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Author: Subject: Movie-style fireballs
rot
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[*] posted on 11-2-2006 at 07:04
Movie-style fireballs


You've all seen it in the movies, when an explosive like C-4 'goes bang' you see a great fireball. but is this true? won't the explosive's powerfull shockwave put out any fire rather than starting it? and how is this done in the movies?
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vulture
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[*] posted on 11-2-2006 at 07:16


Movie explosions are not real HE explosion, rather they're small BLEVEs.

A flammable liquid is superheated and rapidly dispersed in air by a small explosion. The boundaries of the vapor cloud come in contact with oxygen and give the typical fireball.

Imitating is easy, put a small amount of HE below a bag or drum filled with gasoline.




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Taaie-Neuskoek
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[*] posted on 11-2-2006 at 19:26


It can even very well without an HE, place some good BP or H3 powder under a plastic bag with gasoline.
For small applications (backyard fun) an HE is too powerfull, the blast will prevent ignition, while something 'slow' like BP is ideal...

I reckon that for a larger scale a good flash cracker will do the job as well...




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Pommie
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[*] posted on 13-2-2006 at 06:32


Google "cremora fireball"

Mike.
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Bert
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[*] posted on 15-2-2006 at 03:29


Pommie has the correct idea. Liquid fuels are dangerous to use near crew, actors and expensive equipment- Flammable dust fireballs are the way to go. And yes, movie effects look nothing like real HE. But they are what the public expects to see...
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Fulmen
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[*] posted on 15-2-2006 at 04:16


Let's not forget naphtalene mothballs, these are safer than liquids and work like a charm. 2 parts finely ground naphtalene and 1 part blackpowder should be a good place to start...
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bomberman41688
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[*] posted on 15-2-2006 at 05:33


Even though the naphthalene fireballs are more safer than the liquid ones, you still have to worry about the fumes. I think the best would be the cremora. this stuff only burns when its scattered out in the air and its non-toxic.
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Fulmen
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[*] posted on 15-2-2006 at 08:19


Bah, as long as you're not doing it for a living and use a bit of common sence I wouldn't worry too much.
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[*] posted on 15-2-2006 at 09:20


I am doing it for a living. My common sense is debateable.

I know a person from Texas who was carrying a 5 gallon bucket size cremora effect when it went off (he let the firing leads drag on the ground behind him on a low humidity day). It scared the shit out of him and his co-workers, but he had only crisped hair and first & second degree burns on some exposed skin. BP/Napthalene of a similar size would have likely been a fatality or permanent disfigurement. A liquid fueled effect would have been "Johnny human torch". Hobbyists and amateurs are well advised to use the safest methods available to them for a given desired effect.

[Edited on 15-2-2006 by Bert]
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[*] posted on 15-2-2006 at 10:42


There are many fuels used for fireball effects. Other than the already mentioned naphthalene, cremora and liquid fuels there is a commercially available product known as Chem F/X. It is said to be some sort of grain chaff product but I don’t know for sure. Other fuels used regularly include saw dust, hexamine, red gum, charcoal, flour and mixes of same with or with out metallic fuels to liven things up a bit. Most any brand of milk replacer used for dairy calves works very well and is cheaper than cremora. Other more exotic and expensive things like lycopodium and cortex (ground cork) are also used more commonly in indoor special effects.
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[*] posted on 15-2-2006 at 11:03


Quote:
Originally posted by Bert
I am doing it for a living. My common sense is debateable.


Oooookey. You might want to stick to to the cremora then :-)
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rot
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[*] posted on 16-2-2006 at 04:18


i think I'll try it with some nitrocellulose as propellant...
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bomberman41688
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[*] posted on 16-2-2006 at 05:44


I don't think nitrocellulose will burn fast enough. try using black powder. I tried it with nitrocellulose and did'nt get very good results.
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[*] posted on 16-2-2006 at 06:25


Lets not forget paraffin as a good fuel source for a fireball..

Your typical mix of charcoal and Ammonium nitrate will pick up a nice firey flash if you add a few percent of paraffin.
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[*] posted on 16-2-2006 at 07:23


Nitrocellulose will not work as it not hot enough to ignite most dusts used for this effect. It will most certainly loft them though. If you provided some sort of secondary ignition you might have some success.
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[*] posted on 18-2-2006 at 09:30


Quote:
Originally posted by bomberman41688
I don't think nitrocellulose will burn fast enough. try using black powder. I tried it with nitrocellulose and did'nt get very good results.

Nitrocellulose burns faster than the best black powder you can get/make. at least the hexanitrate does, it's gone in a flash. When mixed with nitroglycerin it's even more powerfull, called double based smokeless powder. it's made by dissolving nitrocellulose into excess of acetone, adding nitroglycerin and letting it dry to a cake and then crushing it up.
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[*] posted on 18-2-2006 at 18:13


oops, I forgot about the hexanitrate. maybe that will burn fast enough after all
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[*] posted on 18-2-2006 at 23:24


smokeless powders will seldom directly replace black powder in low pressure applications. They have very different pressure exponents for a start.

;) Black powder- the original propellant!
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[*] posted on 22-2-2006 at 11:32


I just tried it with nitrocellulose,
I did it as following:
Took a paper tube, sealed the bottom with several paper discs and lots of tape.
then I placed an electric igniter on top of the discs, and a gram or so cellulose hexanitrate. On top of that a plastic bag, in wich I poured some (100mL or so) gasoline. I fired the nitrocellulose, and the gasoline just shot out of the tube without igniting, no fireball. the second time I poured the gasoline into the bag, lit it and than fired it. succes! it made a great fireball and shot burning gasoline everywhere, pretty spectacular, but also dangeroes as the gasoline does not stop burning after it's shot out of the tube! it's stays burning on the ground for a while, be carefull

[Edited on 22-2-2006 by rot]
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[*] posted on 25-2-2006 at 20:53


Quote:

pretty spectacular, but also dangeroes as the gasoline does not stop burning after it's shot out of the tube! it's stays burning on the ground for a while, be carefull


I repeat: flammable dust clouds are a MUCH safer way of achieving this effect...
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