Help with T-Shirt Cannon

[ThePoopieBandit]

I worked on my team's cannon a lot and our website has a lot of good information that you can use for ideas. https://www.team254.com/shockwave/

The distance the shirt will be fired can be changed dramatically by how you roll up the shirt and if you add rubber bands not. For short shots we roll them up loosely so they open up almost immediately after leaving the barrel, another is to roll it up and add a single rubber band at the back of the shirt to hold it together in flight but after firing the front of the shirt will mushroom out to slow it down and for long shots we add two rubber bands on the shirt, one on the front and one at the back. Another way to adjust distance is to change the amount of time that you leave the valve open.

[Steven Smith]

Originally Posted by Why_A_Username?

The gist of what i got from your post is that basically flow rate is equal too, if not greater than PSI in importance, because it is hard to maintain a constant PSI. We want a fast-fireing solinoid with a wide flow in between the resovior and the barrel tube to minimize the pressure drop between the two. Our current design has multiple barrels so thst presents a challange for multiple resovior connections. Thanks for the great reaponse!

What you really care about is PSI at the point of delivery (base of the barrel). If the T-Shirt could magically travel down the barrel without flow, you would essentially try to pull a vacuum on the tube, as an extremely small volume of starting air expands to try to fill the tube. The more air you flow into the tube, the more pressure you will have throughout the travel of the T-Shirt, and the more total force imparted.

Flow and starting pressure are linked of course, such that a higher pressure can drive a higher flow through the same size solenoid/tubing, etc. However, the pressure loss is exponential as tubing and orifice sizes decrease, and you are ultimately limited by what is called "choked flow". Simply put, air can not travel faster than the speed of sound in the gas system, so if there is a tight enough restriction, increasing pressure further doesn't add more flow.

Therefore, your solenoid does need to be "fast firing" but also have a large flow orifice (specified as a Cv value, which is basically the total flow across a valve for a unit pressure drop... bigger is better for this). A typical FRC solenoid will not work well for this application.

Multiple barrels and multiple connections is not as much of a design challenge as a cost challenge, if you wanted to have a solenoid valve per barrel. You could also consider (if you really wanted to push the envelope) of a revolver style design with a single barrel and a t shirt magazine that advances through it. There you still just need a single solenoid, and the main challenge is getting a good seal between the barrel and magazine.

Feel free to keep asking questions. Gas system design was my day job for a number of years.

[Mattb706]

Originally Posted by Why_A_Username?

Hey CD,

My team has been working on a couple of modules for our outreach robot. Another student and I requested to be in charge of the T-shirt cannon. At first we thought this was fine but now we're a little but stuck (He's in charge of CADD and i do Drive Train and PR). Niether of us has taken physics yet and so we arent entirely sure how to do the calculations for pressure required. We did them on our own, and got some results that didn't seem right, so we where wondering if CD could give us a hand. Below is the poster on which we wrote our answers:



Thanks CD!

We have one, trying to find some video for you......