T-Shirt Cannon

My Team has constructed a T-shirt Cannon, which got tested for the first time today. We had a very anticlimactic result has the T-shirt basically just fell out of the end of the tube. We are using the KOP pneumatic system connect to about 1.5 foot ABS tube. The tube has a inside diameter of 3". I know we do not have enough pressure behind the T-shirt. I was wondering if a smaller diameter tube would fix this issue? We have our pressure built up to 110psi, which I thought would be enough to at least get 20 yards. Any help on this would be greatly appreciated!

Check to make sure there aren’t any leaks and that you are releasing the air pretty quickly so there’s a fast burst of air

How are you introducing the air to the shirt? You need a fast acting valve in order to ensure the full psi is applied to the shirt in a small time frame rather than slowly over time. Look up designs online of potato guns (or other t-shirt cannons) and see what kind of valves they use. Sprinkler valves are very popular.

The valves and tubing specified by FRC are specifically chosen to LIMIT flow rate… you want to maximize it.

When we built a t-shirt cannon we used a sprinkler valve https://www.homedepot.ca/en/home/p.1-inch-npt-jar-top-valve.1000769546.html

Note that these valves are designed for water pressure, not air pressure. A small crack with water pressure causes a small leak. A small crack with air pressure (compressible fluid) causes an explosion.

The same thing applies to your compressed air storage tanks. Don’t assume that a 200psi rating on plastic pipe makes it safe for holding 100psi of air pressure. You will likely be using some components outside their designed applications… it can be made to work (you’ll see lots of people doing it on online videos and instructables), but we chose to use air-pressure rated pipe black iron pipe as our storage tank. Maybe a bit conservative… but just be aware that the same energy that goes into launching the shirt can also go into launching shrapnel if your tank or valve lets go.

Another feature to consider adding would be an IR or ultrasonic rangefinder on the tip of your cannon barrel that disables firing if an object is detected in front of the cannon barrel.

I assure you, the results will be pleasing. We had to dial back the pressure on ours because we were bouncing the shirts off the gym ceiling, rather than arcing them nicely into the stands.

Jason

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. (1.5^2pi110psi).

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= 0.5900000.001^2 (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 1.5^2pi0.5 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 0.015100060, 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.

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We use one of these valves on our cannon:

A little bit more expensive than the sprinkler valves, but it’s rated for air pressure.

Beside what is posted above 3" is to big. I think we use 1.5" or 2". It works great.

-Hugh

I wish I had found that one when my team was making our t-shirt cannon. We used a fairly similar valve, only 3 times the price.

http://valvesandinstruments.com/solenoid/asco-8210g4-8210g004-12dc-2-way-brass-1-in-solenoid-valve-normally-closed-general-service.html

this may seem stupid but how are you folding the shirts? if you just sort of shove them in the barrel it will open in flight like a parachute. However, there is a way to fold a $@#$@#$@#$@# to make it more rigid, if folded this way you can toss it, throw it, or shoot it and it will stay together. (PS, working on pics)

I’ll save my lecture about using plastic as a barrel for a later thread. :wink:

Your pressure is not the issue. 110PSI is overkill. What your issue is … is flow and reserve air.

Gas (in this case, air) expands to fill the area (universal gas law). It only takes a few pounds of force to overcome the friction if the t-shirt to the barrel. Once that happens the t-shirt begins to move up the barrel, increasing the volume that the air needs to fill and dropping the pressure in the barrel. You need a flow rate that is higher than the expected volume increase as the t-shirt moves up the barrel and enough of an air reserve to sustain that flow rate throughout the entire firing time.

If you don’t have enough air flow, or enough reserve air to fill the entire barrel at the pressure you want, the t-shirt will not fire very far … and will tent to just drop out of the barrel rather than fire out.

We have a 3/4" brass solenoid on our air cannon quite similar to the one posted by SamM. We’d probably go 1" if we were buying today. We can launch a t-shirt over our stands and press box with 60 psi. Our launch tube has an inside diameter of 2-1/2".

Folding shirts: for large and XL shirts in our 2-1/2" pipe, we lay a shirt out flat, then fold the left third of the thorax over on to the center third, then the right third over the center third, leaving the right sleeve extended on the left.
We then roll the shirt up from the bottom hem, until it is just above the bottom seam of the sleeve. We then flip the sleeve around the cylinder rather like rolling a pair of socks to create a “T-shirt burrito” that we can launch. Smaller shirts need to be folded into fourths; larger shirts only in half. Your breakpoints may vary with your pipe diameter.

Let me reiterate the points about using parts rated for water but not air. Water is essentially in-compressible, which means that not that much work is required to increase water pressure to 60 psi. Compressing air to 60 psi means reducing its volume to less than 20% of its ambient volume, which is a whole lot of work (aka energy). If an air filled tank/tube ruptures, there is a lot more energy for the shrapnel. Do NOT use anything not rated for air pressure, especially the valve, or upstream of the valve. There’s just way too much ugly potential there.

Thank you all for the help. We have purchased a steel tube rated for air for our barrel to replace our ABS. SamM I do think we are going to purchase the solenoid that you posted. I was wondering if we can purchase an adapter to get it to hook up to the 1/8" tubing we have? Or would it be better to find a way to use 1" tubing throughout? Thanks again everyone for the great advice and help!!

What you want to have is a “buffer” tank connected to the barrel by large tubing or pipe, i.e. at least 1/2", but 3/4" or 1" is better. The tank should be at least the volume of the barrel.

The buffer tank can be pressurized by small tubing, but you need large tubing from the buffer tank to the barrel.

We used an inline 3/4 sprinkler valve with a piece of 3 inch drain pipe and got about 100 120 feet with it. We are using a 1 gallon tank at 110 psi.

This. Keep in mind that smaller tubing will restrict flow, so while your air compressor can be connected using standard 1/8" tubing, everything between the tank and the cannon should be 1". Consequently, you’re probably not going to want to use a standard air tank like you might find on AndyMark, look for one with 1" ports (or 1/2" ports and use several of them to merge to a 1", since 1" port air tanks are really hard to find).

My students discovered that a shirt folded to be tight in the barrel shoots much farther than a loose one - although the tight shirt has more friction on the way out, it seals in the barrel much better.

Plastic tanks and barrels have always made me nervous (and I’m not an ME, and I’m sure this has been discussed elsewhere.) An industrial arts teacher at a school 668 is helping suggested copper. It’s readily available, and tears rather than breaks up into shrapnel. Yes, it’s also more expensive, but it takes a good shine! And it’s a good way to prepare future homeowners to solder their water lines. :slight_smile:

This is great info. Thank you! Do you know of any websites that would be good to order from for the tank?

I would suggest just checking out this link if you want to make a relatively cheap T-shirt cannon (mostly out of PVC):
http://www.instructables.com/id/T-shirt-Cannon/?ALLSTEPS

Ummm, yeah. Do Not do this with PVC. Please.

I will second this. Flying plastic = very dangerous projectiles in a crowd.

=Daniel_LaFleur;1565142]Ummm, yeah. Do Not do this with PVC. Please.