You run to your connecting flight, flash your ticket just in time and scramble into your seat. The plane pushes away from the gate, and the engines power up. As you breathe a sigh of relief, you peer out the window and wonder: Just what will be propelling me into the troposphere? For virtually all commercial airliners today, the answer is "turbofan engines," the latest in a decades-long evolution. First came the turbojet, now obsolete, then the turboprop, which is still found onboard small planes. The big advance for turbofans is that the propeller is traded for a fan inside the nacelle (housing), and much of the incoming air bypasses the guts of the engine, providing thrust simply by being compressed by the fan's shape.
A jet engine exploits famous physics principles. According to Newton's third law of motion--for every action there is an equal and opposite reaction--air streaming out the back faster than the aircraft's speed will thrust the plane forward. The fan adds energy to the airstream, increasing velocity.