Why engines run so hot
A jet engine turns heat into thrust, and the hotter the gas entering the turbine, the more work and thrust each kilogram of air gives. The Brayton cycle calculator shows the effect: raise the turbine inlet temperature and the work per kilogram climbs. So engine designers have spent decades pushing it up. In large modern engines it is well above 1,500 °C, while the nickel alloys the blades are made of start to melt at roughly 1,300 °C.
The blades are not just hot. Each one spins at the edge of a disk turning thousands of times a minute, so it is also pulled outwards by a force of several tonnes. Hot metal under that much load slowly stretches, a process called creep. A blade that creeps too far rubs on the casing or breaks. Three things keep it alive: cooling, coatings and better metal.
1. Cooling air
Some of the air from the compressor skips the combustor entirely. It is hot by everyday standards, several hundred degrees, but far cooler than the combustion gas. It is piped through the turbine disk and into each blade, where it winds through internal passages and leaves through hundreds of tiny holes. The air leaving the holes spreads over the blade's surface as a thin protective layer, called film cooling, so the hottest gas never quite touches the metal.
Cooling air is not free. It has been compressed at a cost in work, and it bypasses the combustor, so the turbine gets less out of it. Engineers use as little as they can.
2. Ceramic coatings
The blades and the parts around them are sprayed with a thermal barrier coating, a thin ceramic layer that conducts heat poorly. With cooling air flowing inside and a ceramic layer outside, the metal can sit well over a hundred degrees cooler than the gas around it.
3. Better metal
Ordinary metals are made of many tiny crystals (grains), and creep tends to happen along the boundaries between them. Turbine blades went from ordinary cast nickel superalloys, to castings with grains lined up along the blade, to single-crystal blades grown as one crystal with no grain boundaries at all. Each step let the blades run hotter for the same life. The newest step is ceramic matrix composites, ceramic fibres in a ceramic body, which are lighter than metal and tolerate higher temperatures. They have started to appear in the hot sections of new airliner engines.
The same choices in the game
The game models all three ideas in simplified form. In the Design office you pick the hot-section material and the share of blade cooling air, and the two together set the turbine's temperature limit:
- Each material has a base limit: steel alloy 1,150 K, nickel superalloy 1,350 K, single-crystal nickel 1,500 K and ceramic composite 1,650 K. The last two are part of Pro.
- Each percent of cooling air lets the blades live about 22 K hotter, and costs some turbine efficiency.
- The metal heats up more slowly than the gas, so a brief spike is survivable but a sustained one is not. Above the limit the turbine takes damage, shown on the TURBINE bar, and if it keeps going the blades melt.
- With FADEC on, the engine computer keeps the gas about 25 K below the limit. With it off, that is your job.
The free Training turbojet uses a nickel superalloy turbine with 4 % cooling air, which gives it a limit of about 1,438 K for a design turbine inlet temperature of 1,400 K: not much room to spare, which is typical of real engines too.
The spinning parts have limits too
Heat is not the only enemy. The rotor's disks carry the blades, and the faster they spin the harder they are pulled apart. The game's rotor materials set how far past full speed the rotor can go: aluminium alloy 105 %, titanium alloy 112 % and nickel disks 120 % (Pro). Push much beyond the limit and the disk bursts, which is the Overspin challenge.
Things to try
- On the Training turbojet, turn FADEC off, ease the throttle to full and hold it. Watch the TURBINE INLET reading and the TURBINE damage bar until the blades let go.
- In the Design office, swap the nickel hot section for steel and run it again with the same settings. Then raise the cooling air and see how much it buys back.
- Raise the turbine inlet temperature with the same material and watch the thrust rise on the dyno, right up to the point where the turbine can no longer take it.