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My first spud gun! Need input 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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My first spud gun! Need input please

Unread postAuthor: AnCap » Thu Feb 06, 2014 12:22 am

Thanks a lot to this website, and a few other sources, I've been motivated to make my first ever spud gun. I decided on a propane injected over/under. I did a lot of research, and put a lot of thought into the design. I just finished mocking up the main portions today. I decided before I start glueing and drilling I'd post a photo of the mockup on here and get some input.

Image

I still haven't decided on a barrel design and how to attach it. I'd like something modular so I can quickly swap different barrels. For now, that's just a leftover piece of 2" pipe for visualization.

The combustion chamber is 3", and I'll be installing a fan on the ball valve end.

I plan on installing that gauge into the metering pipe and using the valve on the propane bottle as a sort of throttle valve to reach the desired pressure. I'm planning on connecting the bottle to the metering pipe using PVC enforced tubing, and connecting the pipe to the combustion chamber using the same tubing. I have a 3/8" barb attached to a 3/8" nipple I plan on threading into the combustion chamber.

Based on my metering pipe size, I'll need about 25# of propane to achieve 4% in the combustion chamber. Will the tubing with hose clamps handle this with no leaks?

When I measured the volume of the combustion chamber (with water) I included everything up to the last 90 degree elbow. Is this correct? I assumed the combustion area was everything up until the projectile.

Any input is greatly appreciated, and no need to be kind. I'm expecting some rookie mistakes and I can handle criticism. Thanks.
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Re: My first spud gun! Need input please

Unread postAuthor: CS » Thu Feb 06, 2014 2:03 am

using the valve on the propane bottle as a sort of throttle valve


That will work, the propane torch valve is precise.

Will the tubing with hose clamps handle this with no leaks?


Yes.

You can always pressurize the system and test for leaks with soap water.

[useless]As a side note, I always wondered how hard you had to tighten threaded connections to create a air tight seal. I used to crank on them in a vise, but after a quick test just past hand tight is good enough.[/useless]

I assumed the combustion area was everything up until the projectile.


Yep.

One question how do you plan on making a spark gap?
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Re: My first spud gun! Need input please

Unread postAuthor: AnCap » Thu Feb 06, 2014 2:26 am

Thanks for the quick reply!

CS wrote: [useless]As a side note, I always wondered how hard you had to tighten threaded connections to create a air tight seal. I used to crank on them in a vise, but after a quick test just past hand tight is good enough.[/useless]


Not useless at all, good to know. I'll obviously use a fair amount of Teflon tape.


CS wrote:One question how do you plan on making a spark gap?


Tomorrow is "research electronic portion" day, so I'm not quite sure yet. Although, as an electrician I think this will be the most fun for me. I do plenty of troubleshooting/maintenance/etc but rarely do I get to fabricate new stuff. I'm looking forward to the sparker and fan power supply. Do you have any suggestions on where to start researching?
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Re: My first spud gun! Need input please

Unread postAuthor: jackssmirkingrevenge » Thu Feb 06, 2014 5:52 am

AnCap wrote:When I measured the volume of the combustion chamber (with water) I included everything up to the last 90 degree elbow. Is this correct? I assumed the combustion area was everything up until the projectile.


You assumed correctly.

One might want to consider syringe fuelling, no fiddling with meters and pressure and all that.

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Re: My first spud gun! Need input please

Unread postAuthor: CS » Thu Feb 06, 2014 7:14 am

Along with the other moderators we approve new users first 5 posts, so I was first to see it. :p

Spark gap information:
spud_wiki/index.php?title=Spark_gap

Main page of the Spud Wiki:
spud_wiki/index.php?title=Main_Page
(a guide covering the basics of potato launchers, with some teasing advanced stuff on the fringes. btw, I don't mean to shoe you away from asking questions, it is just a good resource, it is obvious your capable of research.)

One question how do you plan on making a spark gap?


Sorry, that is sort of tantalizing in retrospect. I've used two long screws through a thick wall on the chamber, pipe+fitting, that meet in the middle with a small gap. With one I made recently for a friend I've had problems with the screws moving. In my own mind I've wondered what would be a better alternative. Again I am ruffling your feathers. A spark plug will work if your not spraying aerosol type fuel in. Liquid will land on the spark plugs small gap and render it useless. Your using propane so it doesn't come out as a liquid. Then again 2 screws are free for any self respecting hobbyist. A spark plug is 5 bucks and a trip to the store.

One might want to consider syringe fuelling, no fiddling with meters and pressure and all that.


WTF?! What kind of anti-progressive, meth addict, butane can collecting guy do you think AnCap is? He has got all the sh*t he needs! While your at it drop your dreams of an ox drawn cart and walk your a$$ home... barefoot. Joking of course, hopefully someone laughs beside myself.
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Re: My first spud gun! Need input please

Unread postAuthor: jackssmirkingrevenge » Thu Feb 06, 2014 8:03 am

CS wrote:WTF?! What kind of anti-progressive, meth addict, butane can collecting guy do you think AnCap is?


bb.jpg


:D :D :D
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Re: My first spud gun! Need input please

Unread postAuthor: AnCap » Thu Feb 06, 2014 1:16 pm

CS wrote:(...btw, I don't mean to shoe you away from asking questions, it is just a good resource, it is obvious your capable of research.)


I appreciate the links. The hardest part of researching this kind of stuff is sorting through garbage to find something useful. Links provided by seasoned vets makes that much easier.

CS wrote: I've used two long screws through a thick wall on the chamber, pipe+fitting, that meet in the middle with a small gap. With one I made recently for a friend I've had problems with the screws moving. In my own mind I've wondered what would be a better alternative.


I'm trying to figure out an alternative but I think the screws will probably be the way I'll go. Designing the power supply to those screws is what I need to research, I'm going to have some fun with it.

CS wrote:
One might want to consider syringe fuelling, no fiddling with meters and pressure and all that.


WTF?! What kind of anti-progressive, meth addict, butane can collecting guy do you think AnCap is? He has got all the sh*t he needs! While your at it drop your dreams of an ox drawn cart and walk your a$$ home... barefoot. Joking of course, hopefully someone laughs beside myself.


Haha, I certainly got a laugh. My goal here is to be able to refuel and fire all while keeping the gun at my shoulder. The propane bottle will be under my armpit, so I can hold the gun with my right hand, reach over with my left and turn it on. The metering pipe gauge will be mounted towards me so I can monitor pressure. Once full, inject it into the chamber, and fire.
A couple quick questions. Is there a preferred spot on the combustion chamber for the propane injection? I assume it wouldn't matter with a fan to mix it up. Which reminds me, I'm a little worried when I turn that fan on, the starting current could be enough to emit a spark, igniting the propane. Anybody have an issue with this?
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Re: My first spud gun! Need input please

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Re: My first spud gun! Need input please

Unread postAuthor: CS » Fri Feb 07, 2014 3:03 am

You can inject from wherever. However you would want to thread into a thick part on your chamber so the fitting has more area to grab onto.

"The most commonly used fans are case fans out of computers. The corners may be sanded down so it fits in the chamber, and sometimes the complete frame around the fan is cut off and even the fan blades need to be cut for fitting it into in small chambers. Instead of PC fans you could also use any other electric motor with fan blades attached. However, you should avoid brushed motors at all times. PC fans have brushless motors and thus do not create any sparks within. A brushed motor could spark with the possibility of suddenly igniting the fuel mixture, which can lead to accidents damaging the environment or even harming people. So now and then someone still uses a brushed engine but sealed the holes in the housing off with glue, making sure no fuel can enter the insides of the engine. The engine wont be able to cool itself anymore, but when used in short periods, that doesn't really matter.

Since PC fans are cheap, brushless and readily available in various sizes, this is the choice of most spudders and the most recommended option. Further characteristics of fans are noise, CFM (the amount of air it displaces) and RPM (how fast it rotates). Noise doesn't really matter for a cannon, and you probably want to have an high CFM, high RPM fan for some good mixing"
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Re: My first spud gun! Need input please

Unread postAuthor: jackssmirkingrevenge » Fri Feb 07, 2014 8:18 am

CS wrote:You can inject from wherever.


Spoken like a true methhead :D
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Re: My first spud gun! Need input please

Unread postAuthor: AnCap » Fri Feb 07, 2014 5:22 pm

Thanks for all the help! I'm going to a local computer repair shop today to pick up a used 80mm fan. Then swing by radio shack and get the stuff I need to power it. I'm also going to pick up the stuff I need to build a driver circuit for a flyback transformer. Then hopefully I can find a flyback transformer. Ideally I will have all the electrical done today or tomorrow and start assembly. I'd like to test this thing out by Sunday. Wish me luck :)
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Re: My first spud gun! Need input please

Unread postAuthor: jackssmirkingrevenge » Sat Feb 08, 2014 8:17 am

I would say a flyback transformer is overkill for a simple combustion, even a piezo ignitor from a cigarette lighter is sufficient to give reliable ignition.
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