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[French questions !]

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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[French questions !]

Unread postAuthor: kornino » Sun Aug 26, 2007 10:05 am

Hi American's friends. I'm a french Spudgun builder and I'm here to drag informations to my french friends.

In first, i don't speak very well english, so excuse my bad english langage, i will do what i can to don't do a lot of mistakes.


I will ask you some questions, because in France we aren't as advanced as you in knowledge of potatoes' launchers.

We have already built a lot of basics Spudguns, with the help of your website SpudFiles, where we have learnt a lot of interesting things about Spudguns.

If you want an exemple of one of our most advanced Spudgun, i can show you my "3 Chambers combustion launcher here :
http://ftpkrono.free.fr/Patator/Trichambre/support%20realisation/DSC00872.JPG
http://patator.frbb.net/users/72/77/74/avatars/195-9.jpg
And if you want to see our forum, go here : http://patator.frbb.net/index.htm
You will be always welcome on our website, and you're always autorised to post even if you speak english :lol:



So now, i'm going to ask you some questions :

->In first, Hybrid Spudguns are nearly unknowns in France, so i have questions about thems :

1) When you say "1x mix", "2x mix", "3x mix", etc... What does it means ?

2) What is the classical material to build an Hybrid Spudgun ? High Pressure PVC is it good ? And don't will explose ?
Or, are we obliged to use metal and other really resistants matérials ?

3) I've understood the functioning of "burst disks", they are totally unknowns in France, nobody hasn't ever use thems. Can you tell me more about these disks ? How to build thems (homemade?) ? And how to keep them between the chamber and the barrel ? Without lose pressure ?

4) Isn't it possible to use valv (like Quick Exhaust Valv) to remplace the burst disks ?

5) What are the classics gases you use in an Hybrid Spudgun ? Butane/Propane/other? And did you inject pure oxygen with these gases ?




Thanks to all of your answers, and a congratulations for what you have discovered in this amazing domain of Spudguns.

Please answer my questions in a basic english langage :)
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Unread postAuthor: Hubb » Sun Aug 26, 2007 10:14 am

1: These indicate at what ratio the fuel / air is mixed at. Example: a 2xmix would be twice injected with fuel and twice the atmospheric pressure.

2: When constructing a hybrid, you should use some really tough pipe, such as metal pipe. Hybrids have a big pressure spike and the pvc could explode. If you do use pvc, consider remote firing.

3: Burst discs are used to contain the pressure in the chamber until it reaches a certain point. Common materials used are aluminum foil (sometimes layered), milk jug cartons, and playing cards. The simplest way to keep them seated is by using a union.

4: It may be possible but, like I said in #3, it is probably easier to use a union.

5: Butane and propane are pretty popular, as well as MAPP.

Also, to not be able to speak good english, you did a very fine job. And, welcome to these forums.
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Unread postAuthor: trae08 » Sun Aug 26, 2007 10:19 am

for your first question.. i dont know that but can easily be found out thru wiki im sure

for the second question most people use sch 80 pvc. but some do use steel pipe

for the third question a burst disk is anything that can be placed in a union. a common one is a coke botle cut into a sheet or aluminum foil . and the way to put the burst disk is by using a union. wiki that to or google it.


for the 4th question. no that is a very bad idea

for the 5th most use propane and air. the pump in the air with a compressor or bike pump and then add propane or mapp gas


hope that helps
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Unread postAuthor: turbohacker » Sun Aug 26, 2007 12:10 pm

Just in case you didnt know what a union was (I had trouble wih pipe names when I first started constructing these things.)

By the way, welcome to THESE forums. :D
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Unread postAuthor: kornino » Sun Aug 26, 2007 12:24 pm

Okay, really thanks turbohacker, i was searching what a union was without success... Thanks !

But, i haven't understand what "1x mix, 2x mix, etc..." means... Sorry, but i have search in Wiki, search in this forum, search by Google, without clear results !
If someone can explain me clearly what does it means, i thank him.


And for an Hybrid, do you, before fire, clear the air in the chamber by pumping to have 0.00PSI absolute pressure, and after do you inject in the chamber explosive gas (butane, propane,...) and(or not?) pure oxygen ?
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Unread postAuthor: Hubb » Sun Aug 26, 2007 12:30 pm

Where I live, the atmospheric pressure is around 14.7psi. With that said, it is assumed that the pressure in the chamber is 14.7psi as well.

Let's use propane as an example for fuel. One would inject a ratio of 4.02% propane into the chamber in order for this fuel / air mix to ignite. This is considered a 1x mix.

Now, up the propane injection to 8.04%. One would then have to increase the pressure in the chamber by 14.7psi. The chamber now has 29.4psi of air, along with 8.04% of propane. This is considered a 2x mix.

Repeat this to gain bigger mixes.

I hope this doesn't confuse you but this is the best way I can explain it. Hope it helps. :)
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Unread postAuthor: boilingleadbath » Sun Aug 26, 2007 12:39 pm

As far as hybrids go, here is my attempt at explaining what is meant by the mix numbers:

Let's say you have a chamber with a volume of 100ml. The size does not matter, I just chose this volume because it makes the math easy.

Now, if you fill this chamber with 96 ml of air (measured at Standard Pressure and Temperature - STP) and 4 ml of propane (measured at STP), we find that the internal pressure will be about 1 bar absolute.
We would call this a 1x mix, because 1 bar is about equal to the pressure of the atmosphere (often, in english, we express we express this by saying "1 atmosphere of pressure").

However, if one where to fill the chamber with 192 ml of air and 8 ml of propane, the internal pressure would be about 2 bar.
Because 2 bar is about 2 atmospheres, we would call this a 2x mix.

A 3x mix would be 288 ml of air and 12 ml of propane; a 4x mix 384/16... and so on.
*********************************************

I am not sure how most users vent their ("clear the air") hybrids, but I know that some just inject clean air into the chamber until they have displaced most of the combustion products.
******************************************

I am sorry if I used more words than you would have prefered, but I feel it is best to provide more context than is needed than to provide too little - and pronouns can be confusing.
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Unread postAuthor: Fnord » Sun Aug 26, 2007 12:53 pm

Alright, maybe I can explain this simply:

When fueling a hybrid, you first decide what "mix" you want to fire at.
A normal combustion potato gun always fires at 1x. If you want to fire your hybrid at 2x, you will need twice as much propane and air than you need for a normal combustion.

You put twice the normal amount of propane in first, then add twice as much air. When you add air for a 2x mix, you should always add air until your chamber pressure is 15 Pounds per Square Inch (PSI).
If you are using kilograms/square centimeter (BAR), then you should have 1 bar in your chamber.

For 3x, you add 3 times the normal amount of propane and air.
The chamber pressure should be 30 psi after you add air for 3x mix.

You must pump fresh air into the chamber after each shot, so that you don't have any of the "burnt" gasses left when you fuel it up again.
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Unread postAuthor: kornino » Sun Aug 26, 2007 1:21 pm

Thanks for all these precisions, all of my questions have been answered and i'm very happy^^
I'm going to report them to my french website, these will be a very good help to french Spudgunners.

But, a last question :
-> How do you know how many propane do you need to put in the chamber ?
4%propane and 96%air are they the stoechiometric proportions (for a perfect combustion) ?

What about other gases (butane, which is very simply to buy in france) ? Is there knowns proportions to respect ?

Thanks !
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Unread postAuthor: Hubb » Sun Aug 26, 2007 1:57 pm

Okay a little math and science:

This first equation is how to determine the percentage of propane.
the chemical formula for propane is C3H8. Combining it with oxygen (combustion) is C3H8 + O2 = CO2 + H2O. The balanced equation looks like this: 1C3H8 + 5O2 = 3CO2 + 4H2O.

According to the balanced equation, one will need 1 propane molecule to combine with 5 oxygen molecules in order to generate combustion.

Air is 20.95% oxygen. With this said, (.2095)(1/5) = .0419 or 4.19%

Now to butane. Using the same equation above, the balanced equation for butane is 2C4H10 + 13O2 = 8CO2 + 10H2O

(.2095)(2/13) = .032 or 3.2%

This equation could be used to figure the percentage of other gasses as well, just obtain the balanced equation for combustion for that particular gas and you're set.

MAPP - 5.24%
Acetylene - 8.38%

Mixes with pure oxygen (not recommended):
propane - 20%
butane - 15%
MAPP - 25%
acetylene - 40%
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Unread postAuthor: jimmy101 » Sun Aug 26, 2007 5:36 pm

You can get the various balanced chemical equations and the percentage of fuel need for air Spud Wiki Fuels.

Folks have done a pretty good job of explaining hybrids. I would add that, as a rough approximation, a 2X mixture has twice as much energy in the chamber as does a 1X. (A basic combustion gun is a "1X" mixture.) A 3X mixture has three times as much energy in the chamber as a 1X, etc.

The peak pressure in a closed chamber, again as a rough approximation, will increase the same way. 1X propane + air in closed chamber ~120 PSIG. 2X ~240 PSIG, 3X ~360 PSIG. So, if the round jams in the barrel a 3X mixture will usually exceed the rating of pressure rated Schedule 40 PVC pipe. Hence the common use of Sch. 80 PVC or steel pipe.

Why not use a valve instead of a burst disk? Mostly because once you fire the chamber the energy in the chamber is stored totally as heat. (Unlike a compressed air gun where the energy is stored as increased gas density.) The gases are very hot, roughly 2200K (~4000F). The mass of the gases, and the heat capacity of the gases, is very small. Much smaller than the mass and heat capacity of the gun. As a result, the gases will cool off, and the pressure drop, very quickly. Indeed, the pressure in the chamber is back near 1 ATM probably within a couple seconds at most. So, if you were to use a valve instead of a burst disk you would have to fire the gun within less than one second of igniting the chamber. For full power you probably would need to open the valve within 0.1 second of igniting the chamber. That would not be easy to do. Much easier to use a burst disk and let the "valve" open itself when the pressure is high enough.
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Unread postAuthor: boilingleadbath » Sun Aug 26, 2007 6:42 pm

Eh, jimmy, there are valves which will open when the pressure rises.

I suspect they aren't used much because they increase the complexity of the device and reduce performance a bit. (and a low-performance hybrid is kind of silly)
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Unread postAuthor: jimmy101 » Mon Aug 27, 2007 2:30 pm

boilingleadbath wrote:Eh, jimmy, there are valves which will open when the pressure rises.

I suspect they aren't used much because they increase the complexity of the device and reduce performance a bit. (and a low-performance hybrid is kind of silly)


Sure, like a pop-off valve. But like you said, the performance is going to suck unless you can find one with a 2"ID.
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Unread postAuthor: dongfang » Mon Aug 27, 2007 6:05 pm

Hi,

I also proposed a gun where the igniter is triggered by the valve opening. Whether that will work well in reality, time will have to show... Of course, some gas will escape the valve before ignition; the question is, how much.

Hot gas is good; compared to cold gas, it is lighter for the same volume and pressure, and it flows easier through everything. It has a higher speed of sound, too. But it will also burn away at whatever it passes by...

Regards
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Unread postAuthor: kornino » Tue Aug 28, 2007 11:08 am

I've found an chamber for my future Hybrid Spudgun, it can hold 60Newton/cm². It is a "cumulus" (sorry i can't find the english translation) like that :
Image


Is it good to make an Hybrid ? Or must it be more solid ?

OR, if it is too fragile, i will use an "extincteur 2KG" :
Image


Thanks,
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