Development of the GpSfl IVa proceeded though with delay. By the time an evaluation pilot vehicle was being actively manufactured, the north of France (including the capital city of Paris) had fallen to the Germans. As such, the requirement for a bunker-busting vehicle dwindled to naught. By January of 1941, the Krupp concern had completed two pilot vehicles for testing though its future would seem in doubt at this point. Its fortunes changed when, in June of 1941, the Germans invaded the Soviet Union through "Operation Barbarossa" and, while initial progress was excellent, a formulated Soviet response eventually introduced the famous T-34 medium tanks and, perhaps more importantly, the IS (Josef Stalin) heavy tank series in the IS-2.
The IS-2 was a monster of Soviet engineering combining thick armor protection reaching 120mm (4.7 inches) and a powerful 122mm main gun in a traversing turret. The vehicle had the capability to engage all manner of German types at range and posed an ever-growing threat as production and availability of the series increased. 3,854 of the type were eventually produced and these were followed by the improved IS-3 models which entered development in late 1944 though failing to see action in World War 2 altogether.
With this new threat, GpSfl IVa development continued though now under the direction that it be tested and completed as a self-propelled tank destroyer. Its tracked nature assured that it could meet cross country challenges required of the fighting in Eastern Europe and mainland Asia and its use of a standardized German Army cartridge proved logistically friendly to a resources-strapped Germany. It was envisioned that 100 vehicles would be completed in an initial batch with production set to begin in early 1942. The two pilot vehicles were placed into active frontline combat groups for formal in-the-field evaluations, this with Tank Destroyer Battalion 521.
The two vehicles were active participants along the Eastern Front against the heart of Soviet Army forces. One of the units fought on into late-1941 before it was shipped back to Krupp factories for refurbishment. Once redeployed it was eventually lost in action as the German Army began suffering through various setbacks and eventual retreats. The other pilot vehicle was not lost to enemy action, however, for an onboard fire caused by the engine consumed a propellant charge and, in turn, the vehicle was lost in a terrific explosion, the crew managing to exit their vehicle prior.
Such ended the short-lived legacy of the GpSfl IVa tank killer as only these two prototype vehicles were ever completed. Testing showcased several shortcomings in their approach: the powerplant was underpowered and made for a slow road vehicle (17mph), outpaced by the Panzer IVs (26mph) it was forged from. The superstructure was fixed in place which required that the entire vehicle be turned to face the intended target area. Not only did this consume precious fuel and oil but it applied stresses to both the engine and running gear over time which made for a less reliable vehicle. Additionally, the superstructure was thinly protected which offered little to the gunnery crew from small arms fire, the elements and air attack and there was no form of protection overhead. From what is known, there was also no active provision for a machine gun for self-defense against infantry and low-flying aircraft. Ammunition storage only allowed for 26 x 105mm projectiles to be carried into battle and this undoubtedly proved a tactical limitation for the series.
The GpSfl IVa's lone saving grace proved to be its 105mm main gun though this was not enough to save the program as a whole. The Germans found better success with the SdKfz 164 "Nashorn" tank destroyer which saw 473 vehicles produced before the end of the war. The developmental 128mm-armed "Sturer Emil" tank destroyer of 1942 also saw only two pilot vehicles produced with one destroyed by the Soviets and the other captured - yet another German tank destroyer program given up for good.
The GpSfl IVa was known under the nickname of "Dicker Max".
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