Jet propulsion is the production of motion in one direction by the release of a high-pressure stream of gas in the opposite direction. Jet propulsion powers rockets, guided missiles, and many airplanes.
The first flight of an airplane powered by a jet engine occurred in Germany in 1939. Since then, jet propulsion has powered aircraft of all types, including supersonic airliners and spacecraft that have journeyed to other planets.
English scientist Sir Isaac Newton first described the principle of jet propulsion in 1687 in his third law of motion. This law states that for every action there is an equal and opposite reaction.
The scientist Hero of Alexandria built a small jet engine about A.D. 60. It was powered by steam escaping from a hollow sphere through two nozzles that pointed in opposite directions. The escaping steam turned the sphere in much the same way that jets of water spin a rotating lawn sprinkler.
The growing tensions that led to World War II accelerated the development of jet engines to propel aircraft.
A turbojet designed by Hans von Ohain, a German physicist, powered the first jet engine airplane, the Heinkel He-178. In Italy, the jet-propelled Caproni-Campini CC2 airplane was built and flown in 1940. Neither of these first two jet aircraft engines proved practical.
Frank Whittlem, an officer in Great Britain's Royal Air Force, soon developed a more successful turbojet. Whittle's turbojet powered the Gloster E. 28/39, which first flew in 1941.
During World War II, Germany pioneered the use of jet propulsion for guided missiles. In 1947, the rocket-powered Bell X-1, built in the United States, became the first airplane to fly faster than the speed of sound
During the 1950's, turbojets and turboprops began to power some commercial airliners. Also at this time, ramjets propelled such early United States guided missiles as the Bomarc and Talos.
During the 1960's, the turbofan began to replace the turbojet on commercial and military aircraft. Because of the turbofan's efficiency and low noise, it came into widespread use
in the 1970's. Research to improve the ramjet as a missile engine intensified during the 1970's and 1980's.