Pneumatic Efficiency

[rcman50166]

Hey, all. Been a while since I posted. Anyway, a quick search yields no results for the efficiency of a pneumatic cannon. According to other sources, combustion yields an efficiency of around 15%. This percentage is the efficiency ratio of the chamber energy vs. muzzle energy. The reason why I ask is I've been doing some math and am getting some really strange answers. I check my math at least 3 times per equation. The cannon in question is a pneumatic coaxial cannon with a comparitivaley (sorry, spelling is bad because of a headache I have) small chamber. With ideal physics (no drag, 100% efficiencies) I've been getting a muzzle velocity of a .6kg projectile at 400fps. Strange eh? The muzzle energy is calculated to be close to 4500 J @ 250PSI. This is for a coaxial piston pneumatic that is designed not to be more than 3 ft long. I know you'll need more information to make a proper analysis but my head is pounding right now. (I wince every time my heart beats) Anyway does anyone know what efficiencies are lost to things like friction, blow by, and isovolumetric reactions? Any help is much appreciated.

[Jared Haehnel]

I don't know all the things involved but a big portion is what type of valve you use...with that said what kind of valve are you using?

[boilingleadbath]

Unless you are just doing the calculations for the lolz... I'm going to recommend you use the GGDT.

In the case that you are doing it just to do it:
1) Your projectile is heavy and muzzle velocity is low. With any type of sane valve you can probably convince yourself to ignore opening time and pressure loss across the valve breach.
2) Friction is ignorable
3) Blow-by is either really hard to calculate or really easy, depending upon how you are modeling the expansion of the propulsive gas. It's best to just eliminate it anyways...

[rcman50166]

Believe it or not I do the calculations for fun. Anyway I was planning to cross reference with GGDT but I haven't had the chance. I could do it now actually. Anyway, pneumatic cannons aren't really my domain so forgive me if I sound like an idiot.

The cannon type is a coaxial piston. The same kind that is used it JSR's pen gun. The chamber is around the barrel and the pressure differential keeps the piston closed until the back side is vented sliding the piston away from the barrel introducing it to the chamber due to a new pressure differential between the chamber and the back side of the piston. Pretty common design. Anyway, calculating only from known measurements of the gun and it's operating pressure I calculated all the way down the specifications list up to it's muzzle energy. The numbers seemed likely until I hit muzzle velocity and energy. 400fps and 4500 J of muzzle energy. .6kg is extremely heavy for those kinds of velocities. So I'm wondering what are typically muzzle energies of pneumatics and what kind of flow restrictions should I expect with an airgun vented piston valve? The way I have it calculated now is completely skipping over flow restriciton and calculating the potential energy of a given amount of pressurized gas.

BTW these are preliminary designs for a cannon that I can rediculously overprice (on ebay) based upon it's aesthetics and performance. A pneumatic is far from certian in my future.

EDIT: wow, I feel like such a noob. There are a lot of terms I don't know, like where the dead volume on a coaxial pneumatic would be and what a seat and flow coefficient are.

[rcman50166]

Well, the forums are very busy... BUMP!!!!

This post wasn't entirely useless look, I scanned my math to show what I'm talking about!