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 Post Posted: Sun Apr 15, 2012 2:52 pm 
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Joined: Sun Apr 08, 2012 1:13 pm
Posts: 3
OK, now that I have decided upon a single valve, I need to figure out my parts.

I am sure I can get most of the PVC fittings here in Alaska, but will get the valve and adapters at the Spudtech Part Store.

To keep the cost/weight down I will go with a simple push-button version.

Is there a formula for figuring the best size pressure chamber to barrel length combo?

I see the 2.5" barrels are offered in 3', 4' and 4'10" version.

I'm thinking to go with the 3 footer.

I know with firearms the barrel length should ideally allow all of the propellent to burn before the bullet exits the muzzle. I am assuming that the same somewhat holds true with a pressure chamber.

In other words, if all of the air has not transferred from the pressure chamber before the ball exits, it would just be a waste of a larger chamber and pumping. Is this an accurate statement?

Thoughts?


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 Post Posted: Sun May 06, 2012 6:57 pm 
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Joined: Wed May 02, 2012 12:33 pm
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What did you decide to use for a valve?? And usually, your barrel/chamber ratio should be about 2 to1. So, if you use a 3 inch by 2foot chamber, your barrel should be 4 feet of 1 1/2 inch, understand??

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 Post Posted: Mon May 07, 2012 3:33 pm 
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Anything between 2:1 to 4:1 is good for pneumatic launchers. Anything over 4:1 and the gain you receive is so small it is not typically worth the size increase.
The chamber pressure drops as the chamber volume increases. As the projectile moves down the barrel everything behind it is consider chamber volume.
When the projectile is at the very end of the barrel the chamber pressure has decreased because the chamber volume increased.
Check out Boyle's law if you want to learn more.

When the projectile leaves the barrel the excess pressure is released into the air and that is what makes the report(bang).

A rough way of looking at launcher design is:
Barrel length determines the time the projectile can accelerate. Longer barrel = More potential acceleration.
Ratio determines the efficiency(pressure at the end), report and launcher size. Higher ratio = Larger size = More waste/report(higher pressure at the end) = Higher muzzle velocity for the projectile.

Typically you will try to find the right balance of size and performance for you.

Here are some rough chamber to barrel ratios for 2.5in barrel 3ft long. They only consider the pipe volume not any fittings.
Ratio - 3in Chamber Lengths
.75:1 - 17.4422726795
1:1 - 23.2563635727
1.5:1 - 34.8845453591
2:1 - 46.5127271454
3:1 - 69.7690907181
3.5:1 - 81.3972725045
4:1 - 93.0254542908

Ratio - 4in Chamber Lengths
.75:1 - 10.0980126757
1:1 - 13.4640169009
1.5:1 - 20.1960253513
2:1 - 26.9280338018
3:1 - 40.3920507027
3.5:1 - 47.1240591531
4:1 - 53.8560676036


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 Post Posted: Tue May 15, 2012 11:20 am 
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Joined: Tue May 15, 2012 11:05 am
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C:B ratios can often be misleading. As opposed to talking about the general case, which is a hopelessly vague approach, you would be better off to use GGDT to model the approximate performance of your launcher. It is a 0-D launcher model (which is to say it essentially models a massless gas with infinite sound speed) and works by "moving" gas through the valve from the chamber to barrel. It has been found to provide satisfactory results up to about 2/3 the speed of sound in air. Whether this is true of other, lighter gases has not been investigated.

I also think it prudent to point out that Boyle's Law models an isothermal expansion process, which is far from the case in a pneumatic launcher. The process occurs so rapidly that, if one seeks to avoid modeling heat transfer, it is in fact better to approximate the process as adiabatic (which is to say P<sub>initial</sub>V<sub>initial</sub><sup>γ</sup> = PV<sup>γ</sup>). Constructing any model more advanced than GGDT is quite difficult, and requires a decent background in computational linear algebra and CFD. For most pneumatics though, the 0-D model is quite sufficient.

In many cases when working with a fixed chamber size, you will find that the "ideal" barrel length is ludicrously long - tens of feet in some cases. This is one of the cases where GGDT is so handy - it allows one to determine easily where the increased performance from lengthening the barrel begins to be outweighed by the decreased mobility and accuracy inherent in long, droopy barrels.


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