[Steven Smith]

What is generating the propelling force isn't just the pressure, but a combination of pressure and flow. Simply put, you need enough flow to maintain a pressure behind the T-shirt as it travels down your barrel.

If you are using 1/4" tubing and a KOP solenoid to feed that giant 3" ID tube, and you imagine your T-shirt as a perfect seal against the back of the tube with negligible clearance, then yes at T=0 you have an impressive ~700 lbs of force to get that T-shirt out of the cannon. (1.5^2*pi*110psi).

Let's say the T-shirt weighs 0.25lb and just use an arbitrary delta time of 1ms.
At t1, let's make an assumption that we had enough air to sustain the force (and acceleration) on the t-shirt. F=ma/gc, so 770/(0.25/32.1) = 90000 ft/s^2. v=at, so the velocity after 1ms is 90 ft/sec. That's a pretty decent speed, about 60 miles per hour, or the speed of a decent pitch by an amateur.

So we're good to go right? We have a lot of pressure, generating a lot of force, T-shirt goes boom!

Let's go back and check an assumption though, our unlimited air flow. Assuming again the constant acceleration (which means we maintain 110psi across the entire 1ms, then x=1/2at^2. So X= 0.5*90000*0.001^2 (ft), or 0.045ft, or 0.5inches. So our T-shirt has traveled 0.5" down our barrel, so in order to maintain the pressure behind it, we need 1.5^2*pi*0.5 inches of air at 110 psi, or 3.5actual cubic inches, or 26 standard cubic inches. (110/14.7). This is 26/(12^3) or 0.015 standard cubic feet. Nice low number right? We can handle 0.015 SCF. Wait a second though, we are doing this in 1ms. So that is 0.015*1000*60, or a whopping 900 SCFM if we want to maintain the pressure behind our T-shirt as it hurtles down the barrel.

If I go to my charts, let's say our feeder tube is 0.1875" ID, 900 SCFM, 1 foot of tubing, 110psi... it tells me I'm losing 8000psi per foot. So hrm, I'm probably not getting that out of it. I have to drop down to 90SCFM just to get to where I'm losing 110psi across the 1 foot limit. If I only flow 90SCFM instead of 900, I'm only getting a 10th of the pressure behind the T-Shirt I thought I had... or something like 10psi. Now I'm not getting 60 miles per hour... I'm getting 6 miles per hour.

So really, what ends up happening is your T-Shirt moves along the barrel at the flowrate of the tubing/valve limiting it, and plops out the front.

Long story short, my math might be a little off, I was just spitballing, but as several posters above stated, to make the system work, you need two things... storage preferably quite close to your barrel, and a large valve capable of providing high flow rate. You could potentially run it all off of a KOP compressor, but you might only get to take a shot every few minutes (do the math), and run it at near 100% duty cycle (which it isn't designed for).

As always, please be careful. It's easy to think that 110 psi isn't a big deal as it is "shop air", but you are harassing a lot of energy. If you try to trap that pressure of air in many containers (plastic), you can blow something up. Many types of plastic turn into shrapnel when they explode, which can seriously injure or kill someone. The preference is to use more ductile metal pipe where available (which won't turn into shrapnel), or at the minimum encase any plastic parts in a shield and don't work on it under pressure.