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An interesting accident

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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An interesting accident

Unread postAuthor: Combustion Monkey » Thu Jan 22, 2009 4:58 pm

First, nothing was damaged.

I was testing my new golf barrel today. My first shot went off without a hitch, but i noticed some pressure leakage around my threaded chamber fittings. I tightened said fittings up a bit and reloaded. Upon charging the chamber for shot number two the golf ball left the barrel at a very high rate of speed!
I used a few layers of duct tape to build up the breach end of the barrel enough to hold the ball snugly, but not nearly as tight as a press fit spud.
The volume of fuel dumping in and pressurizing the chamber was enough to fire the gun like a pneumatic.
So this begs the question, can a combustion gun fire golf balls without the aid of some sort of burst disk? I know that Starman uses burst disks on his combustions with golf balls, has any one found a way to do it without them?
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Unread postAuthor: CS » Thu Jan 22, 2009 5:03 pm

Odd, never had a problem with metered propane moving the projectile. First of are you even sure that you are injecting the right amount? And if you are perhaps release it slower into the chamber.
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Unread postAuthor: Combustion Monkey » Thu Jan 22, 2009 5:12 pm

The volume of propane was correct. I will try and release it slower on my next attempt, but it will add the same amount of pressure to the chamber no matter how slow i add it, assuming the chamber has no leaks. Once the chamber pressure exceeds the friction on the projectile, the gun will fire unless I am missing something.
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Unread postAuthor: D_Hall » Thu Jan 22, 2009 5:26 pm

You first shot answered your question for you.

If you've some way to safely release excess pressure, everything is cake and pie. During your first shot, the leak in your pipe fitting did it.

If you don't have some way to safely release excess pressure, you get what happened on your second shot.
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Unread postAuthor: Combustion Monkey » Thu Jan 22, 2009 5:33 pm

Ok, I figured that, but having read many of your posts, I would guess that you have a really fantastic idea for this that i cant quite grasp at the moment. Although it just struck me that i could crack my fresh air ball valve wile fueling to vent pressure. Thanks D_Hall for kicking my imagination in the pants!
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Unread postAuthor: frankrede » Thu Jan 22, 2009 6:17 pm

Just open the valve slowly, its real simple.
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Unread postAuthor: starman » Thu Jan 22, 2009 7:33 pm

frankrede wrote:Just open the valve slowly, its real simple.


I'm not sure that's the answer either. It's pretty hard to open a ball valve slow enough that the pressurized fuel won't just slip on by. Even if you manage to do it slowly, the final pressure could still pop the ball out if it's snugly fit into the breech.

D_hall's comment about a pressure release opening is probably your best option short of setting up a burst disk. Klugeboy here installed a fueling pressure release on his cannon. It just becomes another valve to open and close for each shot, but it does seem to work well for him.

Your idea about keeping the fresh air vent open during fueling will obviously work as well.... :wink: Just prepare to close it quickly after fueling.

This was an excellent topic Monkey!!
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Unread postAuthor: Combustion Monkey » Thu Jan 22, 2009 8:44 pm

Ok, so tomorrow I'll resume testing with a cracked fresh air intake valve during fueling. By letting air escape during fueling, I'll offset the air/fuel mixture. This should be easily compensated for by adjusting the fuel charge. But just for the sake of overcomplicating a relatively straight forward operation, anyone want to give ideas on how much i should offset it by? Sometimes i like to make the simple complex (heck just look up my gun!) just to see what happens , please bear with me and play along! BTW I use a 12in x 1/2in meeter tube at 75psi. And the chamber is about 350cubic in, need to do another water volume after the mods.
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Unread postAuthor: frankrede » Thu Jan 22, 2009 8:47 pm

If I were you, I would just get rid of the tape and instead put more of a dent in the pipe.
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Unread postAuthor: Combustion Monkey » Thu Jan 22, 2009 8:52 pm

I cant think of a good way to dent an almost 1/2 in thick wall of pvc without really deforming it, and I would still need to find a way to seal the projectile to the bore to prevent fuel and air venting past it. I was initially going to use a bearing headed set screw till i recognized the fact that i would have no seal at all on the combustion chamber.

Starman, thanks for the great link, i've seen some of his work floating around the forum. And as usual you add great insight into a problem as well as feedback. I always look forward to hearing from you because I know thats it's going to be worth the read!
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Unread postAuthor: starman » Thu Jan 22, 2009 9:52 pm

You can just dimple the SDR21 with a screw and a sched 80 sleeve. Drill a hole in the sched 80 and tap 6-32 threads in it. For a dimple dont drill the hole through the SDR21. Then just screw a short 6-32 screw into the threads and tighten enough to cause a small bump or dimple to appear inside the barrel where you want to hold the ball. You can fine adjust the screw tightness to allow just enough hold. SDR 21 1.5" is soft enough to allow some deformation this way.

Image

I just installed another screw all the way through the sched 80 and SDR 21 to become the rear ball stop. Notice it's on the other side of the sched 80 sleeve to help balance out screw forces....and holding a golf ball just perfectly.

Image

Thanks for the good words!
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Unread postAuthor: Combustion Monkey » Thu Jan 22, 2009 10:05 pm

Great idea and thanks for posting back. The setup you show provides enough of a seal on the golf ball to prevent blow by? I guess it must, but it looks like it just wants to leak pressure.

I may go back to my original idea of using a bearing head setscrew, or a few of them to locate the ball if blow by is of no real concern. And if i get a little bit of blow by i don't have to worry about venting pressure upon fuel filling.

Thanks again for the post, I love anything that gets me thinking in a new direction.
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Unread postAuthor: starman » Thu Jan 22, 2009 10:11 pm

No I wouldn't say it is "sealed" by any stretch. All you have here is a ball holding scheme. However, your blow-by with a golf ball in an SDR 21 barrel is very minimal. They fit extremely well in there. The barrel is only a couple of hundredths of an inch larger than most balls...one reason the single small dimple is effective as a holder.

I wouldn't put any more than 2 dimples at the most and then only for balance preventing spin.
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Unread postAuthor: Combustion Monkey » Thu Jan 22, 2009 10:21 pm

I really like that idea. I'll give that or something like it a try in the very near future. Do you think there is any significant velocity loss by using this method as opposed to a sabot or my duct tape seal? My first shot with the leaky chamber penetrated 1/2in of high end shop board (almost as strong as plywood), but i know the gun is capable of much more.
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Unread postAuthor: starman » Thu Jan 22, 2009 10:44 pm

There really no need to sabot the golf ball and this holder won't be any real hinderance to performance.

I don't know if you caught this vid or not but witness the awesome power of a sabot-less golf ball...here's the burst disk coupler write-up...

[youtube]http://www.youtube.com/watch?v=8tkOpiFHSac[/youtube]
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