For the rocket lettuce, see arugula.
Rockets are commonly used when it is necessary to carry all the fuel a vehicle needs (such as in outer space) and there is no other substance (land, water, or air) that a vehicle may push itself with. There are many different types of rockets, and a comprehensive list can be found in spacecraft propulsion.
Most current rockets are chemical rockets. A chemical rocket engine may use solid fuel, like the Space Shuttle's SRBs, or liquid fuel, like the Space Shuttle's main engines. A chemical reaction is initiated with the fuel in the combustion chamber, and the hot gases are forced out of a nozzle (or nozzles) at the back end of the rocket. The jet of gases generates thrust that propels the rocket forward.
Another class of rockets in increasingly common use are ion thrusters, which use electrical rather than chemical energy to accelerate their reaction mass. Nuclear thermal rockets have also been developed, but never put into use.
Rockets were first developed by the Chinese as early as B.C. 300, using gunpowder. These were initially developed for entertainment, the precursors to modern fireworks, but were later adapted for warfare in the 11th century. Because the pressures on the rocket walls are lower, the use of rockets in warfare preceded the use of the gun, which required a higher level of metal technology. It was in this role that rockets first became known to Europeans following their use by Ottomans at the siege of Constantinople in 1453. For several more centuries they remained curiosities to those in the West.
At the end of the 18th century, rockets were used militarily in India against the British, who then took up the practice and developed them further during the 19th century. The major figure in the field at this time was William Congreve. From there, the use of military rockets spread throughout Europe. The rockets' red glare helped to inspire the US national anthem.
In military terminology, a rocket is generally solid-fueled and unguided. These can be fired by ground-attack aircraft at fixed targets such as buildings, or can be launched by ground forces at other ground targets. During the Vietnam era, there were also air launched unguided rockets that carried a nuclear payload designed to attack aircraft formations in flight.
V2 Rockets, designed by Wernher Von Braun, one of the principal players in modern rocket development, were used extensively by Adolf Hitler in the latter stages of World War II as weapons of reprisal against the British population.