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Problems with my very first combustion 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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Problems with my very first combustion spudgun.

Unread postAuthor: 50bbx » Tue Apr 19, 2011 3:44 pm

Hi,
I'm new here at the forum, I'm not English or American, so I don't speak very good english, but here's my situation: I'm new building a spud gun and I'm having loads of trouble making it woking. I do not follow a specific guide, I've just used my common sense, but I am not an expert at all.
As you can see in the pictures attached is just two tubes glued (badly) and a electric lighter. I put the potato on the small side and the hairspray through the bottle neck glued on the bottom. I was having trouble closing little holes that allows air to escape, than I finally success in closing them. So, I went into the wood, in a remote place to try my spudgun. But it didn't work at all.
The hairspray didn't burn at all. So, I've tried to light it with a lighter from the bottle neck. And it works: it did a *big* bang when I tried to put out the fire blowing on it (I didn't try with the potato).
There are 2 possibilities: I sprayed too much hairspray or the electric lighter is too weak. (I'm more for the second one).
My questions are:
1. How many seconds do I have to spray the hairspray?
2. Is correct using the hairspray insted of, for example, deodorant?
3. Which is the best glue to make the spud gun durable?
4. I've heard that is better using a kind of electric "ignition" of a barbecue or something like that: where can I find one to replace the electric lighter?

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Thanks for the answers, I hope I can shoot spuds soon :)
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Last edited by 50bbx on Tue Apr 19, 2011 3:51 pm, edited 1 time in total.

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Unread postAuthor: warhead052 » Tue Apr 19, 2011 3:47 pm

Use pvc cement and primer to make it stick together. Deodorant, hairspray, if it sprays, it should work. Other wise it looks pretty good, can we get a video or something up?
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Unread postAuthor: saefroch » Tue Apr 19, 2011 3:51 pm

1. That depends on the size of your chamber. The design you're using, we call a spray 'n' pray. I recommend upgrading to a metered design. Look through the showcase for inspiration

2. You're just looking for the combustible propellant, butane. I recommend using straight propane or butane instead.

3. Not glue, per se. Read.

4. If your ignition source is sparking, it's perfectly adequate for the use.

I recommend resizing your pictures before putting them into a thread, the huge file sizes are making my connection laggy, and it took a while to load (others will probably have the same issue, just a courtesy suggestion).
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Unread postAuthor: 50bbx » Tue Apr 19, 2011 4:28 pm

Images reduced, sorry :)

well, the ignition source is sparking but the hairspray doesn't burn at all , not even if the chamber is open. Why? maybe the spark is too little? Is this a good alternative?

when I shoot with the spudgun the chamber has to be perfecty closed, hasn't it? not little holes or else, right?

a video will be made soon, even a little one showing the non-working supdgun, maybe it will help find a definitive solution to all my problems:)
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Unread postAuthor: warhead052 » Tue Apr 19, 2011 4:32 pm

Spray the hairspray in for about 10 seconds, then allow 2 seconds of just air to get in for a pretty good mix. I did this with a friends combustion, the bang was louder than 10 seconds of flat hairspray with no air... Plus we found the ammo he used split up into fragments :D
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Unread postAuthor: jimmy101 » Tue Apr 19, 2011 4:54 pm

The most likely problem is too much fuel. Most sprays use propane as the propellant in the can, and that is what you are trying to use as the fuel.

Propane will only ignite and burn if the volume of propane in the chamber is between about 3% and about 8% of the volume of the chamber. Too much propane, or too little, won't burn.

Rummage up a 50cc or so syringe and either a disposable butane lighter or a butane refill cartridge and get the fueling right on every shot.

Take a look at the spudwiki pages:
http://www.spudfiles.com/spud_wiki/inde ... Fuel_meter
http://www.spudfiles.com/spud_wiki/inde ... le=Propane
http://www.spudfiles.com/spud_wiki/inde ... tle=Butane
http://www.spudfiles.com/spud_wiki/inde ... n_Spudguns
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Unread postAuthor: Insomniac » Tue Apr 19, 2011 5:42 pm

Yeah, it's a very specific amount of spray you need. Also, try shortening the wires between your igntion and the gun... It will help the spark along.

One last thing, remember to vent out the chamber and purge it with fresh air before each attempt... Otherwise you'll just have a buildup of fuel and no oxygen.
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I wonder how much deeper the ocean would be without sponges.
Right now I'm having amnesia and deja vu at the same time. I think I've forgotten this before.

Add me on msn!!! insomniac-55@hotmail.com
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Unread postAuthor: saefroch » Tue Apr 19, 2011 7:35 pm

50bbx wrote:doesn't burn at all
That usually indicates a fueling problem. I've attempted to used a very large spray 'n' pray before, and I can tell you that fueling correctly is incredibly difficult to get the hang of. So long as your ignition source is producing a spark at all under atmospheric conditions, it should function perfectly well for this build.

I suggest syringe fueling, JSR's favourite method. It really works, and is quite simple. You may also be having an issue with mixing. What warhead052 talks about with allowing air to get into the chamber isn't really that, it's allowing the gases to diffuse together and become a uniform mix. If the mix isn't uniform, you may end up with a region of gas around the spark that isn't combustible.

If there are holes in the chamber, it will reduce performance, but you will still be able to ignite a combustible mix. However, be aware that if there are any holes a jet of very hot gas will shoot out during ignition, and can burn through duct tape, hair, and clothes.
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Unread postAuthor: Technician1002 » Tue Apr 19, 2011 8:27 pm

Very carefully check if the hairspray is flammable. I don't recommend holding the lighter in your hand. If you manage to spray your hand it could be very bad.
http://www.youtube.com/watch?v=H8BfP6zrUBY
Some hairspray is more water and less butane or propane.
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Unread postAuthor: shiny455 » Tue Apr 19, 2011 8:38 pm

if you have a air compresser you can spray the hair spray in and use the air compressor to fill it with air to get a good mix.. worked for me
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Unread postAuthor: mark.f » Tue Apr 19, 2011 10:00 pm

If you have a source of propane or butane (which should be available in just about any country), you should have no problems with ignition.

Use a syringe and fill with ~4% of the chamber volume with propane, or ~3% of the chamber volume for butane. Inject through the bottle neck and cap quickly.

Also, if you have a torch designed for your gas (plumbing torch for soldering copper and brass pipe and fittings, butane torch for soldering electronic or small components, etc.), use it to fill you chamber with premixed fuel/air, just allow time for the mixture to displace the air already into the combustion chamber.

If none of these work, get a butane refill cylinder (used to refill butane lighters), and take the spray nozzle off of a can of deodorant, trim the tube coming out of the canister to the right diameter, and press fit the nozzle onto the tube. Then, use a system of trial and error (counting) to determine the spray time which yields the best combustion.
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Unread postAuthor: Technician1002 » Wed Apr 20, 2011 1:41 am

In re-examining the photos, does the chamber have a clean out cap? The breech looks too short to contain a threaded and glued ends. Lack of fresh air between shots will prevent igniton.

You need a way to open the chamber wide open for full venting of spent gas and replacing it with all fresh air.
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Unread postAuthor: mark.f » Wed Apr 20, 2011 7:31 am

Tech, the bottle neck you see on the side of the cannon is for fueling and venting.
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Unread postAuthor: Technician1002 » Wed Apr 20, 2011 10:07 am

Unlesss you are venting with a shop compressor, it is too small.
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Unread postAuthor: warhead052 » Wed Apr 20, 2011 10:15 am

Oh I just realized this. Dont use bell reducers because they are weak. They cant handle pressure the way bushings do, just a tip! I dont know if they work better in combustions, or if they are safer, but I know I wont use them unless its for decorations.
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