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Increasing velocity

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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Increasing velocity

Unread postAuthor: john bunsenburner » Sat Dec 20, 2008 4:47 pm

I just got GGDT to work on my PC and put in my gun specs and i am horrified at the results! Ill give you all the specs, maybe some one can help me out. I changed the barrel length because GGDT said so but what ever number i typed in the velocity did not change. To my guns specs(all in metric):

Barrel: 100cm long Aluminium pipe with a ID of 2.4cm and a OD of 2.5cm
Chamber: Iron pipe with two end caps, Volume of 41cm3
Valve: Hammer valve(explosive air release) made from 25cm of 0.8cm copper pipe
Projectile: Lead balls/pellets weight still to be decided
Pressure: 200psi


I hope that this is all the info you need, if not say so. Please give me some recomendations of how i could increase projectile velocity(which is now 59fps). the only thing i build yet is the valve and barrel, the chamber can still be modified.
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Unread postAuthor: VH_man » Sat Dec 20, 2008 4:51 pm

what is the hammer mass and hammer spring strength/distance?

That will make all the difference. That and increase the diameter of your valve a little bit.
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Unread postAuthor: john bunsenburner » Sat Dec 20, 2008 4:54 pm

The spring is 10cm long and is pushed about 6cm back. I am unsure of the acctual force exerted. And the hammer is activated by my hand (i said i made a new trigger mech) which mayb make things more complicated...
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Unread postAuthor: Pilgrimman » Sat Dec 20, 2008 5:30 pm

Did you enter 2.4 for the ID? That entry is meant for coaxials. That might be the problem right there.
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Unread postAuthor: ALIHISGREAT » Sat Dec 20, 2008 5:31 pm

a hammer valve is a low powered valve when compared to a piston as it releases a small amount of air compared to a piston... that is why your power is bad... so if you want good power... you'll need to up the pressure...
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Unread postAuthor: btrettel » Sat Dec 20, 2008 5:33 pm

If you know the dimensions of the spring and its material, approximating the spring constant isn't too difficult. Here's a calculator I found quick in Google: http://www.efunda.com/DesignStandards/s ... signer.cfm
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Unread postAuthor: cheeseboy » Sat Dec 20, 2008 5:58 pm

the most likely problem you have is that you have entered the inner diameter of the chamber, unless you are building a coaxial the inner diameter is zero this is pretty much what pilgrimman said except he said the barrel inner diameter.
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Unread postAuthor: scavies » Sat Dec 20, 2008 6:28 pm

Wait. Is air going from .8cm to the 2.4 cm barrel? That's about a 3:1 ratio, so you can expect expansion to slow down 3x after it leaves that part.

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Unread postAuthor: Ragnarok » Sat Dec 20, 2008 7:39 pm

Have you had the chamber OD/ID confusion?

Outer diameter should be the ID of your chamber pipe. (Slightly counter intuitive, but that's how it is)
This is because some cannons have barrels running through the chamber, which obviously take up chamber volume.
ID is the outer diameter of any pipe that may be running through the chamber. I'll assume you don't have one, so set this to zero.
Hence, OD is the which is the inner diameter of the outer pipe.

Also, that's a very long and high diameter barrel for a hammer valve cannon. I suggest bringing it back to no more than half that diameter. (More like 1/2", not 1")

You also can't get a meaningful velocity reading if your projectile mass is unknown, nor your hammer mass, spring or draw distance. Work them out.

Fix those issues, then you might get the right numbers.

@Scavies: No, because it's area, not diameter. That makes it a 9:1 ratio.
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Unread postAuthor: john bunsenburner » Sun Dec 21, 2008 12:27 am

Ok, then ill forget about shooting snow and ill just use bbs as bullets. Ill take a 6mm ID ali tube as a barrel, i hope that will solve the problem. Thanks alot all of you.(oh and i had the ID/OD confusion!)
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Unread postAuthor: john bunsenburner » Fri Dec 26, 2008 5:23 pm

I just got to NYC and have now gained internet access. After playing around with GGDT during the 8 hour flight with united and enjoying their lovely food I noticed that if i use a barrel suitable for pellets (.45mm diameter) then i get a velocity of around 180m/s(590fps). This seems like a very large number but here big is better(obviously the builders of NYC had the same idea-looks out the window). I have three questions now:

-Is this velocity realistic or is something wrong/messed up?
-The diameter of the hammer is 8mm meaning the flow is chocked, is this bad?
-A copper pipe with a ID of 4.5mm is not too stable, i wondered if i could put the small pipe into the 15mm pipe and then pour something around it to hold it in place, what meterial would be suitable for pouring?
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