SpudFiles

would this make a suitable combustion chamber? i have one spare lying around and its made of pretty tough stuff obviously because its army issue so needs to be strong, i'm unsure of the plastic its made of though.

its 1000cc and i'd epoxy a 1.5" or maybe 28mm copper barrel to the cap and i'd have a ready made burst disc holder!

Thoughts?

Is it rigid plastic of sort of bendable?

No, not a good choice.

I've tested similar shapes and materials, and they don't fare well.
Seriously, I've said it before, and will no doubt say it again, things that have a non-circular cross section are not suitable for pressure use.

How come rag? I have figured it because i have not seen many rectangular chambers but i never understood quite why...

Ragnarok wrote:No, not a good choice.

I've tested similar shapes and materials, and they don't fare well.
Seriously, I've said it before, and will no doubt say it again, things that have a non-circular cross section are not suitable for pressure use.

ahh well...

john bunsenburner wrote:How come rag? I have figured it because i have not seen many rectangular chambers but i never understood quite why...

It creates uneven forces around the circumference.

Take a balloon, and blow it up. It automatically takes a circular shape, because that's where the forces are balanced. Squash in the sides, it will spring back.

You might argue that's because the balloon's made that way (To which I say, if that's the case, then why don't they make oval balloons as well?), but if you take say a plastic milk jug, which is naturally near rectangular or square and blow into it (or alternately, fill it with water and freeze it), you can see the sides bowing out.
Any vessel with pressure in it naturally tries to expand out to a circular shape.

If that vessel was not originally circular, then this creates stresses on it as the forces try to bend the sides.
With a circular vessel to begin with, the only forces involved are the stretching of the sides under pressure, thus reducing the load the parts are put under.

It depends on the exact shape and dimensions of parts, but I've seen vessels very similar in shape, but with relatively thicker walls than army water canteens fail somewhere under 100 psi.

This vessel had walls about 3-4mm thick and was about 2" across it's widest diameter, and in a parallel hydro test against a PVC bottle for a non-carbonated drink, which had walls about 0.5mm thick, and a diameter of about 3", the PVC bottle won out, in spite of the much flimsier walls and higher diameter.

That gives you an idea of how much the strength of a non circular pressure vessel is compromised.

That is what i like about you(and most spudfiles members) you sit down and explain things properly. Thank you that helped me!

ALIHISGREAT wrote:its made of pretty tough stuff obviously because its army issue so needs to be strong

Nope, it means it's made by the lowest bidder

jackssmirkingrevenge wrote:Nope, it means it's made by the lowest bidder

The man makes a fair point.

And that means that it's just (but usually by a loose definition) capable of what it was designed for, and little more.
Take assault rifles. They usually work just enough that the army doesn't bother to take the time and money replacing them.

I would use a smiley, but it's a rather more serious issue than deserves that.

@john bunsenburner: Not a problem. I quite enjoy explaining things.

C'mon we have used plastic drink bottles for ever, it's not like its going to blow up in his face. It might shrink a bit but thats about all...

What makes you think it would shrink?

hehehe...that rhymed...

Eddbot wrote:What makes you think it would shrink?

hehehe...that rhymed...

mkay!

Eddbot wrote:What makes you think it would shrink?

hehehe...that rhymed...

Have a heat gun? Try it on a PET (water, coke, etc.) bottle and see what happens.

It's a useful technique for making plastic parts using a plug mold.

CasinoVanart wrote:C'mon we have used plastic drink bottles for ever, it's not like its going to blow up in his face.

We seen use of plastic drinks bottles for a long time, but generally advised against it.

And I assure you, the uneven shape of this canteen will make it a considerably less able pressure vessel than a normal plastic bottle.
I'm serious when I say that off circular vessels are not suitable for pressure.

It'll be fine as long as that burst disk isn't to make it a hybrid chamber of any note.

I doubt it would shrink from heat, those bottles might well have hot liquid put in them during normal use. A fizzy drinks bottle on the other hand crumples as if it'd been punched in the stomach if you pour a kettle of boiling water into it.