T-Shirt Cannon
 

Hi Chief Delphi community,

I've been working on a T-Shirt cannon base for the past few days and I was wondering about how to plumb it. I'm using 2 scuba tanks as storage and firing chambers and I'm planning to tap threads into a custom pressure block that attaches to the barrel. Thanks in advance to anyone who has insight :)

Specs:
4 Cim drive
3/16in center drop WCD
360 degree rotation (shoot in any direction relative to wheels)
Angle adjust from 45 to -45 (barrel facing down)
2.875in ID Barrel
Magazine fed with cylinder ejection


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Re: T-Shirt Cannon
 

]There have been a number of T-shirt cannon threads in recent years; you can find plenty with some searches, including ideas for rolling t-shirts and working through various problems. Here are a few key items:

1. Safety first! Do not use PVC, or other hoses/tubes/pipes that break in a brittle fashion; make sure that all components are rated for the appropriate PSI of air (not just water). Air at a given pressure has a lot more stored energy than water and while air may or may not be more likely to cause a rupture than water would, the consequences would be much worse.
2. Large airflow at relatively low pressure with a brief open interval for the solenoid is the favorite way to go. We use about 50 psi through a 3/4" brass solenoid valve, and 1+" pipes most of the way, open for 40-50ms. If I were building now, I'd get a 1" brass valve so we could take the pressure down a bit more. (We have a 3" ID barrel so its similar to yours).
3. It takes a lot of energy to launch a t-shirt. Don't plan to power the compressor from an FRC battery; either use air compressed off-robot (our method) or get a larger (marine deep-cycle) battery.

Re: T-Shirt Cannon
 

Scuba tanks? 2500-3500 psi. Please tell me that's not what you're using.

Re: T-Shirt Cannon
 

I would assume their working pressure is much, much lower than that. There are several other teams that have used Scuba tanks in the past without any issues.

Re: T-Shirt Cannon
 

I would avoid a "custom pressure block" and just go for 100psi (or whatever you're running at) metal pressure fittings. Your barrel should also be thin-wall metal rather than thick-wall plastic.
Scuba tanks for storage sounds pretty standard from what I've seen here; just make sure you run a regulator to the output. :P

What is the small cylinder in the back of the barrel for?
I really like your design, it looks very neat. You could remove the twist axis if you wanted to to add stiffness (just turn the robot instead) but your way could be more interesting.

Re: T-Shirt Cannon
 

That appears to be the firing mechanism. We used the same design for our cannon. It pushes the cartridge forward and activates the release of stored pressure at the same time. This also provides a good deal of safety with our cannon because it cannot fire over the pressure that is used to push the cartridge forward. In our case that pressure is 30 PSI, I don't see a lot of others running that low of pressure however.

Re: T-Shirt Cannon
 

They referred to using scuba as "storage and firing containers". We use scuba as "storage", but some cast-iron tanks as accumulators at low (50-60psi), and I am guessing this is what they mean by "firing containers".

In our case, we get the scuba tank charged at the fire house or dive shop. It has a standard scuba regulator which regulates the pressure down in the 110 psi range. (I don't recall the precise number, but it's fixed as far as the consumer is concerned, so we don't have any control over it.) We feed that into an FRC standard regulator to get down to 40-60 psi to store in our cast iron tank, which has an outlet that can handle up to a 2" pipe IIRC - I know that we use a reducer to get down to the 1-1/4" galvanized pipe we're using.

I fully concur that trying to use 3000psi is much higher than I am comfortable with professionally, much less with FRC. We had a diesel engine and a 3000psi compressor in a "CON-X" shipping container on an acoustic survey in the late '90's when my office experimented with air guns for seismic surveys to determine the acoustic behavior of the seafloor. (Yes, I pay the bills as an underwater acoustician for the US Navy.) We had an ME aboard who was hired specifically to do maintenance on this unit, and I was the senior representative of my office for the survey. At the start-of-survey safety brief, I explained just how much pressure 3000 psi is. I believe I said "That hose that snakes across the deck is about two inches in diameter. A quarter-inch hole in this hose will generate enough force to toss you across the deck, and possibly overboard". Eyes got big, and everyone looked to Les (the ME), and he just nodded. It was a slight exaggeration for the largest members of the survey party who weighed over 200 pounds, but that element of the safety brief had exactly the right effect as far as getting the survey party to respect just how dangerous 3000 psi can be.

Re: T-Shirt Cannon
 

Yes, from quick tests, it seems as though 50 psi (working) would be adequate to launch t-shirts.

Re: T-Shirt Cannon
 

Hi, thank you for your helpful posts, would pressure would you recommend that we store air at?

Re: T-Shirt Cannon
 

Haha, I hope the twist will be interesting in a good way. As for the pressure block, I just couldn't think of a solution to transfer air from standard piping to the firing tube. Do you have any suggestions? I figured a properly welded on block of steel would suffice at 50 psi. (Please correct me if I'm wrong)

Re: T-Shirt Cannon
 

Oh wow, a T-shirt cannon that's not the usual "gathling" type. One thing that concerns me is the rod that the cannon system is mounted on. Was there a reason for such a mounting system?

My reasoning is behind... I believe Newton's Third Law: With an action force comes a reaction force. As cannon fires the shirt, the shirt "fires" back with the same force to the cannon. Therefore after several shots out of the cannon, that rod its mounted on will bend significantly.

So yeah, I would strongly advise to mount the cannon onto something sturdier to stop this from happening.

Re: T-Shirt Cannon
 

Charging the accumulator to 40-60 psi should be adequate and relatively easy to find fittings and hoses, apart from the swivel.

Re: T-Shirt Cannon
 

Well the main reason was to allow for a 360 degrees of motion and to allow perpendicular shooting to the wheel base. However at second glance, I can see many ways to raise the second mounted bearing in order to lessen the moment on the steel rod. I have thought about the "recoil on the cannon" as 50 psi on a 3in diameter item is not a small amount of force.

Re: T-Shirt Cannon
 

Are you using one tank as the HP side and the other one for the accumulator tank to provide the volume of air at LP to fire the shirt? If so you will not want to use the valve in the LP tank to gain the volume of air flow. You can use 3/4"
steel pipe with NPT threads.

A typical scuba regulator will drop the HP pressure down to about 170psi and I would suggest an additional regulator that is adjustable and will drop the pressure again to 150 psi max.

You can view ours at prhsrobotics.com under outreach. It is built on the same principle that you are using.

Re: T-Shirt Cannon
 

The recoil will not be nearly this large, unless you're tossing chain mail t-shirts. The 50 psi air will be rushing through a 3/4" or 1" solenoid valve and quickly expanding to push on a half pound projectile over 7 square inches of area. My back-of-the-envelope calculations based on how far a half-pound T-shirt is tossed tells me that the recoil force for our no-sabot system (we roll the shirts into their own sleeves and muzzle load them) is more like 40 pounds than 350 pounds. If you are feeding the shirts in cylinders, you may require more pressure and will generate more recoil.

Edit:
Not a bit surprising after the teaser video; I made a bit of chain mail myself in the 80's. I'm rather impressed with how much our student leadership team is focussing on game mechanics, strategy, and grand strategy while ignoring the stylistic hints we've been given. I wish I could ignore teasers and game hints as well as they can.