A Prospective Standard Chassis

The E-50 and E-75 are a special topic within the history of German tank building. These tanks were not finished before the end of the war, and the work that was performed is still not entirely known. As a result, the E-50 and E-75 became a viable platform to "build" fictional tanks that would have fought the T-44, IS-3, Centurion, and Pershing. Because of this, many now know about the existence of the E-50 and E-75, but few have any understanding of what these tanks were really supposed to be like.

Unfortunately many materials pertaining to the E-50 and E-75 have been lost. Only drafts of the hull, running gear, and suspension have been preserved, so it's impossible to reconstruct how these vehicles would have looked. However, we have enough information to give a general idea.

This article is based on three interrogations of Heinrich Kniepkamp, the mind behind the E series concept and author of their requirements, and correspondence of Karl Jenschke, the technical director of Adlerwerke, who directed the development of the running gear and transmission of the E-50 and E-75. Some information was taken from the interrogation of Gerd Stiele von Heydekampf, head of the Tank  Commission, as well as the works of Walter Spielberger, Thomas Jentz, and Hilary Doyle. Since in interrogations most information was given from memory with the resulting mistakes, the information was cross referenced and interpreted wherever possible.

Simplification and standardization

The new series of prospective tanks was thought up by engineer and ministerial advisor Heinrich Kniepkamp, an employee of the Ordnance Directorate who had been responsible for development of new tanks since 1936. Initially, the design of tanks was driven by the desire to reach maximum characteristics, which led to excessive complications of the design and difficulty during service and repairs. More and more attention was dedicated to the end user's needs starting in 1941. As a result of this change in priorities, Kniepkamp began thinking about a new series of prospective tanks known as the E series (Entwicklung - development). Work on the designs of E series tanks began in April of 1943.

Drafts of the running gear of the E-50 and E-75 made by Adlerwerke. These tanks differed in armour thicknesses and number of bogeys. The weight given is the sprung weight, rather than the weight without a turret as is sometimes claimed.

The new series of tanks included the E-50 and E-75, replacements for the Panther and Tiger respectively. Like other tanks of the E series (with the exception of the E-100), they were designed according to the following principles:

1. An external spring suspension instead of a torsion bar suspension to make repairs easier and free up room in the hull.
2. Use of single steel rimmed road wheels instead of dual ones as well as track links with a single guide horn.
3. The transmission was located in the rear to allow for the front armour to be thickened without unbalancing the tank. Additionally, the tank remained mobile if it hit a mine. The track could be shortened and the forward intact road wheel could act as an idler.
4. The gearbox, turning mechanism, and final drives were joined into one unit to make production and repairs easier.
5. Elements of the running gear, engines, and transmissions were common across the series.

The lighter running gear and transmission on the E-50 and E-75 freed up about 4 tons (if not more). This reserve allowed the designers to improve the armour and armament without further weight increase. Production and assembly of the running gear and transmission became much simpler.

Parts commonality was a very important aspect. The E-50 and E-75 would use the same engine, transmission, fuel tanks, cooling system, idlers, and track tension mechanism. The E-50 had six suspension bogeys, three per side, the heavier E-75 had eight bogeys. The E-50's tracks were used as transport tracks on the E-75. The hulls of the two tanks were very similar and differed only in armour thickness. Since their external dimensions were the same, the E-50 had slightly more room inside due to thinner armour plates. This design allowed the E-50 and E-75 to be assembled at the same factory with the same tools.

A drawing of the mysterious E-75 designed by Weserhütte.

Adlerwerke worked on the running gear and transmission of the E-50 and E-75 projects, so the two tanks were also called Panther (Adler) and Tiger (Adler). The Germans often referred to prospective projects by the name of the vehicle they were supposed to replace. Aside from Adlerwerke, some work was done by Weserhütte, but the author is not aware of what the work was. A drawing of a strange vehicle called Tiger (Weserhütte) survived. Its dimensions are different from the Adlerwerke project, and the number of bogeys and front armour thickness match the Panther, rather than the Tiger.

Engine

The E-50 and E-75 would have been equipped with the Maybach HL 234 engine. It was based on the HL 230, but put out 900 hp at 3000 RPM at the same dimensions. This was possible due to direct injection and other improvements. The issue with unreliable carburettors that was never solved on the HL 230 was therefore sidestepped. An analogous engine (perhaps the HL 232) with direct injection supercharged by a secondary 2-cylinder motor could put out 1200 hp. The use of a second motor for compression is explained by a sharp deficit of heat-resistant materials to build turbocompressors. The secondary motor was also used as a starter.

Kniepkamp revealed that diesel engines were also considered. Maybach was working on the HL 234 R based on the HL 234. It could put out 650 hp or 900 hp with a supercharger. An alternative was developed by Klöckner-Humboldt-Deutz as of late 1943 and was expected by the fall of 1945: the 32 L water cooled  two-stroke TM118 V-8 engine. This engine had good torque and was simple to produce. As a bonus, it took up a similar volume to the HL 234. The cylinders were drained by a pump driven by a small auxiliary motor. Two one-cylinder prototypes put out 86.5 hp at 2000 RPM. An order was given to build three 8-cylinder engines, but just one was finished before the end of the war.

In choosing the engine, Kniepkamp aimed to attain the most power at the cost of the least space taken up by the engine, its cooling system, and fuel tanks, which is why he preferred compact gasoline engines with high power like the HL 234. In addition, Kniepkamp considered powerful and fuel efficient two-stroke diesel engines.

In two-stroke engines, every other stroke produces power, which increases the effective power compared to a four-stroke engine but introduces a problem with cylinder drain. German engineer  Adolf Schnürle patented a looped three port drain system that drastically improved drain. Engineers from Klöckner-Humboldt-Deutz developed two-stroke diesel engines for aircraft and armoured vehicles under the direction of Adolf Schnürle and Emil Flatz. While information on German air cooled diesel engines has been discovered, development of two-stroke diesels still remains a mystery.

Transmission

Unfortunately, the blueprints of the Adlerwerke transmission were destroyed, but Karl Jenschke described its design. It was analogous to the Tiger's transmission and included an 8-speed semiautomatic shaftless gearbox and two-loop two-radius turning mechanism, but was laid out much differently. The E-50 and E-75 had their gearbox, turning mechanism, and final drives joined into one unit, which helped reduce size, save up to a ton of weight, and simplify production and repairs. The top speed of the E-50 would be 60 kph and the E-75 40 kph. The speed changed by altering the gear rations in the final drives. Otherwise, the transmissions were interchangeable.

Diagram of the Maybach Mechydro automatic transmission. Similar transmissions were used on diesel locomotives. Kniepkamp considered the Mechydro transmission to be promising.

Adlerwerke produced the OG 40 12 16 gearbox for the Tigers and was well familiar with its design, which is why it was not surprising that it received the order for an analogous transmission, but for a rear transmission compartment. As an alternative, Kniepkamp selected the Mechydro hydromechanical transmission, which consisted of a torque converter and Maybach 4-speed shaftless automatic gearbox, or the Voith hydromechanical transmission.

Note that since the final drives were moved inside the hull, the hubs of the drive sprockets would have to align with the transmission shafts. Since the drawings of the E-50 and E-75 have the hubs lined up with the floor of the hulls, there would be no way to supply them power. This means that these drawings reflect only the shape of the hull and location of running gear elements, and that it is impossible to reconstruct the look of the tanks with them. There is a suspicion that the E-50, E-75, and E-100 were initially designed to use an existing transmission in the front of the hull, and that a rear transmission would be reserved for future developments.

Suspension and running gear

Ernst Lehr from MAN, a specialist in mechanical properties of materials and designer of the Panther suspension, proposed a suspension using Belleville springs. Adlerwerke designed a new suspension for the E-50 and E-75 based on his work. The suspension bogey had a casing made up of two parts (upper and lower) as well as two suspension arms. The teeth of each arm shifted a plate, which moved a rod that then pushed on the Belleville spring and shock absorber. The casing was filled with oil, which was also used to lubricate the ball bearings. The dynamic travel was 170 mm and the full travel was 300 mm. To compare, the Tiger H1 had 220 of full travel and the Pz.Kpfw.III with a torsion bar suspension had 210 mm of dynamic travel and about 250 mm of full travel.

Cutaway drawing of the Adlerwerke suspension.

The design was made with the estimation of 40.8 tons of sprung weight for the E-50 and 60 tons for the E-75. This is the weight that loaded the suspension springs. The author does not have precise methodology for these calculations, but from the looks of it the Germans excluded the weight of the road wheels and partially the tracks, since some weight of the tracks is transferred through the drive sprocket and idler. The overall weight of the E-50 tank was estimated to be 46 tons and the E-75 to be 67.5 tons. The suspension had enough reserves for a 74 ton SPG. The Germans could increase the sprung weight by lightening the suspension, then use the reserve to either strengthen the tank's armour and armament, or just reduce the weight of their tanks. The weight of the Panther with steel rimmed wheels was 46.5 tons, 500 kg more than the E-50, but the sprung weight was 900 kg less: 39.9 tons vs 40.8. Note that the potential upgrades were purely theoretical and there was not enough time to draw them out.

The Adlerwerke suspension is often compared to the Porsche suspension, which was used on the Tiger (P) and Ferdinand. Indeed, both suspensions were mounted outside the hull and both had bogeys with two road wheels, but that is where the similarity ends. The Porsche suspension had its wheels in pairs, with the road wheels attached to one casing that was linked to the hull through a swing arm. The Adlerwerke suspension was independent, where each road wheel had its own swing arm that linked it to its spring. Two independent suspension elements were joined into one bogey. The Adlerwerke and Porsche suspensions were completely different.

Adlerwerke bogey with two single road wheels.

The Porsche suspension was rigid, with short dynamic travel, which was limited by a rubber bump stop. The road wheels could move a considerable distance, provided that its counterpart made the same journey in the opposite direction. The entire weight of the tank was spread out between 6-8 cams, which required the use of highly robust hardened steel. Adlerwerke's suspension looked much better in comparison, since it could use Belleville springs made from lower quality steel. The weight was spread out between 12-16 rods and the travel was much softer.

Porsche's suspension is visually similar, which introduces confusion.

The E-50 and E-75 used single 800 mm road steel wheeled road wheels. One of the E-50's road wheels carried 3400 kg of weight, and each of the E-75's carried 3750 kg. Porsche created a similar design for the Jagdtiger. It also had single wheels and an external suspension. During trials, it turned out that the single wheels do not evenly load the wide tracks, as a result of which the track links bent and track pins snapped during off-road driving. Considering that the weight per road wheel of the prospective SPG on the E-75 chassis was similar to that of the Jagdtiger with the Porsche suspension (4160 kg compared to 4100 kg), it would not likely be reliable. At the very least, it would not be able to use the early type of track links.

Hull and turret

The hull of the E-50 and E-75 were similar to those of the Panther Ausf.G and Tiger II, although the front was designed to be closer to the E-100. The slope of the upper front plate was 60 degrees (compared to 55 on the Panther and 50 on the Tiger II), and the lower front plate was sloped at 45 degrees. Drafts show that the E-50's upper front plate was 100 mm thick and the E-75's was 150 mm thick, similar to that of the Panther and Tiger II, but at a larger angle. This data should be treated carefully, since we don't know how precisely these drafts reflected the actual planned armour thickness. Additionally, the hatches were greatly reinforced. There was also added protection against mines in the front part of the hull (which can explain the height difference between the front and rear of the floor).

Hilary Doyle's reconstruction of the E-50 and E-75 hulls. He chose 100 and 150 mm as the thickness of the upper front plates, just like the drafts.

Karl Jenschke revealed that the E-50 and E-75 had the same turret rings to accept the same turret with an electric traverse, but that the armament would be different. This turret was designed by Krupp, but no data survives. Due to the chaos of the late war, Jenschke did not know how far the work progressed. There is no surviving information on what armament was planned. Modern reconstructions that used the Panther Ausf.F or Tiger II turrets are incorrect.

Fuel for fantasies

The E-50 and E-75 are often presented as prospective tanks that would have replaced the Panther and Tiger in 1945, competing with the T-44, Centurion, and Pershing in case the war continued. In these fantasies, they are usually given "bonuses" from the Panther Ausf.F and late Tiger II, such as a rangefinder, new engine deck with a three-piece hatch, Schmalturm turret with an 88 mm gun, and even a gas turbine engine and Tiger II turret with a 128 mm gun. Some fantasies progress even past these bounds and lead to 80 ton SPGs on the E-75 chassis.

Another example of a poor reconstruction of the E-75. Its author decided to use dual road wheels despite the available information, as a result of which they did not fit and the inner row still had to be single. (https://www.facebook.com/TigerClaw89)

Reality is far more boring. The head of the Tank Commission and director of Henschel Gerd Stiele von Heydekampf described the prospective developments at the end of the war, even presenting a plan for further production. The Germans places their bets on further developments of the Panther Ausf.F and Tiger II. He did not even mention the E-50 and E-75. Kniepkamp revealed that work on the E series ceased in August of 1944. The Germans had individual components and assemblies laid out, but they were still far from a final well thought out design. Some present a photograph that they claim is an E-50 prototype as an attempt to bring these tanks closer to being a reality. In truth, this was an experimental Panther designed to test steel rimmed road wheels and two-part track links. As for the SPGs, Jenschke mentioned the possibility of creating an SPG on the E-75 chassis. However, he spoke merely of the suspension's weight reserves, rather than any actual design for such an SPG. It is also not clear where his 80 ton estimate came from, since the suspension blueprint stated a limit of 74 tons.

A Panther built to test new road wheels and tracks. It would not be possible to install the E-50's suspension on it, as the inner row of road wheels would not fit.

Another common myth is that the E series had an influence on post-war tank building. Allegedly, E-50 and E-75 designs were used on the AMX 45 tank. A closer look at the blueprints shatters this myth. On them, we can see tracks with two guide horns, dual rubber-rimmed road wheels, a torsion bar suspension, and the Zahnradfabrik transmission developed for the French had separate final drives. All of these elements are contrary to the E series designs.

Note that the first draft of the AMX 45 was dated August 1st, 1945, while Karl Jenschke's message describing the E-50 and E-75 was dated August 28th, and Kniepkamp was only interrogated on August 31st. It's possible that the French started their work without knowing about these tanks at all. It is not surprising that the first AMX 45 was closer to an ordinary Panther. True, the HL 295 engine was developed by Maybach for the French based on the HL 234, but the latter was not a part of the E series and was also going to be used on regular Tigers and Panthers. The claim that the HL 295 engine was going to be used on the E series is absurd, as it did not exist at the time these tanks were being developed.

The end?

There is another way of looking at the situation. According to this point of view, German tank building hit a dead end, since the E series were no longer considered viable, and in case the war continued the Panther and Tiger II would face off against the T-54, IS-3, Centurion, and Pershing. The Germans were stuck at the same level they reached in 1943. The only nation to widely use German experience was France, but the AMX 45 program was a failure. When the Germans began working on the Leopard, they began working from a clean slate. This point of view can be reasonable compared to the proponents of the E-75 with a 128 mm gun and gas turbine, but there are also some nuances.

An early AMX 45 blueprint. The Panther style suspension and transmission with separate final drives have absolutely nothing to do with the E series.

Let's start with the fact that the dead end at the end of the war was a failure of production. Even the most rational and reasonable project proposed by German engineers would have had zero chances at life due to a total deficit of raw materials and rapidly collapsing industry. As for the prospective tanks, it is hard to evaluate them since materials on the E series are so fragmented. How can one compare the E-50 and E-75 to the T-54 and IS-3 if we know nothing about the former's armour and armament? The French can only blame themselves for what they did with what they were given. They could have had a quite modern highly mobile analogue of the Centurion, but tried to produce a tank with 180 mm of front armour or a massive oscillating turret with a 120 mm gun. One has to freeze water first before walking on it. This is a good saying to think about before constantly changing requirements and chasing a moving target.

Can we also fairly say that post-war tank building began with a clean slate? Yes, the Germans radically revised their concept of what a tank should look like and learned from their mistakes, but the result was still a quintessentially German tank. The group A Standardpanzer used a transmission with a two-loop two-radius turning mechanism tested on Henschel heavy tanks. The group B Standardpanzer used a Belleville spring suspension and a hydrostatic turning mechanism that was initially supposed to go on the Panther. The Pulsgetriebe planetary gearbox was based on an analogous design developed for the Tiger. Thanks to Kniepkamp, a Hanomag two-stroke diesel was considered as an alternative to the four-stroke Daimler-Benz.

Group B's Standardpanzer on trials.

Let's sum up all that we learned. The scraps of information on the E-50 and E-75 that survive teach us not about specific tanks, but about the way German ideas on tank building changed before the end of the war. The designers aimed to lighten the running gear and transmission in order to put the freed reserves towards armour and armament, as well as simplify production and repair. The armour of the E-50 and E-75 surpassed that of the Panther II and Tiger II, let alone the ordinary Panther. The more powerful engines would have improved mobility. On the other hand, the running gear with single road wheels was quite dubious, the location of the transmission in the rear is under question, and nothing at all is known about the armament.

Kniepkamp gets a lot of criticism, but he arrived at the concept of a modern tank during the war: an individual suspension, a powerful compact supercharged engine, an isolated cooling system with a minimized air ducts, a cooling fan with a fluid coupling to control the power consumption depending on temperature, two-loop hydromechanical transmission (preferably with a hydrostatic drive) as well as an engine and transmission in one unit to make it easier to remove. The Germans made a lot of mistakes when it came to incorrect or premature ideas, but overall they were going down the right road.

Original article by Dmitry Zaitsev.