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Panzerwaffe Between III and IV

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German tank building during WW2 ended up in a situation where the similar Pz.Kpfw.III and Pz.Kpfw.IV tanks as well as SPGs and other vehicles on their chassis were built in parallel in large numbers. The tank chassis were similar in many of their characteristics. They were equipped with the same engines and transmissions of the same type. At the same the designs were incompatible in many ways. Let us try to figure out how the Germans arrived at two solutions instead of one and why multiple attempts to unite the two designs ended in failure.

The Pz.Kpfw.III and Pz.Kpfw.IV's main difference was armament.  However, different guns didn't need different chassis. The 50 mm tank gun fit perfectly well into the Pz.Kpfw.IV Ausf.D and the casemate from the StuG III was reused on the StuG IV. One can say that the Pz.Kpfw.IV chassis became the standard, as it was easier to produce and operate. The Pz.Kpfw.III could also have become the standard. The 75 mm KwK 37 fit on the Pz.Kpfw.III Ausf.N, making it an analogue of the early Pz.Kpfw.IV, although the chassis would have to be changed for the long 75 mm gun to fit.

A Pz.Kpfw.IV Ausf.D tank with a 50 mm KwK 39 L/60 gun. Production of these tanks could have began at the Niberlungenwerk factory, but in the fall of 1941 these plans were cancelled in favour of the long 75 mm gun.

For the sake of convenience, we will use the term "20 ton class tank" to refer to the Pz.Kpfw.III, Pz.Kpfw.IV, VK 20 series, and the Pz.Kpfw.III/IV, even though the weight of these vehicles could have been greater or less than 20 tons. The German desire to fit their tank into the 18 ton weight limit can be explained by the 18 ton capacity of the standard mobile bridge.

From the "tractor" to a 20 ton tank

The Germans were actively building up two medium tanks at the start of WW2: the Pz.Kpfw.III Ausf.E/F/G with a 37 mm gun and the Pz.Kpfw.IV Ausf.B/C with a 75 mm gun. These tanks had similar characteristics and the only large difference was armament. Both tanks had the same Maybach HL 120 TR and TRM engines, a crew of 5 men, 30 mm of front armour, and weighed about 19 tons. The characteristics of the two tanks became even more similar when the decision was made to remove the 10-speed semiautomatic Maybach Variorex gearbox from the Pz.Kpfw.III and replace it with the simpler 6-speed SSG 77 gearbox based on the Pz.Kpfw.IV's SSG 76 gearbox.

The result was truly odd. The engine was the same, but the engine compartments and cooling systems were completely different. The SSG 76 and SSG 77 gearboxes were almost identical, but not interchangeable, since the Pz.Kpfw.III had its main clutch on the engine and the Pz.Kpfw.IV had it as a part of the gearbox. The turning mechanisms functioned the same way and were controlled in the same way, but the Pz.Kpfw.III had its planetary mechanisms and backing brakes integrated with the gearbox in one transmission block, while the Pz.Kpfw.IV had its planetary mechanisms and brakes joined with the final drives. Again, the same concept was implemented in two completely different ways.

One can only ask: why produce tanks and SPGs on two similar but incompatible chassis at the same time? Is it not better to produce one tank with two different guns? This is what other nations did quite successfully. To understand the origins of this separation, we must go back to the predecessors of the Pz.Kpfw.III and Pz.Kpfw.IV tanks: the Leichttraktor and Grosstraktor

A commercial tractor on the Leichttraktor chassis. In this case, the name accurately reflects the vehicle's purpose.

The Germans were working on two tanks in the late 1920s: the Leichttraktor with a 37 mm gun and the Grosstraktor with a 75 mm gun. The two vehicles were developed to satisfy two completely different requirements. The Leichttraktor was a light tank with a crew of four with a front engine compartment. If necessary, the chassis could be used as an artillery tractor. The Grosstraktor was an amphibious 6-man 2-turret medium tank.

All three variants of the Grosstraktor were failures, and one drowned during trials to boot. Taking this experience into account, the Germans produced the Neubau Fahrzeug, a multi-turreted tank with 75 and 37 mm guns, but one with a more traditional layout and no amphibious capability. Despite its humble armour thickness of 13-20 mm, it weighed 23 tons, a whole five tons more than needed. The Germans then transitioned towards a layout with a front transmission that they copied from the British. The tank was left with just one 75 mm gun. First one machine gun turret was dropped, then the other. As a result, the Pz.Kpfw.IV Ausf.A had just one turret with a 75 mm gun.

Rheinmetall B.W. tank prototype created to replace the Nb.Fz. tank. A spot for a machine gun turret is left free to the right of the driver. The Krupp B.W. was built with one turret from the start.

The Leichttraktor was also a failure. The Germans decided to use the layout with a front transmission they copied from the British and add a fifth crewman who would sit in the turret. The future Pz.Kpfw.III tank was initially envisioned as a 10-ton vehicle, but then the weight limit was raised by two tons, and the final product weighed a whole 16 tons. It turned out that different tanks became closer to one another. The Nb.Fz.'s descendants shed their extra turrets and weight, while the Leichttraktor's descendants grew bigger. The Germans approached the ideal 20 ton tank from both sides. One must then ask: if the Pz.Kpfw.III and IV are so similar, why even have two tanks?

Third time's the charm

Heinrich Kniepkamp, an engineer from the 6th Department of the Ordnance Directorate responsible for mechanization of the Wehrmacht, thought of a new line of high speed tanks in early 1937. He wanted to radically improve mobility of tanks using technical novelties like the torsion bar suspension and semiautomatic gearboxes. It was also already obvious to Kniepkamp that Germany only needed one 20 ton tank.

Pz.Kpfw.III Ausf.E, the first Pz.Kpfw.III produced in any significant numbers. With the exception of a simpler turning mechanism and ordinary tracks, it match Kniepkamp's vision.

Krupp, the company working on the Pz.Kpfw.IV, did not want to follow the new requirements and rejected torsion bars outright. Daimler-Benz went with the flow and designed a new chassis for the Pz.Kpfw.III called Z.W.38 with a torsion bar suspension, halftrack style tracks with rubber pads, a more powerful Maybach HL 120 engine, a semiautomatic 10-speed gearbox, and a three-radius two-loop turning mechanism. The new vehicle's top speed was expected to grow from 35 to 70 kph!

A proponent of standardization, Kniepkamp wanted to get rid of the Pz.Kpfw.IV, as it did not match his vision. Instead, the Pz.Kpfw.IV turret with a 75 mm gun would be installed on the Z.W.38 chassis. Meanwhile, Henschel was working on the D.W. heavy tank that had the same engine and transmission as the Z.W.38. The Germans ended up with a unified 20 ton chassis with two armament options coupled with a heavy tank. The Pz.Kpfw.IV would be removed from production. There were high hopes for the new chassis. Previously Pz.Kpfw.III tanks were only built in small series, but production of the Z.W.38 was planned out for three entire series (Ausf.E, F, and G). The 6th Department informed Krupp in June of 1937 that production of the Pz.Kpfw.IV would end with the Ausf.B series and the turret with its 75 gun will be installed on the new standard chassis (Einheitsfahrgestell) from Daimler-Benz.

Maybach Variorex gearbox. Five pairs of gears for 10 speeds suggest that the switching mechanism must have been complex.

These grandiose plans failed. First of all, an 8 month pause in production was anticipated, and so Krupp still got the order for the third series or Pz.Kpfw.IV Ausf.C. Second, trials of the Z.W.38 prototype revealed many issues. The rubber pads on track links wore out quickly, the three radius turning mechanism did not work properly, and the complex gearbox needed a lot of improvements. Even the simplified tank with regular tracks and simpler gearbox was difficult to produce. The initial plans called for delivery of 96 Pz.Kpfw.III Ausf.E tanks by September of 1938 with gearbox production starting in July. In reality, 23 gearboxes and just one tank were finished in 1938. To compare, 42 Pz.Kpfw.IV Ausf.B tanks were delivered between May and October of 1938 without any issues.

Kniepkamp made a risky bet on a difficult design without any preliminary trials, which led to a failure to meet production quotas right on the eve of the war. It was simply not possible to cease production of the Pz.Kpfw.IV in these conditions, and Krupp kept its contract.

Head transplant

Kniepkamp's plan with a new unified chassis was not the only attempt to install a Pz.Kpfw.IV turret on a Pz.Kpfw.III hull. Krupp began working on the Pz.Kpfw.III Ausf.K in December of 1941. This was a late model Pz.Kpfw.III with a Pz.Kpfw.IV turret and new long 75 mm gun. The increase in weight needed a more robust suspension and wider tracks, since the Pz.Kpfw.III had a shorter contact surface than the Pz.Kpfw.IV and needed wider tracks to maintain the same ground pressure. This idea had some merit, but Hitler spoke out in favour of maintaining Pz.Kpfw.III production with 50 mm guns, especially since the Ausf.K had no advantages over the Pz.Kpfw.IV Ausf.F2

The Pz.Kpfw.III Ausf.K with the 75 mm L/43 gun would have looked something like this.

That was not the end to this idea. A new turret was designed for the Panzerbefehlswagen III Ausf.K command tank in April of 1942. Its size and shape were similar to those of the Pz.Kpfw.IV's turret (specifically the B.W.40 that we will mention later) and the turret ring was taken from the Pz.Kpfw.IV Ausf.E. 50 Pz.Bfw.IV Ausf.K were built with these turrets and 50 mm guns from December to February of 1943. This shows that the Germans could have made a unified tank on the Pz.Kpfw.III chassis if they wanted to and that the argument that the turret ring could not have been expanded has no merit. The problem was that the Pz.Kpfw.IV tank was still much more suitable for this role and didn't need any changes to its chassis.

As for unification of the chassis, the Germans didn't stop with the Z.W.38. Even before production of the Pz.Kpfw.III began to slip behind schedule, Daimler-Benz was tasked with developing the VK 20.01 (III) to replace it. A chassis design was ready by December of 1938. It was equipped with the new HL 116 engine that put out the same 300 hp as the HL 120, but it was lighter and more compact. A new engine compartment was designed to accommodate it. The vehicle had a torsion bar suspension and used large interleaved road wheels.

The Pz.Bfw.III Ausf.K can be distinguished by the observation port in the turret next to the gun mantlet. 

On September 15th, 1939, Kniepkamp discussed the creation of the VK 20.01 (IV) that used some solutions developed by Daimler-Benz to replace the Pz.Kpfw.IV. Unfortunately, this attempt at unification ended before it could really begin. First of all, Erich Wolvert, Krupp's chief tank engineer, accepted the new engine compartment with the HL 116, but refused to use the running gear since it did not meet either the weight or width limitations. He proposed a Pz.Kpfw.IV-like running gear with six road wheels combined into three bogeys per side.

Second, Daimler-Benz learned about the mass breakdowns of Pz.Kpfw.III Ausf.E transmissions and running gear. The company formed the opinion that the 6th Department insisted on using untested components despite the engineers' suggestions. This was the last drop for Daimler-Benz, and in October the company obtained permission to independently develop the VK 20.01 (D) medium tank. It dropped the torsion bar suspension and used DB's own MB 809 diesel engine. One could forget about any kind of unification. In May of 1940 the 6th Department killed off the VK 20.01 (IV) project (which by then was called B.W.40). 
B.W.40 chassis. Later Krupp would return to the six-wheel running gear on the Pz.Kpfw.III/IV.

It's impossible to cover all VK 20 projects in one article to the depth that the topic deserves. Instead, let us discuss two important episodes from this development. The first took place on September 6th, 1941, when requirements for the Pz.Kpfw.III and IV n.A. (neuer Art, "new design") were discussed. The new tanks were supposed to be unified and only differ in armament. The Pz.Kpfw.III n.A. received the 50 mm L/60 gun and 50 mm of armour all-around. The width of the hull was reduced to 1.65 m to meet the 23.5 ton weight limit. The Pz.Kpfw.IV n.A. used the 75 mm gun, which required a 1.8 m wide hull. To compensate, the side and rear armour were reduced to 40 mm in thickness.

At a meeting with representatives of the army and industry held on November 29th, 1941, Hitler limited tank development to four vehicles:
The revised role of the tank with a 50 mm gun was linked to the recent study of Soviet armoured vehicles. It was clear that the German army needed one medium tank with the 75 mm L/43 gun in a unified turret (Einheitsturm). 

Three quarters

The Germans could very well have put into service a unified medium tank with a long 75 mm gun based on the VK 20.02 (K), VK 23.01 (K), or VK 20.02 (M). It's also possible that the VK 24.01 (M), MAN's precursor to their Panther tank, also could have become such a tank, but little information on this project remains. History went in a different direction. A decision was made in December of 1941 to abandon the 20 ton tank class in favour of the 30 ton Panthers. The VK 30.02 (M) grew from 30 tons to 44 during development and could not fully replace the 20 ton tanks. Production of the Pz.Kpfw.III and Pz.Kpfw.IV continued, but now a third chassis joined them.

Installation of the Pz.Kpfw.III transmission into the hull of a Hummel.

Alkett began working on a new 150 mm SPG in the spring of 1942. Until then Alkett produced only the StuG III, but its chassis was too weak, and so a decision was made to use the suspension elements from the Pz.Kpfw.IV tank. The Geschützwagen III/IV was a hybrid vehicle: the cooling system, running gear (minus the drive sprockets) were taken from the Pz.Kpfw.IV tank and the SSG 77 gearbox, turning mechanism, final drives, and drive sprockets came from the Pz.Kpfw.III tank. This chassis was used to build the Hummel and Nashorn as well as ammunition carriers. 

Panzer Tracts has two explanations for why the transmission and drive sprockets from the Pz.Kpfw.III were used. First of all, the Geschützwagen III/IV hull was as wide as the Pz.Kpfw.III hull, which was slightly wider than the Pz.Kpfw.IV, so it was natural to use the transmission from the former. Secondly, the turning mechanism and final drives of the Pz.Kpfw.III were considered more reliable. Both explanations are doubtful. The width of the hull was likely not an issue, since the Hummel prototype was built with Pz.Kpfw.IV drive sprockets and transmission. The Pz.Kpfw.III transmission had its advantages, but it also had a considerable weakness: the main clutch. The Geschützwagen III/IV suffered from failures of the final drive and main clutch.

Drawings of the Pz.Kpfw.III/IV. 5 mm thick spaced armour that covered the lower side is not shown.

Another attempt at unification was undertaken under Gerd Stiele von Heydekampf, who replaced Ferdinand Porsche as the head of the Tank Commission in December of 1943. Requirements for a  new Pz.Kpfw.III/IV tank were composed at a Commission meeting held on January 4th, 1944. As the name suggested, its chassis would be made up of the best components from either tank. The turret and cooling system came from the Pz.Kpfw.IV, SSG 77 gearbox and turning mechanism from the Pz.Kpfw.III. The HL 120 engine remained unchanged. Reinforced Pz.Kpfw.III style final drives were used.

Reserves of the Pz.Kpfw.IV suspension were exhausted, so it was designed anew. The new running gear consisted of six 660 mm steel rimmed road wheels per side combined into three bogeys on leaf springs. The 540 mm wide track links were influenced by the Tiger II. The clearance was raised to 520 mm. The new hull had thick sloped armour: the front was composed of 60 and 80 mm thick plates at angles of 45-60 degrees, and the upper side was 30 mm thick at 36 degrees. Ammunition capacity was increased to 100 rounds. To simplify the design, the Germans discarded the auxiliary motor and replaced it with a 300 L fuel tank. The turret was powered by a flexible electric cable, hence it could not traverse a full 360 degrees (the traverse was limited to 270 degrees to each side).

Simplified Pz.Kpfw.IV turret.

Krupp also proposed a simplified turret for the Pz.Kpfw.IV. It lost the commander's cupola and observation devices in the sides, but had thicker armour: 80 mm in front and up to 42 in the sides. Perhaps this turret was also considered for the Pz.Kpfw.III/IV, as the turret was cancelled shortly after the vehicle was.

A unified chassis

In parallel with the Pz.Kpfw.IV auf Einheitsfahrgestell (one of the names of the Pz.Kpfw.III/IV) Germany worked on a tank destroyer on its chassis. This project was known by several names: Sturmgeschütz auf Pz.Kpfw.IV, then leichte Panzerjäger III/IV (German tank destroyers were separated only into light and heavy categories). The casemate from the Jagdpanzer IV with a 75 mm Pak 42 gun and 80 mm of front armour installed at an angle of 50 degrees would be installed on the unified chassis. Surviving early blueprints produced by Krupp show several variants with 500 and 540 mm tracks, transmissions from the Pz.Kpfw.III and IV, as well as different real hulls.

Reconstruction of how the Panzer IV lang (E) tank destroyer would have looked like.

The 6th Department signed contracts to produce three experimental Pz.Kpfw.III/IV in March of 1944. On April 7th Hitler ordered trials and production of the new Einheitslaufwerk running gear. Discussions were held in June on putting the tank into production at the Grusonwerk factory in February of 1945. On July 12th the Pz.Kpfw.III/IV was cancelled in favour of the Panzerjäger III/IV. A week later the simplified turret was also cancelled.

Production plans for the Panzerjäger III/IV were composed in May of 1944. Alkett and MIAG would switch over to the new tank destroyer in November of 1944, Krupp would switch in January of 1945, VOMAG and Nibelungenwerk would switch in March. The  Pz.Kpfw.IV, StuG III, StuG IV, Jagdpanzer IV, Panzer IV/70 (V) and Panzer IV/70 (A) would all be replaced with the Panzer IV land (E) that had the same powerful Pak 42 gun as the Panzer IV/70 but without the weak final drives and overloaded suspension.

Demonstration of new vehicles. An ammunition carrier with the experimental running gear can be seen in the background.

The Germans tested several types of running gear before settling on one variant. A photo exists of a Geschützwagen III/IV ammunition carrier with the Einheitslaufwerk running gear, presumably it was used to test the new components. Deutsche Eisenstahl produced a hull for the Panzerjäger III/IV in September of 1944, but the Einheitsfahrgestell III/IV program was stopped a month later.

Not too long before that, the Germans discussed production of vehicles on three chassis:
  • The 38(t) chassis
  • The 25 ton Einheitsfahrgestell III/IV chassis
  • The Panther and Tiger chassis
The second group was cancelled and a decision was made to concentrate on the 38(t) chassis with the Tatra diesel engine. As a result, on October 4th the Tank Commission decided to leave only three chassis in production: 38(t), Panther, and Tiger II. The Panzer IV lang (E) was cancelled in favour of the Jagdpanzer 38(t)

Finally, let us mention the SPGs planned on this unified chassis. Discussion of moving the le.F.H.18/40/2 auf Geschützwagen III/IV to the Einheitsfahrgestell Pz.Kpfw.IV (the name used to refer to the Pz.Kpfw.III/IV, presumably to avoid confusion with the Geschützwagen III/IV that was sometimes called Fahrgestell Pz.Kpfw.III/IV) began while the vehicle was not yet finished. Once the Geschützwagen III/IV was replaced with the Einheitsfahrgestell III/IV then a whole lineup of vehicles could be built on the same chassis.

Mittlerer Waffenträger with a 150 mm gun,

Krupp went even further, designing the Mittlerer Waffenträger SFH 18/Gw.IV with a 150 mm gun in an open rotating turret. The mass was estimated at 28 tons, of which 20.5 went to the chassis, 5 for the gun and turret, and 2.5 tons for 40 rounds of ammunition weighing 55 kg each. Interestingly enough the blueprint is dated October 13th, 1944, or 9 days after the Einheitsfahrgestell III/IV was cancelled. It is likely that a Waffentrager with a 128 mm gun was also developed on this chassis, but it is impossible to find out since the blueprints were lost.

Let us tally up the results. In developing the Pz.Kpfw.III and IV, the Germans received two tanks with similar characteristics, but different designs. The first attempt to unify them and leave just one tank in production was undertaken in 1937, but the Ordnance Directorate placed its bets on a new and complex chassis before it was even tested, which ended up killing the project. The Germans undertook several attempts to unify their medium tanks during the VK 20 program, but constant changes in requirements, fighting between the Ordnance Directorate, Krupp, and Daimler-Benz, as well as Kniepkamp's love of complex designs led to the work being delayed for years and years. Just as the VK 20 could have turned into a unified medium tank with a long 75 mm gun, it was cancelled in favour of the Panther.

The Germans began the rational Pz.Kpfw.III/IV universal chassis program in early 1944. It had a simple design and made maximum use of existing components, but shortly before it would enter production it was cancelled in favour of the Jagdpanzer 38(t). In the meantime, the good old Pz.Kpfw.III and IV chassis remained in production until the end of the war.

The author thanks Alexandr Sotnikov for his help in working on this article.


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