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Comment by osigurdson | Hacker News Reader

I used to work in this industry. One thing that might be interesting for people is the metals do not actually withstand the temperatures directly. Instead cooling vanes are needed throughout various parts of the engine. This is why shutting a gas turbine (aka jet engine) down from full power will destroy it. It is necessary to take the engine down to a lower power setting first and then continue to spin the engine (calling motoring the engine) for quite a while even after it is turned off.

Another interesting thing is some engines cannot withstand certain RPM ranges as the compressor and power turbine can get into a catastrophic resonance. A good example is the T700 (used in the Blackhawk).

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I've always wanted to ask...

Why do turbines have a static duct and micron tolerances for the blades (and creep requirements) instead of a rotating (attached to the blades) duct that can be tensioned separately, and (presumably) no creep/micron tolerances?

Look up "blisks". These are used for ceramic turbines, because those are stronger in compression than tension.

Not an expert here, but afaik a turbine section consists of alternating spinning blades attached to the shaft and stationary vanes attached to the duct, which de-spin the air coming off the blades and prepare it for the next set. I'm not sure why the vanes are often hidden in cutaway views.

If you had a spinning duct, you'd presumably need a stationary shaft in the middle for mounting the vanes, and would have similar tolerance issues between the tips of the stationary vanes and the rotating duct. There's reasons that it might be easier to solve (the duct can be lower temperature) and reasons it's harder (bearings for a giant spinning duct). Not sure if anyone has tried such a design.

a full duct spinning at 10k rpm seems like it would massively increase stress on the blades

Your comment is really interesting, but I didn't fully understand.

What do you mean by "metals don't actually withstand temperature"? As in the raw metal would melt were it not for the cooling vanes?

'If powered down, the engine would destroy itself' - from what? Overheating?

The lower power setting on shutdown does what? Spin it at a low RPM so it doesn't decrease in temp too quickly?

> What do you mean by "metals don't actually withstand temperature"? As in the raw metal would melt were it not for the cooling vanes?

Metals don't need to melt to fail. Increasing the temperature leads to gradual reduction of yields limits. For example, the yield stress of steel drops to 50% if it reaches around 500 degrees.

> Metals don't need to melt to fail.

Another example: “jet fuel can’t melt steel.”

but also yes, the metal would melt if it somehow managed to not fail. Often the turbine blades are operating in an environment above their melting point and only don't melt because of the internal cooling.

The blades are hollow and have air injected from where they attach to the outside edge and fin of the blade, so when it’s spinning the blade doesn’t contact the exhaust stream because it’s coated with a layer of relatively cold air. Same thing happens with your car pistons but using an inertial layer.

Image search for a turbine blade and you’ll understand as soon as you see it.

The reason you can’t shut the engine down or power off suddenly is because the blades and housing cool at different speeds, the clearance between the blade tips and housing is as close as possible.

To help with this, hot air from the turbine is sprayed onto the outside of the casing via a hot bleed air bypass when the ecm determines its necessary.

If you shut down suddenly the tips of the blades can contact the housing and best case rub, worst case break.

There’s another problem along these lines which really exemplifies how tight these tolerances are, on the a320, you need to do a bowed rotor procedure if you’ve been sitting with the engines off for 45 minutes before you restart. This involves turning the engine over with the apu to equalize the cooling throughout the engine because the core of the engine cools slower but there’s two shafts running through the middle. These shafts “bend” because the outside is cold but the middle is hot, they can then rub against each other ruining bearings etc.

This also applies to high performance car turbo engines, a “turbo timer” is used so the ignition can’t be shut off until the turbo cools down.

Your china charger doesn't have clearances that tight.

Turbo timers are a legacy from the days when turbos were primarily oil cooled and synthetic oil wasn't common and shutting down a glowing hot turbo would tend to create sludge if done habitually.

This doesn’t seem to be true on modern turbos.

This is amazing yet again that they can ingest rain and snow so the inside can be, what, close to 3000F yet you can come into land in Minneapolis when it's -30F and everything Just Works. Imagine how different aviation would be if in an alternate universe we had modern jet engines but under no circumstances could they ingest water?

Note that at cruising altitude it would be more like -80F. The engine would be more efficient at sea level at -30F as the mass flow rate would be higher. Ingesting water vapour actually improves things for the same reason. The downside is it can cause corrosion over time.

>What do you mean by "metals don't actually withstand temperature"? As in the raw metal would melt were it not for the cooling vanes?

This is similar to the rocket engines where the thrust nozzle and its extension are cooled by the fuel otherwise they would melt or fail structurally.

> What do you mean by "metals don't actually withstand temperature"? As in the raw metal would melt were it not for the cooling vanes?

They creep. Have you seen, for instance, Blu-tac or glue fail? It doesn't go at once, but slowly, over a period of time. At high temperatures most metals (others on this thread have mentioned single-crystal blades) behave a bit like that.

Although steel is also weaker at temperatures far below its melting point, yes. A simple observation of a blacksmith at work should tell you that. And a think some new jets may be running hotter than Tm for steel now?

> The lower power setting on shutdown does what? Spin it at a low RPM so it doesn't decrease in temp too quickly?

Yup, or more relevantly evenly, although those tend to be related. Given almost all materials expand as they get hotter and contract as they cool, different cooling rates between parts -> different contraction rates -> different relative shape -> Very Bad in precision machinery.

So basically metal gets rubbery when hot, and stopping something all off a sudden could have inertial forces(moving blades, gears etc) wreck the structure?

You have to shut things down step by step, so that rigidity is supplied to the metals as the inertial forces are reduced.

> What do you mean by "metals don't actually withstand temperature"? As in the raw metal would melt were it not for the cooling vanes?

A small addition to the sibling comments: Combustion temperatures in modern turbines are around 1400C, if I recall correctly, but the best nickel superalloys go up to 1050C or thereabouts (for long-term operation). To close this gap, the use of high-temperature alloys is supplemented with active cooling and ceramic coatings, as stated by GP.

Many rocket engines, especially the reusable sort, require active cooling of the throttle and combustion chamber. A portion of the fuel is split into channels which run through the combustion chamber, throat, and the nozzle. Generally it is a close loop system, so the fuel makes back to be injected into the combustion chamber.

To get max performance modern engines run hot, aka ox rich, and the regen cooling is generally not enough. So in addition to that, critical surfaces such as nozzle also get protected by injecting a thin layer of fuel. This biases combustion to be fuel heavy in localized areas which is less hot. Of course all of this happens in an extremely dynamic environment where gasses are moving at 2km/s+.

Was actually going to post a similar comment re: NASA and the SSME engines for the Space Shuttle. This graphic shows the coolant system circulation that pumps cold fuel through the outer casing to warm it up to proper temperatures before use. [1]

[1] https://en.wikipedia.org/wiki/RS-25#/media/File:Ssme_schemat...

Not just the reusable ones. Almost all of them do. Exception are monoprop ones where the temperatures are just not high enough.

> To get max performance modern engines run hot, aka ox rich

Oxygen-rich means you have extra oxygen which doesn’t combust. That necessarily reduces performance. Most rocket engines run fuel rich because hot oxygen is a chemical terrorist.

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