English: 21 cm Nebelwerfer 42 and Land Mattress at the Canadian War Museum. See also Image:Land Mattress plaque.jpg.

The Nebelwerfer 41 (fog-launcher in English) fired six 150mm rockets, which each carried an explosive charge comparable to the projectiles of the 105mm howitzers of the time. The six rockets were fired in quick succession by the 4 man crew, which had to take cover from the rocket back blast in a small trench further away. Although a very potent weapon, the Nebelwerfer had a big drawback - the rockets it fired left a large smoke trail in the air, which could be visible from miles away. This meant that the Nebelwerfer batteries could only stay in the same place for a short time after firing, before the enemy artillery would have zeroed in on their position. When the rockets were fired they made a howling pipe-organ sound, which led the allied soldiers to nickname them 'screaming mimis'.

Also look at this link: http://www.lonesentry.com/articles/nebelwerfer/index.html

Resources to find more info about Nebelwerfer:

Emde, Joachim. Die Nebelwerfer: Entwicklung und Einsatz der Werfertruppe im Zweiten Weltkrieg
Husemann, Friedrich. In Good Faith: The History of the 4. SS-Polizei-Panzer-Grenadier-Division: Volume 1: 1939-1943
Geheimen Tagesberichte der deutschen Wehrmachtführung im Zweiten Weltkrieg, 39-45
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Hinze, Rolf. Frost und Pulverdampf: Der Schicksalweg der 20. Panzer-Division
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Lehmann, Rudolf. The Liebstandarte; Vol. III
Mark, Jason D. Death of a Leaping Horseman: 24. Panzerdivision in Stalingrad 12th August - 20th November 1942
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Ruef, Karl. Odyssee einer Gebirgsdivision: Die 3. Geb.Div. im Einsatz
Stadler, Silvester. Die Offensive gegen Kursk: II. SS Panzerkorps als Stoßkeil im Großkampf
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Vopersal, Wolfgang. Soldaten, Kämpfer, Kameraden: Marsch and Kämpfe der SS-Totenkopf-Division

Zetterling, Nikolas. Normandy 44: German Military Organization, Combat Power and Organizational Effectiveness