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Big Explosions!

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: starman » Wed Jan 14, 2009 10:23 pm

MrCrowley wrote:
a few layers of foil?

For 1.5", 2" and smaller diameters. You will need several hundred layers for 8" i'd imagine. That's why very thing sheet metal may be best. Some members might know of some very thin and flexible sheet metal that would work.


Hundreds indeed. I believe the tossed about pressure rating in a 1.5" union is 10 psi per sheet of foil... if that's not accurate, we can use that number for example purposes.

An 8" opening is going to have about 32 times more surface area than a 1.5" opening will. Therefore a 10 psi burst disk of aluminum foil on your 8" opening will need to be 32 layers. You can do the math from there.

My 3" burst disk noise stumps need over twice the masking tape layers than the 2" stumps at the same burst pressure.

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Unread postAuthor: Hotwired » Wed Jan 14, 2009 10:52 pm

No shrapnel eh?

Rules out aluminium sheet then, The entire section in the path of the air flow will end up flying out in shards.

I'm thinking rubber/cloth composites which are used for gasket material.

http://www.allstategasket.com/info_gasket_material_style-1175.asp

Its not a "hard" material but still, when punctured under pressure it should rip back to allow air through. With any luck it's tough enough to hold onto it's own ripped sections too so you won't find much of it coming out.
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Unread postAuthor: kissass78 » Thu Jan 15, 2009 12:33 pm

Two questions:

What material (hollow cap?) do i put around the opening to the barrel to hold the burst disk in place?

And would the presence of dirt and debris on top of the burst disk material compensate for some extra "resistance" or would that make no difference?

Essentially, If I use foil and put a heavier layer of dirt on it, would the weight of the dirt make a significant difference?

Foil doesn't seem that bad. It's relatively easy to wrap foil over itself again and again. I would not get out scissors and cut 200 10" squares. Hell no.
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Unread postAuthor: MrCrowley » Thu Jan 15, 2009 2:53 pm

What material (hollow cap?) do i put around the opening to the barrel to hold the burst disk in place?

A 'union' fitting.

And would the presence of dirt and debris on top of the burst disk material compensate for some extra "resistance" or would that make no difference?

Probably.

Essentially, If I use foil and put a heavier layer of dirt on it, would the weight of the dirt make a significant difference?

It would probably make the burst disk burst at a slightly higher pressure, how much higher, I wouldn't know.
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Unread postAuthor: Hotwired » Thu Jan 15, 2009 4:12 pm

A union is certainly what is used for your average burst disk which might be a couple of inches diameter.

However when you're talking 2-300 layers of foil and a 10" port there are new issues.

a) will the union fitting actually hold 2-300 layers securely

b) can you actually GET a 10" union


I'm not sure about (a) but as for (b) I've yet to find a US or UK supplier of pipe and fittings that sells any threaded plastic fitting larger than 4"

This might possibly be due to industry not really wanting to find themselves having to unscrew a hulking great section of 8" pipe that weighs a few hundred kilograms.

It would appear that for non-permanent connections of larger than 4" you have to get:

Two stub flanges - one for the chamber side and one for the other side

Two backing rings, one over each stub flanges flange and bolt the lot together with some kind of gasket - or in this case the burst disk - sandwiched between them.

Still, that's only up to 8" on the sites I've found and it's a UK one.



On the matter of stuff on the disk itself. Unless there's something heavy and spiky in the debris I really don't think some loose material will bother the disk much if it's been set to withstand the cannon pressure.

10" and 100psi will be 7854 pounds of force it's resisting in the other direction. Still, a cardboard cover to keep things from being directly on the burst disk wouldn't hurt.


*edit:

Could be in luck depending on your point of view.

Charlotte Pipe is US based and openly sells Sch80 flanges for up to 12" pipe.

A Van Stone flange* for 10" Sch80 pressure pipe weighs in with a list price of $387.33 and 6.5kg (the price and weight is for a pair). Clickety.



*A Van Stone flange just means the ring part of the flange can rotate about the rest of the fitting so the holes can be lined up more easily.
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Last edited by Hotwired on Thu Jan 15, 2009 4:48 pm, edited 3 times in total.
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Unread postAuthor: starman » Thu Jan 15, 2009 4:31 pm

The more I think about this, the idea of trying to configure 8 or 10" burst disks for this one shot application doesn't really make much sense. I think you would find the sledding a little easier down in the 4" range...still larger than most anything built around here, but enough to do the job you're looking for. Even then, 4" unions will be hard to get and expensive..you might think about 2 bolted flanges as a burst disk holder.
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Unread postAuthor: MrCrowley » Thu Jan 15, 2009 4:34 pm

BTW, for the 8" diameter chamber pipe, try scouting out some construction sites. You might find some laying next to a tip or dumpster.
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Unread postAuthor: kissass78 » Thu Jan 15, 2009 7:06 pm

Well since someone was cool enough to post that info on the Big Blaster XHV cannons....I emailed for a price quote and heard nothing back.

Does anyone know how much they cost?

Holding the burst disk to the barrel is the only real problem i see. The rest is feasible.

I might be able to get some 8" pvc for free.

The Charlotte pvc company is really stingy and it's had to get in touch with them. But I'll try again.

I'm so excited about building this!
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Unread postAuthor: Hotwired » Thu Jan 15, 2009 7:57 pm

No, none of these silo blaster cannon companies have a price list, its all quotes on request :?

Doesn't even seem to be any chat of prices around either.
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Unread postAuthor: Insomniac » Thu Jan 15, 2009 8:13 pm

Is it possible you could use cleaver camera angles on a smaller-scale set to use your current setup to simulate a larger explosion? Take a look at many of the explosions on 'Team America'. They were all small scale pyro explosions slowed down with a high-framerate camera. Makes them look like they are full size without the hassle of actually doing it. Of course, this would require a few things to work properly:

That the explosion you need could be done on a small scale set (Ie no people etc in view)

A camera with a high enough frame rate to slow the explosion down enough that it looks realistic.

A miniature set that looks very similar to the full-scale one.
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I wonder how much deeper the ocean would be without sponges.
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Unread postAuthor: kissass78 » Thu Jan 15, 2009 10:10 pm

Well the shot I want is essentially this....

Bad guy runs into frame with RPG. He aims, fires, and the camera does a jerky pan right to see a large explosion roughly 200 yards away.

So basically the explosion needs to be large enough to look like an RPG explosion....and since the camera is 200 yards from it....it will need to be big enough to take up a large portion of the frame.

The other option is chroma keying an explosion in. This would be extremely hard to do because the camera will be moving constantly. Also not as fun.

So that's why the burst disk is a problem. The impact needs to be right on cue.

If the bad dude fires the RPG and there is a delay it will look quite silly.

There's got to be a way!
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Unread postAuthor: kissass78 » Thu Jan 15, 2009 10:14 pm

8" pvc is ridiculously expensive.

$280 for an 8" cap.

I want something slightly smaller than this:

http://www.youtube.com/watch?v=GBUBH-jZ ... re=related
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Unread postAuthor: clide » Thu Jan 15, 2009 11:13 pm

Hotwired wrote:No, none of these silo blaster cannon companies have a price list, its all quotes on request :?

Doesn't even seem to be any chat of prices around either.


They had some price lists up back in 2003 (remembered making a post about it and found it on the archives). I believe the retrofit kit is just the valve, although I'm not sure. $2500 for the 6"

http://web.archive.org/web/200311051540 ... php?cat=21

For that kind of money you may be better off finding your own tanks and pipe and getting a welding shop to weld something together for you. A coaxial setup could probably be done fairly easily.
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Unread postAuthor: Insomniac » Fri Jan 16, 2009 5:00 am

kissass78 wrote:Well the shot I want is essentially this....

Bad guy runs into frame with RPG. He aims, fires, and the camera does a jerky pan right to see a large explosion roughly 200 yards away.

So basically the explosion needs to be large enough to look like an RPG explosion....and since the camera is 200 yards from it....it will need to be big enough to take up a large portion of the frame.

The other option is chroma keying an explosion in. This would be extremely hard to do because the camera will be moving constantly. Also not as fun.

So that's why the burst disk is a problem. The impact needs to be right on cue.

If the bad dude fires the RPG and there is a delay it will look quite silly.

There's got to be a way!



To me that would make it seem quite simple to have a scaled-down version of the set for the explosion:

You film the guy firing the rpg as usual, and quickly snap the camera across to the site of where the explosion should be.

Then you get a section of land where you add a few miniature things like trees etc (use branches of small-leaved trees to make 1/2 to 1/4 scale trees) and a few other smaller scale objects so that it looks like a full-scale landscape (minus stuff in the background) Set up a green-screen in the background so you can later edit in your own scale background (either that, or make it so that there is a dirt ridge etc that blocks out the horizon.)

Now, quickly pan the camera from where the rpg guy should have been standing, to the site where your smaller air morter is buried, and fire.

Then just edit the two clips together, taking advangage of the motion blur at the end of the first clip and the beginning of the second clip to make them match up perfectly.
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Unread postAuthor: kissass78 » Fri Jan 16, 2009 10:16 am

Well the set is an abandoned warehouse complex and I really don't have the skills to recreate even a portion of the location in a "diorama".
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