The hwacha (화차) is a wheel-mounted launcher that simultaneously fires between 100 and 200 gunpowder rockets (신기전, singijeon) at an adjustable angle. Developed from Choe Museon's work on Korean gunpowder during the Goryeo-Joseon transition, it represents the first documented multiple-rocket artillery. The singijeon rockets were classified into four sizes according to range and explosive charge. The system was decisive at the Battle of Haengju (1593) during Toyotomi Hideyoshi's Japanese invasion, where 3,400 Korean defenders repelled an assault estimated at 30,000 soldiers. Europe did not develop comparable rocket artillery until the Congreve rockets (1804), four hundred years later. The technology remained practically unknown in the West until 20th-century military-historical studies.