How does a jet engine push a plane forward?

A jet engine pushes a plane forward by throwing a large amount of air backwards, and Newton's third law pushes the plane the other way. In a modern turbofan, the big fan you can see at the front does most of that throwing. It is powered by a gas turbine behind it, which burns fuel, and the fan forces air rearwards.

The core works like a turbojet: it compresses air, mixes in fuel, burns it, and passes the hot gas through a turbine and a nozzle. The turbine powers the compressor. In a turbofan, some of the incoming air bypasses the combustion chamber and turbines altogether and is simply pushed backward by the fan.

The bypass ratio compares the two streams. At a ratio of 10 to 1, ten kilograms of air go around the core for every kilogram that passes through it. Most commercial jet engines are of this high-bypass type, while most modern fighter engines are low-bypass.

Why move so much air gently rather than a little air violently? Bypass trades exhaust speed for extra air, giving the same thrust with less fuel. Frank Whittle, the inventor of the turbojet, called it "gearing down the flow", like a bicycle's low gear. A side benefit is quiet: turbofans are significantly quieter than pure jets of the same thrust.

The idea was ready at the right moment. The high-bypass turbofan was one of the technologies that made the Boeing 747 possible, and was thought to give twice the power of earlier turbojets using a third less fuel.