Why do jet engines not melt?

The combustor of a modern turbofan such as the GE9X or the Rolls-Royce Trent XWB delivers gas to the first turbine stage at temperatures in the region of 1,600 °C, with peaks higher still. The blades in that stage are cast from nickel-based superalloys whose melting point is about 1,300–1,350 °C and which begin to lose strength well below that. On paper the blade should fail within seconds. It does not, because the design ensures that the metal never sees the gas temperature. Three layers of protection work together: internal cooling, a ceramic coating and a crystal structure chosen to resist deformation under load.

Each high-pressure turbine blade is hollow, with a serpentine network of passages cast into it. Air bled from the compressor, at around 650 °C, which is cold by comparison, is pumped through those passages and then out through hundreds of laser-drilled holes, each a fraction of a millimetre wide, on the leading edge and surfaces of the blade. That air spreads into a thin film that clings to the surface and insulates it from the combustion gas. Between 15 and 25 % of the compressor's airflow is used this way, a substantial penalty that designers accept because every 50 °C of extra turbine inlet temperature buys measurable gains in thrust and efficiency. Over the blade sits a thermal barrier coating, typically yttria-stabilised zirconia, a ceramic a few hundred micrometres thick applied by plasma spray or vapour deposition; it drops the metal temperature by a further 100–150 °C. Underneath, a bond coat rich in aluminium forms a protective oxide against corrosion.

Even cooled, the blade runs at around 1,000 °C while spinning at more than 10,000 rpm and carrying a centrifugal load equivalent to several tonnes hanging from its tip. Ordinary cast metal creeps under those conditions, stretching along grain boundaries until it fails. Since the 1980s blades have been grown as single crystals in directional-solidification furnaces, eliminating grain boundaries altogether and allowing higher temperatures and longer lives. The next step is ceramic matrix composites, silicon carbide fibres in a silicon carbide matrix, which weigh a third of a superalloy and tolerate about 1,300 °C without cooling; GE introduced them in the shrouds and nozzles of the LEAP and GE9X engines. The combustor liner and the nozzle guide vanes face similar loads and use similar cooling, which is why a running engine can glow visibly through its tailpipe while the cowling next to it stays cool enough to touch.