While the German Nebelwerfer was the first multiple rocket launcher used operationally in the Second World War, the Soviet 130mm Katyusha rocket was the most widely used, and has become a generic term for a medium-sized unguided rocket used in indirect fire. In the original use, the rockets were fired from 12-rocket mounts on trucks, which operated in battalions of 18 trucks. The rockets had a unitary high explosive warhead; firing large numbers achieved area coverage comparable to that of cluster munitions.
They could, however, be fired individually, with very little accuracy but great portability; guerrillas could put them in a crude trough launcher, connect the rocket igniter to a time fuze, and be long away from the launching site when the rocket fired into the general area of the target. This technique is used by Palestinian guerrillas attacking Israeli targets, and by the Viet Cong in harassing bases.
Individual rockets of this type are quite inaccurate in comparison to howitzers, but in conventional military use, the inaccuracy became an advantage. Typically all the launchers of a battalion would fire at once, and the wobbling rockets would spread out to put blast and fragmentation over a large area. Traditional cannon artillery were more accurate and put more intense blast in a smaller area; the functions were complementary.
Eventually, the Katyusha was replaced by the more efficient 122mm GRAD rocket, which gave longer range and higher density of target coverage.