T-shirt Cannon - 1/2" NPT

[Ginger Power]

I currently have a limited budget and an air tank with only one port which is for a 1/2" NPT fitting. I'm aware that most people here reccomend going no less than 3/4" NPT as the smallest point from tank to barrel. So can I make a 1/2" NPT fitting work? What other aspects of the T-shirt cannon can I modify to increase shot distance? Obviously increasing the barrel length will help, but how long should it be? Is there a way to calculate how the barrel length will affect shot distance?

For reference, the T-shirt cannon will be mounted on a drivetrain with 4 pneumatic wheels, and will be designed to be used at high school football games. The air tank is a 10 gallon tank, and it will be filled with an off board compressor.

[PAR_WIG1350]

Full disclosure: I haven't personally worked with air cannons, but I have a basic understanding of the physics behind them.

For the restricted tank opening, you could precharge a smaller, separate volume outside of the tank from the 1/2" NPT port and have a larger outlet from that volume to the firing valve. It might even be beneficial to further reduce the size of the opening between the main tank and the precharge tank to allow for a smaller, and possibly less expensive, valve between them. If you reduce it far enough, you might not need a valve there, but it might slow down your firing rate.

For calculating the effects of barrel length, I recommend testing a few different lengths and fitting a curve to the result.

[juju_beans]

Originally Posted by PAR_WIG1350

For the restricted tank opening, you could precharge a smaller, separate volume outside of the tank from the 1/2" NPT port and have a larger outlet from that volume to the firing valve. It might even be beneficial to further reduce the size of the opening between the main tank and the precharge tank to allow for a smaller, and possibly less expensive, valve between them. If you reduce it far enough, you might not need a valve there, but it might slow down your firing rate.

That's exactly what I have done and it works pretty well.

https://www.youtube.com/watch?v=vC1d51Z6fuY

One of the big things you have to remember is to make sure that your t-shirts are not rolled up too tight or too loose or it will not shoot as far.

[GeeTwo]

I'm one of the ones who suggested a 3/4" port minimum. This is based on a 60 psi working pressure, and looking for "top row of high school bleachers (home side) from the running track" performance. If you can settle for less performance than this, or can boost your pressure, you should be able to make 1/2" work for you.

The suggestion by PAR_WIG1350 and endorsed by juju_beans is essentially a second tank. You may want to consider using a length of large steel pipe that is capped at one end and has reducers down to 1" or 3/4" at the other end AS a tank. For purposes of calculating the needed volume, our accumulator tank is roughly the size of a scuba tank, though it's a cast-iron beast with a working pressure (from the painted-over plate) of 150 psi. I suspect this is actually a good bit more than we need; we only need to open our solenoid valve for 50ms to get our good shot. Opening for longer provides only modest improvement.

While I don't have any specific numbers handy, according to our tests a few years ago, barrel length is a "optimal length" parameter -- a barrel needs to be long enough to apply the air pressure to the shirts for a suitable duration, but if the barrel is too long (at least given a constant tank size and pressure), friction causes a long barrel to give a reduced range.

OK, specifically, but based on memory not notes -- IIRC, for a 2" i.d. barrel, 60 psi, 3/4" valve and larger lines, the optimum length was between 30" and 40; I believe we used 36".

When we increased our barrels diameter to 2.5" i.d., we reduced our barrel length to about 24" to keep the barrel volume roughly constant (based on work being equal to PΔV, not calibrated by measurements, but it worked OK for us).