A rocket is a vehicle, missile or aircraft which obtains thrust by the reaction to the ejection of a fast moving exhaust from within a rocket engine. The exhaust is formed from propellant which is carried within the rocket prior to its release. The thrust is due to Newton's 3rd Law of Motion. Often the term rocket is also used to mean a rocket engine.
In military terminology, a rocket generally uses solid propellant and is unguided. These rockets 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.
In military terminology, a missile, by contrast, can use either solid or liquid propellant, and has a guidance system.
Rockets must be used when there is no other substance (land, water, or air) that a vehicle may push itself with and so it is necessary to carry all the propellant a vehicle needs (such as in outer space). 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 propellant, such as the Space Shuttle's SRBs, or liquid propellant, like the Space Shuttle's main engines. A chemical reaction is initiated between the fuel and the oxidizer in the combustion chamber, and the resultant hot gases accelerate out of a nozzle (or nozzles) at the rearward facing end of the rocket. The pressure of the gases pressing on the interior of the combustion chamber and the nozzle(s) generates the 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.
Historically, 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 by the Mahrattas. The British 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 USnational anthem.
Early rockets were highly inaccurate. Without any spinning up of the rocket, nor any gimballing of the thrust, they had a strong tendancy to veer sharply off course. The early British Congreve rockets reduced this tendancy somewhat by attaching a long stick to the end of a rocket (similar to modern bottle rockets) to make it harder for the rocket to change course. The largest of the Congreve rockets was the 32 pound (14.5 kg) Carcass, which had a 15 foot (4.6 m) stick. Originally, sticks were mounted on the side, but this was later changed to mounting in the center of the rocket, reducing drag and enabling the rocket to be more accurately fired from a segment of pipe.
The accuracy problem was mostly solved in 1844 when William Hale modified the rocket design so that thrust was slightly vectored to cause the rocket to spin along its axis of travel like a bullet. The Hale rocket removed the need for a rocket stick, travelled further due to reduced air resistance, and was far more accurate.
Early rockets were also remarkably inefficient. Modern rockets were born when, after receiving a grant in 1917 from the Smithsonian Institution, Robert Goddard attached a de Laval nozzle to a rocket engine's combustion chamber, doubling the thrust and enormously raising the efficiency, giving the real possibility of practical space travel.
This technique was soon used on the V-2 rockets, designed by Wernher Von Braun who became one of the principal players in modern rocket development. V2s were deployed extensively by Adolf Hitler in the latter stages of World War II as terror weapons against the British population, each successful launch reaching high up into the vacuum of space and auguring the beginning of the Space Age.