T-Shirt Cannon

[snotsdelta]

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!

[JR0405]

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

[Monochron]

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.

[dtengineering]

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...000769546.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

[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.

[SamM]

We use one of these valves on our cannon:

http://www.electricsolenoidvalves.co...olenoid-valve/

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

[Hugh Meyer]

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

-Hugh

[ASD20]

Originally Posted by SamM

We use one of these valves on our cannon:

http://www.electricsolenoidvalves.co...olenoid-valve/

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

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/sole...l-service.html

[The Ginger]

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)

[snotsdelta]

Originally Posted by SamM

We use one of these valves on our cannon:

http://www.electricsolenoidvalves.co...olenoid-valve/

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

When we wired it to our PCM we keep getting a solenoid fault on it. We changed our program to a single solenoid as we were using a double. Just wondering if we had to do something different wiring wise as there is no errors in our program

Thanks!

[SamM]

The PCM may not be able to supply enough current for the larger solenoid. You may need to use a victor or another speed controller to supply power to it, which is what we did. You could also probably use a small automotive relay.

[snotsdelta]

Great Thank you! So then we will wire the solenoid though the Victor to the PDP then correct? Thanks again!

[SamM]

Yes, just treat it as a motor and run it at full power when you want it to open and no power when you want it closed.

[GeeTwo]

We switch the solenoid valves on our air cannon with a 12 relay module similar to this one. Our first iteration used a spike relay, with one valve powered by M+ and the other by M-, with a common 0V (ground) return.

[Daniel_LaFleur]

Originally Posted by snotsdelta

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!

I'll save my lecture about using plastic as a barrel for a later thread.

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.