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Ignition past valve / Ignition by fuze

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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Ignition past valve / Ignition by fuze

Unread postAuthor: fogus » Tue Nov 04, 2008 10:37 pm

Hello,

I have a question for the hybrid gurus. I would like get some input on the ignition of a hybrid in two very non orthodox ways: either by fuze (fuze into the combustion chamber) or by igniting the mix after a valve (reusable, non-burst disk) opens.

First, I would like to build a style of hybrid cannon with a fuze. This would be like the medieval cannons which were fired by igniting a short fuze that lead into the combustion chamber. I would like to do the same thing. I thought about perhaps eopxying a sparkler into a hole in a PVC dowel to air seal it and then lighting one end. The seal would break just as the mix ignited. Would this work?

The second question is actually similar in the ignition process. I would like to create a spark in the barrel of the cannon before opening a valve from an otherwise very regular pneumatic cannon (like this one: http://www.youtube.com/watch?v=krWHRFpL7No ), charged with a high pressure hybrid mix. As the valve opens, the fuel air mix flows from the chamber, through the valve and touches the spark. My question is, would this avoid burst disks (given that I were able to start the spark)?

The valve in that video is very common in spud guns it seems. Is it a diaphragm valve? If so, why does it open when the blow gun valve opens? Is there a link that air flows across between the pressure chamber of a pneumatic cannon and the space inside the valve behind the diaphragm?

Thanks all
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Unread postAuthor: DYI » Tue Nov 04, 2008 11:07 pm

I'll address the fuze question first: it could be done, if shear visual effect is what you're going for.

I envision something like a hollow steel bar that screws into the chamber via straight threads and seals with an o-ring or similar. One end (the one inside the chamber) is sealed with a custom machined micro-union that doesn't protrude on the sides (and thus doesn't interfere with threading into the chamber). The thickness and strength of material required to seal this tiny hole against 100 psi or so ignition pressure is very low, it could be tape or some kind of plastic film.

Most of the tube would be filled with whatever delay mix you were using (preferably one that left a solid structure behind, thus increasing containment), and the very end would be filled with extremely fine, densely packed flash powder or similar (just make sure it isn't anything that detonates, as that wouldn't be conducive to safe operation of a normal hybrid), to rupture the little disk. Alternatively, something like thermite could be used, incinerating the disk rather than exploding it, although thermite is oft unreliable even in "normal" applications, being notoriously difficult to ignite, and it may not work well at all on such miniscule scales.

Either way, efficiency would obviously not be as high as a normal hybrid, and unless you're mass producing the ignition cartridges with a CNC operation, they'd be time consuming to make, and would need to be reused.

As to your second question: yes, you could avoid burst disks in this way. It would be more like an extra-loud combustion than anything particularly powerful though, as HGDT sims with weak burst disks will show you (because the burn rates of air/propane. or even oxy/propane, are very low in comparison to the muzzle speeds you'll be wanting).
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Unread postAuthor: psycix » Fri Nov 07, 2008 4:09 pm

I would not go with a fuse. 1 Leaking problems (and when it leaks past the fuse, it could go off as soon as you light it).
Also, when the fuse gets short, it COULD blow outside the chamber before it ignites the mix.

Hmm, maybe you could also just tape a fuse against a burst disk. The heat of the fuse will weaken or burn your burst disk.
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Unread postAuthor: SpudBlaster15 » Fri Nov 07, 2008 6:43 pm

DYI's delayed triggered burst disk idea is pretty much the only way you are going to get a "fuse" to work with a hybrid cannon, and even so, it will be much more trouble than it is worth. What's wrong with any of the numerous methods that use electrical energy to ignite the mixture?
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Unread postAuthor: fogus » Tue Nov 11, 2008 6:42 pm

SpudBlaster15 wrote:What's wrong with any of the numerous methods that use electrical energy to ignite the mixture?


Nothing, I'm just having fun. We'll see if I ever get something like this off the ground.

DIY told me that a 2X propane/air mix will get me a maximum of 250 PSI for a short time and then the pressure would lower to be 1.1X the starting (before ignition time) pressure.

How did he calculate that? How could I do the same calculation for different pressures and different gases? I am thinking of hydrogen/oxygen mixes in particular.
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Unread postAuthor: Ragnarok » Tue Nov 11, 2008 9:27 pm

To be honest, 250 psi peak is optimistic. I'd work assuming 150 to 200 psi peaks, the 200 psi figure for safety calculations, and the 150 for power calculations.

In this case, the general rule is assume 75-100 psi peak for each mix number.
Assume 20-30 psi AVERAGE for each mix number. This however is a more complex matter, for predicting muzzle energies when HGDT or similar is not available.

On the other matter, with propane/air, you have the following chemical equation:
C<sub>3</sub>H<sub>8</sub>+ 5O<sub>2</sub> ---> 3CO<sub>2</sub> + 4H<sub>2</sub>O

A mole (mole being a term for a given number of particles of a molecule) of gas has a set volume at a given temperature and pressure. In this equation, for each mole of Propane, you need 5 moles of Oxygen (from the air).

After Ignition, these total 6 moles become 7 moles (3 of Carbon Dioxide, and 4 of water).

However, there is also 78% Nitrogen in the air to be considered.

So the mix before ignition is approximately 78 parts Nitrogen, 21 parts Oxygen, 1 Part Argon and 4.2 parts propane.

After ignition, the mix is 78 Parts Nitrogen, 1 Part Argon and 29.4 parts combustion products (Water and CO<sub>2</sub>). So we now have 108.4 parts to the 104.2 we started with, so once the mix has cooled again we have approximately 0.6 psi per mix number extra pressure, assuming a closed system where no gases can leak away.

It should be noted that nothing is coming from nowhere here - the large propane molecule is rearranged and split up to create smaller molecules, so the fact that you have more molecules at the end means nothing, the MASS is unchanged.

For Hydrogen/Oxygen, the maths is much the same, but I'm far too tired to explain it.
A good port of call would be to download the program GasEq, which does this sort of calculation for you, assuming adiabatic systems where nothing leaves or enters.

This is not the case in the real world, so common sense must be applied, but with practise, you'll be able to work these things out.
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Unread postAuthor: fogus » Wed Nov 12, 2008 12:32 am

Thanks, Ragnarok.

Would that 70-100PSI rule of thumb apply to oxygen/hydrogen mixes as well?
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Unread postAuthor: jimmy101 » Wed Nov 12, 2008 1:00 pm

Ragnarok wrote:C<sub>3</sub>H<sub>8</sub>+ 5O<sub>2</sub> ---> 3CO<sub>2</sub> + 4H<sub>2</sub>O

A mole (mole being a term for a given number of particles of a molecule) of gas has a set volume at a given temperature and pressure. In this equation, for each mole of Propane, you need 5 moles of Oxygen (from the air).

After Ignition, these total 6 moles become 7 moles (3 of Carbon Dioxide, and 4 of water).

However, there is also 78% Nitrogen in the air to be considered.

So the mix before ignition is approximately 78 parts Nitrogen, 21 parts Oxygen, 1 Part Argon and 4.2 parts propane.

After ignition, the mix is 78 Parts Nitrogen, 1 Part Argon and 29.4 parts combustion products (Water and CO<sub>2</sub>). So we now have 108.4 parts to the 104.2 we started with, so once the mix has cooled again we have approximately 0.6 psi per mix number extra pressure, assuming a closed system where no gases can leak away.

Also have to take into account the possibility of some of the water condensing, particularly if the combustion mix cools back to near 70F. If all the water condenses the molecule count goes 104.2 to 108.4 to ~92.

edit: engrish
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Unread postAuthor: fogus » Wed Nov 12, 2008 11:10 pm

I played around with gasEQ. Does someone mind checking to see if the following would be reasonable for an insane 12X accident (maybe I should stay away from this stuff):

[Conditions] 3
Adiabatic T and composition at const V


[Reactants] ------------ No.moles ------------ Mole fraction
H2 ---------------------1.0000e+00 ------------ 0.66667
O2 --------------------- 5.0000e-01 ------------ 0.33333

[Products] --------------------- No.moles ------------ Mole fraction
H2O --------------------- 7.2559e-01 ------------ 0.61427
O2 --------------------- 4.7970e-02 ------------ 0.04061
OH --------------------- 1.4948e-01 ------------ 0.12654
H --------------------- 5.9053e-02 ------------ 0.04999
O --------------------- 2.8995e-02 ------------ 0.02455
H2 --------------------- 1.7015e-01 ------------ 0.14404
N2 --------------------- 0.0000e+00 ------------ 0.00000
CO2 --------------------- 0.0000e+00 ------------ 0.00000
CO --------------------- 0.0000e+00 ------------ 0.00000
NO --------------------- 0.0000e+00 ------------ 0.00000

[Properties] --------------------- Reactants ------------ Products
Temperature,K ----------- 300. ------------ 3957.8
Pressure,atm -------------- 12. ------------ 124.67
Volume_Ratio --------------------- 1 ------------ 1.0
Moles Prod/React --------------------- ------------ 0.787486
Stoichimetry,Phi --------------------- 1.000
H0, kcal/mol --------------------- 0.013 ------------ 7.125
S0, cal/mol/K --------------------- 38.453 ------------ 68.342
Cp, cal/mol/K --------------------- 6.938 ------------ 11.772
Gamma, Cp/Cv ------------------ 1.401 ------------ 1.203
Mean Molecular Weight, g ---------------- 12.01 ------------ 15.25
Density, kg/m3 -------------------- 5.8544 ------------ 5.85439
Sound speed, m/s ------------------ 539.3 ------------ 1610.7
Enthalpy, H, kcal/kg ------------------- 1.10 ------------ 467.16
Entropy, S, cal/kg/K ------------------- 2790.55 ------------ 3852.28
Intern Energy, U, kcal/kg ----------------- -48.54 ------------ -48.54
Free Energy, G, kcal/kg ----------------- -836.07 ------------ -14779.31
Cp, cal/kg/K --------------------- 577.69 ------------ 771.90
Volume, m3 -------------------- 2.0515 ------------ 2.605
Molecules/cc --------------------- 2.94E+20 ------------ 2.31E+20
Moles/cc ---------------------- 4.87E-04 ------------ 3.84E-04
Viscosity, kg/m/s ------------------- 1.81E-05 ------------ 1.17E-04
KinematicVisc, m2/s ------------------ 3.10E-06 ------------ 2.00E-05
ThermCond,cal/m/K/s --------------- 2.14E-02 ------------ 1.20E-01
ThDiffusivity, m2/s ------------------- 6.32E-06 ------------ 2.65E-05
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