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Pneumatic air cannon without proyectile

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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Unread postAuthor: jackssmirkingrevenge » Mon Sep 07, 2009 2:38 pm

Does it have to be air that transfers the energy? Can you use the air to power a piston that physically smacks into the can, without shooting off itself?
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Unread postAuthor: suburban spudgunner » Mon Sep 07, 2009 2:39 pm

Hotwired wrote:
suburban spudgunner wrote:To throw a can from 20 feet (empty, I presume), you might want a somewhat large chamber, decent pressure (100+ psi), and a long, thin barrel to concentrate the pressure.


Concentrate the pressure on what? A very small point of the can?

If you have 100psi coming out of a 20mm valve port into a long 5mm barrel you don't increase the pressure but what you have done is decrease the force as the air blast is now a thin 5mm jet instead of the larger blast from the 20mm opening and thus transfers a lot less energy to the can.

If the cannon cannot fire a projectile then you do not want to use a barrel. You may want to use a nozzle of some kind to manipulate the blast but a barrel is for acceleration of a projectile and since you can't have it in the barrel... forget the barrel.

If this is to have repeatable results you may also want some kind of holder in front of the valve port/nozzle for the can, perhaps a trough made of half a pipe or something.


Okay, I've been proven wrong. It was late and I had a brainfart. Ignore my previous statements.
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Unread postAuthor: tomthebomb137 » Mon Sep 07, 2009 3:18 pm

As stated before, those vortex cannons do not displace enough air to knock anything over at 20 feet. Also keep in mind, the farther you go, especially with unstabilized air (air connon muzzle blast), the force will decrease exponentially, not linearly. So if you have twice the nessisary force to knock the can over at 10 feet, it does not mean you will have enough force at 20 feet.
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Unread postAuthor: jarm0591 » Mon Sep 07, 2009 3:51 pm

Does it have to be air that transfers the energy? Can you use the air to power a piston that physically smacks into the can, without shooting off itself?


I can use a piston, but I have to smack the can with the air.


I saw a video, can I put in a pneumatic gun a barrel like the one of the first gun in the video (the little one, in the second 10)?
http://www.youtube.com/watch?v=hDx-mQqzYNw
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Unread postAuthor: POLAND_SPUD » Mon Sep 07, 2009 6:57 pm

wow I haven't seen this video...it worked with a simple combustion...

so you can either build a combustion cannon like the one he used (if they are allowed) or experiment with a pneumatic...

I can't really think why a pneumatic won't work here as it seems it's all about the right shape of the 'barrel'...

if the above assumption is right you should go with a pneumatic cannon... you wouldn't even need a piston valve but just a burst disk put in an union...

jsut a leanght of 1.5 - 2" malleable iron pipe, union, a couple of reducers to connect an air hose and a ball valve to control filling of the cannon and that's it.. oh yeah you would need a barrel too
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Last edited by POLAND_SPUD on Mon Sep 07, 2009 7:03 pm, edited 1 time in total.
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Unread postAuthor: Technician1002 » Mon Sep 07, 2009 6:59 pm

tomthebomb137 wrote:As stated before, those vortex cannons do not displace enough air to knock anything over at 20 feet. Also keep in mind, the farther you go, especially with unstabilized air (air cannon muzzle blast), the force will decrease exponentially, not linearly. So if you have twice the necessary force to knock the can over at 10 feet, it does not mean you will have enough force at 20 feet.


With a proper vortex cone nozzle on an air cannon, the energy in a vortex blast can travel with fairly high energy over considerable distance. A straight barrel as you mentioned does lose energy rapidly with distance. I am considering building a cone barrel for my little launcher to test this.
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Unread postAuthor: geardog32 » Mon Sep 07, 2009 7:07 pm

i want to build one also. what does everyone think of this design. (not trying to hijack the thread).
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Unread postAuthor: POLAND_SPUD » Mon Sep 07, 2009 7:07 pm

@tech
I am thinking about the same thing right now... if it wasn't for this thread I wouldn't even realise it is possible (if it is :wink: )

now we just have to convince D hall to put one on his :shock:

EDIT
have a look at 2:24 and 1:25 on that vid... the cannon isn't even sealed... lol that's high-tech (but not for an average spudder 8) )
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Unread postAuthor: Technician1002 » Mon Sep 07, 2009 7:18 pm

geardog32 wrote:i want to build one also. what does everyone think of this design. (not trying to hijack the thread).


I would eliminate the smoke chamber and directly drive the cone from the piston for higher energy in the vortex to knock things over.

If you put in a little cologne, you might be able to selectively surprise some ladies. Who needs a viewable ring?

My brain hurts. :roll: I was trying to find what I could about high speed gas flow that I would need to design the flow shape from the chamber thought the valve and into the expansion zone to release a blast of air without losing much energy. I came to a NASA site on rocket nozzle design. Wow, this stuff is rocket science.

This page covers some of the stuff, but nobody but devout engineers will understand all of it.

http://exploration.grc.nasa.gov/education/rocket/isentrop.html

Edit; Testing was in order. I took a few minutes this morning to run a quick test. Results posted below. I had just returned cans, so I didn't have enough, but I did have a collection of bottles. :D

All shots were taken with the end of the barrel at the edge of the cement.
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Attachments
Blast Tank.jpg
Bare tank blast zone. I held the tank a little high and missed the closest bottle. Valve outlet is 1 inch.
Blast Barrel 1.jpg
Barrel test 1 long 1 inch barrel I use to shoot gumballs and jawbreakers.
Blast Barrel 2.jpg
Barrel test 2. Sorry I forgot to put it in the picture. This one blasted a bottle into the air and over several bottles and knocked one over in the back. This barrel is my red 1-1/4 inch marshmallow barrel.
Blast Barrel 3.jpg
Test barrel 3. A short barrel that didn't do too bad. This is a 1 inch barrel.
Blast Barrel 4.jpg
The barrel that shouldn't have had high velocity. It did a very nice job. Maybe this thing will work well with a nozzle cone. Still didn't reach 20 feet. Barrel ID is 2.7 inches.
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Unread postAuthor: geardog32 » Tue Sep 08, 2009 11:19 pm

i revised the drawing. im thinking for the coke if i can find some sheets of plastic to make a basic cone then maybe doing a fiber resin shell. any other ideas for cone material?
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Unread postAuthor: Technician1002 » Wed Sep 09, 2009 1:00 am

geardog32 wrote:i revised the drawing. im thinking for the coke if i can find some sheets of plastic to make a basic cone then maybe doing a fiber resin shell. any other ideas for cone material?


I'm thinking along the lines of galvanized iron ductwork. Tin snips will be needed to cut. The seam will need folded. Check with an AC guy. Then the narrow end can be soldered or brazed to a copper fitting. For experimentation, Poster board and duct tape comes to mind.

I have been studying more on rocket motors in the quest for supersonic and some of what I am finding makes sense.

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http://en.wikipedia.org/wiki/File:De_laval_nozzle.svg

Do not copy these dimensions. This nozzle is for very high temperature and pressure. A low pressure pneumatic nozzle will be much more like a straight pipe.

From the NASA site I was studying, the combustion chamber is subsonic, but high pressure and high temperature. The neck is Sonic.. Mach 1 for the gas temperature. So far that's not too hard to understand. This is the yellow zone.

The tail cone is where it gets interesting. The gas cools as it expands. it's already burnt. It also goes supersonic in speed. :D The pressure at the outlet of the cone X area is thrust. This large area provides high thrust as it is supersonic instead of just sonic at the outlet of the combustion chamber.

The cone is sized to provide the proper pressure after gas expansion.

The part I missed at first is the HIGH combustion pressure exiting the sonic area is how the LARGE volume of gas get's through this narrow area. The red area is high speed but colder than the neck.

With that in mind, (my head hurts) the chamber pressure in bar can be calculated to the flow rate at sonic speed through the nozzle. Then allowing for cooling from expansion, the exit diameter can be calculated at the desired pressure in bar. It would be desirable to make the cone take 11 bar or so air and reduce it to say 2 bar at the colder pressure.

I'll have to play with the math to find just what diameter it will be for my cannons. The cone at these low chamber pressures may need to be only 1.5 to 2X the valve seat diameter to make a vortex cannon. The math makes my head spin.. :geek:

I thinking with a proper transition from a large valve into a smaller barrel, a supersonic golfball launch is possible due to the expansion after the sonic transition zone. It will require a relatively high chamber pressure. I'll play more with online calculators later.

If you want a headache, some of the math is here;
http://science.howstuffworks.com/framed.htm?parent=rocket.htm&url=http://www.gramlich.net/projects/rocket/

In the case of spudding and air, we would use room temperature for the temperature of the gas in the "Combustion Chamber" From there things cool. The nozzle = atmospheric pressure is fine for a vortex ring cannon. For spudding, it will need to be adjusted for driving a golf ball or marshmallow in a barrel.
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Last edited by Technician1002 on Thu Sep 10, 2009 4:24 pm, edited 1 time in total.
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Unread postAuthor: Brian the brain » Wed Sep 09, 2009 3:42 pm

I can't believe some of you doubted me.
:D
If an airzooka type launcher, or vortex gun produces a stable, far travelling blast of air, surely a cannon would enhance the principle...


After seeing the vid, I'm glad to see I was right.


Where can I find an emoticon that pads itself on the chest...? :wink:


Well...I think you have a winning concept..now go for it!! :)
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: Biopyro » Wed Sep 09, 2009 5:00 pm

http://www.youtube.com/watch?v=IyAyd4WnvhU

This might be more relevant to you, depending on how far away the can is. If it's very close you won't need a vortex, but if not you will probably want one to transmit the energy better.
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Unread postAuthor: Brian the brain » Wed Sep 09, 2009 5:06 pm

He was referring to the same cannon himself..pay attention Biopro... :D
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: Northir » Wed Sep 09, 2009 5:17 pm

the one i built today will toss plastic bottles about 20 feet (Top one)
Bottom one will do a soda can to about 20 feet, Specs are
1X24" Chamber 1" Rainbird sprinkler valve, 3/4X24" barrel, and 110 psi in the chamber
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