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Hybrid Precautions

A place to ask general spud cannon related questions.
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Hybrid Precautions

Unread postAuthor: frogy » Mon Jul 24, 2006 12:48 pm

BoogieMan wrote:If you mess up on a combustion or a pneumatic, you are out a few dollars. If you mess up on a hybrid, you are dead.


It's unlikely, but it can and will probably happen sometime if people don't know the precautions for hybrids... I would like experienced members to help make a DataSheet \ FAQ here...

I'd also like someone to help explain why DDT happens and what is really happening...

This list is not only for me when I make my hybrid, but it's for everyone that uses \ or makes a hybrids...

Please post all the valid Building and Usage precautions that you know of...

They don't need to be numbered, but it helps...

#1. If the hybrid is made out of PVC, use all Sch 80 Parts

#2. Remote firing of a hybrid is recommended, anything else and you are endangering youself to many failure screnarios like DDT, or a union exploding even at very low "mixes" of propane and compressed air
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Unread postAuthor: SpudStuff » Mon Jul 24, 2006 1:09 pm

what is DDT i have heard it many times but dont know what it means
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Unread postAuthor: mark.f » Mon Jul 24, 2006 2:16 pm

Deflagration to Detonation Transition. It is what occurs when the normal deflagration of a fuel transits into the detonation of that fuel, or the burning of the fuel above the speed of sound. This produces a very high pressure spike.

It is most likely to happen in high mixes, but certain things can make it more likely to happen even in low mixes, even as low as 1 ATM, (though very, very unlikely unless you have a 50 foot long chamber).

"Run-up time" is the distance inside of a pipe filled with a fuel/air, fuel/oxidizer mix required until the transition occurs. This is the time it takes the fuel burning to accelerate to where DDT occurs. Just so you know, decreasing run-up time = bad.

Run-up time is decreased by a few things...

1.) Pipe diameter: as pipe diameter goes down, the run-up time decreases.
2.) Turbulence: turbulence along the flame front shortens run-up time.
3.) The mix: reactivity in these areas are judged by flame-front cell size. Propane is a stable fuel, relatively compared to stuff like ethylene and acetylene. So, basically, don't use sh** like that.
4.) The mix 2: Obviously, as your mix becomes more compressed or rich in oxygen/fuel, (enriched mixes), the run-up time is decreased.
5.) The mix 3: lean mixes shorten run-up time, because the initial speed of the combustion is increased.

DDT is a real threat in hybrid cannons, but many people forget that it is dangerous any where where you have a fuel/air mixture with sufficient distance to accelerate the combustion. DDT has been recorded to have happened, (but I forgot the link :? ), in a 1 atm setting in an open situation with natural gas. I believe an oil rig...



EDIT: and onto construction and safety precautions. All pipe and fittings, SHOULD, be SCH 80 PVC with an ASTM rating. Rated for fluid pressure. Don't just trust an NSF rating, get the ASTM rating.

Solvent welding should be done PROPERLY. Use a PROPER sized applicator and proper techniques and wait 24-72 hours for the joints to cure.

Threaded joints are bad. Reduce to a minimum. Also, don't drill and tap any more holes than you have to.

<font>DO NOT BREECH A 4X/4X ATM MIX!!!</font>
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Unread postAuthor: dan116 » Tue Jul 25, 2006 6:56 am

I think people are becoming to afraid of hybrids.
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Unread postAuthor: mark.f » Tue Jul 25, 2006 7:16 am

People should "fear" and respect and kind of launcher. If you don't your liable to get too comfortable and kill somebody or yourself, (it always seems to be somebody else).

Indeed, people are becoming more respecting of hybrids, because they have their brain screwed on right. Anybody who isn't nervous building or using a hybrid shouldn't be making em, because they probably don't know what their doing. Remember that even with a 3x mix you are igniting a fuel which will produce about 336 PSI in your chamber.

And last, if you're referring to the way people discourage "newbies" from making them, it's all in good intentions. We don't think their stupid, we just think they lack the knowledge to safely build and use such a potentially dangerous device. Obviously, there are exceptions to the rule. Harmon, (on SpudTech), is relatively new, and he's almost finished an all steel hybrid.
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Unread postAuthor: CS » Tue Jul 25, 2006 9:48 am

This as always is just my opinion.

I think such a list shouldnt be made. With this list isnt the intent to make something that users can reference to when making a "hybrid"? Now what if there is a flaw or "weird" wording that confuses the user? Or even leaving some info out on accident. The users should take the usaul route they always have and learn the "why and how", rather then "this does this". When you know the "inner workings" you better understand what is going on and can react to problems that may devolop, faster and with better results.

AKA- ADHD newbs will come on here, "think" that they know it because they read Frogys, hybrid how-to. (I think we all have ADHD to some degree) Yes, most such discussed people are detered in cost, and construction time, but again that doesnt mean everyone.

Mark, where did you get 336? I got 180 in my head after some rough guesstimation. Are you figuring the DDT or whatever? If so, how? Ive never learned or heard on how to figure it...
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Unread postAuthor: mark.f » Tue Jul 25, 2006 11:13 am

I got 336 from ProMeter HX, (from ProMeter Suite).
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Unread postAuthor: dan116 » Tue Jul 25, 2006 12:20 pm

well, that seems way off, lucky13 got 250 with a 3 times.
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Unread postAuthor: mark.f » Tue Jul 25, 2006 1:14 pm

That seems about accurate. ProMeter calculates in "shockwave" pressure from the flame front. 1x, (normal combustion), registers 102 PSI in ProMeter HX.
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