Sunday, 5 March 2017

Pz.Sfl.IVb: Halfway to the Hummel

German wartime SPGs are well known. However, it took some time to develop the "selbstfahrlafette" concept of a large open casemate, like the one used on the Hummel. Initially, the idea was to built medium SPGs instead of light ones, and their layout differed noticeably from the vehicles that showed up on the battlefield in 1943. Even though German SPGs developed along a different path, the Pz.Sfl.IVb was built and even got to fight.
Byproduct

The first designs of a 105 mm howitzer on the PzIV chassis appeared back in 1935. The vehicle was called Rauchwagen (R.W.), "smoke vehicle", armed with a 16 caliber long howitzer. Essentially this was an artillery support variant of the B.W. rather than an SPG.

The project reached the practical stage. According to correspondence, a B.W. I Kp. turret arrived at a Krupp factory in the winter of 1938 and conversion began. In April of 1938, the turret was installed on a B.W.II Kp. which was also equipped with a Pz.Kpfw.II Ausf. B turret platform. According to a report from November 17th, the turret was removed from the B.W.II Kp. in the fall and installed on a special stand for gunnery trials, and the tank itself was converted to a bridgelayer.

The 10.5 cm leFH 18 was Germany's primary light howitzer during the war. Its large mass, almost 2 tons, was one of the reasons that a self propelled version was needed.

German medium SPG design continued to evolve along the "bunker buster" path. Development of an SPG with a 105 mm K 18 gun began in 1938. The vehicle was designed to assault heavy fortifications like the Maginot line in France or the Beneš line in Czechoslovakia. 

The first drafts were ready by April of 1939, titled 10.5 cm K. L/52 Selbstfahrlafette: 105 mm K 18 gun on a self propelled mount. Even at this stage, there were disagreements between Krupp engineers and the client, the 6th Department of the Armament Directorate.

Krupp insisted on a vehicle whose layout differed little from that of the B.W. (PzIV). The engine would be placed in the rear of the tank in order to better distribute the weight and reduce height, but the 6th Department insisted on putting the engine under the gun. This made the engine more difficult to service and raised the bore axis, but the fighting compartment was larger as a result.

Pz.Sfl.IVb at the factory. The vehicle uses elements of the PzIV Ausf. F chassis.

Putting the engine underneath the gun radically cut the choice of engine. Instead of the Maybach HL 120, the stock engine of the B.W., a 300 hp Maybach HL 116 linear engine was used. It was also too large. The final version of the 10.5 cm K. L/52 Selbstfahrlafette received a 188 hp Maybach HL 66 engine. The power to weight ratio fell by over 1.5 times compared to the B.W. In order to somehow compensate for this unfortunate result, the engineers tried to reduce the mass of the 10.5 cm K. L/52 Selbstfahrlafette to 20 tons.

The suspension layout changed several times during the design process. Several variants of the vehicle with 6 wheels per side and a torsion bar suspension were presented in May of 1939. After dealing with torsion bars on the B.W. II, Krupp engineers were less than enthusiastic about the idea. In August, when the 10.5 cm K. L/52 Selbstfahrlafette began taking its final form, the torsion bar suspension was removed as it weighed 430 kg more than the spring variant.

The 6th Department approved the production of two complete Pz.Sfl.IV (10 cm) on the PzIV chassis and one experimental chassis with six wheels per side.

Even though the Pz.Sfl.IV looked similar to the PzIV, it had less in common with it than the Pz.Sfl.IVa.

The six wheel vehicle with a 105 mm K 18 gun was never built. This project had a different fate in store. Krupp likely began working on an SPG designed to support tanks in the summer of 1939. Two technical solutions found themselves joined in this project: six wheels with a spring suspension per side and a rear engine.

Krupp presented blueprint W 1324 on September 14th, 1939. It contained a vehicle initially called Pz.Sfl.IV (leFH 18). This was only the beginning of the path to a new tank, since the concept they presented had many flaws. It wasn't possible to simply convert a 10.5 cm K. L/52 Selbstfahrlafette, as the howitzer had to fire at a rather steep trajectory, which the chassis could not support. A proposal was made to raise the bore axis height to 1900 mm to compensate. The 6th Department also demanded that the ammunition capacity should be at least 40 rounds.

It's easy to spot the differences from a PzIV from the rear.

Another requirement was thinner armour. The 6th Department rationally decided that bulletproof armour was enough for this class of vehicle.

The characteristics were revised during a meeting that took place on October 10-11, 1939. The components of the Pz.Sfl.IVb would be the same as on the Pz.Sfl.IVa. The bore axis was raised to 1800 mm, which increased maximum gun elevation to 40 degrees. The deadline for the Pz.Sfl.IVb was also given: January 1941.

Half-Tank

The Pz.Sfl.IVb project began taking its final shape in February-March of 1940. A wooden model was inspected by Senior Lieutenant Herbert Olbrich from the 6th Department, and he prepared a series of complaints. In the meantime, discussion continued about the required amount of armour for the SPG. There were debates between 20 mm and 30 mm. As a result, protection against 20 mm autocannons was deemed excessive for the SPG, since extra armour is extra weight, and a power to weight ratio of 10 hp/ton was desirable.

A wider turret ring made changes to the sides necessary.

The result was significantly different not only from other German SPGs, but from the Pz.Sfl.IVa. Instead of a typical SPG with an immobile casemate, Krupp engineers created a tank-like turret. Overall, the design was similar to what would later become common for American tank destroyers.

The Pz.Sfl.IVb had the same engine, transmission, and a number of other components as the Pz.Sfl.IVa, but the two SPGs were visually distinctive. One could easily see the PzIV as an ancestor of the Pz.Sfl.IVa, but the resemblance was less obvious with the Pz.Sfl.IVb. Only the overall layout, some suspension elements, and the observation devices remained unchanged. Overall, this was a completely different vehicle, only somewhat resembling a PzIV. The width of the tank was the same, but the Pz.Sfl.IVb's hull was 35 cm shorter. The diameter of the road wheels grew from 470 mm to 520 mm (the same size as PzIII tanks starting with the Ausf. E), and the length of the track contact surface decreased from 3.5 to 2.9 meters.

The shorter hull and thinner armour helped reduced the mass of the SPG to 18 tons, giving it a higher power to weight ratio than  Pz.Sfl.IVa. The same engine and transmission allowed the SPG to reach an acceptable speed of 35 kph.

The radio was installed in the turret bustle to optimize available space.

Like in the Pz.Sfl.IVa, the driver's compartment only had room for the driver. The designers didn't make him a separate casemate, but installed a fake observation device in the front of the tank. As for the fighting compartment, it had a real turret instead of a fixed casemate. The turret had nothing in common with the turret of a PzIV, and the turret ring was so large that the sides had to be widened. The turret of the Pz.Sfl.IVb was larger than on the PzIV.

The turret was open from the top, which resolved the issue of ventilation. The commander, gunner, and loader were housed inside. The turret could not rotate fully and its rotation was limited to 35 degrees to each side. This was enough for an SPG that mostly fired indirectly.

Gunner and commander positions. It was cramped, but overall there was enough space.

The use of a turret was not the best solution. Even though the turret ring was wider than on the PzIV, it was still not enough for an SPG with a two-piece round. Not to say that it was cramped, but the working conditions were worse than inside a vehicle of similar size with a casemate.

On the other hand, the SPG could boast a large ammunition capacity, including 60 105 mm rounds and 576 MP-40 rounds. In addition, the Pz.Sfl.IVb had a maximum gun elevation of 40 degrees, according to the client's wishes. A large muzzle brake was added to reduce recoil. Of course, it would give away the vehicle's position when firing, but that was not critical for indirect fire.

Interrupted Production

On November 2nd, 1940, the 4th Department of the Armament Directorate signed contract #004-1520/40 with Krupp for the assembly of two experimental Pz.Sfl.IVb prototypes. Around the same time, Rheinmetall-Borsig received contract #004-1039 for four guns.

As it happens in these cases, deadlines were missed, especially since the factory was busy with PzIV production. Both experimental prototypes, which were renamed to leFH 18 (Sfl.) on August 13th, 1941, were ready only by early January of 1942. After gunnery trials in Meppen, the vehicles were sent to Kummerdorf. The second prototype was damaged during trials, which required slight repairs.

Meanwhile, preparation began for mass production before the prototypes were even completed. Contact #SS-016-4253/40 was signed with Krupp to produce a trial batch of leFH 18 (Sfl.). According to revised plans from October 3rd, 1941, the first vehicle was awaited in December, two more in January of 1942, three in February, and four in March. Vehicles from the experimental batch received serial numbers from 150631 to 150640. The same plans requested the first two mass production vehicles in October of 1942, five in November, ten in December, and 15 in January of 1943.

Mass produced Pz.Sfl. leFH 18 Ausf. A during exercises. The function of the suspension can be seen. The tactical number suggests that it was the second gun in the battery.

Revised plans from December 1st, 1941, laid the foundation for contract #SS-210-8902/41. According to this contract, the total order for leFH 18 (Sfl.) vehicles would be for 200 units. These vehicles were also called Pz.Sfl.IVb3 in documents. The new index had a good reason behind it: the SPG's suspension was altered and a change in the engine was planned. The new "heart" would be a Maybach HL 90 P20K. Since by that time production of the Pz.Sfl.IVa was cancelled, it was no longer necessary to use the same engine. The 9 Liter 12 cylinder Maybach HL 90 P20K increased the power to weight ratio to an impressive 17.7 hp/ton. Both experimental prototypes would be converted in the same way.

Increasing pressure on Krupp related to PzIV production had a noticeable effect on the production of the experimental batch of leFH 18 (Sfl.), which was renamed Pz.Sfl.IVb Ausf. A in January of 1942. The name didn't last long and changed to Pz.Sfl. leFh 18 Ausf. A in March. Initially, the project was on schedule. Hulls numbered 150631-150634 were ready by the end of November of 1941. The first turret platform and turret were ready in December. Then the project was put on hold, as the factory was overloaded and too busy to begin assembly.

The remaining 6 hulls were finished by early February of 1942, all turret platforms were ready by March and all turrets by May 1st. The assembly of the first vehicle at Grusonwerk was expected in April, but in reality was only completed by August.

It was clear in the summer of 1942 that the load on Grusonwerk would only grow and that a different manufacturer of the Pz.Sfl. leFH 18 Ausf. A would have to be found. This manufacturer was Stahlindustrie GmbH in Mülheim an der Ruhr. The factory was close to the Krupp factory in Hessen. Contract #SS210-8902/42(H) was signed for the production of the Pz.Sfl.IVb3. Correspondence dated July 20th, 1942, states that a series of changes were made to the SPG, the most important of them was the ability to traverse the turret by a full 360 degrees.

The turret, turned as far to the left as possible.

Production of the SPG suddenly suffered a serious blow, and it had nothing to do with Germany's enemies. An SPG on the PzII chassis entered trials in July of 1942, armed with the same leFH 18. Trials showed that it was suitable for the objectives that it was designed to achieve, and the fighting compartment was roomier than in the Pz.Sfl. leFH 18 Ausf. A. At the same time, the SPG used the familiar PzII hull (the PzII was removed from production). Production of a light SPG meant that medium tank production would not be impacted, and the price was much lower.

Hitler could not ignore such an obvious optimization. Plans for medium SPG production were altered on July 25th, 1942. Instead of the leFH 18, they now had to use the more powerful 15 sFH 18. This meant the death of the SPG. Even though Krupp presented a blueprint of the s.FH.18 auf Sfk.IVb Fahrgestell on August 14th, this was only a failed attempt to rescue the SPG. It was clear that a serious conversion would have to be made, as the maximum gun elevation was only 25 degrees. In early November of 1942, the design was killed completely and development of German medium self propelled howitzers went in a different direction. 

Four Pz.Sfl. leFH 18 Ausf. A ended up in a training facility. These vehicles have idlers from the PzIV Ausf. E.

Even though mass production of the Pz.Sfl.IVb was cancelled, production of the trial batch continued. Grusonwerk delivered three SPGs in September of 1942, four in October, one in November, and another in December. A number of production Pz.Sfl. leFH 18 Ausf. A vehicles had slightly different running gear.

Six vehicles were sent to the 16th Artillery Regiment of the 16th Tank Division. The authorized strength of the regiment was 6 light and 6 medium SPGs. In reality, the regiment received no light SPGs, since production did not begin yet. The regiment was sent to Stalingrad as is, where it met the same sad fate as the rest of the 16th Tank Division. As for the four remaining Pz.Sfl. leFH 18 Ausf. A, they were used as training vehicles. They served alongside the leFH 18/2 (Sf.) auf GW II, the design that sealed their fate.

The fate of the Pz.Sfl.IVb was a tragicomedy. German military-political management sabotaged the production of one of the best SPGs in its class. Interestingly enough, development of a very similar SPG with the same howitzer began in the spring of 1943. The number of projects later grew to two, and both reached the prototype stage. Work continued until late fall of 1944 and ended with nothing. It's difficult to be sad about their fate, since this failure only played into the hands of the Allies.

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