Jet engines, though capable of producing immense amounts of power, are actually based on very simple principles. The tens of thousands of horsepower and thousands of pounds of thrust that can be produced by the various types of jet engine are all based on the same concept: throw something back, and something else has to be thrown forward. This follows from Newton’s Third Law of Motion, and it can also be derived from the Law of Conservation of Momentum.
Jet engines use this law by forcing some sort of “fluid”-which can actually be air or burning hydrogen, among other things-out their nozzles; this propels the engine (and, if everything is working properly, the aircraft to which it is attached) forward.
There are a variety of ways to do this; among the most efficient is the use of a series of fans and turbines, as in a turbojet engine. Air comes in the front of the engine at the intake, its motion turns the turbines, and the motion of the turbines turns the fans to force the air out the nozzle in the back. This way, the motion of the engine through the air provides part of its propulsion (the rest is of course provided by the fuel). An added advantage of this is the ability to burn fuel using the oxygen in the air, rather than having to carry a separate oxidizer.
A modification of the turbojet produces the turbofan, which slows down the air of the nozzle to just slightly faster than the speed of the aircraft. This makes for better efficiency at “slower” (that is, less than the speed of sound) rates of travel, and it also reduces noise. For this reason, most commercial airliners use turbofans.
Rockets are a type of jet engine; a rocket is distinguished by the fact that it is self-contained, carrying its own fuel, oxidizer, and all-this makes rockets perfectly suited to travel outside the Earth’s atmosphere. Rockets can be solid-state, burning like a firecracker, which is cheap, but dangerous (as firecrackers, and, more poignantly, the solid rocket boosters on the Shuttle Challenger clearly demonstrate); or they can be liquid-fueled, which increases both expense and control.
Jet engines aren’t just used on aircraft; they also can be used on light ships, in place of heavy diesel engines or expensive and risky nuclear power plants. Jets readily provide the huge amounts of energy required to maintain a ship, and do so with less weight and comparable fuel usage compared to diesel.
Jet engines are a modern marvel; they are what enables us to travel around the world in hours instead of weeks or months, even what allowed human beings to set foot upon the Moon. They allow us to travel faster and further than ever before, leaving us with one question: where should we go?