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Experience with C:B Ratio

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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Experience with C:B Ratio

Unread postAuthor: Xxplosive42o » Tue Dec 09, 2008 6:29 pm

I am constructing a propane metered combustion cannon, and have viewed several opinions on proper C:B ratios. I have searched and have found people using anything from .8:1 to 1.5:1. I am here to ask what ratio's you guys have used and what has had the best results. I appreciate all imput you guys could give.
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Unread postAuthor: Hubb » Tue Dec 09, 2008 6:33 pm

I use anywhere from .8:1 to 1.5:1...

JK. The norm is to go with the .8:1 ratio with a metered system and the 1.5:1 ratio with a spray-n-pray.
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 09, 2008 6:34 pm

Some C:B testing can be found here.
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Unread postAuthor: jook13 » Tue Dec 09, 2008 6:35 pm

I used closer to a 2:ratio. I like it because of the big bang. I dont mind the ineffiency(spelling?) because propane tanks are so cheap and one tank gives off hundreds of shots.

Go with around .8:1 if you still want lots of power but dont want the extra noise. Any larger of a chamber wont give much more performance and will be louder. At least thats how I understand it.
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 09, 2008 6:43 pm

jook13 wrote:Go with around .8:1 if you still want lots of power but dont want the extra noise. Any larger of a chamber wont give much more performance and will be louder. At least thats how I understand it.


Larger chamber will probably give worse performance:

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Unread postAuthor: Ragnarok » Tue Dec 09, 2008 6:47 pm

jackssmirkingrevenge wrote:Larger chamber will probably give worse performance.

Actually, large chambers, within reasonable limits give higher performance - you have to remember that Burnt Latke's testing changed barrel size and thus acceleration distances.

If you scale for that change, the larger chambers are a benefit.
Personally though, I'd hang around 1:1 for combustion chamber sizes.
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Unread postAuthor: Xxplosive42o » Tue Dec 09, 2008 7:58 pm

Ok update, I just measured and got a pretty close estimate of chamber volume and its 236.4 square inches. The chamber will be attached to a 2" sched. 40 Barrel. The length of the barrel needs to be determined now.
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Unread postAuthor: jackssmirkingrevenge » Wed Dec 10, 2008 1:53 am

Ragnarok wrote:If you scale for that change, the larger chambers are a benefit.
Personally though, I'd hang around 1:1 for combustion chamber sizes.


My latest sounded like a 12 bore with a 0.8:1 ratio, shortening it considerably made didn't appear to make it any quieter or less powerful.
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Unread postAuthor: Xxplosive42o » Wed Dec 10, 2008 2:23 am

Wow, everybody has a different answer. I was kind of expecting this would happen.

There must be a correct answer because...

Too short of a barrel and combustion has not reached peak performance before spud has left barrel.

Too long equals complete combustion occurs leaving no driving force to push spud thus creating drag for the remaining length.

HELP! :roll:
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Unread postAuthor: starman » Wed Dec 10, 2008 4:45 am

The barrel length usually ends up being a compromise for sheer usability, especially for 1.5" potato barrels. The PVC pipe starts to bend much beyond 5'- 6'. You can build one longer of course but you'll need to mount the cannon and externally support the barrel.

It's easier to get a good ratio with 2" pipe but 2" isn't ideal for potatos unless you have some big ones. 1.5" is considered the ideal all around potato caliber.

Don't worry much about coming in with absolutely ideal c:b numbers, especially shooting potatos. Range will be limited by drag and inconsistancy, no matter how powerful and optimized your gun is. I've found a 5' barrel give good potato power but still handles and stores easily.
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Unread postAuthor: psycix » Wed Dec 10, 2008 7:34 am

Most of the times, the optimal barrel length is longer then practical if you have a juicy chamber.
5-6' is about the maximum length that still allows you to carry the thing around.
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Unread postAuthor: jimmy101 » Wed Dec 10, 2008 1:40 pm

Yes, there is always controversy and/or just plain wrong answers when this question is asked. The correct answer is really not controversial and has been a settled matter, in general, for a while now.

The oft quote 0.8 value is from Latke. That is the CB that maximizes the efficiency of a propane + air at 1x gun using the barrel ID and ammo of Latke. Latke's studies looked at changing just the barrel length, the chamber volume was held constant. Nobody knows how the 0.8 number changes with significantly different ammo or significantly different barrel IDs. Given Latke's data, for example the graph Jack posted, it looks like small changes near the 0.8 CB don't make too much difference. 0.7, 0.8, 0.9 are really all the same.

Now, the key is that efficiency is NOT the same as performance. Performance can be defined in many different ways (so can efficiency). Usually though, "performance" means maximizing the muzzle velocity. If that is what you want to do then a chamber larger than CB 0.8 will increase performance for a fixed barrel length. Exactly how big the chamber can be for a fixed barrel before performance starts to suffer is not very well defined. It appears that up to a CB of perhaps 2 or so (larger chamber on the same barrel) the muzzle velocity will continue to rise. The heavier the ammo and the greater the friction between the ammo and the barrel the higher the CB can go before performance starts to drop off. For very light ammo and very low friction (for example a wad of paper) the optimal CB may actually go the other way, 0.5 or so might be better then 0.8, or 1.5 or 2.

Usually though people run into practical restriction before they get to the optimal geometry. If you want to boost performance by say 10% it might be easier to make both the chamber and the barrel bigger and getting the increased performance by "brute force" instead of trying to tweak the individual characteristics of the gun to their optimal values.

All this can be quite easily worked out with HGDT. For a fixed barrel just do a sensitivity study on the chamber length. For typical ammos you'll find the CB (varied as the chamber length varies) that gives the maximum velocity is up in the 2 or more range. The resulting design is not terribly practical, and is very inefficient, and the difference in performance between the optimal CB and a more practical CB is fairly small.

Going back to Lakte's studies, he was trying to maximize efficiency where efficiency is defined as getting the most kinetic energy into the selected ammo from a fixed amount of energy in the fuel. To most spudders this is really pretty irrelevant. Propane is cheap and if more fuel gives better performance, even though it is less efficient, then most spudders would go with the increased performance. So what if it costs $0.0015 instead of $0.0013 per shot?

("I before e except after c, and in the word efficiency")
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Unread postAuthor: Ragnarok » Wed Dec 10, 2008 2:34 pm

jimmy101 wrote:"I before e except after c, and in the word efficiency"

A regrettably over taught and flawed spelling rule - the errors aren't limited to "efficiency". Take the perfectly common words "Science", "Society", "Ancient", "Species", "Foreign", "Height", "Counterfeit", "Weird", "Feign", "Their", "Weigh" "Leisure" "Neighbour", "Veil", "Albeit", etc, etc.

A more complete form is "I before E except after C, and where the sound is a long E", which discounts some of it's errors.
Even then, it still has problems, like Protein, Mercies, Theist/Atheist, Either/Neither, Financier, Caffeine, Being, Tendencies and Coefficient - which all have a Long E sound, but break the rule.

To be frank, it's best to just forget the rule altogether really. :roll:

Anyway, off topic over.
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Unread postAuthor: Xxplosive42o » Wed Dec 10, 2008 6:03 pm

Rag, that was... interesting. lol

Back to Jimmy's post. It was interesting you compare efficiency to performance. In a refutation, I said I am looking for what’s best. By best I mean which barrel "will shoot the darn potato the furthest" to be frank. I understand the graph Jack posted was muzzle velocity, so wouldn’t that show approx. how far the projectile will travel given the trajectory was controlled? Also, that test was done with a 1.5" barrel; will a 2.0" differ? I stated I will be using a chamber with a volume of 236.5" and a barrel of 2" with an unknown length. I just want the best "performance" per say, while not giving a darn about cost of propane.

Therefore, if somebody could help give a straight forward answer based on their results I would appreciate it. I want performance NOT efficiency to clear the air. I am going for distance baby...distance! haha.

(p.s I know potatoes have flaws, create diff. drag, etc. This cannot be controlled, but I am looking for closest number I can get)
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Unread postAuthor: starman » Wed Dec 10, 2008 7:59 pm

You can have as much distance as you want....as long as it's about 250 yds... :wink: Really though, big spudguns, little spudguns, big chamber medium chamber, long barrel medium barrel, propane, hairspray, fan/no fan.....etc etc. You're wasting your time optimizing for distance....it isn't worth the trouble....250 is the limit.

That being said, if you were optimizing for say blasting holes in plywood with potatos from 30', then cannon performance can make a difference on how much plywood thickness can be blasted through.

As Jimmy said, don't worry so much about efficiency. A larger chamber will always provide more power with a given length of barrel...and...a longer barrel will provide more power with a given chamber size.

Go with a comfortable, practical size that includes about a 5' 1.5" barrel and a c:b of around 1.2 - 1.5:1 and run with it.
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