The sIG 33 auf Geschutzwagen became several combat tank-based mobile howitzer platforms for the German Army in World War 2. This collection of Self-Propelled Artillery (SPA) expedients evolved along with the availability of the various tank chassis used throughout the conflict. At their core, the vehicles fielded the sIG 33 infantry howitzer of 150mm (15cm) caliber and were used in the ranged fire support role. The line began with the chassis of the Panzer I Light Tank and were evolved with the Panzer II Light Tank and, lastly, the Panzer III Medium Tank. Another conversion included the Czech Panzer 38(t) series.
The original 15cm sIG 33 heavy infantry guns were short-barreled, towed artillery pieces utilizing a two-wheeled carriage that incorporated a small gun shield, hydropneumatic recoil mechanism, and a horizontal sliding breech block. The weapon entered service in 1927 and was in play with German forces through to the end of the war in 1945. Manufacture of the guns happened through the storied concern of Rheinmetall primarily with additional production encountered under other brands as well - total production became about 4,600 guns by war's end.
Early-war actions by the German Army showcased a need for speed due to the evolution of mechanized warfare - towed artillery systems simply did not keep pace well alongside mobile armored corps which limited fire support tactics during a given assault. This put the focus on making the howitzers more mobile and the decision was made to mate the gun equipment to the existing, outgoing chassis of the Panzer I Ausf. B Light Tank line. The conversion process gave rise to the 15cm sIG33(sf) auf Panzerkampfwagen I Ausf B which also became known under the name of Sturmpanzer I.
The end result was just that, a Panzer I hull and chassis (complete with its running gear) and the sIG 33 series gun (complete with wheeled carriage) fitted over the vehicle - the original tank's turret accordingly removed. To this was added an open-topped, open-rear fighting cabin which essentially was made up of front and side walls. Sloping was only found along the front panel for basic ballistics protection. Overall weight of the new vehicle was 9.4 tons (short) and dimensions included a length of 2.7 meters, a width of 2 meters, and a height of 2.8 meters. Armor protection reached 13mm along the most critical facings and power was provided through a Maybach NL38TR 6-cylinder, water-cooled engine of 100 horsepower. The engine was coupled to a transmission system yielding five forward and one reverse speeds. Operational range was under 90 miles with a road speed up to 25 miles per hour. The crew numbered four and included a driver, commander, and two loaders.
Production of the Ausf. B model totaled just 38 units under Alkett GmbH and examples were available as soon as 1940. Once in action, limitations proved plenty for the vehicle held a high center of gravity making it an awkward and cumbersome battlefield sight. The lack of armor coverage readily exposed the crew to all manner of battlefield dangers as well as inclement weather. Onboard storage space was also at a premium with only three ready-to-fire 150mm projectiles carried. This latter quality required a trailing SdKfz 10 half-track to serve as ammunition carrier and to ferry three of the four crew into battle.
On the whole, the vehicle was an overweight design in which both the frame and motor works were stressed to their limits leading to frequent mechanical breakdowns. However, the sIG33 150mm guns were as lethal as ever, capable of supplying a hefty ranged punch against soft target areas through indirect fire. Range of the weapon reached out to 3.5 miles and a rate-of-fire of four rounds-per-minute could be achieved. The gun carriers saw combat service during the Belgian campaign and then on to the Battle of France (May-June 1940). They then followed with service in the invasion of the Balkans/Greece (April 1941), and the attack on southern Russia (June-November 1942). By the middle of 1943, the vehicles held little battlefield value and were either given up for good or lost through general wartime attrition.
Despite the limitations of the early mark, authorities considered the sIG 33 gun carrier design more or less a success as a quick-to-produce and effective mobile support platform. The 15cm sIG 33 auf Geschutzenwagen II Ausf C (SdKfz 121) (Sturmpanzer "Bison II") followed by mating the howitzer component to the Panzer II light tank chassis and this vehicle first appeared in 1942. Some changes were instituted to help address the failings of the original design such as a lower center of gravity - the roof line now equal to that of the original Panzer II tank. The 15cm sIG 33 FGST Ausf. PzKpfW II (sf) "Verlanget" appeared in 1943 and featured a lengthened and widened hull for better weight displacement. Power to the Panzer II-based marks was through a Bussing Typ GS 8-cylinder, liquid-cooled engine of 155 horsepower.
The most promising sIG33-armed Geschutzwagen vehicle form was a Panzer III-based model - the 15cm sIG 33 Ausf PzKpfW III. The vehicle was under consideration as early as 1941 and, while the medium tank chassis proved more than a viable gun carrier, further work on the line eventually lost steam in that only and only twelve examples were manufactured and these saw service along the Eastern Front.
The most successful of the sIG 33 conversion vehicles was the one based on the Czech PzKpfW 38(t), a design which was taken over by the conquering Germans. The newly-realized model of 1942 became the 15cm sIG 33(sf) Ausf. PzKpfW 38(t) SdKfz 138 "Bison" and SdKfz 138/I "Grille" was its perfected form of 1943. The PzKpfW 38(t)-based systems became the standardized sIG 33 gun carriers of the war and used in the greatest number.