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I noticed that most jet engines are made out of aluminum or metal components. However, carbon fiber has a melting point of 3500 degrees celcius, way above metal or aluminum. So, can I build an entire jet engine out of carbon fiber?

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    $\begingroup$ Carbon fibre burns quite easily. So while it melts at a very high temperature, it will not get anywhere close to that temperature under normal atmospheric conditions - you'll just get a lot of expensive carbon dioxide, which doesn't hold shape very well :) $\endgroup$ – Luaan Apr 18 '16 at 13:18
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    $\begingroup$ I don't know about you, but I can't build an entire jet engine out of any material. $\endgroup$ – immibis Apr 19 '16 at 1:34
  • $\begingroup$ @immibis hey, all chocolate is always an option. $\endgroup$ – yankeekilo Apr 19 '16 at 13:30
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No, you can not. Carbon fibre composites always contain a matrix component which serves different functionalities, but (in a micro nutshell) bonds and keeps the fibres in place. What you get is a very complex material with lots of lightweight potential, but also very different property profiles compared to metals.

Most matrix materials (thermoset, thermoplastics) will not work up to engine (combustion chamber) temperatures and either degrade/burn or melt/vaporize/burn long before reaching even a meagre few hundred Kelvin (say, 500K). In fact, carbon fibres will not work as well in an oxygenated high-temperature environment, and degrade/burn up as well.

One composite material that works and is used already in engines are ceramic matrix composites, like (amongst others) e.g. C/C-SiC structures, but those are very special animals (and even there the carbon fibre parts are susceptible to burn). Especially in terms of mechanical properties, these cannot compete with superalloys.

But also for many other, ambient temperature parts, metals are much better suited. Think of example of a ball bearing - doing that completely in composite will not work really well (if at all).

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    $\begingroup$ So the engine itself would burn up as fuel for the engine? "You see Ivan..." $\endgroup$ – corsiKa Apr 18 '16 at 17:51
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    $\begingroup$ @corsiKa lol - I forgot a suitable preamble: No you can not... lest you be fond of self-devouring turbmachinery with a dangerous taste for spontaneous, violent and messy disintegration. $\endgroup$ – yankeekilo Apr 18 '16 at 19:00
  • $\begingroup$ You assume you MUST run a jet engine at temperatures above the decomposition temperature of the carbon composite. Although that is not true, one normally endeavors to maintain a high combustion temperature due to thermodynamic efficiency. $\endgroup$ – Aron Apr 19 '16 at 8:30
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    $\begingroup$ @Aron to be sure, I am presuming good-ole fashioned combustion. Anyway, if nit-picking, the question is even only asking about carbon fibres (not composites), so the answer to that is definitely no (unless you invent a turbomachine hammock or self-propelling carpet), but I added some common-sense assumptions. $\endgroup$ – yankeekilo Apr 19 '16 at 8:47
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What is relevant is not the melting point, but the temperature at which the material loses its mechanical properties (such as stiffness or elasticity).

The epoxy (the material keeping together the fibers) will usually degrade much sooner than the fibers, basically voiding the benefit of the material.

There are studies to increase the resistance to high temperatures, but I have no idea how near we are to see them applied to critical components such as the engines.

Also note that usually the blades of the turbine and the combustion chambers are made of alloys designed specifically to resist the high temperatures in those areas of the engines.

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    $\begingroup$ Actually, Epoxy as a thermoset will not melt, but degrade / crumble / burn above its maximal operating temperature, although the overall effect is the same: loss of function/stability. $\endgroup$ – yankeekilo Apr 18 '16 at 7:14
  • $\begingroup$ @yankeekilo thanks, edited. $\endgroup$ – Federico Apr 18 '16 at 7:15
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When I was beginning my university studies in engineering 15 years ago our professor told us that they actually think of building jet engines of that material!

Carbon-based materials seem to be the perfect material for jet engines in nearly all points (heat resistance, mechanical strength at high temperatures, weight, ...) with one important exception:

These materials burn when they get in contact with hot air!

The professor told us that a lot of effort and money is spent in search of a coat or varnish that would protect carbon-based parts from the hot air. He told us that jet engines would be built of that material if such a coat is found.

He also told us that up to then no suitable coat was found - obviously this has not changed in the last 15 years.

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  • $\begingroup$ Interesting, so why doesnt a normal flame retardent coat work on carbon fiber? $\endgroup$ – eli Apr 18 '16 at 16:10
  • $\begingroup$ If your question is why some carbon fiber material does not burn down when it gets in contact with a burning piece of paper: It can! Some schoolmate accidentally threw some burning paper into a bottle bank made of carbon fiber and it burned down... However our professor told us that the main problem is the temperature difference. Finding some coat that works at high temperatures seems not the problem but the coating must ahere when the engine is heating up or cooling down... $\endgroup$ – Martin Rosenau Apr 18 '16 at 16:50
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    $\begingroup$ @eli A normal flame retardant will probably itself not survive the temperatures in a jet engine core. In general, assume that nothing is 'normal' when discussing jet engines. $\endgroup$ – reirab Apr 18 '16 at 19:21
  • $\begingroup$ @reirab lol, good advice $\endgroup$ – eli Apr 18 '16 at 23:05
  • $\begingroup$ @reirab A metalized spray coating? $\endgroup$ – curious_cat Apr 19 '16 at 13:26

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