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A rocket launcher is any device that launches a rocket-propelled projectile, although the term is often used in reference to mechanisms that are portable and capable of being operated by an individual.
The earliest rocket launchers documented in imperial China fired fire arrows with launchers constructed of wood, basketry, and bamboo tubes. The rocket launchers divided the fire arrows with frames meant to keep the arrows separated, and were capable of firing multiple arrow rockets at once. Textual evidence and illustrations of various early rocket launchers are found in the 11th century Southern Song Dynasty text Wujing Zongyao. The Wujing Zongyao describes the "long serpent" rocket launcher, a rocket launcher constructed of wood and carried with a wheelbarrow, and the "hundred tiger" rocket launcher, a rocket launcher made of wood and capable of firing 320 rocket arrows. The text also describes a portable rocket arrow carrier consisting of a sling and a bamboo tube.
The rocket launchers category includes shoulder-fired missile weapons, any weapon that fires a rocket-propelled projectile at a target yet is small enough to be carried by a single person and fired while held on one's shoulder. Depending on the country or region, people might use the terms "bazooka" or "RPG" as generalized terms to refer to such weapons, both of which are in fact specific types of rocket launchers. The bazooka is an American anti-tank weapon which was in service in 1942–1957, while the RPG is a Soviet anti-tank weapon.
Other forms of shoulder-launched rocket weapons include anti-tank guided missile, a guided missile primarily designed to hit and destroy heavily-armored vehicles, as well as Man-portable air-defense systems (MANPADS), which provides shoulder-launched surface-to-air missiles. A smaller variation is the gyrojet, a small arms rocket launcher with ammunition slightly larger than that of a .45-caliber pistol.
Recoilless rifles are sometimes confused with rocket launchers. However, the recoilless rifle merely fires a large projectile, not a projectile that continues to propel itself after leaving the barrel of the weapon.
Rocket pod 
A rocket pod is a launcher that contains several unguided rockets held in individual tubes, designed to be used by attack aircraft or attack helicopters for close air support. In many cases, rocket pods are streamlined to reduce aerodynamic drag. The first pods were developed immediately after World War II, as an improvement over the previous arrangement of firing rockets from rails, racks or tubes fixed under the wings of aircraft. Early examples of pod-launched rockets were the US Folding-Fin Aerial Rocket and the French SNEB.
Large scale 
Larger-scale devices which serve to launch rockets include the multiple rocket launcher, a type of unguided rocket artillery system; the transporter erector launcher, a vehicle with an integrated prime mover that can carry, elevate to firing position and launch one or more missiles and various launchers for guided missiles, including ones for surface-to-air missiles, anti-ship missiles, and antisubmarine warfare guided missiles (see ASROC, Sea Lance, etc.)
The largest scale rocket launcher device currently in existence is a missile launch facility.