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Benefits of a large Cannon

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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Benefits of a large Cannon

Unread postAuthor: Rob748 » Tue Mar 17, 2009 2:24 pm

I want this cannon to be the loudest, strongest, fastest of all the ones I built.


So I have this question, I have two 4 inch 220 rated psi chambers laying around that are that combined would make 30 inches in length. Is a larger cannon with a burst disk, metered mapp more powerful than a smaller one ? as in the velocities of a projectile? lets say a 3 ounce hunk of potato ? Would the Size make it louder ? Id have to use a a humongous barrel but I want this one to be the largest and best of the 3 ive built.

Id use 2 chambers fans, 1 optional just to experiment with, Metered Mapp gas, Over and under Configuration with the burst disk right after the 180 bend, A .8 to 1 cb ratio, and a spark strip I built compliments to Starman for his ingenious idea, That has 7 sparks powered by a electronic grill sparker. Thanks
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Unread postAuthor: SEAKING9006 » Tue Mar 17, 2009 2:27 pm

There's a combustion cannon simulation program lying around here somewhere, search around a bit and plug your numbers into it and see what comes out.
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Unread postAuthor: Rob748 » Tue Mar 17, 2009 2:29 pm

I have it shoots heavier objects faster wich is always a benefit lol, But would it be louder ?
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Unread postAuthor: Biopyro » Tue Mar 17, 2009 2:31 pm

Yes. A larger cannon will have a (somewhat) proportionally larger muzzle energy. It will not necessarily be louder, or have a higher muzzle velocity though.

What you want is basically a hybrid...

However, if you don't want to go that route, then lighter projectiles will go faster and be louder.
A shorter barrel will give a louder cannon.
A stronger burst disk will improve all of these characteristics.

Edit: the program you want is [url]HGDT[url=http://www.thehalls-in-bfe.com/HGDT/index.html][/url]

Could someone fix that ^ I can never remember how to do it :@
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Unread postAuthor: Rob748 » Tue Mar 17, 2009 2:38 pm

What about multiple Burst Disks ?, On a over and under, For example a 50 psi burst disk, on the first 90 degree bend, then a 65 -70psi burst disk after the last bend ?. What would be the effects of that ?
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Unread postAuthor: Biopyro » Tue Mar 17, 2009 2:42 pm

That would still fire at 50psi. Giving, if anything a loss in performance. I assume you want over under for convenience, but an inline design will have more power.

Putting both burst disks together in the union will give a better performance (higher pressure), but ideally you'll want something that isn't higher than 50 psi in a 1x cannon.
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Unread postAuthor: King_TaTer » Tue Mar 17, 2009 2:54 pm

As you increase the chamber volume of a combustion you will indeed reach higher velocities. This is also dependent on the volume of your barrel.

If you'd like it to be very loud then your options are a short barrel. On the contrary if you'd like it to be more powerful than your average combustion you'd need quite a long barrel.

Spark gaps only increase performance when they are spaced throughout the chamber. 7 spark gaps clustered together on a tiny circuit board will yield almost no noticeable performance increase. You'd be better off placing a gap in the front, middle, and back of the chamber.

Spark gaps increase the number of places the fuel is ignited in a chamber, allowing more of a "BANG!" rather than a "FOOM!" thus increasing velocity.
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Unread postAuthor: starman » Tue Mar 17, 2009 3:54 pm

7 gaps is overkill, even optimally spaced in a 30" chamber, but if you can get it to reliably fire, more power to you brother.

The shorter the barrel, the louder it will be.

While you are certainly welcome to experiment with dual, multi pressure burst disks, I can't imagine right off the bat what advantage it would offer, especially for the significantly extra trouble it would take to implement and use.

A 50 psi burst disk will still allow pressures of around 100 psi to build up in the chamber and barrel. I have found that playing around with HGDT, you see that higher and higher psi burst disks bring diminishing returns, especially for the trouble they are. Hint hint Rob, if you haven't loaded HGDT yet, drop what you are doing and get that done..... :wink:
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Unread postAuthor: SpudBlaster15 » Tue Mar 17, 2009 4:04 pm

To answer your question, yes, assuming all other factors are held constant, a cannon with a larger chamber and longer barrel will accelerate a projectile to higher velocities.

If you use an ideal C:B ratio, a burst disk will have no effect on performance. Using a disk will give you a higher initial barrel pressure, but the average pressure will be the same as without.

As starman said, 7 gaps if overkill for your 30" long chamber. 2 or 3 would work well in a chamber of that length.
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Unread postAuthor: psycix » Tue Mar 17, 2009 5:14 pm

Is a larger cannon with a burst disk, metered mapp more powerful than a smaller one ? as in the velocities of a projectile? Depends on how long the slug is. Basically, for the same length, the slug will go just as fast in a large diameter or small diameter. (when barrel length is equal)
lets say a 3 ounce hunk of potato ? If the weight is the same, the slug will be longer in a smaller barrel. Larger barrel will shoot faster.
Would the Size make it louder ? Defenitely!


7 gaps are overkill, but having one or two in the elbows will surely help performance on a over/under.
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Unread postAuthor: jimmy101 » Wed Mar 18, 2009 2:22 pm

For sparks gaps, figure you want one every chamber diameter. More than that and you are well into the "diminishing returns" domain. Even with one per chamber diameter you are up near the flat part of the curve.

As to performance, do you want high muzzle velocity or high muzzle energy? They are completely different things and require different designs.

To maximize muzzle energy you probably want a CB in the range of 1.5 to 3. This gives poor efficiency but greater velocities for a given barrel and ammo.

Not sure if I agree with spudblaster.
If you use an ideal C:B ratio, a burst disk will have no effect on performance. Using a disk will give you a higher initial barrel pressure, but the average pressure will be the same as without.

A burst disk that actually works properly will increase the performance of the gun, regardless of the CB ratio. The only time a burst disk doesn't help (again, assuming the disk bursts at a high pressure) is when the mass and/or static friction of the ammo is large.

There is no such thing as an "ideal CB ratio". The best ratio depends on what you are trying to do. 0.8 or so maximizes efficency. CB > 0.8 maximizes muzzle velocity for a fixed ammo and barrel. (Actually, there might be an "ideal CB ratio" for maximizing muzzle velocity, but if it does exists it is probably out at CB's greater than 3.) A low friction + low mass ammo will require a different gun geometry for maximum muzzle energy than would a high mass + high friction ammo.
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Unread postAuthor: SpudBlaster15 » Wed Mar 18, 2009 2:36 pm

By ideal, I mean a C:B ratio that maximizes efficiency. If you have a barrel that is sufficiently long to transfer all of the available energy in the chamber to the projectile, a burst disk will not help performance. It will increase the initial barrel pressure, but the average pressure will be the same.

However, a burst disk will have a significant effect on a launcher with an oversize chamber/undersize barrel. It will raise both the initial and average barrel pressure, which means means greater force on the projectile, greater projectile acceleration, and higher velocity/energy at the muzzle.
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Unread postAuthor: Biopyro » Wed Mar 18, 2009 2:45 pm

I think that a barrel which is long enough to not benefit from a burst disk, will suffer from friction, and variations in the mass of the projectile. Better just to have a burst disk really.

Also, a higher pressure inside the chamber during combustion, increases the burn rate and so, peak pressure. No burst disk means the barell acts as a buffer to the pressure, keeping it low.
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Unread postAuthor: starman » Wed Mar 18, 2009 2:58 pm

SpudBlaster15 wrote:By ideal, I mean a C:B ratio that maximizes efficiency. If you have a barrel that is sufficiently long to transfer all of the available energy in the chamber to the projectile, a burst disk will not help performance. It will increase the initial barrel pressure, but the average pressure will be the same.


Actually if HGDT is to be believed, assuming a ridiculously long 200" barrel and golf ball ammo, the barrel peak pressure in my trip thunder cannons will top out at around 80psi if no burst disk is installed. With my default 65 psi burst disk installed, a peak of 119 psi barrel pressure is produced even with a 78" barrel.

Assuming the barrel length is a reasonable length and all else being equal, a burst disk offers about a 50ft/sec improvement over a best case non burst disk setup.
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Unread postAuthor: SpudBlaster15 » Wed Mar 18, 2009 3:12 pm

Biopyro wrote:I think that a barrel which is long enough to not benefit from a burst disk, will suffer from friction, and variations in the mass of the projectile. Better just to have a burst disk really.


As long as your projectiles are reasonably consistent in mass and static/dynamic friction, there will be no issues with using an 'optimized' setup. From experience, a 0.8:1 ratio combustion gun firing spuds shows no noticeable increase in performance when a burst disk is used.

Also, a higher pressure inside the chamber during combustion, increases the burn rate and so, peak pressure. No burst disk means the barell acts as a buffer to the pressure, keeping it low.


Yes, the peak pressure will be higher when a burst disk is used. However, that's not the issue. To compare setups, we need to look at the average barrel pressure.

Assume we have 2 identical chambers, and 2 identical, efficiency optimized barrels. Both chambers contain the same amount of energy, but one has a burst disk that is designed to rupture near peak chamber pressure. The other relies on the static friction of the spud, about 5PSIG. When the burst disk in the first chamber ruptures, the pressure in the barrel spikes to about 120PSIG. However, as combustion has completed, it only drops from here as the projectile moves and expands the volume of the chamber. The spud in the 2nd gun will begin to move at around 5PSIG, long before combustion completes. However, as combustion continues, the pressure builds behind it, peaking when the spud is somewhere in the middle of the barrel. the result is a more uniform, but identical acceleration. When these 2 projectiles reach the ends of their barrels, the pressure has dropped to the dynamic friction of the projectile. So although the peak pressure in the 1st launcher was much higher, the 2nd launcher had a higher barrel pressure at a certain range of projectile positions, and the end result is identical average pressure values.

Actually if HGDT is to be believed, assuming a ridiculously long 200" barrel and golf ball ammo, the barrel peak pressure in my trip thunder cannons will top out at around 80psi if no burst disk is installed. With my default 65 psi burst disk installed, a peak of 119 psi barrel pressure is produced even with a 78" barrel.


See the above explanation.

Oh, and see these tests, which showed that a burst disk reduces the performance of an optimized (.8:1) launcher.
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