Rocket is a type of engine that pushes itself forward or upward by producing thrust. Unlike a jet engine, which draws in outside air, a rocket engine uses only the substances carried within it. As a result, a rocket can operate in outer space, where there is almost no air. A rocket can produce more power for its size than any other kind of engine. For example, the main rocket engine of the space shuttle weighs only a fraction as much as a train engine, but it would take 39 train engines to produce the same amount of power. The word rocket can also mean a vehicle or object driven by a rocket engine.
Rockets come in a variety of sizes. Some rockets that shoot fireworks into the sky measure less than 2 feet (60 centimeters) long. Rockets 50 to 100 feet (15 to 30 meters) long serve as long-range missiles that can be used to bomb distant targets during wartime. Larger and more powerful rockets lift spacecraft, artificial satellites, and scientific probes into space. For example, the Saturn 5 rocket that carried astronauts to the moon stood about 363 feet (111 meters) tall.
Rocket engines generate thrust by expelling gas. Most rockets produce thrust by burning a mixture of fuel and an oxidizer, a substance that enables the fuel to burn without drawing in outside air. This kind of rocket is called a chemical rocket because burning fuel is a chemical reaction. The fuel and oxidizer are called the propellants.
A chemical rocket can produce great power, but it burns propellants rapidly. As a result, it needs a large amount of propellants to work for even a short time. The Saturn 5 rocket burned more than 560,000 gallons (2,120,000 liters) of propellants during the first 2 3/4 minutes of flight. Chemical rocket engines become extremely hot as the propellants burn. The temperature in some engines reaches 6000 °F (3300 °C), much higher than the temperature at which steel melts.
Jet engines also burn fuel to generate thrust. Unlike rocket engines, however, jet engines work by drawing in oxygen from the surrounding air. For more information on jet engines, see Jet propulsion.
Researchers have also developed rockets that do not burn propellants. Nuclear rockets use heat generated by a nuclear fuel to produce thrust. Ion rockets, expel ions (electrically charged particles) to produce thrust.
Military forces have used rockets in war for hundreds of years. In the 1200's, Chinese soldiers fired rockets against attacking armies. British troops used rockets to attack Fort McHenry in Maryland during the War of 1812 (1812-1815). After watching the battle, the American lawyer Francis Scott Key described "the rocket's red glare" in the song "The Star-Spangled Banner." During World War I (1914-1918), the French used rockets to shoot down enemy observation balloons. Germany attacked London with V-2 rockets during World War II (1939-1945). In the Persian Gulf War of 1991 and the Iraq War, which began in 2003, United States troops launched rocket-powered Patriot missiles to intercept and destroy Iraqi missiles.
Rockets are the only vehicles powerful enough to carry people and equipment into space. Since 1957, rockets have lifted hundreds of artificial satellites into orbit around Earth. These satellites take pictures of Earth's weather, gather information for scientific study, and transmit communications around the world. Rockets also carry scientific instruments far into space to explore and study other planets. Since 1961, rockets have launched spacecraft carrying astronauts and cosmonauts into orbit around Earth. In 1969, rockets carried astronauts to the first landing on the moon. In 1981, rockets lifted the first space shuttle into Earth orbit.
