SpudFiles

I'm interested in building a simple pneumatic cannon to use for special effects. I would use it primarily to throw dirt, flour, and other small debris to give the effect of an explosion on camera (Which I would enhance with digital effects in post).

The only problem I'm running into is what materials to use for the air chamber. The design I'm looking into is very simple . You take a reservoir of whatever size and connect it into a sprinkler valve (The one I got has a 1" thread on both ends). The other end has a 90 degree elbow that will project the debris upward.

The only problem is I don't feel comfortable using PVC to hold compressed air. I'm also not entirely sure if a small air tank would do what I want. I need the air stored to be released as fast as possible, not a slow steady stream. The idea is to get a large quantity of air out the end as fast as possible.

I'm just stuck on what to use that would hold enough air but would still be portable and hidden easily enough on location. Most designs I've seen use a 3 or 4" diameter pipe about 2-3' long, which is pretty easy to hide in this configuration.

Here is an example of the design that I'm trying to make:
http://www.alliancemediaproductions.com ... ticsv2.jpg

And an accompanying video:

First off, beware ofshooting fine powder.

If done well and used within its limits, PVC is perfectly safe.

You can go for metal fittings and higher pressures, say using a propane tank or fire extinguisher as an air chamber, and go for a slightly more serious valve.

Important question, what's the budget?

Have a look at this FX launcher from spudtech.com if you need design ideas...




edit:
On the topic of special effects and sawdust explosions...

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The valve they are using actually looks like a Supah valve.

I'm looking to keep the cost less than $75 for this. I don't plan on using this for any fire, just smoke. I've spent a lot of time learning how to create fairly realistic 3D fire simulations. Since smoke takes forever to simulate and render, this air cannon seems like a logical choice. It would also make my post workflow much faster.

I'd assume my typical working pressure would be somewhere between 60-120 psi, but that depends on the size of the container I think. Is it wrong to assume that a smaller container at a higher psi (And better valve) would produce a similar force of a larger container at a lower psi? Forgive me if that's a stupid question. I haven't personally worked with compressed air before.

Edit: I already bought all the parts for this. the $75 is just for the air chamber.

For a high flow chamber for larger sizes, modifying an existing tank rated for the pressure is not that difficult. I have modified propane tanks, which work well for your pressure range, for 1 inch, 2 inch and 2.5 inch outlets.

The sawdust cannon in a post above is an example of a modified propane tank.

Is it wrong to assume that a smaller container at a higher psi (And better valve) would produce a similar force of a larger container at a lower psi? Forgive me if that's a stupid question. I haven't personally worked with compressed air before.

Not a stupid question at all. In terms of energy, you'd be right that for example 100 cubic inches at 50 psi and 50 cubic inches at 100 psi are identical, but in practice when it comes to propelling a projectile, this is not the case.

Something I always drag up in this case, have a look at this data for a virtual launcher modelled in GGDT - in each case, we're talking about the same quantity of air, but in a reduced volume and therefore at higher pressure. The barrel length, projectile weight and size, valve efficiency and specs etc. all remain the same, all that changes is chamber size and pressure.

20 inch chamber at 50 psi - 406 feet per second

10 inch chamber at 100 psi - 537 feet per second

5 inch chamber at 200 psi - 684 feet per second

2.5 inch chamber at 400 psi - 830 feet per second

1.25 inch chamber at 800 psi - 958 feet per second

0.625 inch chamber at 1600 psi - 1006 feet per second

Yes, it certainly gets more efficient as the pressure goes up and the volume goes down.

A huge tank of air at 1 ATM will have much less than the same volume of air at 100 ATM. This is not rocket science, maybe.

I hate to point out a mistake in the above example. All pressures measured against Gauge pressure are not the same volume of air. Each one has +1 ATM already in the tank, so the larger tanks at lower pressure actually contain more air.

Technician1002 wrote:I hate to point out a mistake in the above example. All pressures measured against Gauge pressure are not the same volume of air. Each one has +1 ATM already in the tank, so the larger tanks at lower pressure actually contain more air.

Fair observation, I'm sure GGDT takes atmospheric pressure into account - still, low volume high pressure wins out in spite of that so I suppose the point stands.

I saw a pretty good deal on a 10 Liter tank on ebay and decided to buy it. It seems like a good fit for me to upgrade in the future if I need to (and it was only $50, shipping included).

http://www.ebay.com/sch/i.html?_from=R4 ... +RESERVOIR

I also went ahead and got some threaded adapters to step down from 1" to 1/4" so I could install an air intake valve and pressure gauge. Will I be ok threading them together with PTFE tape or should I use something else?

Looks like it will fit the bill

Will I be ok threading them together with PTFE tape or should I use something else?

Yes that will work.

Cool, thanks for the suggestions and info everyone. I just kind of accidentally stumbled on that tank out of nowhere lol.

I'll post pictures and a video once I get everything put together and make sure it works. Should be done by next week hopefully.

or maybe I'll come back crying for alternatives when it fails miserably!

Just out of interest what dust/powder will you be using to simulate the smoke?

I'll start using flour. Depending on how that turns out I may or may not try fullers earth. I also got a sponge I ripped into some jagged pieces that I'm going to paint gray/black to act as fake rocks. They should help push all the flour and other small debris out of the end (Dirt, sticks, leaves). Should be fun...but I gotta build it all first. 8)

Be careful with that flour, it's just as flammable as the sawdust shown above. And especially don't shoot coffee creamer:
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