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1:1 C/B Ratio

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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1:1 C/B Ratio

Unread postAuthor: Kilash » Sun Jan 05, 2014 12:36 pm

I have a build in mind that involves the barrel and chamber being of the same pipe. The "projectile" would be located halfway in the pipe, creating the separate barrel and chamber. 3 inch diameter, no above ambient mix, propane/butane fueled, no burst disc. What would my internal performance be like?

I'm only interested in the internal ballistics, the drag resulted from such a large projectile is not a concern for me in this endeavor, I just need to know if I can get a good average combustion pressure from a 1:1 C/B ratio.
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Re: 1:1 C/B Ratio

Unread postAuthor: jimmy101 » Sun Jan 05, 2014 1:58 pm

1:1 will behave just fine though with the setup you describe you can actually get any C:B ratio you want at anytime you want, you just move the starting point of the ammo.

HGDT will tell you how sensitive the performance will be to changes in the starting position of the ammo. I suspect the performance will have some particular optimum C:B (for a given ammo and overall barrel+chamber length) but the actual difference in the range of say 0.8 to 1.2 is probably going to be less than the shot to shot variability of a typical combustion gun.

Edit: Seems to me someone used HGDT to study this exact question but I can't find it on the fourm.
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Re: 1:1 C/B Ratio

Unread postAuthor: Kilash » Sun Jan 05, 2014 2:04 pm

Thanks for replying Jimmy. I actually did attempt to model the build with HGDT, but I keep getting mixed or irregular results with it, for example my pressures are very low unless I specify a burst disc pressure. Setting BD pressure to zero results in a peak pressure of only 5 to 7 psi, which can't be the case or none-hybrid combustions cannons would never shoot. Maybe I'm configuring the app wrong?
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Re: 1:1 C/B Ratio

Unread postAuthor: jimmy101 » Mon Jan 06, 2014 4:46 pm

Everything in the HGDT parameter set has to be set right. Here's an example for a 3" x 36" "monogun". The ammo mass and friction values are WAGs. On the right is a screenshot of HGDT, on the left is an Excel sheet where I collected data as I swung the chamber to barrel composition with a total length fixed at 36".
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Re: 1:1 C/B Ratio

Unread postAuthor: Kilash » Sat Jan 11, 2014 12:36 am

jimmy101 wrote:Everything in the HGDT parameter set has to be set right. Here's an example for a 3" x 36" "monogun". The ammo mass and friction values are WAGs. On the right is a screenshot of HGDT, on the left is an Excel sheet where I collected data as I swung the chamber to barrel composition with a total length fixed at 36".


Thanks Jim, seems that initial position of projectile has to be a none zero for some reason, or it all goes haywire. Any idea why?
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Re: 1:1 C/B Ratio

Unread postAuthor: jimmy101 » Sat Jan 11, 2014 4:18 pm

I noticed that as well. Perhaps because of how the model treats the burst disk. The disk has to deform to fail and even though we've set the burst pressure to zero (i.e., no burst disk) the model still needs to deform it.
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Re: 1:1 C/B Ratio

Unread postAuthor: Kilash » Sun Jan 12, 2014 2:06 am

jimmy101 wrote:I noticed that as well. Perhaps because of how the model treats the burst disk. The disk has to deform to fail and even though we've set the burst pressure to zero (i.e., no burst disk) the model still needs to deform it.


Ay, that makes sense. Thanks for weighing in :).
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Re: 1:1 C/B Ratio

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Re: 1:1 C/B Ratio

Unread postAuthor: sparta401 » Sun Mar 23, 2014 10:22 pm

jimmy101 wrote:Everything in the HGDT parameter set has to be set right. Here's an example for a 3" x 36" "monogun". The ammo mass and friction values are WAGs. On the right is a screenshot of HGDT, on the left is an Excel sheet where I collected data as I swung the chamber to barrel composition with a total length fixed at 36".


jimmy101:

Is the spreadsheet in the screenshot available? I'll buy it if I need to.

Thanks!
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Re: 1:1 C/B Ratio

Unread postAuthor: sparta401 » Mon Mar 24, 2014 11:05 pm

Jimmy:

I found the HGDT application, it doesn't seem to work in 64bit computers as it has components compiled for 32bit computers
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Re: 1:1 C/B Ratio

Unread postAuthor: Zeus » Wed Mar 26, 2014 8:38 pm

Just right click the program, and select "Run as administrator", I run it all the time on 64 bit computers
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