Combustion spud gun problem

[da007man]

HI, I just made a combustion type spud gun with a piece of 80mm PVC (combustion chamber) and a long piece of 32 mm PVC(barrel). I use a piezoelectric sparker to which I attached 2 wires that go into the combustion chamber. I use deodorant as a propellant.
First I put the potato in, then I put the deodorant in, and I spray for about 2 seconds. I have to spark like 15 times before it launches the potato. When it does, it makes a huge BOOM that kills my ears and the potato goes flying out, as well as the end cap on the other side....
Could somebody please help?

[jackssmirkingrevenge]

I spray for about 2 seconds. I have to spark like 15 times before it launches the potato.

Spray less.

Even better, use a syringe

[da007man]

How can I use a seringe???

[jackssmirkingrevenge]

...

[youtube][/youtube]

Also, try making your spark gap bigger, say around 5-6mm

[da007man]

If I make the spark gap bigger it doesn't spark... Is that normal because it is a small sparker or is it just me?

[jackssmirkingrevenge]

It looks fairly small in the picture, but it could just be the picture.

The problem then is probably too much fuel, try spraying less.

[da007man]

Well the space is really small, about 3mm maximum. I'll try spraying less next time. However, I need to change the end cap, because the one I have screws on easily, it's a lock kind (screw to the right once and it will go click). I will get a better one this afternoon and see whether it will fly off or not.

[mark.f]

When it does, it makes a huge BOOM that kills my ears and the potato goes flying out, as well as the end cap on the other side....

First things first, your endcap shouldn't fly off when firing. Are you using a threaded endcap? Because I don't see threads in that picture showing your spark gap.

As to the spark gap, I would recommend a distance of around 1/8" between the electrodes.

EDIT: didn't see the last post, good luck finding an endcap that doesn't blow off...

[da007man]

no it isn't threaded, so it can easily be removed by the blast, and it FLIES and makes the combustion a LOT louder... i use duct tape for now to hold it in place for now until i buy a new endcap.
Or, should I glue the end cap permanently and drill a hole for the top of a coke bottle and use that to insert the deodorant? I could then use the bottle top to screw on and off... Please tell me what you think

[Hubb]

You want to be able to lock the end cap in place. The most common method is to use a threaded plug with the appropriate adapter.

Try not to glue the end cap into place. You want to be able to properly exhaust the launcher after it is fired. You can accomplish this with the coke bottle deal you described, but it is going to be much easier with a bigger opening.

As far as the issue with the fuel, you are using too much. It may not seem like it to you, but that is most certainly the problem. You see, in order for a combustible gas to ignite, it has to have a stoichiometric concentration. The window for this is usually pretty small. Metering the fuel supply, as was recommended above, is an accurate way to get the mix right. Fuel above or below this will result in a misfire. In your situation, it seems that some of the fuel is bleeding off (probably due to the end cap) until the proper mix is achieved, which then allows you to fire it.

And on a side note, if you get that end cap to where it doesn't come off, your potato is probably going to go even further.

Lastly, don't give up on it. You are on to a good start and your spudding adventures are only going to get better with time.

[mark.f]

I think we need a picture of the rear of the cannon. Can't really tell what type of cap you are using and what you can do to replace it or lock it into place...

[Moonbogg]

...

[youtube][/youtube]

Also, try making your spark gap bigger, say around 5-6mm

Dude, that is some badass music in that video. I really like that. Good demo on the syringe as well.

[da007man]

OK thanks you guys. I have tested with less fuel and the potato has launched about every first spark. It already goes really far with the cap flying off. I did however try duct taping it to see if that would hold it in place... For now it works, but I'm afraid it won't last long....
Here's a picture of the end cap part.

[mark.f]

Yeah, that's pretty flimsy looking.

If you can't find a threaded cleanout cap, you may have to use the bottle cap idea your posted before. If you do, it would be a very good idea to install a chamber fan as well to aid in venting the chamber.

Another thing is, if you can find a smaller cleanout cap and spigot x FNPT adapter (female threads on one end and a small piece of "pipe" on the other), you could theoretically drill through the permanent endcap (before installing) with a hole saw the same size as the "pipe" on the adapter, push the adapter through, and then glue a slice of coupling on the other end of the cap.

None of this is the way the pipe and fittings were intended to be joined, though, so it would be much safer and better if you were to find a normal threaded cleanout cap and adapter.

[da007man]

Yeah i'll do that. I think I'll still use the coke bottle cap to insert the fuel with a syringe 'cause the hole is smaller than the end cap so less fuel will be lost.