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Using PTFE or other flouropolymers as a barrel material

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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Using PTFE or other flouropolymers as a barrel material

Unread postAuthor: BigBang J » Wed Jul 14, 2010 10:56 am

Do you guys think that using PTFE or other flouropolymers is a good idea for barrel material on a 10x hybrid?
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Unread postAuthor: Heimo » Wed Jul 14, 2010 11:51 am

I am still busy with my first hybrid also, but if I were you I would go with aluminum or copper or some seamless tubing
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Unread postAuthor: mobile chernobyl » Wed Jul 14, 2010 11:51 am

I think using Butter would be better :D

Check PTFE's tensile strength, and do the math from there.

If you want slick just use a nice stainless barrel with a coating of BAM on the inside walls :wink:
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Unread postAuthor: Zeus » Wed Jul 14, 2010 6:14 pm

JSR needs to post here, self-lubricating durable plastic barrel, puns galore.

I won't recommend any plastic in a 10X hybrid, PTFE is likely to
split (maybe I'm getting my plastics mixed up) like ABS, so
it won't turn into shrapnel, but I havn't seen it applied
yet, so if you try it the results should be interesting.
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Unread postAuthor: rp181 » Wed Jul 14, 2010 7:02 pm

PTFE is soft, and probably will split. I could bend a 1/8" thick piece any way I wanted. a PTFE liner is good though, great [lack of] friction.
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Unread postAuthor: Hotwired » Wed Jul 14, 2010 7:36 pm

The temperature of the hot gases from a 10X hybrid are much higher than needed to decompose PTFE.

Needless to say the gases are not good for you.


Low friction isn't all a barrel has to have.

Especially if you can make do with say, polished stainless steel which would give a harder, stronger and more temperature resistant barrel.

To improve muzzle energy, increasing chamber size, barrel length and density of the mix are all things that you can do with better results unless you've maxed all of that out and are looking to tinker with the details.
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Unread postAuthor: jimmy101 » Wed Jul 14, 2010 7:59 pm

Hotwired wrote:The temperature of the hot gases from a 10X hybrid are much higher than needed to decompose PTFE.

Really? The temp in a 1X gun is nearly 10x the temp needed to decompose PVC but a 1x gun of PVC works just fine.

The question isn't the temperature, it is the heat. The two are not the same thing. A 10X has ten times the heat of a 1X but still, the chamber and barrel aren't going to warm up more than 100C or so. (A typical PVC 1X warms up less than 10C.)

Still, using any type of plastic in a 10X hybrid doesn't sound like a great idea. The barrel will be exposed to the same high pressure as the chamber.
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Unread postAuthor: BigBang J » Wed Jul 14, 2010 8:27 pm

oops, I totally was not thinking about the pressures, I was so tied up in the smooth lubricating material aspect of it. But I was thinking about a ptfe barrel say 1/4" ID 3/8" OD with a stainless steel sleeve over that, 3/8" ID x 5/8" OD, or something like that. So basically having a ptfe liner inside your steel barrel.

What do you think? Still no good?
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Unread postAuthor: Ragnarok » Wed Jul 14, 2010 8:42 pm

With a decent barrel and not overly tight projectile, friction in most cannons is what - the equivalent of 10kPa at most? Pressure from a 10x hybrid mix is more in the region of 7MPa.

That's nearly three orders of magnitude. You'll have to excuse me if I'm not overly bothered about the difference between 1kPa and 10kPa of friction - most people can't even get their chamber pressures that repeatable.

I do like keeping a clean and polished barrel, but that's to do with laminar flow in the barrel (thus improving transonic performance) not because I think friction between barrel and projectile is worth worrying about.

And on that note, in hybrids, the speed of sound in the propellant gases mean that you're unlikely to be in a state where you need worry about transonic losses. I can only recall two hybrids which have recorded velocities that I'd consider transonic.
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Unread postAuthor: BigBang J » Wed Jul 14, 2010 8:59 pm

please excuse my ignorance, but what does transonic mean?
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Unread postAuthor: Ragnarok » Wed Jul 14, 2010 9:27 pm

Where parts of the flow are supersonic, and parts are subsonic. It's a very inefficient region, as the shockwaves caused from the supersonic flow cause turbulence, thus greater drag on the airflow and more energy lost.

The actual range of Mach number that it occurs at vary depending on the flow paths and the smoothness of any surfaces involved, but generally (and this is not specific to internal ballistics), it occurs between about Mach 0.8 and Mach 1.2.

Some things have a wider transonic range - the transonic range of airflow over a sphere, for example, stretches almost from Mach 0.5 to over Mach 1.5 - but some things a narrower range - a good modern bullet design might have a transonic range of only Mach 0.9 to Mach 1.1, or maybe even less.

In a cannon, it can also vary, but in hybrids, I usually work from about 800 m/s as being transonic. Most hybrids are quite some way from those velocities.

Still worth having a smooth barrel (as it's less likely to damage a projectile, and it does actually have a better flow coefficient), but you don't need an ice-slick barrel.
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Unread postAuthor: ramses » Wed Jul 14, 2010 9:27 pm

approaching the speed of sound. Some of the flow is sonic, and some subsonic. A sphere is transonic at around mach .8. That is to say that, at mach .8, some air moves around the sphere at mach 1
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Unread postAuthor: inonickname » Wed Jul 14, 2010 10:05 pm

Using PTFE ONLY as the barrel is a poor idea. It's not terribly strong. It's also very expensive. You'd be much better off buying an aluminum barrel.

Also, when heated enough the PTFE compounds will decompose into flourine compounds eg. HF. For the amount of time it's actually exposed to heat, it shouldn't decompose much.
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Unread postAuthor: BigBang J » Fri Jul 16, 2010 7:40 pm

O.K. but I was thinking of using ptfe as the inner barrel for a better fit and then sliding a steel tubing over that for strength. What do you think? Still no good?
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Unread postAuthor: Hotwired » Fri Jul 16, 2010 8:58 pm

Well what do you hope to gain from using a teflon liner that won't be more easily achieved by a more powerful combustion.
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