easiest clue is the ideal gas law:

pV=nRT
p is pressure
V is volume
n is the number of moles
R is the universal gas constant
T is temperature (K)

So higher temperature, ceteris paribus, yields higher pressure. A regular explosion has an overpressure wave too, it's very sharp but very short. For a thermobaric weapon, the pressure builds, builds, and just doesn't stop, until much, much later. It's a bit like the difference between getting punched and a very close encounter with a 300lbs lineman. See a Chinese weapon in action:

thermobaric bomb

Jane's dryly describes it thus:

Its effectiveness against buildings, bunkers is noted, as well as the fact that because the blast takes oxygen from the air, personnel in the airtight space suffocates because of the oxygen deficit.

These weapons were initially developed by the Soviets, who developed man-portable versions such as the РПО-А Шмель RPO-A Shmel and the ТОС-1 - тяжёлая огнемётная система TOS-1 Heavy Flamethrower "Buratino" missile launcher. These were used by the Russians to level Grozny in 1999. Russian pinnochio

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  • $\begingroup$ These are the fuel air explosives I briefly mentioned in my answer. They occur accidentally when flammable gas, aerosol, or fine dust (fuel) is mixed with air. Hence the "fuel air". Thermobaric weapons work a bit different than the accidental versions from gas leaks, coal mines or grain silos, though. $\endgroup$ – Ville Niemi Feb 7 '15 at 14:22
  • $\begingroup$ @VilleNiemi, Yes, FAEs are a subclass of thermobaric weapons. $\endgroup$ – Serban Tanasa Feb 7 '15 at 14:29
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What you are describing is essentially what happens in steam explosions. The word to note is explosion. The damage done is relative to the speed the pressure increases since otherwise the rate of gas escaping and the rate of gas increasing find a balance at some much lower level. So if you want to weaponize air pressure, you pretty much have to use an explosion of some sort. A practical example of a weapon like that is a concussion grenade. Fuel air explosives also have some of the features you want.

Similar effects can happen without explosion if the pressure outside falls fast enough. This can happen with tornadoes. An artificial effect that can cause the same effect is a firestorm.

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This already exists (although not exactly the way you described it). It is called a sonic weapon, and it uses sound.

Sound is just a pressure wave. These weapons shoot sound at opponents. Really big ones knock people over; big ones cause weird effects because of the vibration in internal organs; small ones make really annoying noises.

AFAIK these are currently experimental and not available to the public. However, they are definitely possible to build and use. They seem somewhat portable (if you have a vehicle to put it on), but as they are still experimental, they will probably shrink over time.

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  • $\begingroup$ I have explored various methods, including (ultra)sonic waves. This isn't really what I'm looking for though; air pressure is key. And really big ones, AFAIK, kill people. I didn't really look into it though. $\endgroup$ – blaizor Feb 7 '15 at 7:12
  • $\begingroup$ @blaizor I think the whole point was that sound waves Are nothing more than pressurized waves of air. Period. $\endgroup$ – Get-HomeByFiveOClock Feb 8 '15 at 13:41
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One weapon I haven't seen mentioned yet that police and others use when storming a house are flash-bangs, they are using a bright flash and a concussion/loud noise (fast change in air pressure) made from rapidly expanding explosion to blind and deafen and disorient those in an enclosed space.

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Since you literally asked for it...

FTL

Take portable wormholes that can be turned on or off. So for example you'd have a circle A that would connect to another circle B, instantaneously at any distance.

The first option is obvious - put one end in space, throw the other at someone and open it. Instant vaccuum as air is sucked out.

Second option - put one in a high-pressure chamber you control, you can now release instant high-pressure atmosphere anywhere the other end is at.

But let's go further down the rabbit hole. It's not strictly air pressure, but put a really tiny one at the bottom of the ocean (a couple of miles down). Now open it, water will shoot at at incredibly speeds and give you a portalable water knife.

Connect one end to the inside of a steam engine, you can now boil people on-demand.

If they're tough enough, let's put one inside the outer edges of the sun...

Ok, enough of that. More realistically you should go with:

Heat / Cold

Air pressure is largely a function of air temperature. Any technology that would let you pull heat from the air, or put heat into the air quickly, will let you also rapidly change air pressure, giving you the effects you describe (bursting windows either out or in, for example).

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    $\begingroup$ Unfortunately, I think the original question wants to use local air pressure, so it might be a bit off. Also the pressure difference between space and somewhere on the ground is really low. Not that much of a sucking will happen. $\endgroup$ – grimmsdottir Feb 7 '15 at 10:18

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