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Unread 07-13-2018, 01:59 PM
FredK FredK is offline
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T-shirt cannon firing pressure not getting reported distance

We have read every post we can find regarding t-shirt cannons and are not getting the distances that other teams are reporting.
We have a 5 gallon storage tank, connecting through an FRC regulator to a 1.5 gallon storage tank. 3/4" pipe out of storage tank leads to a 1" solenoid followed by 1" nipple, stepping up to a 3" barrel. We are firing at 60psi but only getting about 40-50 ft, not the 100+ that other teams have reported. Have tried various barrel lengths and various t-shirt packing diameters from very loose to very tight. Our best results are with a t-shirt that slides easily but does not have gaps between it and the barrel.
We are thinking of trying a different accumulator tank so we can have 1" pipe from it to the solenoid but we keep reading about teams that are getting much better distances with 1/2' or 3/4' solenoids. We don't want to buy the new tank if we don't have to.
Question - are teams lying about their cannon pressures or are we doing something wrong?
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Unread 07-13-2018, 02:13 PM
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Re: T-shirt cannon firing pressure not getting reported distance

My guess is your launching pressure. T-shirts cannons I've seen fire at a much higher pressure than 60 psi. For example, if you look at the specs for 254's Shockwave, they regulate their pressure to 120 psi when they fire.

https://www.team254.com/shockwave/
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Unread 07-13-2018, 03:20 PM
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Re: T-shirt cannon firing pressure not getting reported distance

What pressure is the 1.5 gal tank? Is that at 60psi? If so, then as the t-shirt travels down the barrel the pressure reduces. For ours, we would store at ~120psi then drop to 40-100psi with a 1" regulator to vary our shot distance.

Our setup:
3000 psi SCUBA tank -> 120 psi SCUBA tank -> 3/4"-1" fittings and hose to 1" regulator -> 1" solenoid valve -> ~2.5-2.75" barrel
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Unread 07-13-2018, 04:24 PM
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Re: T-shirt cannon firing pressure not getting reported distance

Can you clariify where the second regulator is and what the pressure is for firing?
Our regulator controls the pressure in the accumulator tank (and distally) and we have it set at 60 psi right now.
What distances do you get with the various pressures? 40, 60, 80 100?
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Unread 07-13-2018, 05:48 PM
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Re: T-shirt cannon firing pressure not getting reported distance

In our experience it's more about how quickly you can deliver air quantity vs pressure. We found that 1" orifice tanks and solenoids were very expensive, so rather we setup parallel accumulator tanks with 3/4" orifices and parallel plumbing to parallel 3/4" solenoid valves -- which then tee into 1" pipe into a 3" barrel.

We use a setup on our plumbing as follows:

scuba tank -> scuba regulator -> 120 psi to tee -> 2 x FRC regulator -> 2 x accumulator tank -> 2 x 3/4" solenoid -> 3/4" to 1" tee -> barrel

We run our accumulator tanks at 2 different pressures, typically about 30-35 psi on one, and 50-55 psi on the other. This allows us to create different "shot lengths" by mixing which solenoid we "fire." We get a 30-40 foot shot with just the "low pressure" solenoid, a 50-80 foot shot with just the high pressure solenoid, and 150+ foot shot when we fire both. Rather than not having enough range, we need to be careful to not fire over the top of the stands. We have accidentally hit the press box or camera sitting on top of the press box at least once each.

We have "test fired" the cannon at higher pressures (up to 120 psi on both tanks) and have a "reach" of easily 250-300 feet at those pressures. We would NEVER recommend running it at such high pressures.

We have found that changing the pressure matters a LOT less than just sheer air volume and how fast you can expel it (i.e. pipe size). Firing from both tanks at 40 psi goes further than from a single tank at even 80+ psi.

A couple of other things we learned:
-- Have big enough air accumulator tank(s). Your 1.5 gallon tank is probably your issue -- i.e. just not enough air to maintain pressure as you expel the shirt from the barrel. We use a pair of 20 lb propane tanks (with propane valves replaced with appropriate pipe fittings) which are 4.7 gallons each so 9+ gallons of air accumulation. Probably overkill, but our tanks hold their pressure fairly well through a shot (about 10 psi drop).
-- Put your solenoid valves as close as possible to the rear of the barrel. This "wastes" less of your air capacity with expansion through the volume of all of your hoses/plumbing.
-- We tried to vary our shot distance by varying the length of time we open the solenoids but that was NOT effective. The solenoids we bought don't cycle fast enough to matter, with about 20ms being the minimum open/close cycle time we could achieve. Opening them for longer than 20ms did not change the length of our shot, it just wasted compressed air.
-- We agree that having a snug fit of the shirt in the barrel is critical! Too loose is worse than too tight.

Safety first! Make sure all your plumbing/fittings/hoses/tanks all are rated appropriately.

Good luck
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Unread 07-13-2018, 08:34 PM
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Re: T-shirt cannon firing pressure not getting reported distance

Quote:
Originally Posted by FredK View Post
We have read every post we can find regarding t-shirt cannons and are not getting the distances that other teams are reporting.
We have a 5 gallon storage tank, connecting through an FRC regulator to a 1.5 gallon storage tank. 3/4" pipe out of storage tank leads to a 1" solenoid followed by 1" nipple, stepping up to a 3" barrel. We are firing at 60psi but only getting about 40-50 ft, not the 100+ that other teams have reported.
Yes, we have gotten 100+ ft with 60 psi and a 3/4" solenoid valve. Our accumulator is rather larger than 1.5 gal; in fact, it's larger than the scuba tank we're using. I'm going to eyeball estimate it at 3-4 gallons. We also keep the pipe run short from the accumulator to the barrel, currently no more than 3 ft with 3 right angles of turns, and in our redesign we're reducing it to about 1 ft and no more than 2 right angles.
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Originally Posted by FredK View Post
Have tried various barrel lengths and various t-shirt packing diameters from very loose to very tight. Our best results are with a t-shirt that slides easily but does not have gaps between it and the barrel.
concur 100% here.

Quote:
Originally Posted by FredK View Post
We are thinking of trying a different accumulator tank so we can have 1" pipe from it to the solenoid but we keep reading about teams that are getting much better distances with 1/2' or 3/4' solenoids. We don't want to buy the new tank if we don't have to.
I think this is it. On our old system, we had 1-1/4" pipe from the tank to just a few inches before the solenoid valve, then 1", and a bushing type reducer to get to 3/4" to mate with the solenoid valve. The valve was as close to the expansion bell as feasible on our old system. Our new one will only have 1" pipe, but for a much shorter run.

Quote:
Originally Posted by FredK View Post
Question - are teams lying about their cannon pressures or are we doing something wrong?
Apart from the Cv of the pipe (a function of diameter, length, and turns), the next big factor once you find the best roll diameter is likely to be the coeficient of friction in the tube. We have used PVC pipe in the past. In our conversion, we are going to something which won't fail so spectacularly, but haven't made the decision yet as to exactly what. If you're using a metal pipe, perhaps polishing or lining [edit] or spraying[/edit] your barrel with something slicker will help.
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Last edited by GeeTwo : 07-13-2018 at 09:49 PM.
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Unread 07-14-2018, 01:37 PM
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Re: T-shirt cannon firing pressure not getting reported distance

5842 Royal Robotics has a similar setup, we have a scuba tank with a scuba regulator that leads to a storage tank regulated to anything between 60-110psi.

Although, the speed of the solenoid does play a large factor when its 3/4in size.
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Unread 07-14-2018, 02:54 PM
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Re: T-shirt cannon firing pressure not getting reported distance

Can you post the distances you get with the different pressures and explain the solenoid speed issue.
Thanks
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Unread 07-14-2018, 03:03 PM
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T-shirt cannon firing pressure not getting reported distance

On ours, we found that pressure was a factor but most importantly was the tightness around the tshirt in the barrel.... the tigher the sirt in the barrell, the farter it goes at same pressure.

This is our first prototype. Using SCBA tank from local fire dept. One is holding approx 3000PSI and second tank is accumulator, we use a 1" valve to release the air from accumulator directly into the barrel.

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Unread 07-14-2018, 03:57 PM
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Re: T-shirt cannon firing pressure not getting reported distance

The fit of the projectile to the barrel is another critical factor. We have used a sabot - a thick/fluffy piece of cloth about 18" circle - that is placed around an end & sides of the shirt as it is inserted into the barrel to make the fit more air-tight. It also gets fired, but flaps away and falls to the ground after about 15 feet. You may consider a piece of stout cord tied to an edge and to the cannon to retain it more predictably.

Basically you put the sabot on the barrel end and push the shirt (and sabot) into the barrel.
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Unread 07-14-2018, 04:25 PM
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Re: T-shirt cannon firing pressure not getting reported distance

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Originally Posted by FredK View Post
Can you post the distances you get with the different pressures and explain the solenoid speed issue.
Thanks
Well sadly I don't remember and haven't used or seen the thing in about a year because I'm not on the team but, estimating 90-120feet at Max pressure but it fluctuates due to how well the shirts are rolled.
But the solenoid speed applies because you may be using 3/4 in piping and fittings and other stuff but because you have a slow solenoid a lot of the pressure is released before the valve is completely open.
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