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Movie special effects spudgun

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Movie special effects spudgun

Unread postAuthor: skyjive » Wed Jul 23, 2008 7:23 pm

So I have this friend who is seriously into making movies and wants me, as the resident guy in charge of things that shoot and blow up, to do some special effects. Specifically, I need to blow up a model building in a dramatic fashion. I thought that a miniature short-barreled combustion spudgun, fired straight up from below the model (off-camera), would probably work. However, I am much more exerienced with pneumatics than combustions, so my question for all you expert combustion people is what type of fuel and gun setup would produce the most flame and bang without being too dangerous for a combustion noob to use or too difficult to make work? Obviously there would be no projectile so the usual gun-ish considerations are not important.
Thanks
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Unread postAuthor: mister pointy » Wed Jul 23, 2008 7:36 pm

You need to beter explain what you're trying to do.

how big is the model building?

I doubt you're going to get much more than a big "poof" with the setup you described. A burst disk might give you a big bang, but no flame.
If your friend is really into making movies, you should be able to explode your model house with a pneumatic gun, and then just edit in an explosion sound effect afterwards.

If you really want a bang and flame, you could always just stuff a really big firecracker in there and film that from a distance.
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Unread postAuthor: VH_man » Wed Jul 23, 2008 8:29 pm

do you want the thing to come clean off the map or more.... collapse......

i reccomend some kind of pyrotechnic charge....... not a spudgun.

and if your trying to get it to collapse, i reccomend making it HEAVY on the top and letting it crumple by taking out some supporting beams on the inside.
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Unread postAuthor: iPaintball » Wed Jul 23, 2008 9:05 pm

Yeah, if its an explosion you want, you'll be better off with pyrotechnics. I would reccomend a small "magic silver dust" salute surrounded by some type of liquid fuel. You'll get a very loud bang and a large fireball.
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Unread postAuthor: starman » Wed Jul 23, 2008 9:11 pm

You could check out my noise cannon concept here.

It shoots no projectile and was designed for noise, has some very fast but low flame output in low light and produces a tremendous and I'm sure devastating pressure wave at anything directly in its way....maybe too much power and not enough flame for your application, but at least you can take a look.
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Unread postAuthor: THUNDERLORD » Wed Jul 23, 2008 9:16 pm

starman wrote:You could check out my noise cannon concept here...


I was just about to recomend that link.
IMO A lot of movie explosions have too much flame.
I was thinking a good explosion scene could be made with something similar because of the high speed and large area of the blast. 8)
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Unread postAuthor: starman » Wed Jul 23, 2008 9:21 pm

THUNDERLORD wrote:
starman wrote:You could check out my noise cannon concept here...


I was thinking a good explosion scene could be made with something similar because of the high speed and large area of the blast. 8)


Especially if it was filmed at high speed...say 100 frames per second could possibly produce a top notch "Independence Day" type effect.
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Unread postAuthor: Ragnarok » Thu Jul 24, 2008 3:57 am

Simple solution - just cut out the middle man (spudgun) and fill the model house with a propane mix... You might need to deal with a few leaks and stuff, but it's the simplest option.
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Unread postAuthor: psycix » Thu Jul 24, 2008 5:43 am

Ragnarok wrote:Simple solution - just cut out the middle man (spudgun) and fill the model house with a propane mix... You might need to deal with a few leaks and stuff, but it's the simplest option.

Yup, a gas explosion :D
Try to seal the model house up as good as you can after you placed some spark gaps here and there and let in a calculated amount of propane or any other combustible gas.
A fan may help mixing for the best result.
DDT will not be a problem, but just something to help the burn speed.

As I assume you will take enough distance from it ofcourse.

But we really need the size of your model house if we want to help you further. If its smaller, you could also try running an powerful hybrid in PVC inside the house. Add gasoline for the visual flames.
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Unread postAuthor: DYI » Thu Jul 24, 2008 8:20 am

An oxy-propane balloon lit with an internal sparkplug and filled with a near-stoichiometric mix will do the trick. It really depends on the size of house how difficult this is, and how spectacular the results are, but a 24" balloon inside something the size of a refrigerator will completely demolish its containment. Just be careful - with larger balloons, the lethal radius can expand to a worrisome diameter.
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Unread postAuthor: skyjive » Thu Jul 24, 2008 1:03 pm

First off, thanks for all your ideas.
Second, the model would probably have a base of something like 30cm by 30cm with an appropriate height
I think most of the ideas put forward would work, but this will be happening while I'm at college away from my tools and facilities (and free time) at home so it has to be simple. So making the noise cannon is out, since I wouldn't have the time or resources to do it. Filling up the house with propane or something similar would be more workable.
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Unread postAuthor: rcman50166 » Thu Jul 24, 2008 1:17 pm

1. You need fireballs to coverup the obvious problem with blowing up a building with a ridiculous sized cannon. (scaled)

2. Make sure your friend doesn't use the cannon boom as the explosion. I would get a professional sound byte.

3. Make sure the video is slowed down to give the building a realistic motion when it explodes or, in other words, making things slower appears to make them larger.

4. The model is to small for an explosion. I would make it around 3 times the size so that a fireball isn't ridiculously out of proportion.
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Unread postAuthor: jimmy101 » Thu Jul 24, 2008 4:46 pm

The balloon is probably the easiest way to go, no need to seal up the model building. Propane or butane in air should work fine (no need to get an oxygen source or acetylene). You can run tests on the ballons without commiting it to the actual model. You can probably get the proper fuel ratio accurate enough just by the diameter of the ballon.

For example, fill the ballon with a butane lighter (additive fuel ratio ~3.8%) until its diameter is 1". Then inflate with air to a diameter of ...

V<sub>2</sub>/V<sub>1</sub> = (4/3)(Pi)r<sub>2</sub>^3 / (4/3)(Pi)r<sub>1</sub>^3
= r<sub>2</sub>^3 / r<sub>1</sub>^3
Rearranging gives
r<sub>2</sub>= (r<sub>1</sub>^3)(V<sub>2</sub>/V<sub>1</sub>)
r<sub>1</sub>=0.5"
and V<sub>2</sub>/V<sub>1</sub>=26 (1/fuel air ratio)

so
r<sub>2</sub> = (0.5"^3)(26)
r<sub>2</sub> = 3.25"
so the diameter goes from 1" to 6.5". That should get you within the combution limits of butane + air.

Add some flour to the ballon and some sand to the inside and top of the building to give the blast something to fling about. You could test the setup with a building made from sugar cubes or wooden blocks.

Might want to use a clear or perhaps brown colored ballon so it isn't to obvious in the video.

Sounds like fun :)

(ya might want to check my math to make sure I didn't hose something.)
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Unread postAuthor: rp181 » Thu Jul 24, 2008 4:53 pm

honestly, that all seems too much of a hassle, pyrotechnic charge would be easiest. Try on another site that deals with this stuff, they can tell you what to use/ how big to make it.
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Unread postAuthor: jimmy101 » Thu Jul 24, 2008 5:15 pm

rp181 wrote:honestly, that all seems too much of a hassle, pyrotechnic charge would be easiest. Try on another site that deals with this stuff, they can tell you what to use/ how big to make it.

In many places in the US, pyrotechnics are illegal unless you have the proper license(s). In the places where they are legal (like Indiana :D ) the largest cracker you can get without a license isn't all that much more powerful than a generic 1" fire cracker. You could try to use a firework mortar round but that is probably overkill (and dangerous).
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