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oversized chamber?

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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oversized chamber?

Unread postAuthor: mikespahn » Mon Dec 21, 2009 5:52 pm

i understand why to small of a chamber is a bad thing but what about too big of a chamber. i know bigger would mean less efficient because the potato has left the barrel before the combustion has finished expanding. what are the negative effects as far as velocity and overall power caused by this. i would like to go for a significantly larger chamber to barrel for a few reasons but if it will take away from projectile speed and total launch distance, then i will be forced to shorten my design.

on a side note, i have only ever seen crappy spud guns made by high school kids that didn't know what they were doing. i haven't even seen one since high school. this got me thinking about how i could use my 10+ years of machinist experience into making one the likes of which i have never seen before. i started coming up w/ ideas like making a clear chamber to see the fire, onboard fuel injection and metering, multiple high voltage spark gaps, pistol grips, pump action, and all kinds of other things. then i found this forum. unfortunately, i found out that the majority of my ideas have been done a million times over by tons of people. however i'm glad i found this and hopefully i can continue to find more valuable info to further my project. the first will be simple in comparison to a lot of the more advanced designs i have seen in this forum but maybe after i get some experience this will become a new hobby and i will attempt some of the more advanced cannons i have seen here.
when i finish my first cannon i will post a few pics and you guys can judge (be gentle i'm a beginner)
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Unread postAuthor: kjjohn » Mon Dec 21, 2009 6:49 pm

i know bigger would mean less efficient because the potato has left the barrel before the combustion has finished expanding. what are the negative effects as far as velocity and overall power caused by this.


Not really, actually. There is no way to really make a chamber "too" big. Just scale up the barrel in proportion to the chamber, the you will be able to reach very high velocities and kinetic energies. Yous sound like you already know a bit about these, so I suggest you go with an advanced combustion.

One thing to take into consideration, however, is that as pipe sizes get larger in diameter, they become weaker. If you want to use metal, this will not be a problem. but if you are using PVC, I would take this into consideration.
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Unread postAuthor: jmadden91 » Mon Dec 21, 2009 6:50 pm

Welcome to the forum mate you have come to the right place 8)

The oversized combustion chamber will not harm your performance in any way, simply make it louder lol. Check out HGDT A spudgun modelling program developed by one of our members to play with different chamber sizes and see how they change performance.

Looking forward to seeing your cannon mate, having access to a machine shop always produces some pretty amazing cannons 8)
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Unread postAuthor: drac » Mon Dec 21, 2009 7:33 pm

The thing about a big combustion chamber is that performance increases become negligible after about a 1.5:1 chamber to barrel volume ratio. A bigger chamber will simply give you more of a boom, but if you don't have a long enough barrel to take advantage of the increased gas volume post combustion, it won't make a difference.

Big chambers are very useful for making the neighbor's bladder happy dog wet itself instead of your porch, may I add. :wink: (And I don't mean shooting the damn thing either, haha)
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Unread postAuthor: SpudBlaster15 » Mon Dec 21, 2009 8:08 pm

A larger chamber will increase launcher performance, but as drac said, the returns start to diminish after a certain point.

My combustion launcher has a 2:1 ratio, and it makes quite a loud bang upon firing, but the muzzle energy is only about 20% higher than what a chamber of half the volume would produce with the same barrel.

However, fuel gas is cheap, and large chambers don't have any other significant disadvantages aside from noise. I would just build the launcher to the dimensions that suit your particular needs, without worrying too much about the ratio. As long as it's .8:1 or higher, performance will be good.
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Unread postAuthor: mikespahn » Mon Dec 21, 2009 10:06 pm

thats what i was hoping to hear. to make my launcher to the dimensions i want will have about a 2.6:1 C:B. i figured it would probably be loud and maybe even belch fire out the end. i was hoping it wouldn't harm any, since all the stuff i'm reading has a ratio of somewhere between .8:1 to 1.5:1.

to elaborate a little more on my design if you would like to hear...
I'm planning on mapp gas metering actuated by a pump action grip, chamber fan behind gas inlet, some LED's, and multiple spark gaps inside a clear PVC chamber. male threaded chamber for easier removal and stronger cap, for cleaning chamber. pistol grip w/switch for stun gun power to spark gaps. clear pvc barrel w/ rifling and muzzle brake. cam lock interchangeable barrels w/ breech loading. i think i might be lazy and just hold most of it together w/ hose clamps. i think i might also be lazy on building the propane meter and just get the one from ultimatespudgun so i don't have to build a cylinder and valves that work like that.
i would consider that beginner for my capabilities. if that works well and i continue to find this hobby fun to pursue (i probably will) i will go more advanced. any suggestions or advice for my first design are welcome. thank you.
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Unread postAuthor: kjjohn » Mon Dec 21, 2009 10:10 pm

Ultimatespudgun's meter is a spool valve. You could make one just like it yourself, but I don't know if it would be very easy.

Your cannon looks like it's going to be very good for a beginner. I can't wait to see it when you are done!

if that works well and i continue to find this hobby fun to pursue (i probably will) i will go more advanced. any suggestions or advice for my first design are welcome. thank you.


Try to outdo Chenslee and make a full auto combustion :D
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Unread postAuthor: mikespahn » Tue Dec 22, 2009 8:44 am

i do like his design and full auto would be cool but i think that is a few cannons away at this point. also i want this one to be clear. i wouldn't be comfortable trying to do something fully auto unless it was metal.
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Unread postAuthor: chenslee » Tue Jan 05, 2010 5:54 pm

kjjohn wrote:Try to outdo Chenslee and make a full auto combustion :D


:evil:

I just calc'd the tactical repeater, and I have a .96:1 comb:barrel. By combustion chamber is actually smaller than the barrel volume.
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Unread postAuthor: jeepkahn » Tue Jan 05, 2010 6:16 pm

Also, Be aware that clear pvc has different(read lower) pressure ratings than standard pvc... But as long as your not going hybrid, it should be fine...
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Unread postAuthor: starman » Tue Jan 05, 2010 8:55 pm

Welcome to Spudfiles!

You should check out Burntlatke's site. It is pretty much the defacto standard "how to" when in comes to advanced combustion cannons.

Keep in mind when you see C:B efficiency numbers that you are talking about just that...efficiency. On an absolute basis, a larger chamber volume will always provide for a more energetic shot for any reference sized barrel. Similarly a longer barrel (within practical reason) will provide higher velocities for any reference sized chamber. You've got to get down to .4:1 C:B ratios and below before extravagant barrel length actually starts hurting performance.
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Unread postAuthor: mikespahn » Tue Jan 05, 2010 9:50 pm

jeepkahn wrote:Also, Be aware that clear pvc has different(read lower) pressure ratings than standard pvc... But as long as your not going hybrid, it should be fine...


already observed that, but thanks for mentioning.


and yes i have checked out bunt latke's site.

what size barrels do most of you use for a portable inline design? i was thinking roughly 3 feet. does that yield reasonable accuracy and velocities? i could go bigger.
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Unread postAuthor: Moonbogg » Tue Jan 05, 2010 11:33 pm

For an easy to transport cannon, I think 3' is a good length for a barrel.
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Unread postAuthor: starman » Wed Jan 06, 2010 2:07 am

mikespahn wrote:what size barrels do most of you use for a portable inline design? i was thinking roughly 3 feet. does that yield reasonable accuracy and velocities? i could go bigger.


Really depends on what you're looking for performance wise. Go as large as you can easily transport. If you go with a detachable (camlock) barrel you can steal a little more length. I can barely carry a 5 1/2 ' barrel in the back seat of my truck...easily carry a 5' barrel outfitted with a male camlock.

If you permanently mount the barrel on your chamber, you'll need to subtract the length of the chamber from your barrel for keep the same transport size.
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Unread postAuthor: mikespahn » Wed Jan 06, 2010 2:39 pm

mikespahn wrote:to elaborate a little more on my design if you would like to hear...
I'm planning on mapp gas metering actuated by a pump action grip, chamber fan behind gas inlet, some LED's, and multiple spark gaps inside a clear PVC chamber. male threaded chamber for easier removal and stronger cap, for cleaning chamber. pistol grip w/switch for stun gun power to spark gaps. clear pvc barrel w/ rifling and muzzle brake. cam lock interchangeable barrels w/ breech loading. i think i might be lazy and just hold most of it together w/ hose clamps. i think i might also be lazy on building the propane meter and just get the one from ultimatespudgun so i don't have to build a cylinder and valves that work like that.
i would consider that beginner for my capabilities. if that works well and i continue to find this hobby fun to pursue (i probably will) i will go more advanced. any suggestions or advice for my first design are welcome. thank you.


w/the breech loading i might extend it a little into the chamber to save on length, since my chamber has more than ample volume to take some reduction.
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