Hello, so my team is trying to build a t-shirt cannon for a summer project, and need some advice on the air release. We have a good portion of it figured out, except how we actually launch the shirt. What would be a good way to store the air, and release the air(solenoid?). Any help would be appreciated. 
For our T-shirt Cannon: It is made entirely out of PVC. We use 3" pvc sewer pipe (rated to 300PSI) with an end cap at one end, reduced to 1"on the other end, through a (manual) ball valve, back up to 3" for the barrel.
The PVC storage tank (behind the ball valve) Has a weird fitting (I don’t even know what to call it) but has a standard 3/8" threaded hole in it, where we connected first a FRC compressor via spare FRC pneumatic fittings, but was upgraded to interface with our 21 gallon shop compressor.
People distrust hand-built pvc as a pressure vessel, but we have successfully fired hundreds of T-shirts at 155 PSI (Firing nearly clear over our stadium [which is huge]), without incident. It was PVC cemented on the day we built it and has had zero maintenance in the 7 years we’ve had it.
If you wanted an electrically fired cannon (probably would fire a t-shirt further than with a manual ball valve), you could use a sprinkler system valve. They are designed to hold back a lot of water pressure, and hence, a lot of air pressure as well. They also actuate quickly and require maybe 12 volts.
What sort of range are you going for? Ours (range of ~70 yards) has a variety of components that make it work.
To start, we have a compressor that lets up go up to as much as 120 psi in our main tank. This goes through a standard FRC regulator to our holding tank (60psi max) made out of PVC. One end of the PVC goes into a sprinkler valve that realeases all the air in our holding tank into the barrel. If you are interested in doing something like this, PM me and I can give you the details on how we control ours.
Ours is much smaller in length than the post above (that robot failed inspection I am sure :P)
It is about 4.5’ long, and is shoulder fired. It looks very much like a bazooka and very much so, as it is one. I don’t actually know how far horizontally far we can fire, but we can fire t-shirts over the top of this stadium (we hit the bottom press box window once) from the track at the bottom
I don’t know if this picture will show:https://fbcdn-sphotos-f-a.akamaihd.net/hphotos-ak-frc3/t1.0-9/27987_3961945411915_2078328293_n.jpg
PVC pipe is NOT RATED FOR AIR PRESSURE!
PVC pipe is rated for hydrostatic pressure only. Using PVC for containing pressurized gas can lead to serious injury or death. When PVC pipe fails due to pressurized gas, it does so catastrophically.
There is a big difference in air and water pressure. Water is incompressible. If a pipe bursts holding water, the water will just leak out. Air on the other hand will expand to reach the same pressure as the atmosphere, releasing an enormous amount of energy, which will propel shrapnel at high speeds in all directions.
Don’t just take my word for it, see this engineering document from Charlotte Pipe, this recommendation from the Plastic Pipe Institute, and this warning from OSHA.
This does not mean it won’t fail the next time you use it. Fatigue cracks have most likely started, which severely reduces the strength of the material.
What is more important, quick and cheap air cannons, or your and your student’s safety?
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That looks like a very nice soccer stadium but what are all of those weird numbers and lines on the field?
If you scroll down you will see video of our prototype. We have fancied it up since these pics/video was taken and have added pistons so that it can change the positions (30 and 45 degree angles approx.). We use PVC cartridges, each containing a tightly wrapped and taped t-shirt. They are then gravity fed to the shooting position, locked tight next to the 120 psi confetti cannon integrated solenoid and tank. Once the shirt has been launched, the cartridge is unlocked and a piston knocks it out the side, making way for the next cartridge to slide into place.
Don’t use sewer pipe. It is not rated for pressure. I would use schedule 40 PVC at least, and wrap it in packing tape (the kind with threads) to contain shrapnel in the event of a failure. PVC will decay after a lot of UV exposure, so don’t leave this thing outside.
Or use materials that are actually rated for pressurized gases. I highly doubt tape will do much to contain the force of a burst.
I think a great parallel is operating machine tools with gloves on. It is entirely possible to drill tens of thousands of holes with gloves on and not lose a finger, but why would you expose yourself to that risk?