I'm in the process of making a potato gun. Its built, and glued, but having ignition problems (i do get spark though)

I posted this on a few forums, so I'll copy and paste what I'd allready written... Yeah, its kinda long

As a disclaimer, id like to mention that this wont be fired in the neiborhood, this will be fired at my friends 40+acre farm. I'll wear safety glasses, and use good common sence... bla bla bla, that sorta stuff

I'm using a 3 foot long 1 1/2" sch40 pvc pipe for the barral, and a 18" long 3" sch 40 pvc pipe for the expancion chamber... The end is threaded female, with a male companion (so i can add the fuel). Theres also a 1 1/5"-3" adaptor in there. I used purple primer, and glue to join them together...

Thesch 40 has a working pressure of 330 psi at 73 degrees. The ignighter is a $10 coleman quickspark...

Before I glued it, I did a quick test fire. It was just a dry fit test fire, and it was pretty awesome... 3 sec of spray using pledge, and it blew the gun apart... went "WOOF", and shot flames... Pretty damn cool imho. That was before I glued it

The pledge worked before I glued it, but doesnt any more. Allthough the cans getting almost empty though, but i was still spraying pledge (not 100%propellant)

I got spark, but no ignition. I degassed the expansion chamber (with a fan) a few times... still nothing. With or without the end cap on

I tried pledge, hair spray, spray deoderant (suprizingly unflamabal), and spray paint (degassing&cleaning after each test) I did get some carb cleaner, and that went... KAPOOF, but .5 sec of spray was a bit scary... I wont use it again.

The ignitor is 6" from the end of the expantion chamber, and the the spark gaps the probes at <1/2" into the barrel. The gap is about 3/8", which is about as long as it can be, If i want a consistant spark.

Any ideas on what else I should use for propellant?
Any ideas on anything else?

Heres some pictures
http://photobucket.com/albums/v644/NathanEbert/Potato%20gun/

Tks alot, I appreciate it
~Nate

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2012 Escape, 'hunter" (5 sp 4cyl)

Propane -

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What position is the ignitor? I think they have to positioned toward the top of the combustion chamber to ignite the good stuff.

Is this happening during a dry fire or are you locked and loaded?

BTDT. Two things may be happening:
1) You arent actually getting the spark in the chamber when the electrodes are wet.
2) You have too much fuel, not enough oxygen.

I used hairspray. Aerosol hairspray is the BEST to use in these. I've tried propane (not explosive enough), I've tried gasoline (if you can get it to ignite it just tends to burn. Now if you could use a FUEL INJECTOR in it... but thats a bit more advanced for a spudgun than I wanted to go). I've tried lots of stuff, hairspray works best. A 1 second blast is more than enough. Sometimes I had to blow out hairspray and put the cap on after to fire it. You need to get fresh air into the gun between firings too, you cant just spray again and go.

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I was getting spark, I verified that by shutting off the lights, and looking at the end of the expansion champer, with the end cap unscrewed... I wasnt looking "down" it either...

Too much fuel? ok, ill try a one second blast next
electrodes on top? ok...

next test fire in a min or so

This is with nothing in the barrel, but its all glued, and the end was closed
tks alot
~Nate

__________________
95 Honda Shadow ACE 1100.
1999 Plymouth Neon Expresso. 2.4 swap, 10.5 to 1 comp, big cams. Autocross time attack vehicle!
2012 Escape, 'hunter" (5 sp 4cyl)

Still doesnt work on pledge any more...

It does work on the carb cleaner, ONLY if the electrodes are at the bottom...
The reason I think this is because I sprayed directly into the chamber. Then screwed the cap on, and flicked the ignitor. Hit it a few more times (5 or so) and nothing. Then, I rotated the barrel 90 degrees, and tried again, no fire. electordes on the bottom, worked first spark.

Mabey that was just the time it took for the carb cleaner to become vaproized... I dont know

it was a very quick shot of cleaner, just as fast as i could (<.5 sec)
tks alot
~Nate

__________________
95 Honda Shadow ACE 1100.
1999 Plymouth Neon Expresso. 2.4 swap, 10.5 to 1 comp, big cams. Autocross time attack vehicle!
2012 Escape, 'hunter" (5 sp 4cyl)

I second that hypothesis. I have built my own tennis ball cannon with similar technology. Use the same sparker for ignition. I use car starting fluid for the propellant, which I find is much cleaner, much less residue, than hair spray or furniture polish. I recommend it.

The thing is that these things are very sensitive to the mixture and can EASILY be overfueled. With the starting fluid, basically I spray just about as little as I can for a shot. You just give it a psst, far less than 1 second's worth of spray. If you overspray, the gun will either not ignite at all, or it will just do a slow woosh with almost no power. A good shot is almost like a gunshot. Sharp and powerful.

And the next advice is to air it out between shots. Gotta get ALL the spent air/fuel mix out and replace with fresh air.

Rgds,
Chris W.

I used a 6" combustion chamber on mine that was about 12" long. White rain hair spray worked the best for some reason.

Along the lines of safety, my friend put a gas "match" on his so that he could get that "trigger" feeling. It worked for about three launches and quit (got wet). He pulled the end cap off and pulled the trigger to see if it was sparking and burned his eyebrows off. FWIW, this was 15 years ago

PVC pipe is pretty damn dangerous for your application. I know that 330psi sounds like a lot, but in the right conditions, higher pressure (approaching 330, say) results in a much faster burn, faster than the projectile can be accelerated. You can blow that stuff up, and if it does blow, the shrapnel can do severe damage. It could easily pierce your safety goggles. Since you're not carefully controlling the mixture within the tube, all bets are off for burn rate.

If I were building a potato launcher, I'd use compressed air with a huge ball valve for the power source.

I once blew the end off of an *OPEN* piece of Sch 40 PVC. I won't go into details of how I did it or what needed to be done as a result, but let me just say that you don't want any of it.

Uhhh...., I hate to second these possibly party pooper remarks, but I had a friend in high school who blew of most of one thumb and parts of other fingers while making bombs out of match stick heads. Less sophisticated than what you're doing, but PVC ain't the same as high grade steel in containing and directing the force of an explosion.

I mean, I've read that they work and they're a hoot. I'm curious though, if there's a dark side to the results on some of these.

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When I built mine I threaded 2 bolts through opposite sides of the combustion chamber to position them in the center of the combustion chamber. Used a barbeque igniter for awhile but it was too inconsistent so I bought a cheap stun gun and wired it on to the electrodes and that worked much better. I would say the best fuel to use was map gas.

Nate shoot a sparrow with you pellet gun and put that in the potato gun.

Yeah, i know that I could loose a finger/eye... but ill be trying to protect myself, with gloves, and safety glasses.

If I were to make it out of steel, it would be more bomb looking, and surly the cops wouldnt like that (if i got pulled over in transit, or they came all the way down off the bridge, through the brush, and into the clearing (15 min walk)

I used two longer screws, at 45 degrees from eachother, with a gap in the middle... Has good spark, and is about 1/2 way into the barrel


I really didnt want any safety speaches, I know what I'm getting myself into... I know SCH 40 pvc isnt the strongest of things to use, but thats the hand i am delt, unless ABS is better... but it felt kinda flimsy to me, and I couldnt find sch 40 (didnt look very hard though)

~Nate

__________________
95 Honda Shadow ACE 1100.
1999 Plymouth Neon Expresso. 2.4 swap, 10.5 to 1 comp, big cams. Autocross time attack vehicle!
2012 Escape, 'hunter" (5 sp 4cyl)

YEAH!

I wouldnt worry about it blowing up. We used oxy/acetelyne mix in mine a couple of times and I lived to tell about it.
Try launching a wad of duct tape or a shop towel. Those can safely land in any strangers back yard.
Feed mice are good too because their fate is already sealed!

i would imagine that since pledge has wax in it its causing the problems. used butane

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