mobile chernobyl wrote:TDOIA - I've worked on a few semi-concepts, and my conclusion is they work best with small volume and higher pressure (paintball gun) with my current air source (compressor). I'd need a high volume air source to make a large scale semi feasible with any reasonable amount of rounds per minute.
Technician - Propane tanks are a good idea, however this is like my "signing off" project for PVC so I won't change anything on it. My next large scale gun, if using metal, will most likely just use a homemade pneumatically actuated 4" butterfly valve, as the butterfly valve alone can be had for like $60, and they are rated for 200WPSI (on the low scale). A good portion of the pumpkin chunkers all use butterfly vales, and when your dealing with large volume, and long barrels, the slightly slower actuation of a butterfly valve versus a piston (which on a 4 inch porting gun will weigh quite a lot!!) will not have any great effect over the length of the barrel (usually 15' or longer)
Propane tanks, water heater tanks, etc are all valid ideas however, and with my current job as a tig welder on pressure vessels (300PSI) I trust my welds. I'd simply just weld a 4" flange to whatever air reservoir I'm using and voila - A big ass spud gun lol.
Nice. I have been reaching the same conclusion that smaller higher pressure is the way to go for many smaller projectile cannons.
On the topic of pumpkin cannons and butterfly valves, they are a good match simply because the projectile is relatively soft. Due to the launch speeds nearing mach 1, most casual observers assume high pressure. This is false. High pressure and high cross section would make mush and produce the "Pie in the Sky" shots that don't count. For this reason pumpkin cannons are low pressure with a very long acceleration distance (Long barrel) To feed this long large volume a large chamber is required. When a valve is first opened, the chamber pressure is at it's peak, so to limit the peak acceleration a slower valve is desirable. Most people do not know that most pumpkin cannons use less than 35 PSI. Higher pressure creates broken pumpkins.
With the pumpkin cannon theory out of the way, is there any reason to stick with a butterfly valve? A high pressure cannon will use a much smaller tank, shorter barrel (under 20 feet), use harder projectiles, and can benefit from a faster valve. At near supersonic speeds the pressure wave off the edge of an open butterfly valve is a concern in terms of valve coef.
Butterfly valve, larger cannon
Are you planning on building a pumpkin cannon?
I was playing with GGDT again an it looks like a 4 inch valve on a 200 PSI vessel feeding a 3 inch barrel (toolie style valve to barrel cone) off a piston valve will be able to exceed SOS by a large margin. I'll play with that more after I collect data off the current 3 inch project. I may even be able to get the small marshmallow cannon supersonic using the 1 inch valve to feed the 3/4 inch barrel with 3 gram projectiles at 200 PSI. (mini marshmallows & sabot) I'll play with it more later.
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