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I can't get it to fire!!!!!!!

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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I can't get it to fire!!!!!!!

Unread postAuthor: theBOOM » Fri Jun 12, 2009 9:49 pm

Hello spudders I recently joined Spudfiles and have been browsing around :lol:

Anyway, I built a mini/small spud gun this week and as fuel I'm TRYING to use propane. I have had a lot of problems trying to get it to ignite... I dunno what I've done wrong. For metering the propane I use a 10 ML syringe which attaches to a hose which then is attached to the propane tank thingy. I'm using an Electric grill ignition to generate the spark ... I have 2 sparks (4 screws) and they are approx 1.5 mm long.

Something that I have noticed is that when I try blowing in the chamber after seeing that it does not work I unscrew the plug and I can smell all the propane coming out.. Do you think that the propane is rising towards the upper part of the chamber? :?: To solve this problem I was thinking a MINI chamber fan but I have no idea where I could get one that is small.... I know that inonickname used a small router fan (I think) on one of his minis but I don't have one just laying around to be destroyed.

Some specs on my cannon

The total chamber length with the fittings is about 7.5 inches long the pipe that I used for the chamber was 5.5 Sch 80 600 pressure rated pipe
Also the chamber is 1 inch in diameter... I measured the volume of my cannon with a measuring cup and it took 160 ML of water and if I am not mistaken 4.2% is the desired propane ??? So I inject 3.8 ML of propane... I have tried from 3-6 ML with no success

Any help would be appreciated :sad1:
Thanks in advance :D

Boom :wave:
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Unread postAuthor: inonickname » Fri Jun 12, 2009 10:15 pm

It's sinking to the bottom.

Assuming you are using a good mixture then try shaking or rotating the cannon to mix. If this helps then you should look into a chamber fan. If not then perhaps your chamber is extremely leaky.
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Unread postAuthor: theBOOM » Fri Jun 12, 2009 10:38 pm

Oh ! your back from your camp :o

I'll try your tips and see if I can get some results :D

Boom :twisted:



UPDATE: I don't know why but the sparks that it creates are almost invisible they are kinda long but they aren't fat OR look powerful... I tried putting some copper tape on the screw but it didn't help a lot.... I tried the electric grill igniter unattached to the potato gun and you can see fat sparks... I don't know why its making them so little and unpowerful inside the chamber :cry: :cry: ANY Suggestions?
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Unread postAuthor: Gaderelguitarist » Sat Jun 13, 2009 12:06 am

How many gaps are you using when you get the more powerful spark?

It might be that your two spark gaps are dividing up the voltage to much...
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Unread postAuthor: theBOOM » Sat Jun 13, 2009 12:16 am

Well without the screws and only 1 gap its about a 3/4 inch powerful spark...
What would I need to do to solve this problem ?? Like you said it might be that the spark gaps are dividing the voltage.... Any solutions? :(


Edit: Is there such thing as a cannon not working because of a weak spark??
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Unread postAuthor: Gaderelguitarist » Sat Jun 13, 2009 1:21 am

Your screws are probably not very conductive then.

Have you tried different screws?

If the spark isn't strong enough i suppose it wouldn't ignite the gasses.
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Unread postAuthor: theBOOM » Sat Jun 13, 2009 1:26 am

Thanks for the fast reply... I'll try sniffing around for some screws although I don't think my dadd has any small ones :/
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Unread postAuthor: jon_89 » Sat Jun 13, 2009 1:30 am

Your math is correct, maybe it could be the length of the chamber? :? You could use a small chain attached to each end of the chamber to help mix if I am not mistaken.
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Unread postAuthor: theBOOM » Sat Jun 13, 2009 2:05 am

Hm I was thinking about that but I think I have discovered the flaw... The sparks are to weak to ignite the propane... So now I need some ideas ... I'll try and get more conductive screws but im not sure if this will help or not. :[
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Unread postAuthor: jon_89 » Sat Jun 13, 2009 2:21 am

You can make a small spark strip. This way you will have two holes and as many sparks as you can get. I use wire coat hangers and they work pretty well.
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Unread postAuthor: theBOOM » Sat Jun 13, 2009 2:32 am

I already have the 4 screws(4 holes).. I tried a spark strip design and it was a total failure... Although I could try coat hangger wire since it seems to be conductive :D LETS hope it workssss :twisted: :twisted:
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Unread postAuthor: inonickname » Sat Jun 13, 2009 2:59 am

When you add your screws, make sure the tips are pointed (sharp). You get much less voltage breakdown through two sharp points than dull ones.
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Unread postAuthor: theBOOM » Sat Jun 13, 2009 3:24 am

Thanks inonickname I'll try that tommorow

I tried the coat hanger as requested ... My results where weird.... When I used a small piece of coat hanger (1 inch) I got the same failure (weak sparks) but when I put the hole coat hanger in I got a fat spark.... I have no clue of what is happening ... any veterans around that might have some theories???? :cry: :cry:
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Unread postAuthor: mic » Sat Jun 13, 2009 4:36 am

try sharpening the tips of the screws and grind away the points where the wires touch the screws with a dremel untill you see shiny metal. thatll make them much more conductive, makeing the spark stronger
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Unread postAuthor: psycix » Sat Jun 13, 2009 1:39 pm

There are some errors in this thread...

Let me correct the wrong information:
-Gas does not SINK. It diffuses and mixes perfectly over time.
-If you can see a spark, then its (WAY) powerful enough to ignite a mixture.

a 3/4 inch powerful spark...

Thats a LONG spark. Way too long. The reason that it doesn't work properly is that your cumulative spark gap distance is too long. Close down the gap up to like 1mm (maximum). Sharp tips help, but you first need to close down the distance.
If you fixed that, and it still doesnt work, it will be your fuel mix. If you are sure that you are injecting the right amount of fuel (and you are not losing half of the mix while pulling out the syringe and closing down the chamber) then you will need to mix it, preferably with a small PC fan.


Oh, and I read that you are using a hose between the propane tank and the syringe. Did you flush the hose with propane before filling up the syringe. There may be a lot of air in your fuel injection.
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