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Llamas with Hats

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Llamas with Hats

Unread postAuthor: DR » Mon Apr 12, 2010 4:17 pm

Sitting in the garage, wondering why Llamas with Hats is so damn funny, I had an epiphany:
If a pulsejet's cycling draws in fresh air to ingnite the new fuel air charge, then why couldn't we do the same to vent a basic metered-propane combustion launcher?

We all pretty much understand; That when the fuel/air mixture is ignited... as the gas expands and the combustion chamber empties, the pressure inside the Combustion Launcher drops. Due to inertia of the moving gas, this drop continues for some time even after the pressure falls back to atmospheric. The expansion stops only when the momentum of the gas pulse is completely spent. At that point, there is a partial vacuum inside the Combustion Launcher.

The process can now reverse itself. The outside (atmospheric) pressure is now higher than the pressure inside the engine and fresh air can rush into the end with the PVC Check Valve.

The words from the last 2 paragraphs, were copied and pasted out of a pdf file about Valveless Pulsejets, which was created by Bruno Ogorelec. The words were edited to fit the current application of a new Combustion Launcher design concept.

It may take quite a bit of experimenting to find an ideal C:B ratio, such as the "general rule-of-thumb 0.7:1" for most basic metered-propane combustion launchers. But, how cool would it be to have a launcher that now requires no (additional) lapse in the reloading sequence, to vent the chamber?

My idea is:
If the check valve were mounted so that the flow was towards the inside of the combustion chamber, then perhaps the 14.7 pounds per square inch of force would be more than sufficient to overcome the spring pressure in the check valve and allow fresh air to be drawn in?

Starting with a 0.7:1 C:B ratio, the barrel can be progressively shortened, until the tester experiences a noticeable reduction in performance... Much like Latke' did in his testing.
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Unread postAuthor: Ragnarok » Mon Apr 12, 2010 6:52 pm

While an interesting idea, I turned up a few problems when I looked at borrowing ideas from pulse jets a while back.

The big part is that pulse jets don't transfer the momentum of their gasses to a projectile. Robbing the gasses of momentum to drive a projectile will lessen venting effectiveness.
And with that considered, I'm not sure the gasses will have the remaining momentum to continue their flow out of the muzzle against the vacuum needed to open the check valve.

Yes, the idea can theoretically work, but it will demand something more sophisticated than a standard check valve. (I'm sure have a design for a valve that might have fulfilled the role, but I can't find the file on my computer at the moment. Meh, it'll turn up eventually.)

... and as you suggest, it'll need a less efficient C:B ratio to leave some reasonable momentum in the gasses.
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Unread postAuthor: jimmy101 » Mon Apr 12, 2010 7:05 pm

It would be interesting to know just how well a generic combustion spudgun ventilates all by itself, without any kind of breech valve system.

I would WAG it that 50% or more of the combustion gases are expelled during firing and replaced with fresh air.

I can't think of any easy way to actually measure the CO2 to O2 ratio after firing, if I just had a mass spec...
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Unread postAuthor: POLAND_SPUD » Tue Apr 13, 2010 3:23 am

someone has pointed out that in a piston valve hybrid there is a slight vacuum inside the chamber after each shot... probably due to the fact that the gases are first exhausted, then the piston closes and the gasses inside cool down
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Unread postAuthor: psycix » Tue Apr 13, 2010 8:09 am

Holes in an endcap, rubber flap on the inside that can open, but also seal depending on the pressure differential.

However, I don't think that the gases have enough momentum to suck in all the air you need.
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