Ceramic Lady

[Al Belli]

Hi PerPlex,

Fiberglass will melt.
It is made by melting glass " marbles " in a high rpm rotating inconel chamber that has many tiny holes around the periphery.
The glass strings are sucked into a duct and accumulated into a mat that is used as insulation.
I lit a small pinch with a match and it melted. ( see attachment )

Al Belli

[Mark]

http://composite.about.com/library/glos ... -q4372.htm

"Although pure silica is a perfectly viable glass and glass fiber, it must be worked with at very high temperatures which is a drawback unless its specific chemical properties are needed. It is usual to introduce impurities into the glass in the form of other materials, to lower its working temperature."
http://en.wikipedia.org/wiki/Fiberglass

[PerPlex]

But maybe that melted glass fiber would be good too. I mean the problem with the ceramic is that it cracks very fast right? so if the structure of the ceramic gets a little more "soft" it may be able to survive longer.

when Iam at home next time i will definetly try this out by making two identical jam jars one out of ordinary clay and one with the glassfiber-stuff mixed in it.But they will eventualy survive both because the pressure differences and the shocks in a jam jar are to small for cracking them.
whatever worth a try
Alex

[tufty]

But maybe that melted glass fiber would be good too. I mean the problem with the ceramic is that it cracks very fast right? so if the structure of the ceramic gets a little more "soft" it may be able to survive longer.

when Iam at home next time i will definetly try this out by making two identical jam jars one out of ordinary clay and one with the glassfiber-stuff mixed in it.But they will eventualy survive both because the pressure differences and the shocks in a jam jar are to small for cracking them.
whatever worth a try
Alex

It's not pressure that will kill a clay or ceramic pulsejet (modulo the odd big bang when starting) it's thermal shock, exactly what kills glass jars. There's a big thermal differential on a pulsejet, from thousands of degrees at the combustion chamber to mere hundreds at the exhaust (and, if you're running on methanol or liquid LPG, potentially close to or even below zero at the inlet).

There's 4 separate issues to deal with here:

Resilience during drying and firing. Similarly to making any other ceramic, there's issues with shrinkage and distortion during drying (which makes building around a core difficult) and cracking during firing.

Then there's the thermal differential between very hot and less hot parts as described above - this will cause stresses between different part of the jet

Also, there's the thermal gradient through the walls, particularly in the CC area, there will be stresses in the fabric of the jet itself.

Then, remember the inlet and exhaust are going to be receiving slugs of gas alternating between "very hot" and "ambient" temperature, there's a massive amount of thermal cycling going on. For something like the chinese, there's also the relatively cold inlet blast hitting extremely hot CC walls.

Once you've beaten all those, there's vibration, "machinegunning" on starting, cooling after use, physical shocks due to landing or high-G manoeuvers for flight engines, etc.

It's a tough environment for any ceramic. Not that I think the problems are necessarily unresolvable, but I don't think it's gonna be terribly easy to do either.

Do try it out, though.

Simon

[pezman]

Per simon's discussion on the various thermal stresses on a ceramic components. While most ceramics seem to be a little more susceptible to thermal shock, they do have the great advantage that they can tolerate incredibly high temperatures, which I think introduces the option of insulating the structure.

Decreasing radiative loss sure seems like it could be the single most significant improvement that could be made in a pj while retaining all of its other elements of simplicity. That heat needs to go somewhere, and by insulating the structure more heat is bound into the motive fluid. I think that Larry's experiments in melting steel indicate that insulating a steel PJ would be a formula for its demise.

Insulation seems like it would have the secondary benefit of smoothing out temperature gradients in the structure -- for sure the transverse gradient and possibly the axial gradient.

BTW, below are some heat-related bits of data that I dug up on alumina vs stainless (I didn't double-check these, so apologies for any gross errors and I realize that the characteristics may be quite different at high temps). The conductivity of alumina was a big surprise to me. An uninsulated alumina jet might be a bit worse than its stainless counterpart in terms of radiative loss.

I was also somewhat surprised by the heat capacity. I wonder if an insulated ceramic pj might heat incoming air up all to well, which I presume would limit the mass being ingested and decrease thrust a bit.

BTW, you can get graphite coated glass gaskets at Home Depot and you can get adhesives and mortars for repairing fire places there as well. You could almost certainly use these materials to make an inexpensive, durable jam jar or maybe even a working FWE.


Alumina
Specific gravity 3.89
Heat capacity 0.85 J/g-°C
conductivity at 20°C (W/m.K). 40.

Stainless
specific gravity 8
heat capacity 0.5 J/g-°C, 0°C to 100°
Conductivity at 100 C, 16.2 W/m-K

[pezman]

How about rock-wool ...

[Eric]

Rockwool melts and turns to slag....

[pezman]

faaaaascinating -- kind of an "ashes to ashes" thing

That is interesting, since its parent material melts at a pretty high temp. According to thei Wikipedia article (link below) it is manufactured at about 1600 degrees C. Most stainless melts at about that temp. In all likelihood, the rock-wool is taken well above its melting point, but it sure seems that it is in the neighborhood o stainless steel's

I wonder if its slaggy form has any fibrous content in it? Maybe it is possible to make something of rock-wool and let the interior develop a glaze -- it should only penetrate so far and the boundary between the glaze and the fiber should be pretty tough. Then coat the interior with a thin layer of something that can really take the heat. Or, alternatively, wrap an alumina slurry with rock-wool. With any luck, the slag/alumina interface would be reasonably durable, and the surrounding fibers would stabilize any micro-cracks.

I suppose alumina wool would be an alternative. I think that it's not too insanely expensive.

http://en.wikipedia.org/wiki/Rock_wool

[heada]

I recall reading somewhere (I don't remember where) but there was a process that would take carbon fiber cloth, form it into the shape you wanted, burn it and then treat it with something (no clue what) that would then strengthen it again. It was now extremely light weight (more so than the carbon fiber was before) could take extreme temperatures (orbital re-entry type temps) and yet be strong (don't know how strong)

Unfortunately, this process was fairly expensive and time consuming.

I think the best idea so far is the cork lining. Its cheap, readily available and fairly well suited for the task. I know it's been used for similar tasks before.

-Aaron

[pezman]

Mike Everman posted some links on the topic here:
http://www.pulse-jets.com/phpbb2/viewtopic.php?t=4468

Apparently just flowing alkanes through a structure at 1100 C or so will cause them to crack and leave carbon in the fiber structure. I guess the carbon kind of sinters on to the structure?

I gather that the resulting structures only have their high-temp resistance when there's no O2 around. Otherwise, they are only good to 600C or so.

I think making a densification chamber might be hard for amateur builders and coating the resulting structure with a reliable barrier against O2 might be non-trivial as well.

I agree, though, that the cork idea has an elegance that is almost impossible not to love.

[Mark]

http://en.wikipedia.org/wiki/Space_shut ... ion_system
http://www.blaylockgasket.com/hi-temp.html#tape-sil
Just a tidbit/tangent.
"Although quartz melts at 1670°C, the hydrothermal technique makes crystal growth possible at much more manageable temperatures between 300°C and 400°C. Seed crystals are hung within the autoclave near the top; the bottom half of the vessel is filled with natural quartz, and the vessel is filled with an aqueous alkali metal hydroxide or carbonate solution. When the autoclave is properly heated and pressurized (up to 90 MPa, or 13,000 psi), a temperature difference develops between the bottom and top halves, carrying quartz in solution from the warmer dissolving chamber at the bottom to the cooler growing chamber. Growing high-quality large quartz crystals can take up to a year."
http://www.laserfocusworld.com/articles/266369

[Irvine.J]

Time for mangykrete fellas. Hell my cork idea seemed pretty close, maybe you could pioneer some cork composite, again its a really crazy idea induced by antibiotics, copious amounts of beer and painkillers...Dont ask and you wont get the gory details.

[metiz]

How about aerogel? it's the lightest sollid known to man, strong, translucent, and heat resistant up to 1200 degrees. for sale on unitednuclear.com :P

[ace_fedde]

mmm, nice!

http://www.pyrotherm.nl/techk/techk.html

Fedde

[ace_fedde]

Which all, b.t.w., reminds me that I still have to remove from my boat a 4" dia asbestos pipe and some asbestos clothes that are wrapped around the boat's exhaust!
(and yes I know that asbestos is dangerous and that I have to handle it when it's soaked or under water)

Fedde