Enough compressed air for an air cannon?

Hi folks,
I hope I can get some advice on a project I have to build an aircannon that can shoot a ball out using compressed air. Thanks in advance for
any tips! The ball is 70mm and made out of light sponge rubber. I plan to put it in a plastic tube with inner diameter 73mm. I plan to connect an air compressor to the 73mm tube via a long quarter inch tube. Between the quarter inch tube and the 73mm tube will be a solenoid valve. The plan is to compress the air in the quarter inch tube to around 8 bar or less, and then open the valve to shoot the ball. The ball only needs to go around a couple of meters.
I have two worries, and I don't have enough engineering skills to work them out. Firstly, will there be enough air in the quarter inch tube to shoot the ball, without the tube being really really long? Maybe I need some kind of reservoir?
My second concern is flow rate. Solenoid valves that I have seen that are affordable only seem to have a flow rate of around 700 litres per minute. It seems to me that when I open the valve, I may get too slow a flow of air to get the ball really going. But I can't be sure. Any guesses?
I have no idea how to work out an optimum length for the plastic tube. Presumably, longer means more accurate but more friction so less far?
By the way, the point of the cannon should be that the ball should travel a quite predictable repeatable path. I figured compressed air should work well for this because the same amount of air will come out every time if the compressor is set to the same setting.
Any other tips?
Thanks a lot!
Ben
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amorphia wrote:

Ben, Every air cannon design I've ever seen used a butterfly valve to control the air. Butterfly valves can be opened quickly and present very little resistance to the air flow when fully open. You can purchase them with motorized actuators. See:
<http://www.mcmaster.com/> and look up "butterfly valve"
Good luck with your cannon but be careful. Dealing with compressed air can be dangerous.
--

Paul D Oosterhout
I work for SAIC (but I don't speak for SAIC)
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Ben, For your application I have a few thoughts, based on a little web watching, no personal experience.
To acheive adjustability, you will need better control. So, let me propose a few changes.
The hose from the air source should NOT be the reservoir. It's too hard to change. With that in mind, I would build an air cannon from plastic pipe, in this manner:
Hose from the air source, to a needle valve, to a resreservoir, made of 3/4" pipe. The 3/4" pipe to screw together without glue, so the volume can be changed if needed, start at 1/4 meter. On a "T", add a pressure gauge, but lets keep it down to 6 bar or so. Now for the solenoid valve. A sprinkler valve is inexpensive, and has a large orfice, and can be easily battery powered. Beyond the valve is the barrel, made of 2 1/2" pipe, (I don't know if that's available in plastic, wherever you are). the ID of 2 1/2" should be 4mm larger than your ball. The length of barrel to be 1/4 meter.
The instantaneous pressure on the ball, before it starts moving but after the valve opens is about 0.46 kg/cm^2. My feeling is that this is more than adequate for a few meters of ball flight.
Be safe, start at a very low pressure and work up to whatever is required for your project.
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Dear amorphia:

Could you use a CO2 cartridge?

CO2 will be too strong. Proabaly blow through the foam.

Go with the same diameter or smaller ID than the ball. Let it act like its own sabot.

What tube material? PVC is bad for compressed air... it fails by brittle fracture.

You'll damage the ball with each firing with that high a pressure and that small an opening.

Ideal gas law. Assuming negligible change in temperature. P1 * V1 = P2 * V2
Assume the 73mm barrel needs some positive pressure left in it to keep the ball moving, so it and the reservoir needs to be at positive pressure, say 1.3 - 1.8 bara (remember to use absolute pressures)
V1 = reservoir only V2 = reservoir + 73mm barrel V1/V2 = (1.8 bara) / (9 bara)

You are absolutely correct. I'd recommend a reservoir closely coupled to the barrel, with a large diameter ball valve in between. Precharge the reservoir, isolate it, then open the outlet valve to fire.

Scale it based on a pistol, length over diameter.

Not going ot happen with a foam ball. Stop now.

Use a hard heavy ball, and a ramp like a ski jump. Start it at the same point each time.
David A. Smith
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Many thanks for all your detailed advice, folks! There is a fair amount for me to chew over there and I clearly need to do some more research into exactly what sort of valve to get. I never realised before but after googling some more it seems there are actually quite a lot of web pages dedicated to building this sort of thing - they tend to be called potato (spud) guns.
There is only one thing that worries me a bit. David wrote that it's just not going to happen to get a foam ball to travel a predictable trajectory. I wonder why? That is actually crucial for me, because of my application. I'm a psychologist of among other things perception and motor skills and I want to test whether it is really true that people can catch balls which are unexpectedly thrown at them. It seems like fairly common sense knowledge that people can but this has never been investigated systematically before. It's quite interesting if it is possible because of the very short reaction time necessary - percieving, consciously processing and then acting on information takes around half a second, so it means that if people can do it, they don't do it as a result of a conscious decision. (The reason sports players can react very quickly is because they have already prepared reactions and well practiced responses).
Anyway, that means I have to use a soft ball - experimental participants will not be pleased if I unexpectedly propel a golf ball at them! They will have volunteered to take part in an unspecified experiment, not to loose their teeth!
Cheers,
Ben
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Ben, As you described the the first draft, I heard nothing about truely controlling the pressure. An air compressor has a deadband between turn on and off, and one reason I included a local pressure gage. Another was to give you an easy way of adjusting the power to a suitable level.
As to the predictability of an air cannon, I think you should be able to get better than 10%,.(see caveat below). Repeatability, should be better than 10% also.
Caveats: Temperature must be the same for calibration and use. Ball must not drag in the cannon. Fast valve is repeatable. Angle of launch is constant.
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Ben, I built an Air Cannon, only it works with ping pong balls.
Bore 1.51" (38.46mm) Length 13.37" (.347m) Air storage same size. Dump valve: sprinkler type 3/4" NPT Launch angle 30 degrees.
I took the following data PSI Bar Dist (M) Deviation (+-) low 100" 2.5 10.6% 20 1.4 159" 4.0 8.5% 40 2.7 224" 5.7 8.9% 60 4.1 289" 7.3 7.2% 80 5.4 298" 7.6 5.7%
Test conditions: Occasional light wind (might have influenced 60 psi range). 12 balls per pressure, distance is flight and bounce/roll on grass. (The occasional super bounce removed.)
There is one observation I should add. There is some noise associated with the air cannon. I wonder if your experiment will be altered by the scaring the subjects at the same time?
You might be better served by launching your rubber balls using two wheels that rotate in opposite directions, pinching the ball between and flinging it.
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Dear amorphia:

... and pumpkin guns too.

Low velocity. low mass projectile, springy projectile, unconfining barrel, no rifling, extremely lossy shape, plus whatever variabililty you impart with a valve choice. A little "fart valve" (also known as an air-pinch valve) could be quick release, and this valve switched with a 3-way solenoid valve.
compressor -> pressure regulator -> solenoid -> reservoir -> air pinch -> barrel + 3way valve-------> air pinch pilot port + open to atmosphere

Tracking a ball in-flight is also a critical skill. It just won't be repeatable to more than a few centimeters. You cannot "hammer a nail" with it. Probably good enough for your needs.
...

"If you can dodge a ball, you can dodge a wrench!" ;>)
David A. Smith
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