oversized chamber?

Boom! The classic potato gun harnesses the combustion of flammable vapor. Show us your combustion spud gun and discuss fuels, ratios, safety, ignition systems, tools, and more.
mikespahn
Private 2
Private 2
Posts: 23
Joined: Mon Dec 21, 2009 5:26 pm

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)
User avatar
kjjohn
Specialist 2
Specialist 2
Posts: 270
Joined: Sat Jul 11, 2009 3:54 pm
Contact:

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.
User avatar
jmadden91
Specialist 2
Specialist 2
Posts: 246
Joined: Thu May 07, 2009 6:03 pm
Location: Queensland, Australia

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)
User avatar
drac
Corporal 4
Corporal 4
United States of America
Posts: 899
Joined: Sun Apr 24, 2005 8:56 am
Location: Avon, NY
Been thanked: 2 times

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)
SpudBlaster15
First Sergeant 3
First Sergeant 3
Seychelles
Posts: 2400
Joined: Wed Oct 18, 2006 11:12 pm
Has thanked: 2 times
Been thanked: 10 times

Mon Dec 21, 2009 8:08 pm

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Cras nec placerat erat. Vivamus dapibus egestas nunc, at eleifend neque. Suspendisse potenti. Sed dictum lacus eu nisl pretium vehicula. Ut faucibus hendrerit nisi. Integer ultricies orci eu ultrices malesuada. Fusce id mauris risus. Suspendisse finibus ligula et nisl rutrum efficitur. Vestibulum posuere erat pellentesque ornare venenatis. Integer commodo fermentum tortor in pharetra. Proin scelerisque consectetur posuere. Vestibulum molestie augue ac nibh feugiat scelerisque. Sed aliquet a nunc in mattis.
Last edited by SpudBlaster15 on Wed Jul 14, 2021 7:27 pm, edited 1 time in total.
mikespahn
Private 2
Private 2
Posts: 23
Joined: Mon Dec 21, 2009 5:26 pm

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.
User avatar
kjjohn
Specialist 2
Specialist 2
Posts: 270
Joined: Sat Jul 11, 2009 3:54 pm
Contact:

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
mikespahn
Private 2
Private 2
Posts: 23
Joined: Mon Dec 21, 2009 5:26 pm

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.
User avatar
chenslee
Specialist
Specialist
Posts: 147
Joined: Thu Jan 29, 2009 2:25 pm

Donating Members

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.
jeepkahn
Corporal 3
Corporal 3
Posts: 747
Joined: Fri Jan 09, 2009 1:11 pm
Location: Triad, NC, USA

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...
My Cannons can be found by clicking the following link.
http://www.spudfiles.com/forums/viewtop ... tml#256896
User avatar
starman
Sergeant Major
Sergeant Major
United States of America
Posts: 3027
Joined: Wed Dec 12, 2007 12:45 am
Location: Simpsonville, SC

Donating Members

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.
mikespahn
Private 2
Private 2
Posts: 23
Joined: Mon Dec 21, 2009 5:26 pm

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.
User avatar
Moonbogg
Staff Sergeant 3
Staff Sergeant 3
United States of America
Posts: 1733
Joined: Mon Oct 13, 2008 10:20 pm
Location: SoCal
Has thanked: 154 times
Been thanked: 100 times

Tue Jan 05, 2010 11:33 pm

For an easy to transport cannon, I think 3' is a good length for a barrel.
User avatar
starman
Sergeant Major
Sergeant Major
United States of America
Posts: 3027
Joined: Wed Dec 12, 2007 12:45 am
Location: Simpsonville, SC

Donating Members

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.
mikespahn
Private 2
Private 2
Posts: 23
Joined: Mon Dec 21, 2009 5:26 pm

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.
Post Reply