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new spudgunner

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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new spudgunner

Unread postAuthor: rp181 » Thu Dec 13, 2007 9:02 pm

hey all, this is my first post, though ive been reading the forums for about a month. Everyone here seems smart and helpfull =)

About two weeks ago, i got interested in electrolysis, and then the gun part. I made a small gun with a BB bottle as a combustion chamber and used a pen tube as a barrell, shot putty against wall with alot of force. Ignition was 2 250v 100uf caps in parrallel sparked on the inside. Now i have a BBQ igniter (not electric) and decided to build a bigger one. Combustion chamber is going to be 2 soda cans with tops connected. I was wondering about the barrel, i like to shoot things not airtight, so i was wondering if making a u and running the barrel on top. I have stiff black tubing (1/2in?) and wanted to do it so gas couldn't escape as well. Thinking like this:

/------------------------------------------ end of barrel
u turn| ___________
\----|___________|combustion chamber

Would this reduce force dramaticlly?

Before you go off on the dangers of hydrogen/oxygen mix, im well aware of the danger and am not collecting large amounts.

EDIT: what are spudbux?
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Unread postAuthor: DYI » Thu Dec 13, 2007 9:35 pm

Bends will always slow and reduce flow, but the difference shouldn't be particularly horrible.

Now, onto the dangers of the O2/H2 mix. I won't tell you not to do it, because I'm working on crazier and more dangerous projects myself, but I will strongly advise you to do your best to avoid a detonation. To achieve this, you need a chamber roughly as long as it is wide, to provide the least run up distance possible.

And spudbux are the root of all evil (or something like that)
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Unread postAuthor: rp181 » Thu Dec 13, 2007 9:52 pm

O.o my chamber lenght is like 4 times the the width. Im collecting the mix directly into chamber and firing right after, no hydrogen storage. Will be igniting well away from it, in case of explosion.
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Unread postAuthor: DYI » Thu Dec 13, 2007 10:08 pm

Its not the storage you really have to worry about, its the flame front accelerating to a detonation, and hydrogen combustion flamefronts accelerate rather quickly compared to most fuels (especially in pure oxygen). If you tell me the actual dimensions of the chamber, someone can probably tell you if it's long enough to be worried about, but I *think* you'll be OK. Remote ignition is a good idea though.
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Unread postAuthor: rp181 » Thu Dec 13, 2007 10:38 pm

Its not the storage you really have to worry about, its the flame front accelerating to a detonation


i don't get what you mean by that, ile get the dimensions now though.

EDIT: Dimesions are:
length 24.5cm
Diameter 6.5cm

in the center, it slants down to a diameter to about 5.7cm, than back up.
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Unread postAuthor: DYI » Thu Dec 13, 2007 10:50 pm

So, slightly over 2" diameter and under 12" long. Considering the slight inaccuracy in fueling caused by preexisting air in the chamber and the fact that atmospheric nitrogen will be present as a buffer gas, I don't foresee any detonation problems. Just make sure that your pressure vessel isn't PVC, and is rated for at least 250 psi, and you'll be good to go.
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Unread postAuthor: rp181 » Thu Dec 13, 2007 10:58 pm

sorry, new to this, but pressure vessel = barrel?
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Re: new spudgunner

Unread postAuthor: Ragnarok » Fri Dec 14, 2007 12:13 am

Would this reduce force dramaticlly?

Before you go off on the dangers of hydrogen/oxygen mix, im well aware of the danger and am not collecting large amounts.

Bends have a little effect on spudgun performance. As a general rule, use only as many as you need, but don't fret about each one.

As for 2H<sub>2</sub>/O<sub>2</sub> mixes, it will need moderate care, but it's not hugely dangerous, as long as you take care and always respect it.

Three simple rules for getting along with a spudgun:
1) Never assume it is 100% safe. Act as if you know it could fail at any second, so always wear goggles and ear defenders.
2) Always treat the thing as if it were a loaded shotgun with an uber sensitive trigger.
3) Don't EVER mess around with it. If you aren't sober, or are tired, put it away, that's when accidents happen.

EDIT: what are spudbux?

Practically useless.
You could try to use them to bribe the moderators though. (Note: May not work)
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Unread postAuthor: Eddbot » Fri Dec 14, 2007 12:39 am

pressure vessel=combustion chamber

i'm pretty sure two soda cans epoxied together at the tops can withstand 250 psi, listen to ragnarok about the basic rules

what would you bribe the moderators to do? give you more spudbux?
not completely useless... you can buy a sig :evil3:
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Unread postAuthor: rp181 » Fri Dec 14, 2007 5:35 pm

1) i didn't epoxy, hot glued together. There are 3 copper tubes(small dia.) across the two cans to help it stay together.
2) how long of a barrel do you think ile need? i read there was a ratio for maximum efficency and a ratio for maximum power, whats the difference? 3/4th inch pvc will be barrel.

thx for all help.
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Unread postAuthor: paaiyan » Fri Dec 14, 2007 5:43 pm

OK, I personally think using H2O2 in two soda cans hot glued together isn't such a good idea. Hot glue isn't well-known as being the strongest of adhesives. I'm siply stating this so that you can't say no one told you it was a bad idea. The chamber-barrel ratio you want is probably about .8:1. That means the volume of your chamber should be about .8 that of the barrel.

Eddbot, I'm pretty sure a soda can can't handle 250 PSI.

And rag, you can't use them to bribe the mods, our resident admin took away the donate ability.
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Unread postAuthor: SpudBlaster15 » Fri Dec 14, 2007 7:13 pm

I can tell you from experience that if a small balloon filled with oxygen and propane situated in an inverted coffee can can blow the bottom of the can 50ft in the air, and peel the sides outward like a bannana, a chamber made from 2 pop cans fixed together with hot glue is going to behave more like a bomb than a cannon when you trigger an even more violent reaction inside it.

H<sub>2</sub>/O<sub>2</sub> mixtures are fine, so long as your chamber can handle the rapidity of the pressure spike.
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Unread postAuthor: rp181 » Fri Dec 14, 2007 7:32 pm

well guess its time to find a better combustion chamber.... mabey one of those stainless steel water bottles? i saw it in instructions for a pulse jet.
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Unread postAuthor: DYI » Fri Dec 14, 2007 7:40 pm

Have you ever considered using a pressure vessel that was actually designed as a pressure vessel?

Spudguns don't need to be ghetto. Use copper or steel tubing, and save yourself loads of trouble.
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Unread postAuthor: potatoflinger » Fri Dec 14, 2007 11:30 pm

Another reason that you shouldn't use hot glue is that the first time you fire the gun, the glue will become soft, so then the second time you fire the gun, it would blow apart easier, which wouldn't be very fun. The only way you could get around this problem would be to wait a few minutes between shots to make sure that the glue was cool enough.
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