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Shock Pumps

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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Shock Pumps

Unread postAuthor: Mr. Potato Head » Sat Jan 01, 2011 5:38 pm

I'm planing on building a coaxial copper canon and was wondering what is the max recommended volume that can be filled to 400 psi with a shock pump. I have searched around and failed to find an actual number.
Edit: I forgot to mention that I would prefer to have to spend less than 5 minutes pumping per shot.
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Unread postAuthor: saefroch » Sat Jan 01, 2011 6:00 pm

You won't find a definite number, as it depends too much on how fast you pump. It would be better to design the build around a bore diameter, generate a reasonable C:B ratio, then work from there and downsize it so you can fill in a reasonable time. I can't say too much else, but I can fill a 5.4in^3 chamber in about a minute with the second stage of my pump, which probably has about 3 times the stroke volume of a shock pump.

Shock pumps are usually used on smaller builds, larger builds at 400psi you're better off with a fridgy, a homemade pump, or a two-stage.
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Unread postAuthor: Mr. Potato Head » Sat Jan 01, 2011 6:36 pm

Yeah "canon" was definitely the wrong term to use haha. I was initially planing on something with a 1/4in barrel however I visited my local home depot a few days ago and found out that they only have 1/4 copper coiled up for refrigerator use. This lead me to scale it up a bit and I've decided that 1/2in copper will be the barrel and 3/4in copper will be the chamber, both 2ft in length since thats what they sell it cut at. The pilot valve will be a 3/8in ball valve. GGDT gives a chamber volume of 5.89in^3 which might be a bit much. I'll probably end up filling fart of it with something to decrease it a little if pumping it is too much of a hassle. GGDT also says that an airsoft bb would go 2067 ft/s which is awesome cause I've always wanted to break the sound barrier. Don't know if that would actually happen though.
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Unread postAuthor: saefroch » Sat Jan 01, 2011 8:27 pm

Well first off, GGDT becomes very optimistic near and exceeding supersonic muzzle velocities. Can you take a screenshot of the data you put into GGDT? It seems rather optimistic to get a supersonic muzzle velocity at 400psi. Have a look at this thread I posted on the subject when I got a similar prediction.

That chamber volume will probably be manageable with a shock pump.
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Unread postAuthor: Mr. Potato Head » Sat Jan 01, 2011 8:57 pm

Yes I though that might be the case with GGDT. I also realize that an airsoft bb isn't .5in in diameter (it would need a sabot, the weight of which I haven't factored in)
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I'm not sure of some of these numbers such as piston mass, pilot volume, flow coeff. ect.
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Unread postAuthor: velocity3x » Sat Jan 01, 2011 11:37 pm

I've found GGDT to be extremely accurate in my testing.

SCREEN SHOT vs. TEST SHOT:
GGDT predicted 1194 fps
Actual velocity as determined by doppler radar was 1194.22 fps
I think GGDT works just fine.
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Unread postAuthor: Mr. Potato Head » Sun Jan 02, 2011 2:55 am

Hmm...
Thats interesting. Too bad I don't have access to equipment that can measure speeds like that. I guess I could just use a projectile that weighs .575g and see if it produces a sonic boom. According to GGDT it should travel at 1127 ft/s, 1ft/s higher than the speed of sound if I'm not mistaken.

I am a little disappointed at how little muzzle energy I'm getting, especially compared to your beast.
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Unread postAuthor: saefroch » Sun Jan 02, 2011 11:31 am

The low mass of your projectile is the reason for the incredibly optimistic muzzle velocity. Keep in mind that an airsoft BB is only about 1/4" in diameter. Scaling up the airsoft BB to a .5" diameter gives a mass of about .96 grams. Try to use a very accurate estimate for the mass of your projectile, that's a very significant factor in muzzle velocity. If anything like a sonic boom occurs, you probably won't be able to hear it over the muzzle blast, but supersonic is still amazing.

The low muzzle energy is probably due to the low mass of the projectile. Throw in a greater mass value and you should see a significant increase in muzzle energy.
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Unread postAuthor: Mr. Potato Head » Sun Jan 02, 2011 4:41 pm

Yeah i realize this and was thinking of having a light sabot or something like that that wouldn't add too much weight.

I've messed around with different masses and even at 1kg the muzzle energy is only 100.1 ft*lbs. A tenth of that, 100g still produces 98 ft*lbs so i think anything that is more massive than that is just a waste since the fps drops horribly low. At 100g it's only 169 fps :(

This might be a stupid question, but should i be more concerned with muzzle energy or muzzle velocity? It seems to me that with my current set up I can't really have a decent amount of both. Now I'm not really familiar with ft*lbs and how much is a lot, but I think even 100 is low, especially when compared to velocity3x's cannon.
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Unread postAuthor: saefroch » Sun Jan 02, 2011 5:19 pm

Most any sabot will add significant mass to an airsoft BB, unless it's something like an aerogel.

There is little use trying to compare your cannon to the golf ball cannon velocity3x posted data for, because it has a much larger bore diameter (more force on the projectile) and is running higher pressure.

Instead of plugging in random numbers into GGDT for projectile masses, try using it to simulate different projectiles you may plan on shooting. Think up a design, make up some reasonable dimensions, and look up density values to come up with an appropriate mass.

You shouldn't be concerned overmuch with either muzzle velocity or muzzle energy, but try to find some sort of balance. If you were to plug in the data for my Trident into GGDT, you'll find that there is a very significant increase in muzzle energy from a small and achievable mass increase. This suggests I should be launching something heavier for better overall performance.

Ft*lbs is a unit of energy, which comes from the equation for kinetic energy = 1/2*mass*velocity^2. It may be useful to compare your muzzle energy to that of conventional firearms to get an idea of what kind of "power" you'd generate.
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Unread postAuthor: Biopyro » Mon Jan 03, 2011 7:04 am

To answer your original question, almost nothing. I had a coax with about 250cm^3 chamber, and it was hell to fill with a shock pump. A homemade high pressure stirrup pump would have worked a treat though.
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Unread postAuthor: Mr. Potato Head » Tue Jan 04, 2011 12:05 am

@Bipyro: I'm relived to here that even 3 times my volume is manageable with a shock pump, although highly unpleasant.

@saefroch: I was thinking just some wading or something, but I'll have to plan it out some more.

As for simulating rounds, I can just use solidworks to get a pretty accurate prediction on mass. My heaviest current round is 25g, consisting of a nail stuck into a piece of rebar and an attached tail for stability and better sealing in the barrel. GGDT predicts 90.3 ft*lbs and 325 ft/s form this which is ok I guess. I currently use lighter nail darts since my parents think I'll blow myself up if I take my launcher up past 40psi :x
That's one of the reasons I'm building this out of copper. Anyways here are some pictures.
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The brackets conecting the barrel to the chamber broke and I haven't gotten around to making new ones. The union is there to act as a breach, with the brackets in the barrel just slides forward and then back. Also the primer job is horid, but o well it was my first.
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