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Need help with school project

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: mafaka » Sat Apr 02, 2011 12:42 am

ramses wrote:If the tank puts out 2 bar gauge, there is 3 atmospheres of air. Eventually the air that was in the meter when you constructed it will be purged out.

At this point, you can fill it up to 2 bar gauge, and the amount that will come out under its own pressure will be 2x the volume of the meter. The last 1 bar (absolute pressure) will be the same as atmospheric pressure, and therefore stay in the tube.


But if I have the fuel meter placed as in the picture, wouldn't the mixture from the tube of propane always go down in the chamber since propane molecules are bigger/heavier than oxygen and nitrogen. Or are my physics all messed up? I'm thinking of the last sentence in your post.
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Unread postAuthor: saefroch » Sat Apr 02, 2011 12:50 am

Gases tend not to settle out, but to diffuse. In addition, turbulence will mix the gas exiting the pressurized vessel with the gas present outside the pressure vessel. Propane molecules are heavier, and have a lower diffusion rate, but will not draw vacuum in their own container like mercury can (because it's a liquid).
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Unread postAuthor: mafaka » Tue Apr 05, 2011 3:58 pm

Labtecpower wrote:

Today we have mounted a 80 mm cpu fan in the chamber, we placed it on the opposite side of the spark plug and fuel intake, slightly above the bottom. Does that sound ok, and do you think epoxy glue will hold it in place or will it eventually come off?

Technician1002 wrote:

Thanks for the link of the kettering ignition system, we tried rewiring everything and the sparks became more powerful. Even got it to ignite the gas once even though we didn't have a chamber fan at the time and the mixture is a tad rich.
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Unread postAuthor: saefroch » Tue Apr 05, 2011 4:05 pm

Is the fan positioned so that its flow is parallel to the axis of the chamber? For an atmospheric combustion, the fan matters little except that it creates turbulence during combustion, stirs the mix, and can help vent combustion gases.
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Unread postAuthor: mafaka » Tue Apr 05, 2011 4:36 pm

saefroch wrote:Is the fan positioned so that its flow is parallel to the axis of the chamber? For an atmospheric combustion, the fan matters little except that it creates turbulence during combustion, stirs the mix, and can help vent combustion gases.


I don't really know where the direction of the flow will be. I've tried to describe the mounting with this picture, one view from above and one from the side. The direction of the fan exhaust is 90 degrees relative to the barrel.

http://img695.imageshack.us/i/flkt.png/

Would you consider Styrofoam appropriate for a sabot? We are going to shoot golf balls, but the barrel is 11,33 mm too wide for them. Would it be possible to make one with ~5,5 mm thickness without breaking it?[/img]
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Unread postAuthor: saefroch » Tue Apr 05, 2011 4:42 pm

That won't be terribly good at venting, but will work perfectly well for stirring and creating turbulence during combustion.

Styrofoam may work for a sabot, but I'd be worried about the friction in the barrel and making a good seal with something that's difficult to compress or deform temporarily.
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Unread postAuthor: Technician1002 » Tue Apr 05, 2011 6:48 pm

The problem with foam sabots is the size is directly related to the volume of gas in the sabot. Volume varies with pressure. If you raise the pressure 10X, the sabot shrinks to about a tenth of the original size. This makes them useless as a sabot during the shot. I have shot foam sabots right past golf balls in a 2.5 inch barrel.

Using a non compressible material for the seal works much better.

Bad idea
Image

Good sabot
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Unread postAuthor: mafaka » Wed Apr 06, 2011 1:02 am

saefroch wrote:That won't be terribly good at venting, but will work perfectly well for stirring and creating turbulence during combustion.

Styrofoam may work for a sabot, but I'd be worried about the friction in the barrel and making a good seal with something that's difficult to compress or deform temporarily.


We won't use it for venting, so that is not a problem. Good to hear that it should work for the turbulence, realized afterwards that it would be quite quite an unnecessary job if would have to remove it and reglue it.

Technician1002 wrote:The problem with foam sabots is the size is directly related to the volume of gas in the sabot. Volume varies with pressure. If you raise the pressure 10X, the sabot shrinks to about a tenth of the original size. This makes them useless as a sabot during the shot. I have shot foam sabots right past golf balls in a 2.5 inch barrel.

Using a non compressible material for the seal works much better.


Ok, don't think we'll be able to make a plastic bottle fit though. Is wood a good substitute? Or can that become dangerous when it gets out of the barrel? I'll look into possible plastic containers to use instead

Thanks for the help both of you, it sure saves us a lot of time!
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Unread postAuthor: Technician1002 » Wed Apr 06, 2011 1:42 am

The good news is plastic containers come in many sizes. Many of them can be customized some with either a knife (cut a tapered drink cup at the barrel diameter) or heat and shrink as needed to fit. The 20 oz soda or 500 mL water bottles just happen to be the perfect size for sabots for one of my barrels.

Wood works, but the lighter the sabot the faster the launch.
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Unread postAuthor: mafaka » Thu Apr 07, 2011 1:19 am

When we were about to test the cannon again yesterday with the fan we failed tremendously.

First off, since the fan runs on DC we thought that if we would switch the cables from plus to minus or vece versa we would be able to run it both ways, and therefore we didn't take much notice of which direction we glued it into.
Murphy's law can be applied to the outcome, the exhaust is in the direction of the wall, and it can only run one way. However, it still creates turbulence in the chamber. But probably not as much as if it would have been turned the other way.

Also, now there is something wrong with the ignition. For all we know we have connected everything just as last time we managed to ignite the gas, but now we don't get the spark plug to work at all. There is a spark between the cables were we connect/disconnect the circuit but the spark plug doesn't even seem connected. You could put your finger on it without feeling a thing. Could the battery have been discharged?
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Unread postAuthor: mafaka » Thu Apr 07, 2011 3:30 pm

Today we went low-tech.

We now suspect that we have actually broken the ignition coil in some way. Tried to wire it again today with no succes.
Therefore we unscrewed the spark plug and installed a piezo lighter sealed with electrical tape instead. I know some of you will facepalm, but since we are quite short on time with the project we want to get started with the trajectory tests.

What we learned today is that the chamber fan is really making a difference, thanks for the advice Labtecpower.

We tried shooting a styrofoam cylinder sealed with tape (diameter: 54mm length: ~100-150mm), which at least I think went unexpectedly far. Even though we suffered quite strong winds from the side it went about 20-30 meters.

Now I have made a sabot for the golf balls made of the cut off top of a plastic bottle glued to the top of a large spray can cap. Going to test it on Saturday, can return with an update and pictures if someone is interested.
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Unread postAuthor: kotabuuki » Fri Apr 08, 2011 3:15 am

Wow, that is really unfortunate, but at least you got it to work. I find your post rather coincidental, because I am doing a project for school that is almost the exact same, except I am building a pneumatic with a 2" piston valve. And I too have run into a Murphy's law situation for everything I do. That being said, I think it is just the nature of school projects to be difficult.

Unfortunately, I only know how pneumatics work, so I cannot provide any assistance. I only posted this because I thought that it is cool that other people do this stuff for school too. But anyways, I wish you the best of luck with your project.
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Unread postAuthor: mafaka » Tue Apr 12, 2011 6:53 am

We tested the cannon this Saturday.
It went surprisingly well. Unfortunately we had made the sabot too weak, and it broke after two shots. However we had a taped styrofoam cylinder under the golf ball instead so it worked anyway.

To our misfortune, we could only retrieve one ball. That was after we had taped eight 8-12 inch plastic stripes onto the ball, so the Cd went way off. We measured that shot to 130 m.

The five other balls we could only track midair for sometime but then you lost them. We also searched the field we were shooting on for them but we didn't find them anyway. We think they either went further than the field (380m) or that they sank into the ground (the field was quite soft).

Is it plausible that we shot further than the field, or is it more likely that we just didn't find them?
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Unread postAuthor: Technician1002 » Tue Apr 12, 2011 10:27 am

It is quite plasuable you overshot the field. I get almost 400 yards with a golf ball on only 50 PSI. I always lose full pressure shots for distance.
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Unread postAuthor: warhead052 » Tue Apr 12, 2011 10:33 am

Technician1002 wrote:The problem with foam sabots is the size is directly related to the volume of gas in the sabot. Volume varies with pressure. If you raise the pressure 10X, the sabot shrinks to about a tenth of the original size. This makes them useless as a sabot during the shot. I have shot foam sabots right past golf balls in a 2.5 inch barrel.

Using a non compressible material for the seal works much better.


I have used my own paper shotgun shell looks sabots for paintballs and bbs, and they work perfectly because when I release the pressure, they dont create a tight enough seal to hold everything in place, just enough of a seal to let it all go flying out of the barrel to 100-150 feet away at only 20 psi... You tell ME if thats a good seal.
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