Bypass

Look at an early jet airliner — a 707 with its slim cigar engines — and then at any modern one, whose engines look almost comically fat. That fattening is the single most important thing that happened to the jet engine after its invention, and lesson 1 already handed you the reason: gently accelerating a lot of air beats violently accelerating a little. The fat engine is that sentence, built.

The fan and the two rivers

A turbofan puts an enormous ducted fan — itself a many-bladed wing wheel, spun by its own extra turbine stages — on the front of the core engine you have spent five lessons building. Air arriving at the fan splits into two rivers. The core stream follows the familiar plot: compressor, fire, turbine, nozzle. The bypass stream — in a modern airliner engine 10–12 times more air — is simply pushed backwards by the fan, gaining a modest, efficient shove, and never sees fire at all. The bypass ratio names the split: bypass air over core air. Military fast jets run 0.3–1; the 1960s' first turbofans ~1–2; a modern GE9X or Trent, 10+. In today's engines, the fan makes ~80% of the thrust — the core's main job has become spinning the fan.

What the fattening bought

Efficiency first: moving the same total momentum with slower, heavier flow wastes far less energy as hot fast exhaust, and bypass — compounding with the compressor lesson's pressure ratios — is why fuel burn per seat has roughly halved since the 1960s. Silence second, and for the same physics: jet noise scales savagely with exhaust velocity (roughly its eighth power), so slowing the exhaust turned the early jets' scream into the modern engine's whoosh — the bypass river literally wraps the core's shriek in a duvet of slower air. The chevron zigzags on modern nacelle trailing edges finish the job, smoothing the shear between rivers.

Why stop at 12?

If fatter is better, why not bypass 50? Because the duct bites back: a bigger fan needs a bigger, heavier nacelle whose drag and weight eventually eat the gains, fan tips start flirting with the propeller's old supersonic problem, and a huge slow fan wants a slower shaft than its turbine (hence geared turbofans, buying another step). Push the logic to its limit — remove the duct entirely — and you have reinvented the propeller, which is exactly what the turboprop is for slower aircraft, and where this course's final lesson picks up the family tree.