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Our Living Language : Fully loaded, a jumbo-sized aircraft weighs nearly 800,000 pounds. Yet its jet propulsion engines are so powerful that it hurtles down the runway fast enough to lift it into the air and climb to 35,000 feet. The turbofan engines now so common on commercial aircraft are descendants of the turbojet, itself a descendant of the ramjet, and all three are remarkably simple devices. In each case, thrust comes from the backward propulsion of hot pressurized air, which—according to Newton's third law of motion—produces an equal and opposite reaction in the body from which it was propelled. The ramjet is a simple hollow cylinder with a central combustion chamber in which incoming air is sprayed with burning fuel and blasts out the rear of the cylinder at high pressure. In a turbojet, greater efficiency and control is obtained by having the escaping exhaust drive a turbine before leaving the engine and by having that turbine drive intake fans that compress the incoming air before it is ignited. Though only a small amount of air is accelerated by these engines, it is accelerated by a huge amount, and extremely high thrust, on the order of thousands of horsepower, can be achieved. The turbojet engine is used in high-performance fighter jets, but if speeds are kept down to a few hundred miles per hour, the aerodynamics of the propeller or fan offers certain advantages; for this reason, the turbofan was developed. The turbofan engine encloses the combustion chamber in a larger cylinder and uses larger intake fans to blow some of the incoming air in an outer path around the combustion chamber, straight out the rear of the engine, without the addition of burning fuel. This way, a good part of the thrust is achieved by accelerating a large quantity of "clean" air by a small amount. This makes the turbofan both more fuel-efficient and considerably quieter than the turbojet design. For the extra thrust needed at takeoff, fuel can even be injected into the outer air path of the turbofan so that all of the air takes part in combustion as in turbojet operation; this fuel flow can then be turned off when the extra power is no longer needed.