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Help with a combustion mortar please

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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Help with a combustion mortar please

Unread postAuthor: rescue341 » Thu Oct 06, 2016 2:05 pm

hey all.

im in a pickle i have a big airsoft game next sat. and thought hey ill build a potato gun mortar. I believe i underestimated these boom tubes. Because i cant for the life of me get it to fire. Here is the design for a combustion chamber i have 1 foot 6 inchs of 2in pvc run into a ruducer that runs into 3 feet of 1-1/2in pvc. ive tried axe, spray paints and carb cleaner and all light up like a flame thrower when sprayed in to a flame. for spark i have the sparker out of a bbq lighter. I originally had it very close to the end cap so that if i needed to i could easily work on the leads. after not being able to get it to fire (it sparks but i cant fire the gun) i thought maybe its too close to the end and fuel isnt getiting to it so i moved it more towards the middle. still i get spark but no igntion. I have tried all fuels at 1/2, 1, 2, 3 ,4 ,& 5 secs of fuel with no luck. if i introduce a flame it the fuel lights but i cant get it to light via spark in the sealed chamber

I would really be thankful for yalls help here i'm stumped
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Re: Help with a combustion mortar please

Unread postAuthor: jrrdw » Thu Oct 06, 2016 2:41 pm

Set your spark at .020, and clear / vent the chamber very good. Use the Axe, about a 3 second squirt since you have it already, put the lid from the Axe can in the chamber. Shove a big wade of TP down the barrel, then shake the cannon real good to mix the fuel. Then yell all clear and fire, hang on tight these things kick.....

Welcome to Spudfiles! :bigsmurf:
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Re: Help with a combustion mortar please

Unread postAuthor: rescue341 » Thu Oct 06, 2016 3:40 pm

Thank you for the reply! I was reading about tuber shooters with a fan to vent the chamber is that what i need and by .20 you mean the spark gap correct?
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Re: Help with a combustion mortar please

Unread postAuthor: rna_duelers » Thu Oct 06, 2016 7:42 pm

Follow as jrrdw says with the fueling,usually less is more I've found though.The spark gaop of .020 should work but try a larger gap if possible gives greater area for a ignition.

Just experiment with it.
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Re: Help with a combustion mortar please

Unread postAuthor: rescue341 » Fri Oct 07, 2016 12:09 am

I tried all of the suggestions and wasn't able to get ignition. I saw a desing on google that had a intact long neck bbq lighter with the tip in the combustion chamber and it worked almost like a on demand pilot light. I was thinking of tring this but was concerned over safety concerns. asuming the lighter is sucured in a way that prevents it from being pushed out by pressure. what kind of risk is there of flame backblowing from the chamber into the lighters butane revere
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Re: Help with a combustion mortar please

Unread postAuthor: jrrdw » Fri Oct 07, 2016 6:43 am

rescue341 wrote:Thank you for the reply! I was reading about tuber shooters with a fan to vent the chamber is that what i need and by .20 you mean the spark gap correct?


The fan is also used to mix the fuel. Watch the video below, it's a more calculated way of fueling:



Yes, experiment with it.
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Re: Help with a combustion mortar please

Unread postAuthor: rna_duelers » Fri Oct 07, 2016 6:49 am

Risk of the lighter on a stem type of ignition failing from a back burn into the butane itself is next to impossible,no oxidizer air/oxygen etc for it to combust. Think of the situation inside the chamber as an example.Give it a try,and if your still having issues try the tried and tested touch hole technique like in an old black powder cannon,small hole in the chamber to directly light the mixture.try it on an endcap so it can be replaced easily :)
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Re: Help with a combustion mortar please

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Re: Help with a combustion mortar please

Unread postAuthor: rescue341 » Fri Oct 07, 2016 4:10 pm

i tried the touch hole and i think im almost there. the touch hole ignites the entire chamber if the barrel is pluged and the end cap is off but if the camber is completly sealed then fuel at the touch hole ignites but not in the chamber. This would mean my Air /Fuel is off correct?
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