Copper Marble Cannon

[cometboy]

I am sorry in advance if this is a well worn topic, but I would like some help in finding a well documented design for a copper cannon that would fire a marble at 500fps or higher.

I am a high speed photographer. I build a very simple PVC cannon that ran at 80psi and fired a broad range of projectiles.

The images I got are here :

http://www.flickr.com/photos/8763834@N0 ... 669945216/

The velocity of the smaller projectiles is about 300fps.

I am interested in trying a higher velocity, between 500-700fps to see what kinds of photographs I could get. I have searched for a copper cannon that would go to higher pressures, but all the designs that I can find are full of dead links or no replies to Email requests for more information.

I am sorry if I sound lazy, but my primary interest is in taking photographs, not figuring out the best way to build an air cannon.

If anyone can point me to a good source of detailed information, that would be great. I am a careful builder and do not need step by step information. Just decent overall pictures and answers to some questions about hidden details, like piston fabrication and internal dimensions.

Thanks for reading this.

Cheers.

[jackssmirkingrevenge]

I've seen your work before, welcome to the forum!

A simple quick exhaust valve cannon will serve you well.

Some examples:

http://www.spudfiles.com/forums/copperh ... t6013.html

http://www.spudfiles.com/forums/dual-3- ... t4650.html

http://www.spudfiles.com/forums/600-psi ... 17298.html

I believe you're in the UK, several members here can help you source parts. If not, eBay is a great cheap resource.

Also, 80 psi is pretty feeble. A simple track pump will take you to at least double that with ease.

[cometboy]

JSR,

Thanks. I will check out the links that you provided.

I am actually is the USofA. The "cheers" is something I liked and picked up from a friend.

And as far as the copper, I am willing to go to 250psi. The safe pressure rating for larger diameter copper pipe is 300+psi, so that gives me a good margin.

The 80psi was for the PVC. The pressure chamber was old pipe, so I was not comfortable going too high.

I like taking pictures, but I am unwilling to get hurt doing it.

Cheers.

Followup : Maybe I am doing something wrong, but none of the three links you provided show any pictures. I am just not confident in my ability to make a safe and useful gun from just a written description.

[Technician1002]

Either high pressure or a high volume large diameter sabot will do the trick.

This thread shows that both a high pressure or a low pressure high volume both work well for high speed launches well over 500 FPS
http://www.spudfiles.com/forums/flat-aa ... 21827.html

This video shot at 1,000 frames/sec clearly shows speeds near 1ft/sec.
[youtube][/youtube]

[jackssmirkingrevenge]

Maybe I am doing something wrong, but none of the three links you provided show any pictures. I am just not confident in my ability to make a safe and useful gun from just a written description.

Odd... the first one at least is loaded with images :-/

Can't you seethis for example?

[cometboy]

JSR,

Thanks.

I can't see a thing.

So I guess my theory is that the blocking software at work is....blocking.

I'll try at home tonight.

Until then, I'll stop babbling since you may have given me exactly what I need, only I can't see it yet.

Cheers.

[jackssmirkingrevenge]

In a nutshell,this would be the heart of your system.



Using compression fittings, you connect your critical parts.

Chamber (closed off copper tube) attaches to the socket marked A

Barrel attaches to the socket marked R

Your pilot valve (ball valve, blowgun, take your pic) connects to the socket marked P

Easily made using standard fittings, there will be some adapters involved but once you get the concept involved one trip to a well stocked hardware store and you're sailing

[mobile chernobyl]

This gun achieved velocities of at least 500fps with higher pressures and marbles as ammo...

http://www.spudfiles.com/forums/like-a- ... 18612.html

[DYI]

I've also seen your work, and found it quite impressive. Welcome to the forum.

There may be a browser issue involved here, because the Copperhead Prime thread's pictures are all still working. You're probably looking for something more along the lines of a tabletop "lab gun", so an ergonomic design like that of Copperhead would be excessive. You'll want minimal pilot volume, so avoid the long pilot lines typical of the more "ergonomic" cannons. Modifying a fridge compressor can be something of a chore, but ~250psi is readily achievable with good quality track pumps.

I suggest threaded steel pipe (available at any hardware store) and malleable iron fittings (also readily available) for the chamber to cut cost. A QEV is the simplest main valve option, and should perform adequately here. Despite their low pressure rating of 150psi, they are routinely used at 400-500psi, and have been tested over 800psi. The failure mechanism at very high pressure is not rupture of the casing, but damage to the internals. Use of the "blowgun" type pilot valve seen on Copperhead is not recommended for higher pressures, as they are often poorly made. A simple ball valve or brass solenoid valve for the pilot would be preferable. For your purposes, the electric triggering inherent to the solenoid valve may be advantageous.

A large bore and sabot will not be necessary - modelling a half litre chamber with a barrel five feet long and of similar diameter to a marble shows that this design should deliver the performance you're looking for at 250psi (note that GGDT is progressively more optimistic as the trans-sonic speed regime is approached).

I look forward to seeing the resulting pictures.

[jackssmirkingrevenge]

If you're willing to play with combustible gas:

http://www.spudfiles.com/forums/mini-hy ... 21655.html

A relatively simple way to achieve high performance especially if you limit yourself to the tabletop "lab gun" setup DYI alluded to.

This would make what's happening at muzzle and chamber level interesting too, for example:

http://www.spudfiles.com/forums/clear-c ... 22067.html

[Gippeto]

Shameless self promotion.

http://www.spudfiles.com/forums/piston- ... 13478.html

Patched 14mm marbles on 400psi co2...750fps over the chrony.

Patched 14mm marbles on 400psi air...830fps over the chrony.

Actually pretty good for "plumbing parts".

[cometboy]

JSR,

So it was the blocking software stopping me from seeing pictures.

I will spend some time with the various options.

One further question : I was looking at the GGDT program and the example HVPL-01 copper gun. I tried loading the file for the HVPL-01 into GGDT to run it and got completely different velocity numbers.

Is GGDT considered to be a useful program? I kind of lost confidence in it when I could not get numbers that were even close.

Thanks so far for all the help.

Cheers.

[DYI]

GGDT is useful for a certain performance range. Being a 0D simulation, it is not capable of modeling effects such as turbulence, gases which are not perfect thermal insulators, pressure and velocity gradients through the barrel (this one is particularly important), non-inviscid flow effects, heat transfer between the propellant gas and barrel walls or projectile, etc...

Despite this, it proves highly accurate for "conventional" airguns below 200m/s or so. As I mentioned, it degrades somewhat as one enters the trans-sonic flow regime. It can be thought of as modeling a gas which has infinite sound speed and no mass. As a result, in performance regimes where the mass of the gas is significant in comparison to the projectile mass, or the flow somewhere is approaching the gas' sound speed, its outputs become much less realistic. For purposes of modeling the launcher you're considering, its output will be sufficient for design purposes.

[cometboy]

JSR,

I printed out all the various designs that you and other posters kindly linked to.

I purchased a 3/4" valve from Hong Kong.

At this point, my understanding is that high pressure will overcome even a non-optimum design. Even If I only get to 500fps, it will about three times more power than my old cannon, which should give some interesting results.

I may be back to clear up some small points. I'd like to get GGDT giving me results that I can trust.

Cheers.

[jackssmirkingrevenge]

At this point, my understanding is that high pressure will overcome even a non-optimum design. Even If I only get to 500fps, it will about three times more power than my old cannon, which should give some interesting results

Pressure is directly proportional to power, the force on the projectile is the pressure multiplied by the base area of the projectile - so double the pressure and there is double the force available to the projectile.

However, there are other parameters that should be taken into account, namely:

- a fast valve

- a long barrel

- a large chamber

The launchers in this link should provide a good guide to what the proportions should be like. Make the barrel as long as you can afford, and make sure the chamber size is at the very least half the volume of the barrel (rough guide for a high-ish pressure launcher, this can be optimised through careful use of GGDT).