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New barrel advice

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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New barrel advice

Unread postAuthor: Moonbogg » Sun Dec 14, 2008 4:23 am

I am making a silenced barrel. My chamber is 4" SCH40 x 17" long. I currently have a 1.5:1 C:B: ratio with a 43" long, 2" SCH40 barrel, and its LOUD.
I was thinking about lengthening the barrel to 60", making it closer to 1:1 and having the last 17" of barrel the silenced part, using 4" SCH40 as the silencer. This would leave me an effective barrel section with no holes in it at 1.5:1 still, with the remaining section used basically for gas and pressure dispersion. What you think? Sound about right?
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Unread postAuthor: john bunsenburner » Sun Dec 14, 2008 4:42 am

If you really want to exchange your barrel why not try to get the true optimum(sp?) ratio of 0.8:1, But i guess 1 is close eanoth, sounds good, oh and are you sure you do not want a loud band, dumb question but many people really like that!
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Unread postAuthor: Moonbogg » Sun Dec 14, 2008 4:51 am

john bunsenburner wrote:If you really want to exchange your barrel why not try to get the true optimum(sp?) ratio of 0.8:1, But i guess 1 is close eanoth, sounds good, oh and are you sure you do not want a loud band, dumb question but many people really like that!


Yeah I love the loud bang actually. But living in the damn city makes it impossible to not have the cops called. Surrounding me are millions of senior citizens with their fingers on their lifealerts 24/7, just waiting to hit the button at the slightest indication of unrest. I drove to the top of a mountain, and got the cops called on me. The cop said the people wayyyyy down in the valley below thought they were being fired upon by a sniper! So you see, its just too loud.

About the ratio, from what i've read 1.5:1 is optimal, going with .8:1 seems to be too little. Won't the spud drag at the end of the barrel?
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Unread postAuthor: john bunsenburner » Sun Dec 14, 2008 4:59 am

http://www.spudfiles.com/spud_wiki/inde ... rrel_ratio

Read child and you shall learn and see it is between 0.6-0.8, ok chances are i am the child but it sounds cool eh?

but there is this little part at the end that goes as follows:

The C:B ratio plays a much smaller role in the design of pneumatic cannons since factors like chamber charge pressure and valve performance play a greater role in determining overall performance. It is advisable to use GGDT to determine the best design for pneumatic cannons.

Retrieved from "http://www.spudfiles.com/spud_wiki/index.php/Chamber_to_barrel_ratio"

But i still it matters in your case as you need it to be quiet...
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Unread postAuthor: Moonbogg » Sun Dec 14, 2008 5:24 am

Well this is a combustion cannon. Sorry if I lost you there, but I am more than willing to check out info I have missed about C:B ratio. I want the best power of course, just with a silenced barrel. I guess I am confused because information differs bewteen two diffrent spud gun sites. I assume both sites know more than I do about spudguns, and if their information contradicts eachother, then I am appearantly in trouble.
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Unread postAuthor: john bunsenburner » Sun Dec 14, 2008 5:30 am

I am some what bias but spudfiles is THE best spudgun site you will find, th knolage here is impressive trust our wiki because it was made by some of the best spud gunners known to man kind. Just out of interest(and maybe for a little laugh) what is the site you found your info on?
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Unread postAuthor: Moonbogg » Sun Dec 14, 2008 6:33 am

I'm afraid to mention because I don't want to start conflict. Its not my goal, so its irrelevant. What is relevant is that the information has been presented to me and beyond that, it makes sense, at least to me. 1.5:1 just sounds right to me right off the bat, but 0.8:1 has intrigued me enough to want to know more. I am not here to start forum wars lol, or take sides. I am also not assuming that this is what you are accusing me of, but I just want to keep it clean. Lets analyze the logic. Maybe not even logic, but good old common sense. What will create more distance? 1.5:1 or 0.8:1.
To be totally honest, I am a bit confused and can see both options could be the best. I really don't know. I don't have the knowledge to know. If no one can tell me in a concrete manner, I WILL drive my ass out to the open desert and do some real world testing of my own. This hobby has enough of my interest and is in tune with me so much that I won't be letting suspicious claims slide past me.
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Unread postAuthor: john bunsenburner » Sun Dec 14, 2008 8:22 am

I love you attitute and i was not acusing you of anything. I am amazed i am the only one talking to you because i am definately far from being the any where near something that i can call my self experienced. Maybe if you explain your gun to me(and all the others here) we can help you a little better, until now all i knwo is it is a cumbustion cannon... If you would of clicked on the link i supplyed and read then you woudl of come across this:
1.A chamber volume that exceeds the optimal value for efficiency will increase performance since more energy is available within the chamber. This theory is based on the "fast combustion model" in which the fuel burns quickly relative to the movement of the projectile. The pressure rises to it's maximum before the projectile has moved a significant distance down the barrel. When designing a launcher using this theory, a larger chamber will produce better performance.

2. The most efficient ratio will usually also produce the highest performance. (Internal combustion engines fall into this category, the most efficient engine speed usually also produces the most power.) This theory is based on a "slow combustion model". A chamber that is too large takes longer to burn the fuel, the pressure rises more slowly, the projectile starts to move through the barrel at lower pressures and the exits before maximum pressure is reached. This results in lower projectile speed. This model is similar to a slow opening pneumatic valve. If the chamber is grossly oversized, combustion may not complete until after the projectile leaves the barrel, further reducing performance. When designing a launcher, the chamber and barrel should be matched to each other for optimal performance. A ratio anywhere between 0.5:1 and 1:1 will be ideal under most circumstances.

Now where you read it you will understand that our advice depends greatly on your gun. It depends really what kind of gun you made if it cumbusts very rapidly or slowly, and to make life even more easy maybe give us your mesurements(chamber volume and barrel volume) and anythign else that might help us help you. I am scared to say much more because, as stated before i am not very experienced(and i know close to nothing when it comes to combustion cannons) but this is simple math and common sense(includes clicking on links and reading them :roll: ) and so i will do my best to help you.
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Unread postAuthor: SP00K » Sun Dec 14, 2008 1:43 pm

So if you really don't use this cannon for other than the joy of shooting a homemade cannon and needing it to be quiet, go with the 0.8:1 ratio.
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Unread postAuthor: jonnyboy » Sun Dec 14, 2008 2:35 pm

Lengthening the barrel for a .8:1 ratio will quit the cannon and give it some more power.

If your still not happy with the noise grab some 4 inch pipe and make a silencer.For the baffles you could try and find some can bottoms or could cut your own out of plywood.
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mobile chernobyl wrote:I can shoot a Canuter Valve off my '82 Chevy Ram F150 AT LEAST 3/4 Mile with 'ma cannon made of soup cans duct taped together, then I just squirt some bacardi 151 in the chamber and hold up my cigarrete lighta and WHOOSH! That thing flies at least 3/4 mile
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Unread postAuthor: starman » Sun Dec 14, 2008 4:03 pm

We recently discussed performance of barrels vs potato ammo in another post so hopefully that issue has been put to rest for the moment.

The longer the barrel and thus the lower the c:b, the quieter the shot will be, and I'm talking in the .5 to .8 c:b range. Unfortunately for common 4" dia chambers, it makes the 1.5" and 2" barrels unreasonably long. 2.5" chambers for tennis ball become reasonable size...5 to 6' or so and 3" veggie can barrels are down in the 3' to 4' range.

Therefore, your only choice for a low c:b and a reasonable barrel length is to build small chambers. If you wanted to stick with a 4" chamber, you could build a small one that was essentially a cleanout and coupler with a small 3" section of pipe to glue them together...at its smallest, the coupler and the clean out touching. you could still put a small fan if you wanted, only 1 spark gap would be needed.

Your other choice would be to go with a 3" dia chamber. It could be twice as long as the 4" chamber and still offer the same volume of space. Radiation's recent Clara Ann uses a 3" chamber successfully.
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Unread postAuthor: Xxplosive42o » Sun Dec 14, 2008 6:56 pm

Well put boys!

I would highly suggest you keep your current chamber and build a couple barrels with different lengths to test this for yourself. Use cam-locks and you will be able to interchange barrels very quickly to compare shots made with the different barrels.

I know it is very confusing when you hear people suggesting different C:B ratios.

One of the major sites that encourage a 1.5:1 is Spudtech. They sell combustion cannons with an available chamber upgrade and quote, "-Step it up to 1.5:1, 150% the barrel volume, 50% bigger than normal, for even more projectile energy and a much louder report!"

I believe this is a marketing scheme to get unexperienced people to pay for an upgrade on their cannons for a higher cost that; consequently, offers no performance gain what so ever in the realm of muzzle velocity.

To sum it up, I believe you should test for yourself to see what you like best. But if you want to base it off an acurate test sample visit: http://www.burntlatke.com/launch.html

They have GREAT test results that are easily comprehendible!

Hope what I have said will aid you in some way! -Xxplosive42o 8)
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Unread postAuthor: Moonbogg » Sun Dec 14, 2008 8:23 pm

Awesome! Thanks for all the replies.
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Unread postAuthor: Xxplosive42o » Sun Dec 14, 2008 8:50 pm

No problem!

Feel free to ask questions anytime no matter how silly it may be. Although, do a bit of researching on the boards to make sure it hasn't already been addressed. This will help you avoid members from coming down on you.

Cheers mate! -Xxplosive42o 8)
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Unread postAuthor: jimmy101 » Mon Dec 15, 2008 1:12 pm

MoonBogg:
The "optimal" CB really isn't controversial or even all that difficult to understand. What is difficult is wading through all the BS and stuff that is misleading or irrelevant or flat-out wrong.

The first thing you need to know about CB is that "optimal" has no meaning. What exactly are you trying to optimize? "Optimal" in what way? The CB 0.8 rule that Latke determined maximizes the efficiency of a gun with a fixed chamber size.

For a particular chamber size the 0.8 CB will maximize the efficiency of the gun. If you are designing a gun based on a particular starting chamber geometry then an 0.8 CB will give you the best performance where performance is defined as muzzle velocity. It also gives the most efficient gun. Efficiency is rarely a significant concern in a combustion spud gun. Fuel is too cheap to be an important design consideration.

If you are designing your gun based on a particular barrel geometry then the "optimal" design, where "optimal" means the highest muzzle velocity, will have a CB significantly greater than 0.8. It might be as high as 3 or even more.

As a practical matter though, a barrel based design will generally perform only a small amount better with a CB 3 chamber than it will with a CB 1.5 chamber. Experienced spudders figure the small increase in muzzle velocity isn't worth having to use a chamber that is twice as big, so you get the oft quoted CB 1.5 instead of a CB of 2 or 3 or 6 or whatever is best for a particular barrel + ammo combination. 1.5 isn't "optimal" in any sense. What it is is a "reasonable" tradeoff and gets you pretty good performance for the chosen barrel without having to use a chamber that is goofily large.

There are other ways to design a combustion spud gun. For example, you might decide that the total length of the gun is some particular value. You would then like to partition the length of the gun into chamber and barrel so that you get the best muzzle velocity. This design approach will end up with an optimal CB that is neither the 0.8 or "1.5 or 2 or 3 or …" values.

To summarize:

1. If you define "best" or "optimal" as that configuration that gives the greatest muzzle velocity and you first choose a barrel geometry then the optimal CB is 1.5 or more.

2. If you define "best" or "optimal" as that configuration that gives the greatest muzzle velocity and you first choose a chamber geometry then the optimal CB is about 0.8 plus or minus about 0.2.

3. If you define "best" or "optimal" as that configuration that gives the greatest efficiency then the optimal CB is 0.8 +/- 0.2.

To the designer of an internal combustion engine, particularly for a car, efficiency is an extremely important design consideration. Spud gunners usually don't give a rat's ass about efficiency (or they shouldn't).
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