In military terminology, a rocket is a self-propelled, generally unguided, weapon-system powered by a rocket engine. Though used primarily as medium- and long-range artillery systems, historically rockets have also seen considerable use as air-to-surface weapons, some use as air-to-air weapons, and even (in a few cases) as surface-to-air devices. Examples of modern surface-to-surface rocket systems include the Soviet BM-27 Uragan and the American M270 Multiple Launch Rocket System. In military parlance, a rocket differs from a missile primarily by lacking an active guidance system; early missiles became known as "guided rockets" or "guided missiles". Some rockets were developed as unguided systems and later upgraded to guided versions, like the GMLRS, and these generally retain the term "rocket" instead of becoming "missiles". Rockets or missiles that travel underwater, like the VA-111 Shkval, are known as "torpedoes", whatever their propulsion system. Early development The use of rockets as some form of artillery dates back to medieval China where devices such as fire arrows were used (albeit mostly as a psychological weapon), and gradually spread to Europe and the Middle East. Rockets became a significant weapon during the 20th century, when precise manufacturing processes made relatively accurate rockets possible. Basic roles Artillery Rockets have been used as an artillery weapon for centuries, and continue to be used in the modern age after being extensively modernized in World War II. Rockets in the artillery role complement traditional field guns, being superior in some ways and inferior in others. Rocket artillery tends to be simpler, lighter and more mobile than guns or howitzers, most of which must be emplaced. Guns tend to have better accuracy, consistency, and range, while rocket artillery is light enough to be employed closer to the front lines and excels at saturation fire, expending its entire ammunition load in a single barrage on a target. The saturation fire produced by rocket artillery is only somewhat approximated in effectiveness with gun artillery via the time on target barrage method. Time-sensitive soft target interdiction (such as personnel or unarmored vehicles moving in large groups) is where rocket artillery is particularly useful. This allows for the shoot-and-scoot method, avoiding the enemy counter-battery fire that is the greatest risk to emplaced artillery pieces, while maximizing damage to the target before it can find better cover. (see Rocket artillery vs gun artillery) Portable anti-tank With the invention of the tank, the infantry required a weapon to