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Oxygen/Mapp questions

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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Oxygen/Mapp questions

Unread postAuthor: Dornep » Wed Sep 19, 2007 8:59 pm

A few weeks ago I was showing my brother my combustion launcher and explained to him the concept of metered propane and my interest in building a hybrid launcher.

He works for a rather large paper company in the research department. He told me he could get me any type of pipe I needed from stainless to aluminum and he could weld and tap any fittings that It might need.

I've been trying to think of a way to take full advantage of his offer. I want to make a launcher that can safely use Oxygen/Mapp gas and possibly Oxygen/Acetylene. I want to use 2" SCH80 seamless steel with the end cap and pipe beveled to 45 degrees and then welded in place. I have seen my brothers welds and they look damn good so I would feel safe using something he welded. This isn't a hybrid but a basic coaxial combustion on steroids.

My first question is.. Do I need to have a way of venting the chamber if I'm using Oxygen? Also how much of a difference in a performance can I get using Oxy/Acetylene compared to Metered propane? Will it be huge or just barely noticeable?
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Unread postAuthor: Pete Zaria » Wed Sep 19, 2007 9:07 pm

I say go for it, great design...

But if you're going this far anyway, you might as well weld on a steel union to the end of the chamber so that you CAN run hybrid mixtures. You're *this* close already.

Yes, you really should have a way to vent the spent combustion gases out between firings, unless you feel like wasting a bunch of oxygen.

As to the difference between oxy/mapp and oxy/acetylene... I can't give you exact numbers but I'd wager that the difference would be significant.

Best of luck.

Peace,
Pete Zaria.
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Unread postAuthor: Dornep » Wed Sep 19, 2007 10:18 pm

Would a Oxygen/Acetylene hybrid be safe even with a heavy duty setup like this?

I really would like the power of a hybrid but I want a high powered launcher with at least a 2" bore. I will work on my design for a hybrid and put some renders up.
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Unread postAuthor: jimmy101 » Thu Sep 20, 2007 10:48 am

Like Pete said, you are so close to a design that can handle hybrid mixtures... go for it!

The difference in energy between propane and MAPP or acetylene is really pretty minor. Based on heats of combustion with air as the oxidizer, the energies are roughly 105, 114 and 120 respectively. (And hydrogen is ~119.) Not all that much difference.

Acetylene and hydrogen burn faster than propane, roughly 3x and 8x respectively. I don't think anyone knows how much better performance the higher flame speed gives. You can get almost the same affect with a chamber fan. My WAG is that the gain in performance is much less than the difference in flame speed. An 8x faster burn speed does not boost the energy anywhere near eight fold.

Acetylene gives a slightly higher peak pressure than propane (9.6 vs 9.2 ATM). Hydrogen gives a lower peak pressure (7.9 ATM).

http://www.spudfiles.com/spud_wiki/inde ... n_Spudguns

The big difference is the old bugu-boo of DDT. I don't think anyone has ever gotten a gun to DDT with any fuel. But then again, hydrogen and acetylene haven't been used all that much and they are the fuels which would be expected to get to DDT the easiest.

Given that you get to fabricate any basic part you want form very high strength materials, I would skip the hydrogen/MAPP/acetylene idea completely. A hybrid will out perform a 1x gun regardless of what fuel is used. You don't even need to go to high hybrid ratios. I would think that a 2x gun fueled with propane + air will outperform a 1x gun with acetylene or MAPP.

One thing to remember when using oxygen is that the gas in the gun is different the first time it is fired compared to subsequent firings. How much different depends on how much fresh air gets into the gun between firings. If you make no effort to get fresh air in the gun then subsequent shots have less O2 in the chamber than did the first shot since the first shot had 20% O2 in the air but subsequent shots have combustion products plus a variable amount of fresh air. (I'm taking about using pure O2 to bring the chamber back to 20% for each shot.) Since combustion guns are pretty sensitive to the fuel to oxidizer ratio is is probably best to fully air out the gun between shots or charge the chamber to 100% O2 for each shot.
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Unread postAuthor: Dornep » Fri Sep 21, 2007 11:06 pm

Ok I've just finished the setup I would like to try if I ever get a chance to build this thing.

This should probably be in the Hybrid discussion but here it is. Everything would be held together with wood or plastic spacers and hose clamps. There are some fittings that I didn't take the time to fully model but it looks good to me. I wanted the chamber diameter to be at least 3" so I could use a 1.5" barrel and maybe a 2" as was my original intention.

I haven't decided on an ignition but when I do I will update the design. So what do you guys think, anything need work?

These aren't to exact scale, just what looked good at the time.

Edit*** BTW Jimmy your reply cleared the venting question up for me very nicely.
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Unread postAuthor: Novacastrian » Fri Sep 21, 2007 11:11 pm

Are you planning remote detonation?
That's a lot of fuel to have under your chamber :twisted:
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