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Help on improving my cannon for competition

A place to ask general spud cannon related questions.
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Unread postAuthor: sjog » Thu Dec 13, 2007 10:21 pm

computer fan is brush-less
easy to buy/salvage
they run on 12v but 9v works fine
computer fan has a frame and can be easly mounted
thru bolts are common for the wires to go thru chamber wallsImage
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Unread postAuthor: Ragnarok » Thu Dec 13, 2007 11:57 pm

jimmy101 wrote:You can use a fan with brushes since many folks have without any problems. Many other folks have tried to ignite spud guns using a brushed fan and have never gotten it to work.

That's still no reason to tempt fate. As computer fans are cheap, easily, ready made and compact, I wouldn't risk using a brushed motor - especially as I have seen more than enough sparking in my time.
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Unread postAuthor: Novacastrian » Fri Dec 14, 2007 1:04 am

Ragnarok wrote:
jimmy101 wrote:You can use a fan with brushes since many folks have without any problems. Many other folks have tried to ignite spud guns using a brushed fan and have never gotten it to work.

That's still no reason to tempt fate. As computer fans are cheap, easily, ready made and compact, I wouldn't risk using a brushed motor - especially as I have seen more than enough sparking in my time.


I tend to agree Ragnarok, although i do believe it is very possible to make a safe setup from a brushed fan. It would cost more though (unless you had the essential bits lying around) so why bother.
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Unread postAuthor: BigGrib » Fri Dec 14, 2007 2:51 am

So here's my input on the whole deal. If you're making a spray and pray for this competition that's cool. Now to make the contest somewhat fair I would make your cannon with a golf ball barrel just to make sure that both of your guys are firing the same ammo because spuds are irregular and if one gets a better spud that the other that's not fair. So keep the ammo the same and aerodynamic so that it will really be a test of the cannon not just luck of the draw on the spud.
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Yea, that's definitely going to get you at least a tazer.

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Unread postAuthor: mauritstheman » Fri Dec 14, 2007 5:26 am

Hmm this is new for me what you need wiht that thing for pressure?
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Unread postAuthor: DYI » Fri Dec 14, 2007 8:11 am

Hmm this is new for me what you need wiht that thing for pressure?


Would you care to try that again, in proper English this time? I have no idea what you're trying to say.
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Unread postAuthor: JDP12 » Fri Dec 14, 2007 8:16 am

Yea i agree with Ragnarok, I would never risk a brushed fan, because you never know what could happen, and bad things could happen, stick with brushless you won't have a problem then
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Unread postAuthor: OldSpice60 » Fri Dec 14, 2007 1:35 pm

So here is what I now know.

1. Install a brushless fan.

2. Try to use an aerosol other than hairspray. (static guard?)

However, the question I largely want answered is: what is the best ratio for a spray n pray if I want the spud to travel 200+ yards?
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Unread postAuthor: Ragnarok » Fri Dec 14, 2007 1:44 pm

OldSpice60 wrote:However, the question I largely want answered is: what is the best ratio for a spray n pray if I want the spud to travel 200+ yards?

That can't be answered, unless we know how long your barrel will be.

I would personally use a 1:1 ratio, and the longest barrel you practically can but there might well be much better things.
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Unread postAuthor: Hubb » Fri Dec 14, 2007 2:07 pm

OldSpice60 wrote:...However, the question I largely want answered is: what is the best ratio for a spray n pray if I want the spud to travel 200+ yards?


I built a spray and pray combustion with a ratio of about 1.1:1. I wanted the distance so what I did was spray 1 second then fire, sprayed 2 seconds and fired, and so on until I maximized the distance. It wasn't nearly 200 yards but was more like 100 to 120 (but it had a 2" sch40 chamber with a 1" sch40 barrel).

Also, I used a product called Oops!. The reason for this is because this would not gum up the chamber like hairspray would and the chamber (before I converted it to pneumatic) was quite clean on its interior. I left the fan running when I fire and had a single spark at the rear of the chamber.
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Unread postAuthor: OldSpice60 » Fri Dec 14, 2007 9:09 pm

*UPDATE* I took my existing spray n pray which is a 3 inch combustion chamber with a 2in barrel (other specs to come), and I installed a fan with a 9 volt battery hookup. The fan was placed on the interior of the blast cap.

Also I re-did my BBQ spark ignition.

I tried it in the dark, and the spud sounded like it traveled a long way, but I won't know for sure until I try it again in the daylight. Thanks for everyone's help.

Pics to come later.

P.S. Anyone know where I can get static guard?
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Unread postAuthor: jimmy101 » Fri Dec 14, 2007 9:59 pm

Novacastrian wrote:
Ragnarok wrote:
jimmy101 wrote:You can use a fan with brushes since many folks have without any problems. Many other folks have tried to ignite spud guns using a brushed fan and have never gotten it to work.

That's still no reason to tempt fate. As computer fans are cheap, easily, ready made and compact, I wouldn't risk using a brushed motor - especially as I have seen more than enough sparking in my time.


I tend to agree Ragnarok, although I do believe it is very possible to make a safe setup from a brushed fan. It would cost more though (unless you had the essential bits lying around) so why bother.

I understand everyones concern about safety but there is really no evidence that a low voltage brushed fan can ignite propane in air.

1. Several people have tried to use a brushed fan as an ignition source for a spud gun. This would be, by far, the cheapest and easiest ignition system. Nobody has ever gotten it to work. A fan that gives visible sparks, with the motor cover removed, will not ignite propane + air even if left running for several minutes. Many people have tried this, including myself.

2. Several guns have been built that use the cheap "personal fans" available at Kmart etc. as the mixing fan. These fans use brushed motors. No premature firings have ever been reported.

So, it may be possible that a low voltage brushed fan can ignite propane+air it is also possible that it is not possible. (It is possible that I could have made the preceeding statement even more obscure than it is.)

Rummaging around in google I can't find the safety spec's for explosion proof switches in explosion prone locations like chemical storage buildings. I know that 120V circuits require a special switch because a 120V switch can throw a spark that'll ignite flammable vapors. I can't find any info on 12V or 24V circuits.

Here are a couple interesting links on sparks in general, and their ability to ignite flamable gas mixtures. These are relevant to both the question of safety of a low voltage DC fan and the more general issue of getting a combustion gun to ignite when you want it to.

http://www.ce-mag.com/archive/1999/novdec/mrstatic.html
http://www.ece.rochester.edu/~jones/demos/MIE.html

These citations say that it is both the voltage and the capacitance of the source that matter. For something with the capacitance of a human body the minimum voltage for ignition of typical hydrocarbons in air is ~1,400V.

If the source has higher capacitance then a lower voltage is adequate for ignition.
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Unread postAuthor: OldSpice60 » Mon Dec 17, 2007 4:29 pm

Sorry guys pics are gonna have to wait b/c of surgery I had. (Non-spud related injury and it was minor stuff)

However, I tried static guard, and it is great. Also after the potato is loaded I found that spraying WD-40 down the barrel reduces friction, and thus the potato travels further and faster.
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Unread postAuthor: BigGrib » Mon Dec 17, 2007 4:43 pm

Hmm the only thing i'd worry about is what if anything would the wd-40 do to the integrity of the barrel over time?? i dont know enough about it to make a guess so maybe someone who knows more about it than I can give some info on what it might do
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Yea, that's definitely going to get you at least a tazer.

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