Staff Writer (Updated: 8/4/2014):
The Katyusha was designed in the latter half of the 1930s and were only beginning to come online by the time the German Army invaded the Soviet Union in June of 1941. At that time, there only a few launcher batteries available and only a small stockpile of 132mm rockets to boot. Regardless, the systems were placed into active service wherever and whenever possible to keep a fragile defense intact until production could keep pace with demand. At any rate, these weapon systems served as quite a wakeup call to the unsuspecting German Army, particularly in July of 1941 during actions at Smolensk. The Germans nicknamed the Katyusha "Stalin's Organ" in response to its arrangement of launch rails (mimicking the pipes of an organ) and the noise the rockets made when launched. The Katyusha was also known to some as "Little Katy".
The rockets themselves, the Rs-82 and Rs-132 series, were in development for some time prior during the 1930s. These weapons were originally intended for use aboard Soviet ground-attack aircraft and ultimately proved effective in the role. It was not until 1938 that Soviet engineers began evaluating the weapon for use by ground-based launchers. Tests revealed the rockets to be viable battlefield implements and, in 1941, the BM-13 launch truck was ordered into production. At their core, these complete Katyusha systems proved relatively cheap to produce and fast to deliver to front line components requiring such a "shock and awe" weapon.
While rocket launching systems such as the BM-13 were largely inaccurate for direct target fire, there was no second-guessing the psychological effect delivered from a release of large, high-explosive rockets against an enemy-held area. If properly amassed in number, a force of such vehicles could lay waste to swathes of enemy-occupied territory and, as such, accuracy proved of little importance in the long run. The end result was usually a dead (or shaken) enemy and a wasteland of debris where trees and structures once stood.
The concept behind the BM-13 was quite simple, it consisting generally of a six-wheeled all-wheel drive truck chassis with a forward-mounted engine and crew compartment followed by the fixed launch rail system itself on a rear-set flatbed. A hydraulic lift elevated the forward portion of the launcher - up to 45-degrees - and interestingly no traverse was afforded on initial models. This meant that the entire vehicle had to be positioned towards the target area. The weapon, as a whole, relied on basic ballistic trajectory that itself relied on adjustments to the firing arc by the crew.
In their early forms, BM-13s were set upon the chassis of the Soviet ZiS-6 - a 6x6, six-wheeled utility truck that was modified to include a 16-rocket launch system. These vehicles fell under the formal designation of BM-13-16. The ZiS-6 offered adequate off-road and on-road capabilities and could reach virtually any battlefield location that a tank or other like-vehicle could. This meant she could participate alongside offensive actions - though most often from behind the direct front line - and launch her deadly salvo before the rush of the initial attack began. Such was the value of these early systems as a weapon to the Soviet Army that measures were taken to ensure their locations were kept secret. Tarps were utilized to shroud their appearance from low-flying enemy reconnaissance aircraft and only proven members (usually Soviet Secret Service - NKVD) of the Communist Party were enlisted to operate Katyushas within the ranks of the Red Army. First actions for BM-13s occurred on July 14th, 1941 in assaults against German Army troops and vehicles.