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barrel length

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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barrel length

Unread postAuthor: Major Collins » Mon Mar 21, 2011 12:18 pm

what is the optimal barrel length for power and accuary , im not too sue if it matters but the projectile is is roughly 17 mm ,about the circumference of your average marble
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Re: barrel length

Unread postAuthor: jackssmirkingrevenge » Mon Mar 21, 2011 12:30 pm

Major Collins wrote:what is the optimal barrel length for power and accuracy


How long is a piece of string?

Optimal barrel length depends on projectile weight, valve efficiency, chamber volume, pressure etc.

In general, a longer barrel gives higher velocity, which means shorter time to the target, less time for the projectile to be affected by environmental factors and gravity and flatter trajectory, reading to better accuracy.

On the other hand, a longer barrel means longer "lock time" - a bigger time interval between you pulling the trigger and the projectile leaving the muzzle, giving the potential for inaccuracy as the launcher would have moved in that interval.
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Re: barrel length

Unread postAuthor: velocity3x » Mon Mar 21, 2011 12:38 pm

jackssmirkingrevenge wrote:How long is a piece of string?


Measured vertically or horizontally?
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Unread postAuthor: saefroch » Mon Mar 21, 2011 2:58 pm

We've gotten questions like this from other members, and the correct response is pretty much exactly what Jack wrote.

Though Technician1002 probably has something to add, but whether or not it'll be useful to you is another matter. Apparently he's actually found that in some cases a barrel too long is a hindrance, for reasons other than what Jack said.
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Unread postAuthor: Secret Squirrel » Mon Mar 21, 2011 3:52 pm

As others have already said, many factors come into play when finding an optimum barrel length. In a pneumatic, there is only so much air in the chamber, and if that air cannot expand enough to fill the entire barrel, the projectile will actually be slowed as a vacuum forms behind it.

If you really want to find the optimum barrel length for your launcher play around with GGDT. It even has an optimization feature that calculates the optimum barrel length for you.
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Unread postAuthor: saefroch » Mon Mar 21, 2011 5:22 pm

The optimum barrel length calculator I think just runs out the barrel length until the projectile stops accelerating, though there may be other factors that come into play. *cough*Technician1002*cough*
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Unread postAuthor: Crna Legija » Mon Mar 21, 2011 7:04 pm

a 1-1 or 1.2 -1 chamber/ barrel ratio and you'll be sweet.
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Unread postAuthor: Technician1002 » Mon Mar 21, 2011 9:31 pm

saefroch wrote:though there may be other factors that come into play. *cough*Technician1002*cough*


OK.. Link

https://inteltrailblazerschallenge.wikispaces.com/Barrel+length+trim+method

The best barrel is shorter than simply a CB ratio may indicate.
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Unread postAuthor: jackssmirkingrevenge » Mon Mar 21, 2011 10:58 pm

Technician1002 wrote:The best barrel is shorter than simply a CB ratio may indicate<sup>*</sup>


<sup>*</sup>for a given projectile weight and friction
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Unread postAuthor: Technician1002 » Tue Mar 22, 2011 12:27 am

More info on flow losses in a spud launcher is on this page..
http://www.crazybuilders.com/item.php?id=000002&type=project_section
Look about 1/2 way down the page.

Designing to minimize losses and maximize flow make the difference between an OK launcher and an outstanding one.
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Unread postAuthor: DYI » Tue Mar 22, 2011 10:25 am

Power is the rate of doing work, F · V ,and the force here is simply the projectile base pressure multiplied by the barrel's cross sectional area (as the force and velocity vectors are parallel, the expression for power is simply the scalar product P = ||F||*||V|| ). Unfortunately, as projectile base pressure and velocity need to be found through the solution of what is, in the very simplest case, a system of three nonlinear partial differential equations in 4 variables, coming up with this power optimizing length is not a simple task. GGDT solves this system numerically, but is not open source, and thus is not adaptable to such tasks.

To optimize your launcher for power, simply find where the expression for power given above is highest, and cut the barrel off just after that. I am, however, left wondering what would possibly motivate you to want to do this, as power is an instantaneous quantity and says almost nothing about the muzzle energy or velocity the gun will achieve, or any other useful quantity. Perhaps you meant you wanted to optimize average power?


Extending to the last section of your question becomes more difficult - optimal for power AND accuracy? Accuracy depends on barrel tolerances, mounting, aiming systems and projectile choice. Assuming all of those are done well, higher muzzle speed increases accuracy for the reasons mentioned by other posters, and the highest muzzle speeds for a chosen projectile result from optimizing the launcher's muzzle energy, not the launcher's power.
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Unread postAuthor: btrettel » Tue Mar 22, 2011 4:30 pm

As has been said, "it depends" is the best answer you can get in general. Just do some tests and/or use GGDT.

You can do some algebra with adiabatic process relationships to calculate approximate ideal CB ratios as a function of barrel friction and starting pressure, but to be honest, unless you know what you're doing and what assumptions are made, you really shouldn't use this.

As for actually writing a simulation, for the low speed case (which most spud guns fit), solving ODEs is plenty adequate (see here). GGDT solves a system of ODEs, not PDEs, to the best of my knowledge. Solving systems of non-linear PDEs is not trivial, especially when the boundaries are moving. The easiest approach seems to be the Lagrangian approach detailed in this book for the curious.
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Unread postAuthor: DYI » Tue Mar 22, 2011 11:17 pm

As for actually writing a simulation, for the low speed case (which most spud guns fit), solving ODEs is plenty adequate (see here). GGDT solves a system of ODEs, not PDEs, to the best of my knowledge.


You know, that might help explain how spectacularly bad GGDT is for high speed flows. However, when I wrote my own toy program a while ago making the same assumptions you do, its performance diverged markedly from that of GGDT at the speeds where compressible effects became important, indicating that GGDT may not be quite as primitive as you suspect. If I were to guess, I'd say it's a quasi- 1-D PDE system solver which loses high speed accuracy due to its lack of boundary layers and shockwave formation on contact with breech parts and the barrel wall. There may also be some problems with the adaptive grid generation, or perhaps it takes a "best guess" approximation of the finite difference solution without one. "Solving" the system directly in the x,t space rather than transferring it into a proper computational space where the steps are constant could be inducing errors like the ones we see.

I was going to contest your assumptions of negligible gas momentum and KE, but after some quick calculations it seems that most of the launchers built here are still low enough performance to get away with that assumption. Assuming constant pressure and temperature throughout the barrel, however, is downright wrong. Even at midrange pneumatic speeds, it just doesn't work like that. Compressible effects are becoming noticeable at even 100m/s, very slow for a modern pneumatic, and at between 200 and 300m/s simply cannot be ignored if reasonable accuracy is desired.
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Unread postAuthor: warhead052 » Wed Mar 23, 2011 10:34 am

I have no experience with pneumatic guns, but my idea is about 12 inches long. I have a valveless rocket that is almost 3 feet long, the air chamber its self is 1ft 5in, and it is 1 1/2 inches round. I use a 1/2 inch barrel and it is about 1 foot long, but I only use around 6 inches of it (because of the way I had to make the sealing bubble...). I am going to buy a coupling for it eventually to make the barrel much longer, big enough to fit a spliced 2 liter bottle. But to the point, my gun is fairly accurate with a diet coke 20oz bottle, and it can hit a target about 100 feet away at only 50 psi.
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Unread postAuthor: Technician1002 » Wed Mar 23, 2011 11:39 am

A shorter barrel works well with higher pressure and a small chamber. Due to the short distance breech to muzzle, the travel time is short. A fast valve is more important with a short barrel, than the long barreled cousins.
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