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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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Big Explosions!

Unread postAuthor: kissass78 » Tue Jan 13, 2009 9:03 pm

New to the forum...first post.

Heres my situation.

I'm making an action movie and I need large dirt explosions. This is not a backyard video, its a war movie with real military guys and real guns.

I made a smaller (4"x4' reservoir) pneumatic cannon which can be fitted under the ground and triggered to create "grenade" blasts. But the craters only end up being maybe eight inches wide.

There is a scene in my film where a bad guy fires an RPG and there's a big explosion of dirt and debris.

How the hell am I going to do it?

Two problems:

First, I'd like to have a very large reservoir, maybe 8" or 12". Maybe even two at 8". I'm thinking from there it goes into a sanitary tee of equal size, and then of course to the valve.

Second problem. What valve is going to be big enough to get that much air moving through? The barrel needs to be an elbow, preferably pretty wide. That attaches to a funnel, or cone, which is in turn filled with dirt and debris.

I'm looking to get explosions 40-60' high.

Is this possible?

Any tips? Help?
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Unread postAuthor: Tidbit77 » Tue Jan 13, 2009 9:14 pm

well for a compatt rig i would use co2 tanks as resevoirs, but if you arent going for compactness, and simple simplicity, id use 2-3 6"x4'-5' pvc resevoirs filled with compresed air and triggered by a piston valve, but im no expert in movie explosions soo.....


(also, if u use pvc pipe, make sure it [b]has a pressure rating and doesnt say cellular core)

thats just my 2 cents

EDIT: I WOULDNT MAKE SURE THE SANITATION TEE ISN DWV (drain waste vent) i dont know but i think sanitation tees are only dwv
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Unread postAuthor: MrCrowley » Tue Jan 13, 2009 9:18 pm

I think you should go combustion, for several reasons:

1: 8" pipe will cost a lot, it's pressure rating will also be quite low so it would be more dangerous with a pneumatic.

2: Some members have built combustion cannons using 8" and 10" pipe quite easily. Making it combustion will save you a lot of money and hassle. Otherwise you'd probably have to make a giant piston valve or burst disk cannon. Try searching the forum for basketball cannons.

3: 40-60' is really pushing it, 20' is probably more realistic but still difficult depending on the quanity of earth you want to displace. It would be just as easy to achieve this with a combustion then pneumatic.

If you made it a combustion, i'd go with a single 8" chamber, 2' long (or even 1'). You would then have your 8" elbow attached to one end and an end cap on the other end. The elbow would lead to the funnel. You block the bottom of the funnel with some paper or anything that will stop dirt from going into the chamber. Load the dirt in the funnel. Use a metered propane setup to fuel the cannon, bury it. Then fire it.

There are a few problems going combustion though:

1: Venting. Combustions need to vent the chamber each shot with fresh air. This would require you to dig up the cannon each after shot and have a 2" ball valve threaded into the end cap on the chamber. You will then need a fan or compressed air to blow air into the chamber and out the ball valve to vent it.

Another option wouldn't require the 2" ball valve but would require a huge fan big enough to fit in 8" pipe. This way you could leave the cannon buried. After each shot, you would switch on the fan so it would blow air out of the chamber. Then switch it in reverse to suck air in.

The fan will also help a lot when fueling the cannon, you can turn it on when you have injected the propane to help mix it.

2: Fueling. If you want to keep the cannon buried for multiple shot, you'l have to figure out a metering system that can be burried or have a hose coming out of the ground from the cannon, so you can fuel it while it's buried.

3: Ignition: This will be pretty simple, just hook a stun gun up to some 30' long wires and you can fire it remotely.

I'm not really a combustion guy, i've probably overlooked some stuff. Other members will help you with more detail.
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Unread postAuthor: Mr.Sandman » Tue Jan 13, 2009 9:31 pm

Maybe if you borrowed jook13's pvc monster? But again as Mr.crowley just stated it is very difficult to do this or at least get 40-60 foot high explosions. Even though i am not allowed to say this......... i would do...........a huge m80 :bom: kinda ironic how i typed that small!
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Unread postAuthor: kissass78 » Tue Jan 13, 2009 9:34 pm

Combustion is out of the question unfortunately. The law enforcement would instantly deem that "unsafe".

See, I have to use compressed air, otherwise the cops won't let me do it. I told them I wanted big explosions and they said 1. find a pyrotechnical who will do it for free (impossible) or 2. use compressed air.

Of course I want to be safe, so if it can't be done I'll go smaller, but I think it's possible.

As far as height is concerned, my 4" cannon shoots peat moss 20' feet up into the air easy.

I'm really looking for wider, bigger explosions.

If 8" pvc is weaker, should I make four or five 4" reservoirs hooked into a 8-12" elbow?
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Unread postAuthor: Hotwired » Tue Jan 13, 2009 9:40 pm

40-60' high and a large crater. That's a big order of flying debris.

Not many cheap commercial valves will be capable of that kind of blast. The only commercial valve I'm aware of that will be of a large enough port would be an "air cannon" which happens to be an industrial name for what we'd see as a pneumatic cannon without a barrel.

Basically it's a piston valve and a chamber which gets discharged into a silo for loose and dry material to clear blockages.

I'm aware of up to 6" valves (outlet port) for that. Heavy metal and cost a bit.


You can also make your own piston valve to whatever size you're after. Sanitary fittings and pipe are not to be considered. Pressure rated pipe and fittings only.


What kind of valve were you using before? A solenoid or sprinkler type valve?
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Unread postAuthor: MrCrowley » Tue Jan 13, 2009 9:40 pm

The law enforcement would instantly deem that "unsafe

Where do you live?

The thing is if you go pneumatic, you'd probably have to go with dual 4" piston valves, but it really depends on how much earth you are trying to displace. If you gave us a rough estimate in weight, that would help.

You could go with an 8" burst disk. No one has done it before that I am aware of. It will provide you with the flow and opening speed that this task would require. The only problem is finding material that will burst at around 100PSI but is big enough for an 8" PVC union.
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Unread postAuthor: Hotwired » Tue Jan 13, 2009 9:53 pm

MrCrowley wrote:The only problem is finding material that will burst at around 100PSI but is big enough for an 8" PVC union.


Sheet aluminium would be fine for that, easily enough get a gauge that will hold over 100psi (it would need to hold more than the firing pressure to fire on demand). For firing a puncturing device to burst the disk such as say a sharp point on the end of a pneumatic cylinder can be triggered.

A triggered burst disk in short. It's been done here with aluminium from drinks cans and an arrow (an actual arrow) on a pneumatic cylinder to rupture the disk.


It would be easier than a piston valve but would take longer to reset for firing. Mind you I don't believe you'd notice the time to reset the cannon compared to resetting the set for another firing.
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Unread postAuthor: kissass78 » Tue Jan 13, 2009 9:55 pm

NC.

A couple of the guys on the cast are cops, and that's why I am allowed to use automatic weapons. Unfortunately they love their jobs, not to mention being extremely anal retentive. No combustion. No fireworks. No bombs.

I generally use light weight material like peat moss, fertilizer, mortar mix, baby powder, flour, grass, etc. for the cone. Mix it together and it's lighter and it covers more space quicker when the button is pushed. Ultimately I'd compress about 4-10 pounds into the cone depending on the material.

If you were to blow up one bottle of baby powder with 60psi the cloud would be at least 5'x15'. That's what I'm going for.

I am not familiar with a burst disk except on air tanks. They are 8"?
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Unread postAuthor: kissass78 » Tue Jan 13, 2009 10:01 pm

I use a sprinkler valve on my 4".

40-60 feet is not that high! If the material is light enough it is possible. I know because my DP was filming from the roof (40') and the cloud from my 4" made it halfway. We was impressed!

I'm really looking to get wider explosions.
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Unread postAuthor: Hotwired » Tue Jan 13, 2009 10:02 pm

kissass78 wrote:I am not familiar with a burst disk except on air tanks. They are 8"?


:D

Heck no. Those things are tiny. This kind of burst disk you make yourself.

You get your burst disk material (I've suggested thin aluminium sheet) and cut your disk from it.

It is then clamped over the opening from your reservoir (It might be off a T or whatever) using a screw union or maybe flanges that bolt together (could be a better choice at 8" diameter).

To fire you either poke the disk with something sharp (such as a pneumatic cylinder with a point) or you pressurise the chamber til the disk blows of its own accord.

Incidentally an 8" burst disk rupturing with a very large volume of 100psi air behind it is going to be incredibly loud.


*edit: you had a 4" porting sprinkler valve?
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Last edited by Hotwired on Tue Jan 13, 2009 10:04 pm, edited 1 time in total.
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Unread postAuthor: MrCrowley » Tue Jan 13, 2009 10:03 pm

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Unread postAuthor: kissass78 » Tue Jan 13, 2009 10:05 pm

Ahhhh!!!

I bet that would be loud.

Here's the problem. I need it remotely triggered on cue. Is that possible?
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Unread postAuthor: Hotwired » Tue Jan 13, 2009 10:07 pm

Yes. You pressurise the chamber then use a pneumatic ram/cylinder with a spiky bit to rupture the disk, it doesn't have to go far, as soon as the disk material is compromised the entire thing peels back like a balloon popping.
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Unread postAuthor: starman » Tue Jan 13, 2009 10:08 pm

You could also build yourself some movie "dirt" by using something called vermiculite. It's not anywhere near as heavy as real dirt clods and you should be able to blow it quite a good distance. They sell it at garden stores and use it to make a very light potting soil. They also sell it for use as packing material. It's very light so you can blow it around very easily but when mixed with some real dirt, maybe some peat moss, and tree bark chunks, should look just like dirt on film.

You could try cutting out a cardboard "piston" to fit in the bottom of a 5 gallon bucket that's had a hole cut in it for your spudgun barrel, fill it with it the movie dirt and attach your spud gun....fire away. Should easily blow it 30-40'. You may be able to scale it up to bigger 55 gallon barrels as well. You'll need a bigger cannon of course.... :wink:
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Last edited by starman on Tue Jan 13, 2009 10:09 pm, edited 1 time in total.
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