The Nebelwerfer (smoke mortar) was a World War II German series of weapons. They were initially developed by and assigned to the Wehrmacht's "smoke troops" (Nebeltruppen). Initially, two different mortars were fielded before they were replaced by a variety of rocket launchers ranging in size from . The thin walls of the rockets had the great advantage of allowing much larger quantities of gases, fluids or high explosives to be delivered than artillery or even mortar shells of the same weight. With the exception of the Balkans Campaign, Nebelwerfer were used in every campaign of the German Army during World War II. A version of the 21 cm calibre system was adapted for air-to-air use against Allied bombers. Weapons 10 cm Nebelwerfer 35 The lower muzzle velocity of a mortar meant that its shell walls could be thinner than those of artillery shells, and it could carry a larger payload than artillery shells of the same weight. This made it an attractive delivery system for poison gases. The U.S. Army's Chemical Warfare Service developed their 4.2 inch chemical mortar for precisely that reason and the Nebeltruppen shared that reasoning. Its first weapon was also a mortar, the 10 cm Nebelwerfer 35, which was designed in 1934. 10 cm Nebelwerfer 40 Almost from the beginning, the army wanted more range than the 10 cm NbW 35's , but troop trials of two prototypes did not take place until May 1940. Neither was entirely satisfactory, but the best features of both were incorporated into the 10 cm Nebelwerfer 40. This was a very advanced breech-loading weapon with a recoil mechanism and an integral wheeled carriage. It had twice the range of its predecessor, but was eight times the weight and cost nearly ten times as much: vs . 15 cm Nebelwerfer 41 Rocket development had begun during the 1920s and reached fruition in the late thirties. This offered the opportunity for the Nebeltruppen to deliver large quantities of poison gas or smoke simultaneously. The first weapon to be delivered to the troops was the 15 cm Nebelwerfer 41 in 1940, after the Battle of France, a purpose-designed rocket with gas, smoke and high-explosive warheads. It, like virtually all German rocket designs, was spin-stabilized to increase accuracy. One very unusual feature was that the rocket motor was in the front, the exhaust venturi being about two-thirds down the body from the nose, with