Half a croissant, on a plate, with a sign in front of it saying '50c'
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Steam bomb

Simple and effective
  (+4)
(+4)
  [vote for,
against]

The energy yield of explosives is disappointingly poor per unit mass.

The objective is to delver a suitably effective quantity of energy at a target location in the shortest practical time.

Authorities expend considerable effort in a futile attempt to deny access to explosives to a small number of individuals.

An obvious solution is to convert an extended low-energy process to a high-speed high-energy process.

The proposed methodology is to employ a sealed presure vessel filled with ultrapure water. This is heated by means of simple burners fuelled by readily available hydrocarbon fuels until the failure pressure is achieved. Provided that the container is suitably tamped above and below, and surrounded in a horizontal plane by a sufficient mass of projectile material, the casualty-causing potential is high.

Given a suitably target-rich environment, and considered placement of the device, kill ratios substantially greater than those achieved by the opportunistic use of unmodified vehicles as weapons are achievable.

8th of 7, Aug 05 2016

http://www.usatoday...explosions/2086853/ [2 fries shy of a happy meal, Aug 06 2016]

Krakatoa tube https://www.youtube...watch?v=drHVT4NHSNQ
This video channel is excellent. [bungston, Aug 17 2016]

BLEVE https://en.wikipedi...ing_vapor_explosion
Background info [notexactly, Aug 18 2016]

Superheating Liquids https://www.jstor.o...e_scan_tab_contents
[bs0u0155, Aug 18 2016]

[link]






       Uh, steam explosions have been used for a long time, for example in mines where conventional explosions might ignite flammable gases. I believe the usual format is a thick-walled insulating container of water, through which an electrical heating element runs.
MaxwellBuchanan, Aug 05 2016
  

       Indeed, but there are no recorded instances of their use as an antipersonnel weapon.
8th of 7, Aug 05 2016
  

       "Right, you lot there; stand still for a few minutes, yeah ? Okay, now group a bit more together and hunch down a bit... maybe around that canister: nonono, don't touch it, you might get a nasty welt..."
FlyingToaster, Aug 05 2016
  

       I'm hesitant to ask due to the high probability of a horrible pun, but why does the water have to be ultrapure?
Voice, Aug 06 2016
  

       //surrounded in a horizontal plane by [...] projectile material//   

       Though he durst not say it, we know: a belt of fractal-cut titanium with edges so sharp 'twill damage one's retinas to look at it; an outer layer of pre-scored high-iron glass of the utmost clarity, though it be as thick as a waiting lobbyist's wallet; and betwixt, a joyous filling of kittens to attract and entertain the victims. The *other* victims, that is.
lurch, Aug 06 2016
  

       //but there are no recorded instances of their use as an antipersonnel weapon.//   

       Do pressure cookers count? [link]   

       No.   

       // why does the water have to be ultrapure? //   

       If the water is ultrapure, it won't boil into steam readily. It becomes superheated. As a consequence, the pressure in the vessel (which needs to be scrupulously clean, and polished on its interior) rises much more slowly than would otherwise be expected. When the vessel finally fails, the release of energy is that much more dramatic.   

       [lurch], you've just redefined" heaven".
8th of 7, Aug 06 2016
  

       //If the water is ultrapure, it won't boil into steam readily. It becomes superheated.//   

       Uh, [8th], would you care to discuss the physics of that in a little more detail? Particle-free water can be superheated by a few degrees above its normal boiling point (whatever that may be, given the pressure), but not enough to make a difference.
MaxwellBuchanan, Aug 06 2016
  

       Every little helps. Yes, it's only a few degrees, but the phase change - when it occurs - is that bit more rapid.   

       If the start temperature is 15C, and the use of pure (degassed) water causes 4C of superheating then the gain in stored energy is 4/85, or nearly 5%. Add that to the faster phase change - yielding a shorter, more intense pressure pulse - and it's worth doing.   

       Even on a small scale, using improvised containers (Kilner jars) and a bunsen burner, the effect is noticeable.
8th of 7, Aug 06 2016
  

       //If the start temperature is 15C, and the use of pure (degassed) water causes 4C of superheating then the gain in stored energy is 4/85, or nearly 5%//   

       That assumes that you are only heating the water to 100°C, which would be pretty feeble. I believe that the water-based bombs they use in mining get up to several hundred degrees (and 100-200 atm pressure).
MaxwellBuchanan, Aug 07 2016
  

       But the improvement in stored energy will be a constant ratio, irrespective of the P, T end point.
8th of 7, Aug 07 2016
  

       //But the improvement in stored energy will be a constant ratio, irrespective of the P, T end point.//   

       Hmmm. This I need to think about.   

       So, you're saying that by having pure water, you can superheat it by a fixed proportion (of temperature) above its boiling point (at that pressure), and can therefore get the water hotter before the bursting pressure of the container is reached?   

       That's plausible, _if_ the statement about proportionality is true.   

       So the question is: if pure water at 1atm can be heated to (say) 377K before boiling (ie, 4K above normal boiling point of 373K); and if the pressure is increased such that the normal boiling point is 2x373=746K, does purity allow you to superheat by an additional 2x4K=8K (ie, to 754K)?
MaxwellBuchanan, Aug 07 2016
  

       Having eradicated the enemy, the squad settled down for a nice hot cup of tea.   

       [+]
BunsenHoneydew, Aug 09 2016
  

       hmm, so, there are records on superheating <link> apparently 279.5C for little drops of water in equal density oil. Pentane was also superheatable by >100C above its boiling point. That's a pretty significant energy storage capacity.   

       //having pure water, you can superheat it by a fixed proportion (of temperature) above its boiling point (at that pressure),//   

       If that scales, can you get water from 279.5C to 125C ish at 2atm + 358C superheating? How does that all work? I clearly don't possess the specific vocabulary to literature search the information in the right way.   

       Is it possible to get a mix of water and mineral oils say, and superheat/pressurize them both? Loss of pressurization would presumably result in a synergistic phase change dependent upon mutual loss of containment.   

       Also, why water? If you can do this with hydrocarbons as well or better than water, why not use them? If you chose a hydrocarbon that's nominally solid, say, a paraffin wax. With superheating and pressure you might get that to say 1000C. You're not gonna want to go above that or your pressure vessel will have to be quite fancy. Then upon vessel failure, you'll get a vast explosion, the vaporizing wax will increase the local pressure, displacing air in a nice explosive pressure wave, which was the point. Now, the cloud will quite quickly cool, then start to condense back to a solid creating a vacuum. You will have air rushing back the other way to fill that. Which I think is considered a desirable feature when in a destructive mood.   

       Even cleverer would be to use hydrocarbons and then ignite them at max cloud size, that way you get the rapid expansion then a big burn, but I guess that's a fuel air bomb.
bs0u0155, Aug 18 2016
  

       // why water //   

       It's readily available at low cost, and it doesn't arouse suspicion.
8th of 7, Aug 18 2016
  

       Neither does heating oil...
bs0u0155, Aug 18 2016
  

       ...and prospective victims can have a nice cup of tea while waiting to be blown up.
not_morrison_rm, Aug 18 2016
  
      
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