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Barrels...

A place to ask general spud cannon related questions.
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Barrels...

Unread postAuthor: octane89 » Sat Dec 01, 2007 5:28 pm

Barrel diameter and lengths, as we know, have a critical impact on how a cannon may perform. However, I was wondering on this fact. Does the barrel's ID have an impact on the "compression" of a cannon firing.
To try to be more specific, I was thinking of making a advanced combustion, and was hoping to use a short barrel (for something different and look), and even thru the idea of a 3" barrel around. I like to use at least a 2"- a 1.5" would cause the barrel to be fairly long and not as portable as I would like.
Just wondering what everyones take on this would be or if they have had expierences doing this.
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Unread postAuthor: BigGrib » Sat Dec 01, 2007 5:46 pm

how long is your chamber gonna be what diameter
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Unread postAuthor: octane89 » Sat Dec 01, 2007 6:43 pm

Well Im still figuring out what I will have exactly, but I will say 3"ID and 20" long. But that doesn't seem to have much to do with my question if barrel ID has effect on how the cannon performs, given the constant is the ratio of 1:1.
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Unread postAuthor: DYI » Sat Dec 01, 2007 8:37 pm

Combustions are extremely complex compared to pneumatics. Someone like SB15 or Jimmy might have a better idea than me, but I can say that, with pneumatics, smaller ID barrels are better with lighter projectiles and higher velocities, and larger diameter barrels (to a point, of course), are better with heavier projectiles. For combustions, the barrel ID is essentially the valve ID, so as long as the barrel isn't larger ID than the chamber there shouldn't be any problem. Theoretically, the most efficient combustion would be a single tube, with a burst disk and the projectile placed in the right spot along it's length to give the proper .8:1 ratio.
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Unread postAuthor: rna_duelers » Sat Dec 01, 2007 9:07 pm

http://www.burntlatke.com everything you need to know about combustions.

And as to how long the barrel is,the longer the barrel the faster the projectile is going to go to an extent,so a projectile that weighs say 50 grams but is 50mm diameter shot out of a large barrel that is 1 meter long would be going slower then a 50 gram projectile out of a barrel with the same volume but with and ID of 25mm and 2 meters in lenght.
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Unread postAuthor: Ragnarok » Sat Dec 01, 2007 10:06 pm

DYI wrote:Theoretically, the most efficient combustion would be a single tube, with a burst disk and the projectile placed in the right spot along it's length to give the proper .8:1 ratio.

Actually, not quite true, in that case it gets more complicated.

A 0.7:1 ratio is most efficient in a single tube launcher for any given length. Not the most powerful (that would be 1.25:1) but most efficient.
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Unread postAuthor: Hubb » Sat Dec 01, 2007 10:12 pm

Ragnarok wrote:
DYI wrote:Theoretically, the most efficient combustion would be a single tube, with a burst disk and the projectile placed in the right spot along it's length to give the proper .8:1 ratio.

Actually, not quite true, in that case it gets more complicated.

A 0.7:1 ratio is most efficient in a single tube launcher for any given length. Not the most powerful (that would be 1.25:1) but most efficient.


Is there anything to back this up? Not trying to say you're wrong because I don't know either way. I'm just wondering if there is any evidence or is this just theory.
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Unread postAuthor: rna_duelers » Sat Dec 01, 2007 10:18 pm

The most efficient combustion chamber is a sphere,not a tube due to the flame front and how it moves.
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Unread postAuthor: boilingleadbath » Sat Dec 01, 2007 11:04 pm

Are you sure that a spherical combustion chamber is superior?

Compared to a cylindrical chamber, a center-ignited spherical chamber would probably take longer to transition from laminar to turbulent flow - and turbulent flame-fronts move much faster.
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But that's kindof off-topic.

Yes, there is evidence that different barrel diameters yield different performance, but it's a fairly small effect, and our dataset is pretty weak, so it's hard to tell how big it is.

There is a theoretical case for such behavior: a skinnier barrel will be longer for the same volume, so the projectile will move through a smaller percentage of the barrel length before the chamber pressure peaks, if it has the same sectional density. (it moves the same distance, but the barrel is longer)

Of course, one could just use a heavier projectile in the larger bore, and get the same result... which leads to the evidence for this behavior: the 1.5" C:B tests used projectiles with roughly twice the sectional density of the projectiles in the other tests, and generated several percent more energy than would be expected for the launcher, based on the performance of the other launcher tests.
****************

And it's entirely possible that this extra energy is just due to lower friction of potatoes.
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Unread postAuthor: Ragnarok » Sat Dec 01, 2007 11:18 pm

hubb017 wrote:Is there anything to back this up? Not trying to say you're wrong because I don't know either way. I'm just wondering if there is any evidence or is this just theory.

I'm just recalling these calculations done a while back:
http://www.spudfiles.com/spudtech_archi ... hp?t=14571

It's theory, but theory with numbers behind it.

The funny thing about that was that although the 0.65:1-0.7:1 is most efficient for any given length of fixed volume launcher, chopping a bit off the barrel to make it 0.8:1 might well make it more powerful. It's a curious quirk of the way that increasing barrel length decreases chamber size (and thus energy in the fuel).
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Last edited by Ragnarok on Sat Dec 01, 2007 11:35 pm, edited 1 time in total.
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Unread postAuthor: octane89 » Sat Dec 01, 2007 11:23 pm

boilingleadbath: That helps actually quite a bit with my question. I want to be able to use a larger bore barrel to reduce the overall length, but I didn't know whether it would sacrifice my performance, even the volume could be the same for a smaller barrel.
Out of curiousity, would a potato be considered to have enough sectional density?

rna_duelers- On how the flame moves, I can't remember who but it was in the archive when I waws researching some stuff. But he broke the sound barrier (with a MAPP injected cannon), and described on one theory he had on why it performed better. He said it was because the ignition was in the back of the chamber, allowing the flame to ignite completly and move in a forward motion. Other theories were based on the MAPP gas, and the fan (about how it moved enough air to cause turbelence). The cannon was called the Crusader I think. A little off topic but I'd thought I share.
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Unread postAuthor: boilingleadbath » Sat Dec 01, 2007 11:54 pm

Ehh, should have defined my terms.

Sectional density is the mass of the projectile divided by its frontal area.

If potatoes were uniform sheets, all cylindrical sections that could be cut perpendicular to the face of the sheet would have the same sectional density, because they would have the same thickness.
************

You can probably consider differently-sized slugs cut from the same potato to have fairly uniform sectional density. It's not quite true (larger slugs will have a lower average thickness, and thus a lower sectional density), but it should be close enough for this comparison.
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Unread postAuthor: octane89 » Sat Dec 01, 2007 11:59 pm

I see what you meant now-I was obviously thinking about their mass, not so much there frontal area.
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Unread postAuthor: jimmy101 » Sun Dec 02, 2007 11:30 am

Boilingleadbath made several important points. A spherical closed chamber burns faster than any other shaped closed chamber. But a spudgun isn't a closed chamber.

A laminar flame front moves very slowly, only something like 15"/sec at atmospheric temperature and pressure for propane + air. There appears to be a pretty significant acceleration in the flame front speed when the projectile starts to move since this starts the gases in the chamber moving. Doesn't take much movement of the unburned gases to get the gas flow to switch from laminar to turbulent. The turbulent gas flow would be expected to give a turbulent flame front which progress much faster than a laminar front. ICE engines appear to take advantage of this affect to get very fast flame front speeds in the swirling fuel+air mixture in the cylinder. (In addition, the fuel + air in an ICE is considerably hotter before it is even ignited than in a spudgun. Flame front speed goes up exponentially in hot gases.)

As to the exact relationship of barrel ID to ideal C:B, I agree with Boiling. Lots of theories, very little hard data.

One thing to consider is the relationship between the mass of the round and the barrel cross sectional area. The force on the round is proportional to the barrel area. The barrel's area scales as the square of the barrel diameter. The acceleration of the round depends on the area and the mass of the round. But the mass of your typical spud ammo scales roughly as the diameter of the barrel, not the square. If you double the diameter of the barrel you get 4x the area and 4x the force accelerating the spud. The mass of the spud only goes up by a factor of 2. So, doubling the barrel diameter gives twice the acceration. But, doubling the area of the barrel means that the pressure drops faster as the spud moves through the barrel. Trying to figure out which is more important would be an intersting study. (I wonder if GGDT could be used to compare the affect of changing the barrel id with a constant chamber volume and projectile mass?)

The above only applies to spud ammo, spuds are unique. The length of the ammo is typically independent of the barrel ID. For other ammo, for example a rubber ball, the mass of the ammo scales as the square of the barrel diameter since you would need to use a larger ball for a larger ID barrel. Doubling the barrel diameter gives 4x the area and 4x the mass of the shell. So, the acceleration would be the same. But the larger ID barrel would give a faster pressure drop as the ball moved through the barrel.

One last thought, the smaller the ID of the barrel the greater the surface area to volume ratio is. That means a small ID barrel looses heat faster than a larger ID barrel if the two barrels have the same total volume. Heat loss appears to be a pretty important detrimental affect in combustion spud guns.

So there may or may not be a significant affect of changing the barrel ID (but keeping a constant CB). I would think that only hard data can really answer the question.
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Unread postAuthor: octane89 » Sun Dec 02, 2007 12:24 pm

What I will end up doing is using a interchangable barrel system and record my data. Since my cannon will be the constant, the barrel will change lengths (and keeping the same C:B ratio) to see the varying results. I can also "match" spuds by their weight and hopefully I will finish this soon. I will record as much as I can: muzzle energy, projectile acceleration (using GGDT perhaps), distance, etc...
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