Burning and Bombing

[Agemegos]

Originally Posted by PPoS

One question that has always been bugging me is why there are gun-cam footage of A6M2's exploding in a great ball of fire when hit. If it's not the fuel, then what is it ?

It is a mixture of fuel and air. And inasmuch as you see a ball of flame, that much of it is burning rather than exploding (an explosion has spread it into the ball). Gasoline vapour mixed with air (in the right range of concentration) is explosive, but you don't see a lot of flame or even smoke when it goes off: a flash maybe, and dust kicked up, but no fireball or black smoke. Visible flame and black smoke come from unburned particles of carbon, indicating that there is not enough oxygen for complete combustion, which usually means a mixture too rich to detonate.

Gasoline by itself is not explosive. Even large drops of it in air aren't explosive. Gasoline droplets suspended in air as a mist, or gasoline vapour mixed with air, are explosive: enough of the octane &c. molecules are next to oxygen molecules for it to all react at once. Methane or hydrogen mixed with air (in the right range of concentrations) are explosive. Coal dust suspended in air is explosive. Hell, flour suspended in air is explosive. Practically any fuel is explosive if you divide it into a fine powder, disperse it into fine droplets, or evaporate it and mix it with air. This is what we call a "fuel-air explosive". But the fuels aren't explosive by themselves. If you suspended or evaporated them in an inert atmosphere of nitrogen and/or carbon dioxide, or a reducing atmosphere of hydrogen and/or methane, they would be no more explosive than fog or a dust-storm.

Conventional explosives are different. Either they consist of a mixture of combustible material and a solid or liquid oxidiser (e.g. carbon and saltpetre in black powder), or they consist of large unstable molecules that disintegrate into a lot of small molecules of gaseous substances when they are violently shocked. Anyway, they don't need air to explode. They will explode in a tightly-sealed cartridge or drill-hole, under water, in vacuum, or in an inert or reducing atmosphere.

The advantages of fuel-air explosives are (1) they are comparatively cheap, (2) you only have to carry the fuel component, since air is ubiquitous, and (3) they are comparatively efficient at converting chemical energy into shockwaves. The disadvantages are that (A) the clouds of dust or mist or vapour take up a lot of room, so you can't pack them into shot-holes for efficient demolition, and (B) said clouds tend to settle out or dissipate fairly quickly, which means that between assembling the explosive mixture and it ceasing to be an explosive mixture might not be long enough to get to a safe distance. Fuel-air explosives have been involved in a lot of accidents such as explosions in mines, exploding flour-mills, and explosions in nearly-empty fuel stores, but actually getting a useful, controllable explosion that goes off when you mean it to is tricky. A fuel-air bomb is a complicated device that first mixes the fuel into the air around it (perhaps blasting it into droplets with a dispersing charge of conventional explosive) and then detonates it with a flame or flare (a) after a long enough time for the fuel to mix with enough air to be ignitable, but (b) soon enough that the fuel-air mixture hasn't blown away, dissipated to below ignition level, or settled to the ground.

A fuel, even a volatile and easily-ignited one like petrol (gasoline) won't in fact explode just because it is heated or compressed, and it won't "join in" the explosion even if you let off a stick of dynamite inside a barrel of gasoline. It can't, because it can't react without an oxidiser to react with. If you let off an explosive inside a full barrel of petrol (gasoline) you will blast the petrol (gasoline) into a mist and rain of drops and droplets, which will just hang there, make a splash, gradually evaporate etc. Unless a spark or flame ignites them.

If you need an REF to determine how big a bang an explosives expert could make improvising a fuel-air bomb from a can of petrol (gasoline), or how big the accidental explosion might be if petrol (gasoline) were spilled in a closed room of about the right size, left to evaporate, and then detonated by someone walking in with a lit cigarette, I suggest that you use the REF listed for a fuel-air explosive listed on p.183 of GURPS High Tech (i.e. 5) and base it on the mass of the fuel alone (do not add in the mass of the included air). That won't be very accurate, but near enough for gaming.

[lwcamp]

In addition to what everyone else has said, I wish to point out that when liquid fuel is heated in a pressurized container, you can get a very dangerous explosion called a boiling liquid expanding vapor explosion (BLEVE). While this doesn't necessarily need a flammable fuel, just a liquid in a pressure container and heat, a flammable fuel can make things worse as the fuel vapors produce a secondary fuel-air explosion.

Plus, BLEVEs look awesome
http://www.youtube.com/watch?v=sl-JgyQA7u0
(if you want to skip the technical description, go to time 1:23).
More BLEVE awesomeness
http://www.youtube.com/watch?v=Xf3WKTwHpIU

Luke

[RyanW]

Originally Posted by lwcamp

In addition to what everyone else has said, I wish to point out that when liquid fuel is heated in a pressurized container, you can get a very dangerous explosion called a boiling liquid expanding vapor explosion (BLEVE).

Here is the horrifying aftermath of just such an explosion.

And by horrifying, I mean delicious. Mmm, popcorn.

[dcarson]

I remember seeing a field expedient FAE in a army manual? that I read for when you didn't have enough explosives to demolish a large building. Set a charge under a pile of flour bags and another charge set to go off a couple of seconds later nearby. Wasn't something that would always work bu if you were almost out of demo charges and had more buildings to destroy was worth a try.