The compressor

The Car Engines course taught that squeezing the charge before burning is where power comes from. A jet has no piston to squeeze with — instead it does something audacious: it compresses air to 40–50 atmospheres using nothing but rows and rows of tiny spinning wings. The compressor is the hardest part of a jet engine to design, and historically the part that delayed the whole invention.

Wings that pump

Every blade in a compressor is an aerofoil — the Aerodynamics course in miniature. A spinning rotor row flings air rearward and faster (adding energy exactly as a fan does); the stator row behind it — fixed blades — catches that swirling flow, straightens it, and by slowing it converts the speed into pressure (Bernoulli, cashed in the other direction). One rotor-stator pair is a stage, worth a pressure ratio of only 1.3–1.4 on its own — modest, deliberately. But stages compound like interest: ten to fifteen stages multiply into that 40–50× total, each one nudging the river politely toward the fire.

Why politeness is mandatory: stall and surge

Why not squeeze harder per stage? Because these are wings, and wings stall. Each blade works at an angle of attack to the oncoming flow, and asking a stage for too much pressure rise is precisely lesson 4 of Aerodynamics: the flow separates. In a compressor the consequences cascade — one stalled blade row upsets the flow into the next, and in the worst case the entire compressed column reverses direction and explodes forward out of the intake: surge, a cannon-bang with flames from the front of the engine, violent enough to wreck blades. Compressor design is a permanent negotiation between ambition (fewer, harder-working stages) and the stall boundary — with bleed valves and variable-angle stator vanes as the diplomatic corps keeping every stage at a polite angle of attack across the whole speed range.

What compression buys

The payoff for all this delicacy mirrors the piston course exactly: the higher the pressure ratio, the more work each kilogram of hot gas can give back, and the less fuel is wasted heating air that never pushes anything. The march of pressure ratios — Whittle's 4:1 in 1937, the 25:1 of the 1970s, over 50:1 in the newest airliner engines — is the efficiency history of the jet age, and it is the single biggest reason a modern flight burns roughly half the fuel per seat of a 1960s one. Feel the trade yourself: