11 ms·

One important aspect of modern jet engines that the article only mentions on the periphery are the materials engineering problems in the hot section. There are

by mppm 2y ago

One important aspect of modern jet engines that the article only mentions on the periphery are the materials engineering problems in the hot section. There are many metals (not to mention ceramics) that can survive 1000C temperatures, but there are not many that can permanently resist creep at these temperatures under high tensile loads. The only really viable class of materials at the moment are Nickel-based single-crystal superalloys that contain rare metals like Rhenium and Ruthenium. This comes with serious supply limitations and rather complex manufacturing, where the molten metal is solidified directly in the shape of a turbine blade from a single seed crystal. Fun stuff, in other words :)

eutectic 2y ago
Silicon carbide fiber reinforced silicon carbide is also being increasingly used.
- mppm 2y ago
  In production?
  - credit_guy 2y ago
    I think eutectic is referring to the ceramic matrix composites (CMC) used in the General Electric's engine LEAP. Here's some quotes from [1]: > The engine has one CMC component, a turbine shroud lining its hottest zone, so it can operate at up to 2400 F. The CMC needs less cooling air than nickel-based super-alloys and is part of a suite of technologies that contribute to 15 percent fuel savings for LEAP over its predecessor, the CFM 56 engine. > GE’s CMC is made of silicon carbide (SiC) ceramic fibers (containing silicon and carbon in equal amounts) coated with a proprietary material containing boron nitride. The coated fibers are shaped into a “preform” that is embedded in SiC containing 10–15 percent silicon. [1] https://www.ornl.gov/news/ceramic-matrix-composites-take-flight-leap-jet-engine https://www.ornl.gov/news/ceramic-matrix-composites-take-fli...
    - Invictus0 2y ago
      This is why I love HN
      - hnax 2y ago
        Me too!
    - pfdietz 2y ago
      From what I understand, shroud linings don't rotate, though. They are fixed to the engine casing. So they are not subject to the high centrifugal force that would make creep really problematic.
      - adrian_b 2y ago
        While you are right about the limited applications for this material, the reason cannot be creep, which should be negligible in this kind of ceramic even at the working temperature. Certainly it must be better regarding creep than the alternative metallic alloys. In a rotating part, subject to high centrifugal forces and vibrations and shocks, I think that the risk of unpredictable fractures may be too high for a ceramic, even a composite one. Silicon carbide ceramic has low toughness. A composite should be better, but still far from metallic alloys. I have seen mentions of research about the feasibility of using silicon carbide composite ceramics for rotating parts, with the goal of reducing their mass and increasing their working temperature, in comparison with metallic parts, but it is unlikely that this has reached the stage of being used in production engines. Ceramics, e.g. derivatives of zirconia, are frequently used for turbine blades, but only as ceramic thermal barrier coatings on metallic blades, not for the body of the blades.
anubiskhan 2y ago
Hell yeah something new to learn about today, thank you.
osigurdson 2y ago
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).
- gameshot911 2y ago
  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?
  - motorest 2y ago
    > 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.
    - xeonmc 2y ago
      > Metals don't need to melt to fail. Another example: “jet fuel can’t melt steel.”
    - thereisnospork 2y ago
      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.
auxym 2y ago
All correct. To add, the main problem with ceramics is their fragility under tensile stresses. Spinning at high speeds puts the blades into tensile stress, which tends to "open up" microscopic defects in the crystal structure and cause complete failure. Some researchers from the academic lab where I work have been working on a turbine configuration in which ceramic turbine blades undergo compressive, instead of tensile, stresses in rotation: https://www.exonetik.com/turbo https://www.exonetik.com/turbo Interesting stuff, but it's a huge challenge to bring entirely new jet engines, as TFA mentions, to certification and market.
datavirtue 2y ago
At GE I kept a few used replacement vanes from a (F414/F110) compressor on my desk. Brand new they run about $4000 a piece. The part is about 1.5x2.0 inches. They don't last long in the desert. Most of the parts we had floating around were from the Saudis' F16s, which had been worn down by the sand.
siva7 2y ago
So we had the chance to get more of these rare materials but trump blew it up?

by mppm 2y ago

eutectic 2y ago

Silicon carbide fiber reinforced silicon carbide is also being increasingly used.

mppm 2y ago
In production?
- credit_guy 2y ago
  I think eutectic is referring to the ceramic matrix composites (CMC) used in the General Electric's engine LEAP. Here's some quotes from [1]: > The engine has one CMC component, a turbine shroud lining its hottest zone, so it can operate at up to 2400 F. The CMC needs less cooling air than nickel-based super-alloys and is part of a suite of technologies that contribute to 15 percent fuel savings for LEAP over its predecessor, the CFM 56 engine. > GE’s CMC is made of silicon carbide (SiC) ceramic fibers (containing silicon and carbon in equal amounts) coated with a proprietary material containing boron nitride. The coated fibers are shaped into a “preform” that is embedded in SiC containing 10–15 percent silicon. [1] https://www.ornl.gov/news/ceramic-matrix-composites-take-flight-leap-jet-engine https://www.ornl.gov/news/ceramic-matrix-composites-take-fli...
  - Invictus0 2y ago
    This is why I love HN
    - hnax 2y ago
      Me too!
  - pfdietz 2y ago
    From what I understand, shroud linings don't rotate, though. They are fixed to the engine casing. So they are not subject to the high centrifugal force that would make creep really problematic.
    - adrian_b 2y ago
      While you are right about the limited applications for this material, the reason cannot be creep, which should be negligible in this kind of ceramic even at the working temperature. Certainly it must be better regarding creep than the alternative metallic alloys. In a rotating part, subject to high centrifugal forces and vibrations and shocks, I think that the risk of unpredictable fractures may be too high for a ceramic, even a composite one. Silicon carbide ceramic has low toughness. A composite should be better, but still far from metallic alloys. I have seen mentions of research about the feasibility of using silicon carbide composite ceramics for rotating parts, with the goal of reducing their mass and increasing their working temperature, in comparison with metallic parts, but it is unlikely that this has reached the stage of being used in production engines. Ceramics, e.g. derivatives of zirconia, are frequently used for turbine blades, but only as ceramic thermal barrier coatings on metallic blades, not for the body of the blades.

anubiskhan 2y ago

Hell yeah something new to learn about today, thank you.

osigurdson 2y ago

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).

gameshot911 2y ago
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?
- motorest 2y ago
  > 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.
  - xeonmc 2y ago
    > Metals don't need to melt to fail. Another example: “jet fuel can’t melt steel.”
  - thereisnospork 2y ago
    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.

auxym 2y ago

All correct. To add, the main problem with ceramics is their fragility under tensile stresses. Spinning at high speeds puts the blades into tensile stress, which tends to "open up" microscopic defects in the crystal structure and cause complete failure. Some researchers from the academic lab where I work have been working on a turbine configuration in which ceramic turbine blades undergo compressive, instead of tensile, stresses in rotation: https://www.exonetik.com/turbo https://www.exonetik.com/turbo Interesting stuff, but it's a huge challenge to bring entirely new jet engines, as TFA mentions, to certification and market.

datavirtue 2y ago

At GE I kept a few used replacement vanes from a (F414/F110) compressor on my desk. Brand new they run about $4000 a piece. The part is about 1.5x2.0 inches. They don't last long in the desert. Most of the parts we had floating around were from the Saudis' F16s, which had been worn down by the sand.

siva7 2y ago

So we had the chance to get more of these rare materials but trump blew it up?