Question to ponder; Pneumatic Cannon

[BillH]

The key with these things is being able to dump ALL the air in the tank as quickly as possible, as close to all at once as you can get. I've been pondering what the best valve is to use to do this. On mythbusters, they used a big butterfly valve that looked pretty good.
An interesting thing to note, one guy that competes at the Pumpkin Chuckin events, was on tv with his giant cannon, 2nd Ammendment. He said that he can get the same distance with 50 psi as he can with 200psi by timing the release of the air.
So Im only thinking about 100 psi max here, any suggestions about what valves to use? It seams that the valves are a closely guarded secret.

[G.A. Ewen]

I would say that a ball valve that is the same size as the bore on the cannon would be the fastest. A quarter of a turn and it's full open.
Maybe not practical for a pukin gun but if you want to toss something the size of tennis balls they should work fine.

[Forrest Addy]

Use a rupture disk. Consider the time the projo is in the barrel Vs time to open the valve.

Rupture disks open in a millisecond or two. You can make you own from scored plastic.

[Mike W]

I am watching the chicken show right now. They called it a butterfly valve. It has a flat disc that opens with a quarter turn.

[Mike W]

50 psi and 120 psi gave the same speed with a chicken. 20 psi also broke the windshield which wasn't rated for bird strikes. One test hit a speed of 132 mph.

[Yankee1]

That ball valve sounds good if it opens fast enough. How about a solinoid actuated
lever on the ball valve and a pressure
regulator on the air supply side.The solinoid would open it fast.
Yankee1

[Thrud]

Bill
What you should be asking yourself, is why don't I have a multiple airchamber gun that can ACCELERATE the projectile down the barrel by constantly increasing the airpressure? The reason it makes no difference now is because the inital impulse is doing all the work! In the case of a rifle the propellant progressively accelerates it by burning until it reaching its maximum CUP - by this time the bullet has reached maximum velocity in the barrel and is beginning to slow down before it even exits the barrel (in the case of complete powder combustion).

In the case of the pumkin, it has be accelerated to its maximum velocity in the barrel by the inital impulse of air, if additional high pressure air is added after the first impulse further accelleration will take place. The opening of these valves would have to be millisecds behind each other to steadily increase pressure behind the pumkin as the barrel volume increases as the pumkin travels up the barrel.

[This message has been edited by Thrud (edited 03-08-2004).]

[barts]

Note that the maximum speed of a projectile is
the speed of sound in whatever is shoving it down the barrel...

Why not use a dump valve? Something like
mcmastercarr's 6646K23 might work....

A 3/4" lawn sprinkler valve might work pretty well,
too...

= Bart

[J Thornton]

In one of the text books I had there was a forging machine that operated on air pressure. It used a system that relied on differential air pressure. it had 2 connected chambers one high and one low pressure The piston was in the low pressure chamber. The pressure on area of the piston on the low pressure side was used to seal a hole in the divider. The machine was triggered by increasing the pressure on the high side enough to over ride the balance. Then the high pressure was applied to the entire area of the piston instead of just the area sealing the hole. That might be quicker than a ball or rotary valve.

[ibewgypsie]

Thrud,

Wasn't it a Super-cannon that got the canadian guy kidnapped by the Mossad? Bringing up old history?

Not just accellerate, but to accellerate without rupturing the projectile. A piston type arrangement is what they use to launch people in the circus. A poly piston that fits the bore and is latched in, you pump it up then release the piston. Doesn't a bb gun work the same way? What a big bee bee..

Pneumatics? How about Hydrogen peroxide? Steam, now there is some expansion for you. Launches aircraft off carriers everyday.

David

[Bill Cook]

I come up with a comparitively large, light weight, hinged trap door.

Firing would require opening a latch that holds the door closed against the air charge. The door would open completely.

The compressed air chamber and valve assembly would be part of the breech - right up close to the projectile - to get the most out of it.

The door would open against a hole in the side (or top or bottom) of the breech. This would:

A) Tend to hold the door open.
A shock absorbing device may be required to keep the door from bouncing on the atmospheric hole.
Or maybe saayy a traveling weight that would delay and tune the bounce.

B) Allow air to be sucked in when the charge pressure in the unit drops below atmospheric pressure. The amount of air sucked through this hole would give some insite when modifying barrel length, charge pressure, and/or projectile properties.

C) Without it, the decreasing pressure may "pull" the cannon forward after the recoil (if it matters).

D) Allow access to the valve area and the back of the projectile.

E) Allow air to escape when ramming that VW down the barrel.

bc

[J Thornton]

David
The Canadian was Gerald Bull and he was murdered in Brussels. Interesting write-up here:

http://www-istp.gsfc.nasa.gov/stargaze/SGbull.htm

------------------
Jesse

[pgmrdan]

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[This message has been edited by pgmrdan (edited 03-12-2004).]

[Forrest Addy]

Listen to the no-load discharge to get an idea of how fast the launch valve on these pumpkin guns is opening. There should be a POW! All the ones I've heard on TV go CHUFF! The CH...! part indicates to me choked inlet air and the video of the air gage subsiding to zero in a couple of seconds bears this out. The gage should snap to zero.

Butterfly and ball valves, unless opened instantaneously, choke the air so you get reduced accelleration in the first part of the barrel. Butterfly valves and ball valves have significant mass in their working parts.

This is apparent in the Mythbuster episode where they were testing aircraft windshields with 100 MPH chickens. Increasing reservoir pressure had little effect on muzzle velocity. The valve was opening too slow. They were opening it by whacking the handle with a 2 x 4.

Look at quick cylinder exhaust valves for air cylinders from Schrader Air. They make them up to 3" IPS which is too small for a pumpkin gun. You need a valve and supply line from the air reservoir having half or more the effective area of the barrel.

http://www.powerdrives.com/sch2.htm

(right column of images 5 down; its a simple thing - three parts and four screws)

Quick exhaust valves designed for use in air cylinders to open rapidly direct to the atmosphere on release of supply air pressure so you don't have to wait for the pressure to bleed off through the connecting line and valve before reverse motion of the piston rod commences. Thanks to their low mass design they open almost instantaneously - explosively it could be said if you've ever been head down close to one that exhausted unmuffled into your ear. I think I still have a bump in my head and it's been years.

They can be used for transfer functions are well like air from a reservoir to the cannon breech. Furthermore they're mechanically very simple being nothing more than a three port housing and a diaphragm and can be scaled up to pumpkin sized.

The beauty of these valves is the active element is a low-mass rubber diaphragm. As I said they scale up easily. Spend $15 and buy a small one and use it to cobble up one having an effective port diameter equivalent to 8" IPS pipe. Remember BIG, HUGE ports. BIG connection lines.

[BillH]

THanks all for the help, Quick Dump valve definetly sounds good to me. The size gun I've been thinking about would be for Tennis ball or potato size loads, use it for a coastal cannon at the lake.
I've done this before with PVC spud guns, fun shooting it, then watching the lake down range to see where it lands.
I also have a little OOpic programmable robot controller that I could easily use to do thruds idea of multiple chambers going off milliseconds apart, even a control for limiting airpressure inside the chamber for precise measurements, perhaps even a gunnery computer. Hmmm...