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Is this a mini?

Posted: Sat Oct 09, 2010 6:04 pm
by Jim Beam
Probably to big to qualify, but I thought I would share anyway.

I built this so I would have something to shoot the leftover potato rings when I'm shooting my jumbo spray-n-pray cannon.

I used roughly the same proportions as my big cannon. The barrel is 1/2" dia. and 21" long. The chamber is a PVC tee fitting 2" dia. by 4" long. I used a peizo sparker from a lighter and managed to rig a pistol-style grip and trigger.

It took a while to find a gas syringe, but once I did I quickly figured out the amount of gas to use.

So now when I'm shooting the big cannon, someone else can have fun with this. And less of the taters is wasted.

Posted: Sat Oct 09, 2010 6:46 pm
by linuxexorcist
I don't know, "mini" is a pretty loose term, generally spray n' pray guns have a >1" bore, so do advanced combustions, I'd say it's a mini.

It certainly looks like a nice build, what do you mean by "gas syringe", to measure gas I just use a syringe with a piece of brass tubing glued on to fit a butane bottle valve-thing.

I'd assume that since you're using a syringe you're getting good stoichiometry?

Posted: Sat Oct 09, 2010 7:31 pm
by Jim Beam
well it's actually a syringe used in a doctors office to clean out peoples ears!

But it slides easy and has graduation marks so I know how much gas I'm using. I plug it in to the gas bottle. When I open the valve slowly the bottle pressure will move the plunger back.

I calculated the chamber volume and use about 6% propane - right in the middle of the explosive range.

Posted: Sat Oct 09, 2010 8:18 pm
by inonickname
Ideally, you should use 4.7% propane IIRC. Not a bad little gun. Clean, safe, and no doubt fun. Good work.

Posted: Sun Oct 10, 2010 7:10 am
by Ragnarok
Jim Beam wrote:I calculated the chamber volume and use about 6% propane - right in the middle of the explosive range.
Mathematical middle of the explosive range doesn't mean the ideal mix. It'll do, but it's easy to show that's not quite stoichiometric.

For the appropriate mix, you inject 4.2% propane by volume - as calculated by 1 molecule of propane to 5 of oxygen, and 21% oxygen content in air.

Posted: Sun Oct 10, 2010 11:14 am
by linuxexorcist
Also be sure you know your exact chamber volume, one way to account for fittings and such is to load it, weigh it (in grams), fill it with water and weigh it again, the difference in grams is the volume in ml (at sea level & room temp, not a significant difference anyway, just thought I'd say)

Posted: Tue Oct 12, 2010 11:05 am
by Jim Beam
Ragnarok wrote:
Jim Beam wrote:
For the appropriate mix, you inject 4.2% propane by volume - as calculated by 1 molecule of propane to 5 of oxygen, and 21% oxygen content in air.

you mean I've been WASTING 1.8% of each 10 cc volume charge on my pistol ?! Oh, the tragedy!

Posted: Tue Oct 12, 2010 11:13 am
by Ragnarok
A properly stoichiometric mix will burn better and provide greater performance. If you want to both use more propane and lose power as a result, be my guest.

Either way, I'd appreciate less sarcasm.

Posted: Tue Oct 12, 2010 2:54 pm
by SpudBlaster15
Jim Beam wrote:OH MY GOD ! !

you mean I've been WASTING 1.8% of each 10 cc volume charge on my pistol ?! Oh, the tragedy!
Ah, arrogance and ignorance, what a lovely combination. Reminds me of a certain member we had here years ago, who went by the name of Affliction.

Anyway, as Rag said, you're wasting more than a tiny molar fraction of propane. Peak pressure occurs very near the stoichiometric oxidizer to fuel ratio. Deviating from this range will significantly drop muzzle energy and velocity, as this graph from Burnt Latke shows:


a little rich

Posted: Tue Oct 12, 2010 4:11 pm
by Technician1002
When you have too little fuel, it is hard to get ignition. When too rich, ignition tends to be low on power. Enjoy this example.

Posted: Tue Oct 12, 2010 6:28 pm
by Gun Freak
LOL I haven't seen that particular video but I saw another one that the potato only went 2 feet lol.

Posted: Wed Oct 13, 2010 11:08 am
by jimmy101
Too much fuel is a problem for reasons other than stoichiometry. From a stoichiometric viewpoint 8% propane should be virtually identical to 4%. All you've done is reduced the effective volume (and theoretical energy) of the chamber by 4%. That's probably too small a change to be noticed.

But there are affects other than just stoichiometry. Get up above 6% or so and the combustion rate starts to drop significantly. Latke's data is showing a clear drop off in performance at 5%. At 7% the flame front moves very slowly and the gun'll fart out the ammo when only a few percent of the fuel has been burned.

In an open tube, propane + air flame front speed starts off at about 1.5 FPS and accelerates to about 30 FPS. In a very rich mixture you don't get the flame front acceleration. Indeed, the flame front only moves at about a foot per second.

Stoichiometric Mix:
(The flame front is marked with the red arrows)

Rich Mix (7% V/V propane)

(The flame front isn't marked but it it the faint diagonal line about 10% of the tube length from the left end in the last image)

A closed chamber will behave a bit different than the tube. The tube is probably pumping hot gases out of the ignition end (left) which is making the movement of the flame front appear slower than it is. Still, in the bottom frame that is the flame front, the faint slightly tilted blue line almost all the way to the left of the tube. (The tube is ignited about an inch from the left end.) It took the flame about 3 seconds to progress to the far end of the 46 inch long tube.

From Propane comnbustion in air in a semi-open poly8carbonate tube.

BTW: The stoichiometric volume depends on how you fuel. With a plumbed meter stoichiometric is 4.2% by volume propane. For a syringe it is 4.0% by volume. The difference is whether or not the injected fuel is added to the air in the chamber or if it displaces air from the chamber. Syringe fueling typically displaces, so for a syringe meter the stoichiometric ratio is 4.0%, not 4.2%.

Posted: Wed Oct 13, 2010 2:57 pm
by SpudBlaster15
Good post Jimmy.

I should clarify that the flame front propagation rate is exactly what I was referring to when I brought up the relationship between stoichiometry and peak chamber pressure.

Indeed, any mixture within the explosive range of propane/air will produce roughly the same energy, ignoring the small volume differences, as well as any changes in reaction enthalpy caused by the formation of alternative combustion products. However, as the system is not adiabatic, and the chamber walls have relatively high heat capacity, the rate at which the energy is released becomes very important in determining the peak pressure.

Flame speed is probably the single most important factor in a combustion launcher, especially one which relies on the static friction of the ammo alone to contain the combustion gases prior to initial projectile movement.