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New guy here. Chamber design question (venturi design)

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: starman » Tue Mar 24, 2009 4:24 pm

No helmet or protective clothing, steel cage...?.. :roll: Please don't do that again.
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Unread postAuthor: TideJoe » Tue Mar 24, 2009 4:29 pm

starman wrote:No helmet or protective clothing, steel cage...?.. :roll: Please don't do that again.


Well, not really. I parked the ATV at the edge of the runway next to a large tree. If the ball's heading was in my direction, I took cover. If not I tried to spot the landing. I doubt I'll do it again..... at least not until I get this tennis ball gun built. It shouldn't be near as dangerous and a lot more visible.
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Unread postAuthor: rcman50166 » Tue Mar 24, 2009 4:36 pm

i was trying to model pressure front and what not with that little animation, but if you're good at analysis, you can see past all the faults of what i did and find some at least little useful information
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Unread postAuthor: TideJoe » Tue Mar 24, 2009 6:01 pm

rcman50166 wrote:i was trying to model pressure front and what not with that little animation, but if you're good at analysis, you can see past all the faults of what i did and find some at least little useful information


I think there's some useful information to be gathered from your animation. It looks pretty clear to me that the venturi design pushes the combustion forward and at a quick pace. The combustion in the conventional chamber appears to be more erratic.
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Unread postAuthor: rcman50166 » Tue Mar 24, 2009 6:21 pm

actually now that I look at it, the flame front does move very fast. Just ignore the movement of air in the Y direction and its crystal clear
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Unread postAuthor: rp181 » Tue Mar 24, 2009 6:29 pm

I think it is the opposite. The narrow opening restricts flow, lengthening the burn time, which means a longer period of pressure.

This was confirmed by advanced simulations.

So what if it was falling sand....
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Unread postAuthor: TideJoe » Tue Mar 24, 2009 6:43 pm

rp181 wrote:I think it is the opposite. The narrow opening restricts flow, lengthening the burn time, which means a longer period of pressure.

This was confirmed by advanced simulations.

So what if it was falling sand....


I'm not denying your claim, but do you have a link or a file with the simulation data?
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Unread postAuthor: Fnord » Tue Mar 24, 2009 6:46 pm

I think it's believable to fire a golf ball 1800 feet. It's tough to do with ggdt, but it can't simulate backspin. And anyone who has ever seen a golf ball travel in a perfect line for over a hundred yards can tell you backspin is a pretty significant variable.

Therefore, the only way to tell if this gun is significantly better is to measure its muzzle velocity/energy. A chronograph or ballistic pendulum is the best option, but a video camera and distance markers can be used to give a rough estimate.

We'll also need to know the chamber volume.
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Unread postAuthor: TideJoe » Tue Mar 24, 2009 6:58 pm

_Fnord wrote:I think it's believable to fire a golf ball 1800 feet. It's tough to do with ggdt, but it can't simulate backspin. And anyone who has ever seen a golf ball travel in a perfect line for over a hundred yards can tell you backspin is a pretty significant variable.

Therefore, the only way to tell if this gun is significantly better is to measure its muzzle velocity/energy. A chronograph or ballistic pendulum is the best option, but a video camera and distance markers can be used to give a rough estimate.

We'll also need to know the chamber volume.


I'll see what I can do. I don't have access to a chrono, but I should be able to get some video, maybe this weekend if the weather cooperates.
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Unread postAuthor: jook13 » Tue Mar 24, 2009 7:19 pm

I am part of the crowd that says 1800 feet is definately possible with a golfball at 1x. My combustion sends golf balls literally out of site. Of course my barrel is about 9 feet long and I use metered propane, as well as induce some serious back spin. That back spin makes all the difference.
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Unread postAuthor: TideJoe » Tue Mar 24, 2009 7:23 pm

jook13 wrote:I am part of the crowd that says 1800 feet is definately possible with a golfball at 1x. My combustion sends golf balls literally out of site. Of course my barrel is about 9 feet long and I use metered propane, as well as induce some serious back spin. That back spin makes all the difference.


How do you induce the back spin. Probably half of our shots have some type of side spin (slice, hook).
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Unread postAuthor: jook13 » Tue Mar 24, 2009 7:26 pm

I screwed in a screw at the end of my barrel on top. It sticks in about 1/8 an inch. The golf ball catches the tip of the screw a bit then keeps on climbing.
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Unread postAuthor: TideJoe » Tue Mar 24, 2009 7:30 pm

jook13 wrote:I screwed in a screw at the end of my barrel on top. It sticks in about 1/8 an inch. The golf ball catches the tip of the screw a bit then keeps on climbing.


So a little sheet metal or wood screw would work? Are you using the thin 1.5" pvc (sdr-21?) or the sch 80 2" with a burst disk?
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Last edited by TideJoe on Tue Mar 24, 2009 7:45 pm, edited 1 time in total.

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Unread postAuthor: jook13 » Tue Mar 24, 2009 7:41 pm

Any screw would work. Just something stinking into the barrel for the ball to catch on. I use sdr sleeved inside of schedule 80 2". No burst disk.
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Unread postAuthor: MrCrowley » Tue Mar 24, 2009 8:30 pm

jook13 wrote:I am part of the crowd that says 1800 feet is definately possible with a golfball at 1x. My combustion sends golf balls literally out of site. Of course my barrel is about 9 feet long and I use metered propane, as well as induce some serious back spin. That back spin makes all the difference.

For the record i'm not saying it is impossible with a 1x combustion. I'm just saying that this venturi effect must make a very big difference to be able to send a golfball that far, as without the effect I believe it couldn't be done with that cannon (no fan, metering, average barrel, bad placed ignition and fuel type). Though I don't see starman's cannon shooting 800m, no offence to him or anything, but that's almost twice as far as my SGA (2" piston, 1.5" port), which managed 420m at 110psi with a 4' barrel.
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