Spark screw placement...

[Pete Zaria]

First, very cool community here. I've been reading the site for a few weeks, first time posting though.

I've built 3 or 4 spud guns, all combustion-powered ABS cannons. My current one has a 4" chamber 2 ft long, bell reducer down to 1.5" pipe, and a roughly 5 foot barrel. I put the screws (with a 1/4th" spark gap) about 3/4ths of the way back on the chamber (close to the end cap), figuring that propane is heavier than air and since most of the time I'll be aiming the cannon upwards, that's the best place for the spark to go.

Needless to say, I'm having ignition problems. The variables are, obviously, fuel to air mixture and ignition.

I haven't rigged up a propane meter yet, so I'm just sticking a torch (unlit, but with the gas on) in the screw-on end cap in the back of the combustion chamber, with the barrel pointing down. I've tried everywhere from 5 to 15 seconds of gas. Higher seems to produce better results.

My question is, where's the best place to put the spark gap? I'm going to install a second set of screws (for dual ignition) but in the past I've had problems getting a BBQ sparker to jump both gaps.

If anyone has any advice for me, I'd appreciate it.

Peace,
Pete Zaria.

[saladtossser]

1st: How the heck did you find a 4" to 1.5" bell reducer

i hear that spark towards the rear shoots better

[spudshot]

using a torch its hard to get the right mixture, just try some aerosol spray for now, it seems to be alot better, starting fluid is my spray of choice, about a 1/2 second spray gets it going good, i dont know how safe it is in ABS though, i've been out of the combustion ring for a while

[Pete Zaria]

Thanks for the input.

My ignition problems seem to be because propane doesn't like to burn very well below 40F (I live in the suburbs of Seattle, WA). The solution to this is to use 3/4ths propane and 1/4th alcohol (in a spray bottle). The alcohol seems to get the propane burning just fine. Is this dangerous?

Anyway, just wanted to say thanks to spudfiles and spudtech. Couldn't have had all this fun without 'em.

Here's my build:
http://img47.imageshack.us/my.php?image=img03607so.jpg
http://img329.imageshack.us/my.php?image=img03614hj.jpg
http://img47.imageshack.us/my.php?image=img03622fg.jpg
http://img47.imageshack.us/my.php?image=img03655sx.jpg

Chamber is 4" x roughly 2'. Bell reducer from 4" > 2" > 1.5" > Female adapter. The valve is a kind of stupid idea I had. On my previous cannons, I'd fuel them up in the garage and then walk out into the yard to fire. Sometimes all the gas would leak out the barrel if my projectile wasn't airtight, so I installed the valve on this one to keep fuel in until I'm ready to fire. One side of the valve says "SAFE TO FIRE" and the other side says "DEATH", because if someone were to fire it with the valve closed, it would undoubtedly explode.

The alligator clip on the ignition wire acts as a safety. I leave it off until I'm ready to fire.
The duct tape on the end of the barrel is to give a tight seal with my home-made silencer. It's a peice of 2" pipe cross-drilled, that vents into a peice of 4" pipe full of cotten batting, like a huge pistol silencer. Cuts down sound and muzzle flash by about half. I'll post a drawing later.

I'm going to build a propane meter this weekend, I just have to do the math to figure out how big my chamber is. Any tips?

Please let me know what you think about using propane and alcohol together.

Peace,
Pete Zaria.

[saladtossser]

cool, i might note that it will not blow up with the valve closed

[Pete Zaria]

If you fired it with the valve closed, the gasses would have nowhere to go... Either the back cap would blow off, the barrel would blow off, or the chamber would rupture... no?

Peace,
Pete Zaria.

[TalonXxX]

If that force doesn't go out the barrel... its gotta go somewhere. I would imagine the cap would blow off, I wouldn't try it if I were you... be very mindful of that ball valve when firing Pete

[Shrimphead]

Ya, but you have to remember that a combustion only creates about 60 psi. I think that abs could hold that pressure for a split second until the gas cools.

[boilingleadbath]

The ABS, although cellcore, will almost certainly hold the 60-30 psi that is generated. You may consider relabling it "safe" an "armed"...

I don't think there have been any good tests done on spark position vs. power, although somewhere between the back and the middle of the chamber seems to fair pretty well.

[SpudStuff]

get the best of both wirlds put one set n the back and one set in the front then wire them in series

[feral_patriot]

greetings! If I may, in response to Pete Z. I am in Spokane so I understand the scenario about cool weather firing... while I haven't yet finished my next project with a chamber size of 279.07 cu.in. ,I can remark that in my 3" 126.04cu.in. chamber I've mounted the screws 10 " from the rear of the chamber (no fan) and use heptane as my fuel with just barely a tap on the nozzle;and gently "scoop" some air in the back as I screw the cap on. I use the bbq lighter method , and alternate between a barrel 51" x7/8 " id & a 32" x 1 1/2 " barrel . both work remarkabely well as far as power AND distance is concerned. Also, Iam seeing a lot about a 1/4" spark gap;I personally have found a precise 3/16" to be muh more effective this year over last with the 1/4"...don't ask me why,I don't know.

[Pete Zaria]

Thanks for the input, everyone.

My ignition problems are solved by using a little bit of alcohol along with the propane. I guess this is a cold weather issue.

Shrimphead and boilingleadbath, as to the valve and potential pressure problem, I think you're probably right but I'm not going to test the theory.

Feral patriot, thanks for the repsonse. Let me know if you get out twards Seattle. I'll try tightening up my spark gap a bit and see if it helps. Where do you get heptane? It sounds very cool.

Peace,
Pete Zaria.

[feral_patriot]

Heptane...www.Sciencelab.com. About $ 12.oo / 500ml... however there are other sources...