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A Few Advanced Combustion 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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Unread postAuthor: Heimo » Sat Aug 14, 2010 2:08 pm

Gun Freak wrote:Dude, get a stick of it, and pump it up as high as you can with that bike pump of yours, and see if it bursts. Give it like 30 minutes full, and it if passes, then you can't go wrong with it in a combustion.


better Idea: fill it with water before you start pumping, put the pipe into a bucket of water also that should keep you from getting hurt when it bursts...
also putting that bucket around the corner will be a added safety step
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Last edited by Heimo on Sat Aug 14, 2010 2:18 pm, edited 1 time in total.
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Unread postAuthor: ramses » Sat Aug 14, 2010 2:15 pm

Gun Freak wrote:
clemsonguy1125 wrote:Thats the worst idea, Ive heard in a while(No Offense), what if it blows while Im pumping.

Dude I didn't mean stand on top of it while you pump, put it behind a corner and a big thing over it so if it DOES blow, then you're safe and nothing gets damaged except the thing over it.


better yet, fill it with water to drastically reduce the stored energy and time required to pump it up.
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Unread postAuthor: SpudBlaster15 » Sat Aug 14, 2010 2:18 pm

As far as the ABS vs PVC debate, I think most of you know where I stand. The exact comparison will depend upon how exactly you're defining safety, but as it stands, solid core DWV ABS is going to be much less likely to cause injury than even NSF-pw PVC.

What most people seem to forget is even though NSF-pw PVC is rated to withstand a certain magnitude of internal pressure, it is not rated for use with compressed gas. Thus, unless you can find the elusive pressure rated ABS pipe, ALL of the thermoplastics we use are technically 'unrated' for this application.

This leaves us to consider only the likelihood of material structural failure, and the failure mode of the material.

There's a common misconception that DWV ABS pipe has little to no quality control, which makes it prone to defects, and thus unsafe. This is absurd, and completely inaccurate. DWV ABS is not rated for portable water transportation because the organizations which regulate these standards have not approved this particular product for such applications. In reality, the injection molding and extruding quality is carefully controlled, and it's extremely unlikely that one would find a defect substantial enough to cause a mechanical failure of a combustion launcher component.

As the tensile and yield strength values of both materials are similar, one is no more likely to fail than the other through means of relatively slow internal pressurization. However, when we consider impact strength, an entirely different picture is painted. ABS as a polymer is largely based on butadiene rubber, which gives it extremely favourable impact strength and toughness properties. PVC on the other hand is very brittle, and has poor resistance to impact. Drop an ABS launcher on the ground from a moderate height, and you can pick it up and continue firing. Do the same with a PVC launcher, and there's a very good chance you've created significant stress cracking in the material, and upon firing the next shot, the launcher will undergo structural failure.

ABS is clearly the winner here.

Next, consider the failure mode of each material. The toughness of ABS once again shines through here. Just as it's very difficult to initiate cracking in the material, it requires a very substantial amount of energy to propagate existing failures. This is why you'll never see a failed ABS chamber/barrel with more than a longitudinal split, even in hybrid applications (DYI's barrel failure at 6x oxy/MAPP is a good example). However, as we all know, PVC behaves much differently in the event of failure. It shatters into small, sharp fragments, even at low pressures. There's no need to elaborate on this; everyone knows that it's highly dangerous.

So getting back to the topic, which material is better for advanced combustion launchers? I think the choice is absolutely clear. When my ABS advanced combustion launcher failed years ago as a result of user error (Overtightened NPT propane meter fitting, and multiple holes in a single layer of plastic), I was uninjured, as even with a longitudinal crack running nearly half the length of the chamber, the material absorbed the stress of an additional shot, and did not produce fragments, or even release the high temperature gases in a dangerous manner. Had this occurred with a PVC launcher (Which is in fact more likely), I might not be sitting here typing this message.

ABS all the way.
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Unread postAuthor: clemsonguy1125 » Sat Aug 14, 2010 2:33 pm

I completly agree SB15, my problem is that in the US where I live it hard if not impossible for me to get Solid Core ABS, I have acess to cellular core ABS which I am leaning towards. I dont trust PVC not to shatter.
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Unread postAuthor: SpudBlaster15 » Sat Aug 14, 2010 3:08 pm

I know Pete Zaria and a few other American members have used cellular core ABS for advanced combustion cannons without issue. If the launcher is constructed well (i.e. No holes drilled through a single layer of material), it will be safe. Even the cellular core material has a much more favourable failure mode than PVC.
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Unread postAuthor: clemsonguy1125 » Sat Aug 14, 2010 3:11 pm

I figure the worst that will happen is it cracks open, Thanks for your help.
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Unread postAuthor: jimmy101 » Sun Aug 15, 2010 4:02 pm

clemsonguy1125 wrote:Yeah but i still dont like that idea, the only SAFE way to test it is hydrotest which I dont have acess to the equipment

Fill it with water then pressurize with air. That'll do the same thing as a true hydrotest. The only significant energy in the chamber is in the small compressed air bubble.
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Unread postAuthor: clemsonguy1125 » Sun Aug 15, 2010 4:53 pm

Ok, thanks, right now I am thinking Ill make the chamber from 3 inch cellcore ABS then sleeve it in 4 inch Cell Core ABS, This should have the thickness of a piece of solid core and if the inner one cracks, I wont get burnt. Also I wont ever drill through just one layer.
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Unread postAuthor: Technician1002 » Sun Aug 15, 2010 11:29 pm

I have pressure tested 4 inch cellular core ABS. See my test cannon thread in my sig. I hope that helps. This is with US Cellular Core ABS DWV pipe.
This year's pressure test was 100 PSI.

What pressure are you expecting?
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Unread postAuthor: clemsonguy1125 » Sun Aug 15, 2010 11:41 pm

I'm making an advanced combustion so under 120, which is the max for milli seconds, so with two layers I think I'll be fine
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Unread postAuthor: SpudBlaster15 » Sun Aug 15, 2010 11:50 pm

clemsonguy1125 wrote:Ok, thanks, right now I am thinking Ill make the chamber from 3 inch cellcore ABS then sleeve it in 4 inch Cell Core ABS, This should have the thickness of a piece of solid core and if the inner one cracks, I wont get burnt. Also I wont ever drill through just one layer.


I'd suggest cementing multiple 3" couplers inside 4" cellular core pipe. They interface quite well, and the couplers are solid, injection molded material. The strength of such an assembly should be more than sufficient.
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Unread postAuthor: clemsonguy1125 » Mon Aug 16, 2010 10:21 am

On the subject of fittings, since Im using ABS DWV fittings, I need to put a few bolts through the fittings and pipe, will this weaken the pipe to a point will it would be better to not use bolts at all
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Unread postAuthor: SpudBlaster15 » Mon Aug 16, 2010 11:29 am

Are you referring to using bolts in an attempt to reinforce the joints? If so, you shouldn't need them. The axial stress on a cylindrical pressure vessel is half the stress around the circumference, so with proper solvent welds, the risk of joint failure is minimal.

The only fasteners you should need are the bolts that pass ignition/fan current into the chamber. These should be threaded into an area where the chamber pipe and a fitting overlap.
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Unread postAuthor: clemsonguy1125 » Mon Aug 16, 2010 11:52 am

Thanks so would a chamber consisting of 5 dwv couplings glued together with abs cement be safer that a chamber of cell core abs? Btw on your advanced combustions was all the pipe solid core with pressure rated fittings?
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Unread postAuthor: Technician1002 » Mon Aug 16, 2010 12:54 pm

For 120 PSI on US 3 inch ABS Cellular Core pipe, a single layer is fine. I have range tested apples with my ABS Cellular Core air cannon next to a propane cannon of the same dimensions.

4 inch ABS holds up to propane combustion launches with a 2 inch barrel just fine.

You biggest hazard is the clean out cap is slightly flexible and tends to blow off.

If you do use couplings, they are solid, not cell core.
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