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Compact Tennis ball "Antenna Launcher"

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Compact Tennis ball "Antenna Launcher"

Unread postAuthor: Lentamentalisk » Sun May 17, 2009 7:29 pm

I have come up with some designs for a super compact tennis ball launcher, whose main purpose is pulling a fishing line over a building/tree/what ever. The barrel is only 8" of 2.5" pvc pipe. What makes it work, is that the chamber, despite being only 8" of 1" iron pipe, is pressurized to 300psi. (all numbers are rather rough estimates, but in the right ball park) I plan on using a McMaster 3/4" QEV, because I need it to be as reliable as possible, so a homemade piston valve will not cut it.

My reason for writing this though, is to get your help on figuring out how best to pressurize it.
Hand pumping is out of the question, unless I go the whole nine yards, and build in a hammer valve, because I need to be able to fire successive shots fairly quickly.
Compressors are also out of the question, because this will have to be used in the field, far away from outlets.
That leaves two options, as I see it:
A 20oz CO2 bottle, with regulator, and preferably, though not necessarily, a remote line, or little 16g CO2 canisters, with the bike tire inflators.
Each has its issues. Paintball rigs are expensive to buy at first, and I don't really know what I am looking for. There are lots of paintball guns and tanks on Craigslist for cheap, but I don't know where to take them apart, to get the regulator, and then how to hook that up to my cannon, and how to regulate the pressure. The Bike tire inflators, on the other hand, have no way of regulating them at all, so pose a slight risk there, though I would make sure to install a pressure gauge.

Any and all suggestions are very welcome.

Oh, and as a bonus, I will get credit for this for my end of year Physics project.
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Re: Compact Tennis ball "Antenna Launcher"

Unread postAuthor: spudtyrrant » Sun May 17, 2009 8:25 pm

i would use 12g co2 coupled with an adapter there is a list of pressure to volume ratios here http://www.spudfiles.com/spud_wiki/inde ... _cartridge since you would need about 14ci to achieve 300psi unregged. with your current set-up you will be getting about 677psi in the chamber and that is with a 12g. i don't know if adjusting the volume or the size of the pipe is out of the question but 12g's seem like a good route to take
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Unread postAuthor: Lentamentalisk » Mon May 18, 2009 8:29 pm

Well I made the prototype, using a 1.5" barrel for catnip tennis balls.
The chamber is 12" of 3/4" copper (cheaper than making it out of iron, for some odd reason...)
Here is the GGDT model:
Image

When I actually fired it though, I didn't get near as good of results. Any clue why?
250psi, I got 200fps
[youtube]http://www.youtube.com/watch?v=iV4tJza1sKg[/youtube]
300psi, I only got 220fps. That is drastically lower than the predicted 375fps
[youtube]http://www.youtube.com/watch?v=JVsU6h49T7o[/youtube]

Alright, here it is, posing next to the 1.5" tennis ball. There is a 2' ruler behind it for scale.
Image


My guess, after discussing with Davidvaini and ilovetoblowthingsup is that the flow coef that I used was too optimistic, so I am actually pulling a vacuum by the end. I am going to drill so wholes in the end of the barrel, to effectively shorten it (as well as for my future suppressor.)

Any other advise is gladly welcome.
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Unread postAuthor: Technician1002 » Mon May 18, 2009 9:21 pm

Lentamentalisk wrote:Well I made the prototype, using a 1.5" barrel for catnip tennis balls.
The chamber is 12" of 3/4" copper (cheaper than making it out of iron, for some odd reason...)
Here is the GGDT model:
Image


Any other advise is gladly welcome.


The main problem I see is you modeled a barrel sealer valve and are using a 3/4 inch QEV as the main valve. It's a whole lot smaller than using a QEV on a barrel seal piston valve.

The 3/4 inch measurement on the QEV may be just the pipe size, Open up the valve and measure the actual size of the valve seat orifice and plug this new much smaller value into GGDT.
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Unread postAuthor: Lentamentalisk » Mon May 18, 2009 9:50 pm

I did. As you can see from the photo, I am using a McMaster 3/4" QEV, and the seat diameter is 1", while the piston itself has a diameter of 1.9". Is there something else I should have been measuring, other than the diameter of the part of the piston that "sees" the barrel, rather than the chamber?

One clarification though, is "Vent Diameter" supposed to be the diameter of my pilot valve?
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Unread postAuthor: Technician1002 » Mon May 18, 2009 10:37 pm

Lentamentalisk wrote:I did. As you can see from the photo, I am using a McMaster 3/4" QEV, and the seat diameter is 1", while the piston itself has a diameter of 1.9". Is there something else I should have been measuring, other than the diameter of the part of the piston that "sees" the barrel, rather than the chamber?

One clarification though, is "Vent Diameter" supposed to be the diameter of my pilot valve?


What is the SMALLEST restriction the flow goes through getting out of the valve. The 1 inch piston seat is not the right measurement to use if it has to exit through a 3/4 inch pipe. Use the smallest diameter as the seat size which may be the ID of the pipe screwed into the valve or the hole in the seat which may be smaller.

Having a 3/4 inch pipe with a hole in the middle of 1 inch is a pretty good trick. :)
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Unread postAuthor: Lentamentalisk » Mon May 18, 2009 10:41 pm

Even when I lower the flow coef to 35%, and the seat diameter to .7", I still get velocities in the order of 300fps, not 220fps. In fact even if I lower the flow coef to 20% it is still at 250fps...
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Unread postAuthor: Technician1002 » Mon May 18, 2009 10:58 pm

Lentamentalisk wrote:Even when I lower the flow coef to 35%, and the seat diameter to .7", I still get velocities in the order of 300fps, not 220fps. In fact even if I lower the flow coef to 20% it is still at 250fps...


Part of that is the valve type you picked is one assuming the piston is "in" the chamber. The over/under QEV has more plumbing and bends to get to the valve which creates a ton of turbulence before it reaches the valve. As the flow tries to approach supersonic, it can't negotiate the corners without a lot of friction.

I don't have GGDT on this computer. Check your valve type selection. You may find a better match as some popular vavles are modeled in GGDT.
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Unread postAuthor: Lentamentalisk » Sat May 30, 2009 11:06 pm

ok, well after tons of work, I have a working cannon... I bought a pressure washer handle and the required fittings, so it has a much better feel. Then I decided to take off the case, to find out what was inside, and it turns out, it is the perfect thing for a hammer valve. All I need is a hammer. I spent all day trying to figure out what to use, but have completely failed at all my attempts. Simply hitting it with a real hammer was ok, but took a very firm hit, so I opened up the valve and replaced the spring with a weaker one, so a very light tap is all that is needed to open it. The trick now, is to figure out how to mount the hammer, which I am having serious troubles with. Any and all suggestions are gladly welcome. Attached are photos of the new layout minus the big handle covering, so you can see the inner workings, and hopefully provide me with some ideas as to how to attach a hammer.

The holes at the end of the barrel are both to make the barrel effectively shorter, so I don't pull a vacuum, and for the slide on silencer I made out of 4 2l soda bottles and some other junk.

The quick connect will soon be replaced by a schrader valve, to use with the CO2 inflator I recently bought, but I have it set up this way to work with my fridge compressor, while I am testing it.

Image
Image
Image
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Unread postAuthor: Lentamentalisk » Mon Jun 01, 2009 11:21 pm

OK, double post, but man up and deal with it. I switched over to CO2 today, and ran into some serious problems. When I empty out the canister into chamber, it gets so cold that it actually freezes over. I tried pouring hot water on the canister to heat it up, which worked for a few more PSI, but when I gave up and unscrewed the inflator, I found a huge chunk of ice attached. Needless to say, the pressure I am getting is dismal... Only about 250psi, and I /NEED/ at least 300. I am considering turning the inflator upside down, to spit out liquid CO2, since I can't over fill the chamber, and that way the entire chamber could be a heatsink/radiator rather than just the cartridge.
This is definitely making a full paintball CO2 setup look a lot more attractive...

Any suggestions about making a heatsink or something will be greatly appreciated!

btw, the inflator is this one:
Image
Image
notice: the CO2 cart in the last photo was actually put in the inflator backwards, so as not to puncture the seal.

edit: videos up in a little bit... Youtube is down for maintenance.
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Unread postAuthor: Technician1002 » Tue Jun 02, 2009 1:13 am

On the CO2 the only comment I have is getting dry ice snow from releasing liquid CO2 is normal. Inverted or not, you will have to wait for the gas to warm to get normal pressure. Pre heating the cartridge is dangerous and doesn't work.

What distance are you trying for with the tennis balls, or are you just trying to reach a particular speed?

For the valve flow issues, I noticed you are using a 3/4 inch diameter chamber and feeding a 1.5 inch barrel. If the gas can get through the valve and bends at the speed of sound, the max speed in the barrel would be 1/4 the speed of sound not counting expansion.

Is it too late to go with a larger diameter and shorter chamber of about the same volume? The fittings up to the QEV could be larger so the flow in this area is slower (same volume of gas flow) so there is less pressure loss prior to the valve.

Pressure loss before the valve results in gas expansion before the valve which means less can get through the valve in any unit of time.

Small packages of high pressure air can get though the valve easier than a huge package of (same mass) of low pressure air as the expanded air must travel faster through the valve resulting in more energy loss to friction. This may be part of your performance loss. You are getting performance in just under 1/4 speed of sound range. This may be due to the max flow rates through your 3/4 inch parts.
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Unread postAuthor: Lentamentalisk » Fri Jun 05, 2009 9:48 pm

Alright everyone, another update here.
My most recent setup:
Image
Notice the current setup is for use with my fridge compressor, but the fitting underneath is for use with the CO2 inflator, also shown under the cannon. For ergonomic issues, the barrel is on the bottom, though that may change in the future.

I need to get rid of the whole plastic junk around the pressure washer handle, because it is a good 6 inches of hollow plastic, tanking up space, and compactness is my highest priority. I am considering using BtB's new valve guy, because of how simple it would be to use, and I could do with out the billion fittings needed to convert from pressure washer fittings to NPT.

I am getting 160fps pretty consistently with 350psi (what I get from a fill with a 12g CO2 cart). I was really hopping for better, but it may be somewhat caused by my largeish pilot volume, and small trigger, because I was getting more like 250 with a 1/2" ball valve. We will see when I try the new valve system.

I had a serious freezing problem with the tire inflator, but turning it upside down while filling fixed that. Now the large metal chamber serves as a heatsink/radiator for the boiling CO2, so that the temperature doesn't drop as much in the cartridge.

I think I am just about ready to demonstrate this to the people interested in a cannon like this...
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