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A new look at combustion cartridge Spud guns

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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A new look at combustion cartridge Spud guns

Unread postAuthor: Cthulhu » Sat May 27, 2017 9:27 pm

I recently viewed a YouTube video showing off a "potowizter" which fired 3D printed projectiles from cartridges that made contact with the electric igniter when the breech was closed.


My question is: is this an efficient design? It seems like the cartridges aren't in ideal barrel to chamber ratio, but does this really affect it's performance to an um-fun standard?

Also, I'm not sure about this, but couldn't you use toy cap gun caps to ignite the propane-oxygen mix inside the cartridge instead of an electric arc from a piezoelectric igniter?

If this proves to be a decent design for a cartridge Spud gun, I plan on building one with a rifled barrel.
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Re: A new look at combustion cartridge Spud guns

Unread postAuthor: jackssmirkingrevenge » Mon May 29, 2017 3:33 am

Interesting find.

First off, their fueling method is a shame when it's so easy to meter:



It seems like the cartridges aren't in ideal barrel to chamber ratio, but does this really affect it's performance to an um-fun standard?


You'll still get decent power from a smaller chamber if the caliber is big enough.

For even more fun, you can use the recoil to eject the cartridge.





but couldn't you use toy cap gun caps to ignite the propane-oxygen mix inside the cartridge instead of an electric arc from a piezoelectric igniter?


Technically, but electric ignition is more practical.
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Re: A new look at combustion cartridge Spud guns

Unread postAuthor: Cthulhu » Mon May 29, 2017 11:09 am

Wow, I had no idea ejection systems like this existed!

I might just be awful at making Spud guns as I've just started out, but I find the electrical ignition to be very finicky and require idealized gas scenarios. On my 1.5" bore 8' long spudgun it generally works as long as you have a good body spray in there, but on my tiny little 100mL syringe Spud gun, it takes a lot of luck to be able to spray and fire. Not sure if it's just my sucky combustion skills or the nature of it, but I thought a cap would give more reliable performance, especially if it was backed by an O-ring plunger.

However, this does seem to reawaken my hope in combustion Spud guns! I was nearly about to give up and move to pneumatic.
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Re: A new look at combustion cartridge Spud guns

Unread postAuthor: D_Hall » Mon May 29, 2017 12:39 pm

> as long as you have a good body spray in there

And that is your problem. Pretty much anything with the word "spray" in the name is an absolute crap fuel choice. Yes, they're popular for some unfathomable reason, but just because it's popular doesn't mean it doesn't suck [insert Nickelback joke here].

Propane.

Propane.

Propane.

Why in God's name anyone other than the most very advanced combustion spud gunners use anything else is beyond me (very advanced folks sometimes move to Methane).
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Re: A new look at combustion cartridge Spud guns

Unread postAuthor: Cthulhu » Mon May 29, 2017 1:01 pm

I attempted to use butane from a lighter but I couldn't get it to combust.
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Re: A new look at combustion cartridge Spud guns

Unread postAuthor: D_Hall » Mon May 29, 2017 5:47 pm

Butane should work OK but "from a lighter" is not a reasonable way to distribute the fuel for anything other than tiny guns using syringes for fueling. I'll say it again....

Propane.

Propane.

Propane.

And not "just" propane, but *measured* amounts of propane. Whether you use a metering pipe or manometric metering doesn't matter, but the point is that you shouldn't be *guessing* about how much fuel to put in or how much you used. This should be an absolutely *KNOWN* property of your gun/procedures and it isn't difficult to do.

Put it this way... If you build a halfway decent gun you should NEVER have a misfire. NEVER. Your gun should fire EVERY TIME.
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Re: A new look at combustion cartridge Spud guns

Unread postAuthor: Cthulhu » Mon May 29, 2017 11:19 pm

Sounds sure enough.

How long do you think the cartridges would be good for after fueling them with propane? Would they indefinitely be"ready to fire" or would the gas eventually settle in the container and become useless.
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Re: A new look at combustion cartridge Spud guns

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Re: A new look at combustion cartridge Spud guns

Unread postAuthor: jackssmirkingrevenge » Tue May 30, 2017 6:13 am

D_Hall wrote:Propane.

Propane.

Propane.


Image

How long do you think the cartridges would be good for after fueling them with propane? Would they indefinitely be"ready to fire" or would the gas eventually settle in the container and become useless.


Nope, they'd always be ready to go.

Syringe metered cartridges are a practical option for good performance and quick reloading.
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Re: A new look at combustion cartridge Spud guns

Unread postAuthor: Cthulhu » Thu Jun 01, 2017 1:22 am

I've been at the drawing board for​ a while and I think I've figured out the elevation system, breech, and even a crude ejection system.
But two major questions arose:
Since the cartridges aren't in the ideal barrel to chamber ratio would it be better to use a 3" PVC (which may have more force) or stick with the 2" rifled PVC (which I imagined would have better accuracy)?
In each cartridge, I'm going to place two screws for the electric igniter to touch, what is the ideal location of these screws?

Some of you may also find it interesting that I'll be making my projectiles out of proto putty (a sort of silicone), which I've never seen done before on a Spud gun.
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Re: A new look at combustion cartridge Spud guns

Unread postAuthor: farcticox1 » Thu Jun 01, 2017 4:26 am

Jim is also into spudguns

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Re: A new look at combustion cartridge Spud guns

Unread postAuthor: Cthulhu » Fri Jun 02, 2017 3:32 pm

Cthulhu wrote:would it be better to use a 3" PVC (which may have more force) or stick with the 2" rifled PVC (which I imagined would have better accuracy)?

In each cartridge, I'm going to place two screws for the electric igniter to touch, what is the ideal location of these screws?


Sorry to push the topic like this, but I'd really like some opinions.
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Re: A new look at combustion cartridge Spud guns

Unread postAuthor: D_Hall » Fri Jun 02, 2017 11:34 pm

Without knowing all the details, it's hard to say. The design. Your budget. Your goals. Your resources. There really is no single right answer here.

That said... 3" projectiles are getting big. I'd roll with the 2". Still plenty of power while keeping logistics to a reasonable burden.
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Re: A new look at combustion cartridge Spud guns

Unread postAuthor: Cthulhu » Sat Jun 03, 2017 12:23 am

D_Hall wrote:The design. Your budget. Your goals. Your resources. There really is no single right answer here.


Design is going to be a breech loaded cartridge based field cannon based on a mix of ww1 field Artillery, so it'll have a breech block and about a 5-6' barrel.

Budget is nearly infinite, I build my Spud guns with a large group of friends, so the cost is always divided to the point of only paying up to 40 bucks per person.

Goal is to make a Spud gun that can achieve a high fire rate (from cartridges) with a distance preferably farther than a paintball gun (so it can be used in paintball/airsoft games), and just enough accuracy to be useable for hitting a general area.

Resources aren't quite a problem either, I have a decent sized hobby shop that should have all the tools I need.

So a rifled barrel of 2" diameter should give good enough results?
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Re: A new look at combustion cartridge Spud guns

Unread postAuthor: D_Hall » Sat Jun 03, 2017 1:06 am

Cthulhu wrote:Budget is nearly infinite, I build my Spud guns with a large group of friends, so the cost is always divided to the point of only paying up to 40 bucks per person.

LOL.... For perspective, my largest gun cost on the order of $30,000 to build and that was using a lot of materials from a junkyard. Using all new materials it would have cost more like $300,000. I say this not to belittle you, but to make you understand that "nearly infinite" is a stupid thing to say. One can spend pretty much as much as one wants to in this world.

Or do you have a lot more friends than I think you do?

So a rifled barrel of 2" diameter should give good enough results?

Given your stated goals, I wouldn't bother with rifled. 2" smooth bore would work just fine and keep things simple.
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Re: A new look at combustion cartridge Spud guns

Unread postAuthor: jackssmirkingrevenge » Sat Jun 03, 2017 5:04 am

2" smooth bore would work just fine and keep things simple.


If he wants accuracy, it's probably worth it, especially with a lightweight projectile safe for airsoft/paintball

I would go with 2" barrel and 3" cartridges.
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