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Preparing to build my first gun

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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Preparing to build my first gun

Unread postAuthor: niglch » Sat Jul 07, 2007 8:54 pm

I'm probably going out to buy materials for my first potato gun tomorrow. I'm using combustion because it seems to be both the simplest and cheapest to make, and I think I'm going to try Axe as a propellant (or some other aerosol, I just happen to have Axe on hand). Naturally, and being the type of person that I am, I have a few questions before I go for it. I am mostly confused by what size PVC pipes I should use (specifically having to do with the C:B ratio). I was looking at both this SpudTech how-to and also this one. Both use relatively high ratios (1.75:1 and 1.90:1 respectively). However, I have heard that different ratios such as 1.5:1, 1:1, and .7:1 are supposed to work the best. I'm also concerned about what diameter the barrel and chamber should be. 4in and 2in, 3in and 1.5in, 4in and 1.5in . . . I've seen all of these used. Since this is my first time doing this, I'm not really sure how to manipulate my design for specific results. Here are my "goals" for my potato gun:
1) Safety- Obviously there is some danger involved. I'd be really upset if my potato gun blew up in my hands . . . This is my primary concern because this whole thing will totally not be worth it if I get hurt.
2) Noise- I'm favoring a quieter gun just because it is less likely to disturb people. I'm planning on firing in an area with at least 250m to the nearest house (I won't be firing in this direction, the next nearest house is at least 1/4mile away).
3) Range- Anywhere between 150 and 200M would be ideal, but I wold be perfectly satisfied with a good football field. Again, I would like power balanced well with safety.
4) Size- Somewhere between 4' to 6'.

Should I just stick with one of the designs in the how-tos or do you recommend any modifications? I was favoring the dimensions from the SpudTech.com design.

Thanks for any recommendations
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Re: Preparing to build my first gun

Unread postAuthor: Modderxtrordanare » Sat Jul 07, 2007 10:13 pm

niglch wrote:1) Safety- Obviously there is some danger involved. I'd be really upset if my potato gun blew up in my hands . . . This is my primary concern because this whole thing will totally not be worth it if I get hurt.
2) Noise- I'm favoring a quieter gun just because it is less likely to disturb people. I'm planning on firing in an area with at least 250m to the nearest house (I won't be firing in this direction, the next nearest house is at least 1/4mile away).
3) Range- Anywhere between 150 and 200M would be ideal, but I wold be perfectly satisfied with a good football field. Again, I would like power balanced well with safety.
4) Size- Somewhere between 4' to 6'.

Thanks for any recommendations


1. If you use proper fittings for your gun, and solvent weld everything together properly, then saftey shouldn't be an issue. Just treat the gun like you would a firearm.

2. The more efficient your C:B ratio is, the quieter it is. The noise is mainly generated from a poor combustion, so if you get as efficient a combustion as possible, then it would be as quiet as it could be. That might be too loud still, in which case you could design and build a silencer for it.

3. Personally, I would be using metered propane, as it's alot more powerful. But I don't know what kinds of ranges you'll get using Axe, or something of the sort.

4. Just make sure your C:B ratio is around .7:1 or .8:1 if you are still going for power and less noise.
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Unread postAuthor: rednecktatertosser » Sat Jul 07, 2007 10:30 pm

Modder seems to have answered your questions, but I highly recommend a chamber fan.

Personally if you want better noise control I'd go with a pneumatic, but thats just me.
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Unread postAuthor: Pilgrimman » Sat Jul 07, 2007 10:35 pm

If you want low noise and a lot of power, build a pneumatic gun! It's not that complex, and the fuel is totally free! Furthermore, it is MUCH more powerful than a combustion cannon. In terms of power, there is no comparison. Also, if the chamber to barrel ratio is over 4:1 (something like that), it won't decrease performance! (I think so anyway, I recall reading this on spudtech somewhere). Even better, it fires every time! This is hard to achieve in combustion guns, but since pneumatics don't rely on stoichiometry for power, they fire 100% of the time. All you need is PVC fittings and pipe, with two threaded male pieces. Then just buy a solenoid valve, (If your barrel is 2 inches or less, otherwise you might want something bigger, like a Supah valve), a schrader valve, and some switches and whatnot for the solenoid. My first pneumatic, (and only one) ran me about 50USD. I built the PSR-01 which can be found at PVC Ballistics. If a techno-ignoramus like me can build a pneumatic gun, then you sure as heck could! It is also (IMO) a lot safer than a combustion cannon. No risk of electric shock, unless you COMPLETELY suck at wiring things, no potentially explosive fuel, no solvent vapors to vent before you shoot. As long as you make good solvent welds, your gun will only be as weak as the tire valve, which for my valve, is around 125 psig. I would suggest going to the Spudfiles Wiki, as there is a lot of reputable information there. Happy spudding! It's a strong addiction!
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Unread postAuthor: mega_swordman » Sat Jul 07, 2007 10:57 pm

Although I too would agree with the facts about pnuematics, he did specify he wanted to build a combustion cannon.
I would go with modders advice. I would first build a simple combustion, then move on to modding it like propane metering and a chamber fan.

Good luck!
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Unread postAuthor: niglch » Sun Jul 08, 2007 10:17 am

Alright, I'm planning to make a 4" x 10" chamber with a 2" x 48" barrel which should give me a C:B ratio of .79:1 according to SpudTool.

One last question:
Since I don't plan on using propane as a fuel, could this potentially change the ratio I should be using? I don't know how much testing has been done for aerosol propellants.

I might try making a pneumatic gun soon. The only reason I shied away from it is because most of the how-to's on building starter-model guns were about combustion cannons. I just assumed the pneumatics were difficult for a beginner to build.
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Unread postAuthor: Binder17 » Sun Jul 08, 2007 10:06 pm

My first four spudguns were aerosol. I used Right Guard but of course they stopped using the volatile chemicals in it. The ratio for this gun should probably be about 1.5:1, I have found this ratio to work best. I much suggest building this one first before using propane so you can get used to how everything works. Axe has never worked well for me. The trick is to find an aerosol with multiple flammable chemicals in it without a lot of extra crap like there is in Axe. The red can of Right Guard seems to work very well except it has talc in it which gums up the electrodes and everything else. Lysol also works if conditions are right, meaning warm temperature. Note that using an aerosol with alcohol only will only ignite in warm temperatures. For assembly be sure to wipe the fitting and pipe ends with acetone or pipe cleaner then apply Purple Primer and Heavy Duty PVC cement. A chamber would make it perform significantly better, just beware the talc in some antiperspirants.
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Unread postAuthor: Flying_Salt » Sun Jul 08, 2007 10:16 pm

Here are some good plans for starter spud guns. My first one was the spud buster. You don't have to use a lantern sparker, grill ignitors work much better.

niglch wrote:I might try making a pneumatic gun soon. The only reason I shied away from it is because most of the how-to's on building starter-model guns were about combustion cannons. I just assumed the pneumatics were difficult for a beginner to build.

Right you are. Combustions can be less reliable, but they are easier and more fun to build because they are loud and don't have to be pumped up. They're also lower pressure and don't hold pressure for too long, so they're safer on the pipe.

Welcome to your new addiction![/b]
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Unread postAuthor: f.c » Sun Jul 08, 2007 10:53 pm

have fun. may i suggest propane ?
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Unread postAuthor: dongfang » Wed Jul 11, 2007 8:54 am

Hi,

Agreed. Propane. Clean, cheap, reliable .. and really, it´s not like super high power dynamite compared to aerosols .. just slightly more powerful, and simply better.

On my old unmetered combustion, I put a small banana jack in the cap to inject fuel through. When you gave the propane (lighter gas) can 3 light taps on the bottom with its nozzle in the banana jack, the gun got fueled correctly.

Regards
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Unread postAuthor: psycix » Wed Jul 11, 2007 9:16 am

I recommend a ratio of .6 to .8 for a quiet gun, the lower the ratio, the more quiet the gun.
Though dont drop it too much or you will lose performance really quick.
Take a look at this test result:
http://www.burntlatke.com/jpg600/15cb-graph.gif
a .5 ratio could also be done for a quiet gun but dont go lower then that

Barrel size: You should take a 2"or 1,5" diameter barrel.
2" will have more power, but does not fit smaller potatos. You will have to throw alot of potatos away.
Barrel length: The longer, the more power you can get. Though a barrel of 8' might be uncomfortable to walk with. If you want to walk around while holding it, dont make it longer than 5', 2' would be a small barrel.

Calculate the barrel volume and by the ratio you may calculate the needed chamber volume.

Take a chamber diameter of about 3" or 4", the larger the diameter, the shorter the chamber will be. Then calculate the needed chamber length.
I prefer a large-diameter chamber so 4" would be nice.
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Unread postAuthor: niglch » Mon Jul 16, 2007 9:51 pm

Wow, this turned out really well . . .
I've gone on two separate spudding expeditions into the woods to small lake so far, and I've gotten really good results for a first gun in my opinion. I went with a C:B ratio of around 1.2:1 (larger than the .8:1 for propane because I was primarily planning on using aerosols). I found Axe to be the best propellant to this point with right guard performing about as well. Old spice didn't seem as good, but i haven't experimented with it much either. An any case, about 1.5 to 1.75 seconds of spray sent a chunk of potato blasting satisfyingly from the 48" barrel to ranges I'm estimating at 175-200M. I was able to out-gun my friend's launcher by at least 80M! My gun is still very loud (it makes you're ears ring a little after firing). I'm not sure if I should try dropping the chamber size a bit smaller for my second gun to see if I can get an even greater increase in efficiency and reduce noise.
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Unread postAuthor: psycix » Sun Jul 29, 2007 12:35 pm

it is so loud because its a 1.2 ratio ;)

indeed, drop the chamber size (or increase barrel length) and it will become silent
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