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Airsoft RPG7: Design Help

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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Airsoft RPG7: Design Help

Unread postAuthor: T-Bone » Thu Sep 09, 2010 9:19 am

all right, here is my concept and initial design, followed by my questions:

Concept: Bike Pump powered launcher, using 2" nerf footballs as the projectiles. pressure chamber and barrel constructed out of PVC, with ball valve.

Design:
2" PVC Cap->2" PVC tube->Ball Valve->2"PVC Tube
-Mini Type Bike Pump

Issues:
1. Ball Valve is incredibly hard to turn quickly, and i am worried i wont get the action i need with the slow turn. i wonder if it is possible to lubricate it, or whether it turns more easily with wearing it in. open to other valve ideas, but cost should be considered.

2. How large should i make the pressure chamber? i want to be able to pump it up using the mini bike pump, and am looking to shoot the football anywhere from 110-180 feet.

3. should i attach a pressure gauge?

Design Notes:
-Must be air powered
-Looking to make 3 to 4 of these, so i am looking to keep costs under $60
-open to CO2 propulsion, if i can find a way to do it with my budgeting and lack of engineering knowlege.
-If anyone has a similar launcher, but of different design, let me know!
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Unread postAuthor: jackssmirkingrevenge » Thu Sep 09, 2010 9:37 am

First of all, since you're limited to a relatively small chamber and lowish pressure since you want to pump it up manually, I fould forget about the ball valve and go for a 1"-1.5" sprinkler valve (have a look- complete with pneumatic actuation mod, less than $20). A smallish chamber - say 6" of 2" PVC - should be relatively easy to get to 100 psi or so and should give you the sort of range you require.
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Unread postAuthor: T-Bone » Thu Sep 09, 2010 10:51 am

never worked with anything like that before. do you have any plans to make a launcher? i am fairly new to the design process, so im trying to make it simple.
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Unread postAuthor: Gun Freak » Thu Sep 09, 2010 11:22 am

We don't give out plans here but there are plenty of tutorials on the internet. For example...
http://www.youtube.com/watch?v=9PKVyzV0aGU
and
http://www.youtube.com/watch?v=A3EOdNP6Iag
It should be about 20 bucks for a good modification and it is the easiest way to get good power out of the specs you provided. With about 2 feet of 2" PVC you should get some decent power.
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Unread postAuthor: Technician1002 » Thu Sep 09, 2010 11:43 am

If it is any help, my Marshmallow Cannon with a tennis ball barrel is easy to pump to 100 PSI with a hand pump. It is able to launch t shirts 200 feet with the 3 foot long 2.5 inch barrel. The pressure chamber is a 1Lb propane bottle modified to use a 1 inch quick dump valve. If you don't count the labor costs, it is in your price range. It is very labor intensive to build.

A 4 inch ABS chamber 4 inches long feeding a 1.5 inch sprinkler valve with a Nerf rocket size barrel about 2 feet long should perform well and be within your budget. I do recommend the sprinkler valve modifications for non electric use.

Large chambers take lots more pumping, but with diminishing returns on distance. For rapid cycle time, a smaller chamber is better.

As for the pressure gauge, you can use the one on the pump. This is what I do.

Image

Field testing with tennis balls provided the results shown below. Points were measured by GPS and visual orientation to the trees on the right.
Image
For less distance, use lower pressure. Note that sprinkler valves are less efficient and will produce shorter ranges than the QDV.
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