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Newbie intro

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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Unread postAuthor: Pactin » Sun Jul 19, 2009 3:59 am

Hi there spudders, I'd like to introduce myself to the forum.

My name is Pactin.......and I have no experience in spud guns haha. However, I have drawn out plans several times after being intrigued by the fun factor.

I have a bit of physics knowledge which I hope would help me with my future projects. I'm more of a car forum guy though :iroc: .

Well enough of that, heres my project:

Image

codename SwORD (Sawn Off Replicated Defense), not really much of a defense but hey I wanted a cool acronym hehe. I originally planned it out to use EPE (elastic potential energy), but htat plan failed due to the fact that I could not find an elastic strong enough to carry out my projectile.

As you can see it looks like a twin barrel but I plan on using a "spray n pray" method (i think thats the correct term?). An alcohol based propellant with a bbq ignitor hooked up to a spark strip. The primary chamber (first 2 inches) is used as the combustion area while the shell itself acts as a secondary chamber for expansion. I was thinking of using copper piping for the barrel, but does anyone here think PVC would suffice/hold up?

I'm having trouble most however with fuel delivery. I was thinking an airbrush, but I want something totally autonomous, maybe some kind of pump when I eject the shell?

Okay, I think I'm getting ahead of myself here. Ehh...just gimme any suggestions or comments you deem necessary and lets hope I don't get flamed. Thanks!
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 19, 2009 4:05 am

Welcome to the forum.

Just a small point, having such a tiny chamber with a combustion at atmospheric pressure the power is going to be rather miserable. Some cartridge research you might want to look into :)
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Unread postAuthor: Pactin » Sun Jul 19, 2009 4:19 am

jackssmirkingrevenge wrote:Welcome to the forum.

Just a small point, having such a tiny chamber with a combustion at atmospheric pressure the power is going to be rather miserable. Some cartridge research you might want to look into :)


Thanks for the input and link. Yeah I realized that my chamber is an insect compared to the other beauties I've seen on here, but I really want this to be maneuverable- maybe some ingenuity could fix my problem? Is it more of a barrel/chamber length problem, or a chamber diameter problem?

Also, what do you mean by "with a combustion at atmospheric pressure"?


Edit: In addition, my diagram is kinda confusing. The shell is actually hollow except for its last 5mm which will house my projectiles. So the actual volume the propellant is able to encompass is about pi x 1"sq x 4"

I dont know how much of a difference that makes, but all help appreciated!
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 19, 2009 4:39 am

I suggest you first take a look around the Image to get a better practical background for your designs.

Yeah I realized that my chamber is an insect compared to the other beauties I've seen on here, but I really want this to be maneuverable- maybe some ingenuity could fix my problem? Is it more of a barrel/chamber length problem, or a chamber diameter problem?


It's a chamber volume problem, there simply isn't enough space for a large enough fuel/air mixture for decent power.

If you are restricted by volume, what you can do is increase chamber pressure, what we call a "hybrid".

Basically, if you have a 100mL chamber, you can only put 4mL or so propane in it for it to work well. If you pump the chamber to 15psi though, you've effectively doubled the mass of air, and therefore you can put 8mL of propane, and so on.

The power potential is impressive, but you're left with the problem of having to seal the cartridge if you're going to pressurise it.

I would recommend you make yourself a simple combustion first to better understand the mechanics of what you have to play with.
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Unread postAuthor: dudeman508 » Sun Jul 19, 2009 1:17 pm

You could reduce the diameter of your barrel and keep the chamber as is.
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Unread postAuthor: jimmy101 » Sun Jul 19, 2009 3:35 pm

Don't use liquid fuels, they are much to hard to measure. Take a look at the wiki page on liquid fuels, it'll tell you about how many drops a particular size chamer will take for various fuels. For you tiny chamber the volume of fuel will be too small to measure. Just get a plastic syringe and a butane lghter. It'll give you nearly perfect fueling for every shot.
Image
For a small chamber the syringe from an ink jet printer refill kit should be big enough.

Or, hack a long handled BBQ lighter. That'll give you both fuel and an ignitor. See here for some ideas.
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Unread postAuthor: Pactin » Sun Jul 19, 2009 9:55 pm

Sorry I couldn't reply fast enough, it's like my internet was having a seizure....

Anyways, after reading over the several links proposed, I've made some revisions.

jackssmirkingrevenge: I will look into your hybrid suggestion. Kind of like a turbo I suppose? where you fill a cylinder with as much air as possible, some fuel and that allows a greater energy potential. The only thing that really kills me on the idea is that I would need to carry some sort of exterior "pump" that would allow me to pressurize the chamber with air before injecting propane. Even a bike pump with a 1 way valve or something is more weight that I don't want.

dudeman508: Okay, so I've looked at jackssmirkingrevenge's cartridge link and I'm guessing that: in exchange of not increasing my chamber volume, I must decrease the barrel diameter so that the expansion will move more laterally than having to fill everything up. I'm looking into a .5" dia. barrel- from shell to tip.

jimmy101: I've looked at the bbq hack link and I've got a question. Would that amount of fuel metered by the butane tank be enough to power my projectiles (6mm bb's and copper bb's)? And that syringe method, would I actually inject the fuel using hte needle? wouldnt it come out a liquid?

Almost forgot. Would I be okay using pressure rated pvc for my project? I could get copper piping, but thats much heavier and difficult to "shape".

Thanks for all the help guys. Really trying to research the stuff myself but my noob side is still looking for answrs.
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Unread postAuthor: Gaderelguitarist » Sun Jul 19, 2009 10:08 pm

Welcome to Spudfiles!

PVC works fine for combustion cannons.

Make the chamber extend further back towards the handle. As long as you're chamber walls are strong enough, your hand will be fine.
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Unread postAuthor: inonickname » Sun Jul 19, 2009 11:57 pm

If you're using a larger chamber than barrel now, why not make a co-axial? That will give a larger chamber volume and keep the gun smooth (and all the same size)
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Unread postAuthor: Pactin » Mon Jul 20, 2009 2:18 am

inonickname wrote:If you're using a larger chamber than barrel now, why not make a co-axial? That will give a larger chamber volume and keep the gun smooth (and all the same size)


Lol okay sorry, I looked up "coaxial" in the wiki and google and I still don't know what that means. All I understand is that the chamber is larger than the barrel- and the barrel extends into the chamber?

Image

Heres a "revised" version of the plans. The blue area is a 1" PVC with a .5" PVC in it as a barrel. I've increased the chamber length to 4". The green canister at the bottom of the handle will be the fuel tank with a green line leading to the hemisphere where the fuel is sprayed.

Edit: As on the car forums, I forgot to state my goals for this project. I'm looking to make it

A. Maneuverable- Lugging it around is a bit difficult if it has an extension such as an air compressor.

B. Respectable power- respectable for a cannon of this size at least. Nothing too crazy like putting projectiles through .25" wood, but maybe like BB's thru soda cans ~20 ft away.

C. Easily reloadable- bb's are easily loaded in the break barrel via a shell and whatever the propellant is.
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Unread postAuthor: inonickname » Mon Jul 20, 2009 4:21 am

Pactin wrote:
inonickname wrote:If you're using a larger chamber than barrel now, why not make a co-axial? That will give a larger chamber volume and keep the gun smooth (and all the same size)


Lol okay sorry, I looked up "coaxial" in the wiki and google and I still don't know what that means. All I understand is that the chamber is larger than the barrel- and the barrel extends into the chamber?
.


Well, seen as your having the barrel smaller that's the point. You can run the 1" chamber the whole length of the gun, then run the 1/2" barrel most of the way through- so you get a slightly larger chamber, and it's the same size the whole way through. It'll take more work to mix the fuel well and load quickly, but it's something to consider..
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Unread postAuthor: jimmy101 » Mon Jul 20, 2009 3:37 pm

Pactin wrote:jimmy101: I've looked at the bbq hack link and I've got a question. Would that amount of fuel metered by the butane tank be enough to power my projectiles (6mm bb's and copper bb's)? And that syringe method, would I actually inject the fuel using hte needle? wouldnt it come out a liquid?

Almost forgot. Would I be okay using pressure rated pvc for my project? I could get copper piping, but thats much heavier and difficult to "shape".

Thanks for all the help guys. Really trying to research the stuff myself but my noob side is still looking for answrs.

To your question above, propane at 1 ATM pressure and anything near room temperature is a gas. When you fill the syringe it'll be full of gas and not liquid propane. And yes, the needle is a convenient way to transfer from the syringe to the chamber. Make a small port in the chamber just a bit bigger than the syringe needle. Use a small screw or stopper to close up the whole when firing. (If the hole is small enough it won't experience very much force when the gun is fired. Just covering it with tape might be enough.)

There is enough fuel in a typical disposable butane lighter for many shots with a gun this size.

Your new design (with the lighter in the handle) should work. A syringe would be better but as drawn it is much simpler. It'll take a few tries to find out how long it takes to supply enough fuel. I would WAG it at 2 to perhaps 10 seconds with the lighters valve fully open. You can hack the lighter to supply gas at a much higher rate than the stock setting range.

Since you don't have a fan in the chamber it'll take a while for the fuel to fully mix with the air in the chamber. Probably at least a couple seconds, perhaps as much as twenty or thirty seconds. If you could figure out a way to include a chamber fan and battery that would eliminate the long wait between fueling and firing and also make it a bit easier to purge the chamber of combustion gases after firing. Perhaps a 9V battery and the tiny little vibrator motor from a cell phone? Rip of the weight from the vibrator and make a small fan blade from something like a plastic soda bottle. If you want a really tiny motor perhaps something like http://www.allelectronics.com/make-a-st ... TOR/1.html. That motor is just 4mm diameter x 8mm long (1/6"D x 1/3"L, that's a really tiny motor) and will probably work with a single AA or two AA batteries.

PVC is fine for this type of gun.

Here is a "popper" made from a long handled lighter and a 35mm film cartridge. Works similar to what you are proposing in your new design.
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Unread postAuthor: Pactin » Mon Jul 20, 2009 4:17 pm

inonickname wrote:Well, seen as your having the barrel smaller that's the point. You can run the 1" chamber the whole length of the gun, then run the 1/2" barrel most of the way through- so you get a slightly larger chamber, and it's the same size the whole way through. It'll take more work to mix the fuel well and load quickly, but it's something to consider..

ahh okay, thank you for clarifying. I am willing to do several hundred tests to find that perfect stoichiometric mix.

jimmy101 wrote:To your question above, propane at 1 ATM pressure and anything near room temperature is a gas. When you fill the syringe it'll be full of gas and not liquid propane. And yes, the needle is a convenient way to transfer from the syringe to the chamber. Make a small port in the chamber just a bit bigger than the syringe needle. Use a small screw or stopper to close up the whole when firing. (If the hole is small enough it won't experience very much force when the gun is fired. Just covering it with tape might be enough.)

There is enough fuel in a typical disposable butane lighter for many shots with a gun this size.

Your new design (with the lighter in the handle) should work. A syringe would be better but as drawn it is much simpler. It'll take a few tries to find out how long it takes to supply enough fuel. I would WAG it at 2 to perhaps 10 seconds with the lighters valve fully open. You can hack the lighter to supply gas at a much higher rate than the stock setting range.

Since you don't have a fan in the chamber it'll take a while for the fuel to fully mix with the air in the chamber. Probably at least a couple seconds, perhaps as much as twenty or thirty seconds. If you could figure out a way to include a chamber fan and battery that would eliminate the long wait between fueling and firing and also make it a bit easier to purge the chamber of combustion gases after firing. Perhaps a 9V battery and the tiny little vibrator motor from a cell phone? Rip of the weight from the vibrator and make a small fan blade from something like a plastic soda bottle. If you want a really tiny motor perhaps something like http://www.allelectronics.com/make-a-st ... TOR/1.html. That motor is just 4mm diameter x 8mm long (1/6"D x 1/3"L, that's a really tiny motor) and will probably work with a single AA or two AA batteries.

PVC is fine for this type of gun.

Here is a "popper" made from a long handled lighter and a 35mm film cartridge. Works similar to what you are proposing in your new design.


Sounds good. I will probably experiment with the diagram's fuel delivery, and a syringe as well. As for the fan, I have a small motor that can be powered by two double A batteries so its light weight as well. Perhaps I will work out the fan to run a 5-10 second cycle with a pressure trigger behind the bbq ignitor.

Thanks again guys.
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Unread postAuthor: inonickname » Mon Jul 20, 2009 6:48 pm

Rather than doing a massive number of tests, you could just use a meter then measure the volume of the chamber.
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Unread postAuthor: Pactin » Tue Jul 21, 2009 2:31 pm

inonickname wrote:Rather than doing a massive number of tests, you could just use a meter then measure the volume of the chamber.

that would b faster haha. but wouldnt there b a certain ratio of air:fuel i would need to test?
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