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New to hybrid spudding and need some help : )

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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New to hybrid spudding and need some help : )

Unread postAuthor: Mecke » Tue Apr 17, 2012 12:14 pm

Hey!
Im pretty new to hybrid launchers, i have been looking around for a while and now i feel that i want to build my own one! :wink:

Theres one thing that i want some help to understand, and that is about mixes. what is for example a 8x mix? and how do you calculate what mix you are using? :?
And is it any other tips that you guys know that can help or improve my build :P

And sorry for my bad english :roll:
Take care,
Melker Jonsson
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Unread postAuthor: jackssmirkingrevenge » Tue Apr 17, 2012 12:22 pm

this thread is a good place to start: http://www.spudfiles.com/forums/hybrid- ... 13602.html
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Unread postAuthor: Labtecpower » Tue Apr 17, 2012 12:24 pm

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Unread postAuthor: Mecke » Tue Apr 17, 2012 12:46 pm

thanks for the fast answears guys!
Seems like there is alot to read before you start ;) I think i get it now :P
Here comes another question, what would be a good starting pressure for me? i will make my chamber out of solid steel with about 6-7mm thick walls so i dont think higher mixes would be a problem :)
And what if i want to use pure oxygen and propane, instead of air/propane. Is it any formula for mixing that?

thanks in advance.
Melker Jonsson
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Unread postAuthor: jackssmirkingrevenge » Tue Apr 17, 2012 12:53 pm

This was my basic first effort if you want to start on a small scale and need some inspiration: http://www.spudfiles.com/forums/mini-hy ... 21655.html

And what if i want to use pure oxygen and propane, instead of air/propane. Is it any formula for mixing that?


I would steer clear of oxygen, especially for a first attempt. That being said, you can make an oxygen/propane hybrid at atmospheric pressure without the need for burst disks, to a maximum of around 5x.
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Unread postAuthor: DYI » Tue Apr 17, 2012 12:55 pm

EDIT (ninja'd by JSR): Firstly, JSR is using my X system, not the old one, when he talks about an "atmospheric pressure hybrid". Second, you gave no chamber dimensions apart from wall thickness. As such, I can definitively tell you that its working pressure will be between 0psig and 500 000psig :roll:
Third: as regards the oxygen/propane system, just use GasEq to get an idea of what you're working with. Remember that lean mixes are prone to detonation and typically generate lower pressures. If you don't have a good buffer gas like helium handy, use more fuel to compensate.

Don't worry too much about the X system - with the increasing trend toward using more suitable mixes than air/propane, it's largely obsolete. In the case of air/fuel mixes, X simply specifies the number of times atmospheric pressure which the chamber is at before ignition. Since only a narrow range or fuel percentages will ignite in air, the system is largely valid for that case.

In the case of oxygen enriched mixes, buffer gases apart from nitrogen, or fuels with high partial pressures in the mix (hydrogen comes to mind), the X system is almost entirely useless. For example, using the above definition of the "X", a 1X 2:1 oxygen/propylene mix would produce over 300psi on ignition. To avert this, the system I took to using was a modified version of the existing system, where X=(partial pressure of oxygen in the mix)/3.087, which gave somewhat better comparison to the old air/propane mixes (it typically gives an exaggerated estimate of performance though).

What I suggest now is simply specifying the fuel mix as a ratio, and the starting pressure (e.g. 5:2:1 helium/oxygen/propylene). This way people can enter it into GasEq immediately and get a feel for its performance characteristics, rather than fiddling about converting from one of the two X scales to useful numbers.

If you don't have GasEq, get it. It takes very little time to learn, and is an indispensable tool to determine optimal hybrid fueling. Stoichiometric mix calculations in the simplified "high school chemistry" fashion provide a good starting point, but typically give mixes leaner than would be desirable, and don't come close to telling the whole story.
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