The components of a stage are a stationary row and a rotating one, in that order:
Nozzle guide vanes (NGVs) — the stationary row: convergent passages that accelerate the gas, converting part of its pressure into velocity, and aim the resulting fast stream onto the rotor blades at the working angle.
Rotor blades and disc — the rotating row: the blades take the stream's energy out as torque on the disc and shaft, the work that drives compressor, fan or propeller.
That is the working principle in one sentence: NGVs turn pressure into directed velocity, the rotor turns that velocity into shaft work. The gas parameters across a stage follow: pressure and temperature both fall (energy is being removed), the velocity rises sharply through the NGVs and falls again across the rotor as its energy is harvested.
Impulse, reaction, and the real blade
Three design philosophies split the stage's work:
Impulse turbine — the whole pressure drop happens in the NGVs: the rotor's bucket-shaped blades merely catch the high-speed jets and are pushed round by their impulse, the pressure across the rotor staying constant.
Reaction turbine — the pressure drops across the rotor as well: the blade passages act as nozzles themselves, and the blades are driven partly by the reaction to the gas accelerating through them.
Impulse-reaction — the practical aviation turbine blends the two along the blade's own length: predominantly impulse at the root, increasingly reaction toward the tip, which is one reason turbine blades, like compressor blades, are twisted.
Living with the heat
Why thrust is limited by turbine inlet temperature: the hotter the gas entering the turbine, the more energy the cycle delivers, so designers push the turbine inlet temperature (TIT) as high as they dare — and what stops them is the first row of NGVs and blades, the blades spinning under enormous centrifugal load, all of it soaked in gas hotter than the metal's melting point. Internal air cooling (compressor bleed air fed through passages inside blades and vanes) and exotic alloys buy margin, but the blade's creep life still writes the limit: available thrust is capped by the temperature the turbine can survive, not by the fuel the chamber could burn.
The same heat drives three more design battles:
Thermal stress — blades and discs live under huge centrifugal load with fierce temperature differences across them, rim hot and bore cool, every start and shutdown cycling those stresses through the metal: creep and thermal fatigue govern turbine life.
Tip losses — as in the compressor, gas leaking over the blade tips does no work; turbines fight it with shrouded tips, interlocking rims on the blades themselves, plus minimal running clearances.
Active clearance control (ACC) — manages those clearances in flight: cool air sprayed onto the turbine casing shrinks it gently around the blades during cruise, closing the tip gap for efficiency, and is shut off when the gap must open again for transients.
One geometric echo completes the picture: the turbine's annulus is divergent, widening rearwards, the mirror image of the compressor's convergent one and for the mirrored reason: as pressure falls the gas expands, its volume grows, and the passage must grow with it to keep the axial velocity within working bounds.
The exhaust unit