We are designing a T-shirt cannon robot (which I feel like every team thinks of doing at some point) and are curious what other teams setups look like for their firing tank.
What is the volume of your team’s firing tank and at what PSI do you pressurize it to? What results (aka distance) do you see with this setup? (Also we know that PVC is not air pressure rated, before this thread turns into that debate)
We built one with scuba tanks. We run a storage tank at 3000 psi and that feeds an accumulator tank at 120 psi. That tank empties after each shot. We have shot probably 40-50 yards normally, if we stuff the shirts down more we could probably shoot some extra distance.
Our tank/manifold is in the 1500 cc range. We pressurize from 50 to 100 psi depending on how far we want to shoot. 100 psi gets the low end of the upper deck at our football stadium. We also use a linear actuator to change the angle of the firing tubes. I’ve never laid out a tape measure on the distance, but if set up for max horizontal distance, I’m sure we can get at least 40 yards. We might be able to get 50 yards. See the linked video for an example. This is a long shot, but not max. If you look closely, it appears that the shirt unrolls a bit in flight, reducing distance. In that parking lot, we’ve gotten to about the far side of the big greenhouse. The shirt roll is a significant factor. Too small of a diameter and air leaks around the shirt. Too large of a diameter and friction reduces initial velocity. We tend to err on the side of a tighter fit.
The current values are definitely not required, I was just messing around with a personal cannon. This is just an Excel calculator that might be helpful for you; it’ll tell you the max length of a cannon barrel based on a few factors. Please note that it does not account for lost air, so I would not go all the way to the listed max length since you will always lose some air around or through t-shirts. Feel free to DM me or ask more questions here, I have quite a bit of experience with air cannons of different sorts.
Cannon length.xlsm (15.9 KB)
(Also, please let me know if this doesn’t work. I made my own function in there, and idk if Excel likes to send the macro file with the Excel file.)
Can confirm that the document was broken. Here’s one without custom functions. Whoops.
The work done on the T-shirt, and thus its change in kinetic energy, is equal to the (gauge) pressure times the volume of air in the tank used to fire the shirt–keeping in mind that the expanding volume must act on the shirt all the way down the length of the barrel. Do some projectile motion kinematics to find out what kind of velocity will give you the range you want, plug that into kinetic energy, and set that equal to PV to get your required combination of pressure and volume for the barrel you’re using.
That’s the ideal case.
In reality, you’re going to have a bunch of lost air, frictional losses, air resistance during the shot, possible spin on the shirt, etc, all of which will make your actual requirement larger than your initial calculated requirement.
My rule of thumb on most ideal-to-real conversions is to use the math to find out what you need, then go for total overkill (double or more) beyond that. (I apply this philosophy to FRC robots, too, because I’m a physicist and not an engineer, so am rather fond of my spherical cows and frictionless surfaces.)
Ours is 2 gallons, we use CO2 at 10-30 PSI and typically launch shirts 50-80 feet depending on our regulator setting and launch angle. Candy will launch 20-30 feet at a parade and tootsie rolls unwrap themselves sometimes.
I’m totally with @pfreivald on how to calculate these things, except that [from my experience] things which happen quickly with pneumatics often need a multiplier of 4 or 5 rather than just 2.
3946 has a tank which is about 3-4 gallons. It doesn’t need to be anywhere near that big (usually over a dozen shots when there’s good time control), but somewhere in the first year of the team, a couple that size showed up rated for 200psi with rather large (1-1/2") end threads cheap. It is rarely charged over 60 psi, but at about 70 psi, it can toss a shirt or football over the home side of the football stadium and put it in the Ninth Street parking lot – provided the solenoid valve is at least 3/4" in diameter, and preferably 1". Keep the plumbing run from your tank to barrel as wide and short as possible - when I left the team a year ago, it was a minimum of 1" wide and less than 12" long from tank to barrel. To do this, we actually rotated the tank with the barrel.
We found that adjusting the elevation (vertical angle) of the barrel wasn’t necessary if you can control pressure and/or time your valve is open. Just hard mount it at 45 degrees (the optimum for a level throw and no air friction) or a bit higher if you can manage the fittings (to better optimize a launch into the stands). In our case, we were also using this at parades, and 45 degree fittings are ubiquitous, so that was our solution.
Hmm… in the Gulf South, we physicists have spherical chickens. Same joke, I’m sure: