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Low mix hybrid vs oxy/propane

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Low mix hybrid vs oxy/propane

Unread postAuthor: Moonbogg » Sat Dec 27, 2008 10:58 pm

I have designed a steel cannon and was going to use basic propane combustion. I now want to make something with a little more power than a regular advanced combustion and get the most out of the steel, but not something so powerful that spuds and other common ammo will vaporize in the barrel.
The goal is to achieve more power while maintaining the ability to take the cannon anywhere you want without having to rely on air compressors or manual pumps. I was thinking about a low mix hybrid, like 2x. A small, refillable compressed air tank could be included on the cannon along with the propane tank to achieve this. Then I got to thinking that the only reason you need twice the air volume is because with double the propane, you need double the oxygen, right? Could similar power be achieved by adding the appropriate percentage of oxygen that doubling the air pressure would provide? This would keep chamber psi at atmosphere and eliminate the need for burst disks, and as long as the air displacement is accounted for, twice the combustion should be achieved.
Am I on the right track here? Would the 14.7 psi of the chamber in a 2x hybrid add any significant pneumatic power along with the combustion of the propane?

I built one successful advanced combustion and I want to take it up a notch. Just a small notch though.
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Unread postAuthor: starman » Sun Dec 28, 2008 12:03 am

It's cool you want to power up some. However, just about any powerup you do, even a burst disk on a 1x combustion is too much power for a potato shooter. You'll want to move to hardened ammo exclusively.

Yes you can double up with O2, just be careful with it and make sure you meter it properly. You would use 21% of your chamber's volume for the O2 shot into the chamber, just like you use 4.2% propane. That would give you 2x worth of O2 counting the ambient fresh air to start. As you can see, you will need a much bigger meter for the O2. Also, you'll need special gauges that certified to work with Oxygen.

Don't do any hybriding with Sched 40 PVC. If you promise to limit to 2-3x you can get by with sched 80. Anything greater, go with metal stuff.
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Unread postAuthor: Moonbogg » Sun Dec 28, 2008 12:51 am

starman wrote:You would use 21% of your chamber's volume for the O2 shot into the chamber, just like you use 4.2% propane. That would give you 2x worth of O2 counting the ambient fresh air to start.


Is this assuming the chamber remains at 0 psi? Injecting 21% of chamber volume with O2 will displace air if the projectile isn't tight fitting and fully sealed. I could do the calcs but thought you may know. Would it even matter that much either way?

Oh, do I have to worry about this DDT thing happening with either a 2x hybrid or doubling up with oxy/propane? I read that multiple spark gaps would help prevent it.

Thanks much for the reply.

edited: I am an idiot. Atmosphere has 21% O2, so to double up no air can escape the chamber. Thanks.
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Unread postAuthor: starman » Sun Dec 28, 2008 1:53 am

Adding another 21% chamber volume of O2 will raise the overall pressure but not as much as if you used air. Be sure your chamber is well sealed.

Don't worry about DDT with this setup. You've still got lots of nitrogen to buffer out your ignitions. Yes, go with at least a couple of gaps... spread equidistant down the center of your chamber.
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Unread postAuthor: SpudBlaster15 » Sun Dec 28, 2008 2:01 am

Using a 83.2% oxygen/16.8% propane mixture will give you an adiabatic pressure of about 260PSI, roughly the same as a 2x hybrid mixture. If you're looking for the best performance without using a pump or compressor, just eliminate the nitrogen from the picture, and use a pure oxygen atmosphere to begin with.
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Unread postAuthor: Moonbogg » Sun Dec 28, 2008 2:09 am

SpudBlaster15 wrote:Using a 83.2% oxygen/16.8% propane mixture will give you an adiabatic pressure of about 260PSI, roughly the same as a 2x hybrid mixture. If you're looking for the best performance without using a pump or compressor, just eliminate the nitrogen from the picture, and use a pure oxygen atmosphere to begin with.


Wouldn't you have to suck out all the air from the chamber to make it a vacuum if you wanted pure oxygen?
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Unread postAuthor: Moonbogg » Sun Dec 28, 2008 2:10 am

starman wrote:Adding another 21% chamber volume of O2 will raise the overall pressure but not as much as if you used air. Be sure your chamber is well sealed.

Don't worry about DDT with this setup. You've still got lots of nitrogen to buffer out your ignitions. Yes, go with at least a couple of gaps... spread equidistant down the center of your chamber.


Well sealed, looks like I need to use a burst disc with anything besides a standard 1x mixture.
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Unread postAuthor: Moonbogg » Sun Dec 28, 2008 2:29 am

Why dont people use pure O2 instead of air with hybrids? You could cram a lot more fuel and O2 into a chamber than you can fuel and air. So, for the same chamber PSI you could have 80% more combustion taking place, right?
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Unread postAuthor: ALIHISGREAT » Sun Dec 28, 2008 5:54 am

Moonbogg wrote:Why dont people use pure O2 instead of air with hybrids? You could cram a lot more fuel and O2 into a chamber than you can fuel and air. So, for the same chamber PSI you could have 80% more combustion taking place, right?


because you get a massive massive boom.
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Unread postAuthor: SpudBlaster15 » Sun Dec 28, 2008 1:32 pm

Moonbogg wrote:Why dont people use pure O2 instead of air with hybrids? You could cram a lot more fuel and O2 into a chamber than you can fuel and air. So, for the same chamber PSI you could have 80% more combustion taking place, right?


Some do.

I built that back in September of 2007. I only used it a few times, because the oxygen bottles ran out too quickly. It was the first oxy/fuel hybrid cannon that I have seen built out of commercial parts. DYI built one last spring also. Someday I may rebuilt it and use an oxygen tank with a larger capacity.
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Unread postAuthor: Moonbogg » Sun Dec 28, 2008 4:04 pm

Oh OK. So compressed air is just cheaper and always available. Seems easier to calculate also. What kind of chamber pressures could I expect from a 2x hybrid?
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Unread postAuthor: starman » Sun Dec 28, 2008 4:07 pm

Moonbogg wrote:Well sealed, looks like I need to use a burst disc with anything besides a standard 1x mixture.


That would be a given.

Moonbogg wrote:What kind of chamber pressures could I expect from a 2x hybrid?


200ish psi, + or - ... :wink:
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Unread postAuthor: Moonbogg » Sun Dec 28, 2008 5:28 pm

starman wrote:
Moonbogg wrote:Well sealed, looks like I need to use a burst disc with anything besides a standard 1x mixture.


That would be a given.

Moonbogg wrote:What kind of chamber pressures could I expect from a 2x hybrid?


200ish psi, + or - ... :wink:


Oh good. I am using Solidworks to run stress tests on the chamber, and I am using 300psi as my pressure for analysis. I am shooting for at least 1.5x FOS @ 300psi. I have to do some changes to the end caps and weld joints. Just because the steel tube can take a ton of pressure, doesn't mean the welds and bolted flange can. Trying to get the most strength out of this design while keeping it as simple as possible for cost reasons. If the design has weaknesses, the steel tube's strength is wasted and I might as well use PVC.

Oh, and 200psi would be totally SICK!!! Thats PWNS any PVC pneumatic i've heard of and takes the advanced combustion gun to a disgusting level.
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Unread postAuthor: starman » Sun Dec 28, 2008 6:28 pm

I can't remember if you are using HGDT or not, but you should if you are going to pursue this. You'll be able to predict your performance and optimize your design.
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Unread postAuthor: Moonbogg » Mon Dec 29, 2008 12:42 pm

Thanks, i'll definately use that. I didn't even know it existed.
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