Both a turbojet and a turbofan burn kerosene to spin a turbine, yet they solve the problem of thrust in opposite ways. The difference comes down to a single principle from Newton’s second law: thrust equals mass flow multiplied by the change in velocity of the air passing through the engine. A turbojet is the purist. Every molecule of air is drawn through the core: compressor, combustor, then turbine. Fuel ignites in the combustor, the expanding gas drives the turbine, and what remains is blasted rearward as a thin, blisteringly fast jet. It accelerates a small mass of air to an enormous velocity. Simple, compact, and capable of supersonic flight, but thermodynamically wasteful at subsonic speeds and deafeningly loud. A turbofan reframes the question. A large fan sits ahead of the core, and most of the air it moves never touches the flame at all. That air is ducted around the engine as bypass flow, generating thrust cold and quietly. The governing metric is the bypass ratio, the mas...