The Entwicklung series (from German Entwicklung, "development"), more commonly known as the E-Series, was a late-World War II attempt by Nazi Germany to produce a standardised series of tank designs. There were to be standard designs in five different weight classes (E-10, E-25, E-50, E-75 and E-100) from which several specialised variants were to be developed. This intended to reverse the favor of extremely complex tank designs that had resulted in poor production rates and mechanical unreliability.
The E-series designs were simpler, cheaper to produce and more efficient than their predecessors; however, their design offered only modest improvements in armour and firepower over the designs they were intended to replace, such as the Jagdpanzer 38(t), Panther Ausf. G or Tiger II; and would have represented the final standardization of German armoured vehicle design. Indeed, nearly all of the E-series vehicles — up through and including the E-75 — were intended to use what were essentially the Tiger II's eighty centimeter diameter, steel-rimmed road wheels for their suspension, meant to overlap each other (as on the later production Tiger E and Panther designs that also used them), abandoning the interleaved Schachtellaufwerk roadwheel system that first appeared on German military half-tracks in the early 1930s.
Jagdpanzer E-10
Said to have been designed by the Klockner-Humboldt-Deutz Magirus AG firm in Ulm, the E-10 project (development name - "Hetzer") was developed as a replacement for the Jagdpanzer 38(t). The name Hetzer was never officially used for the 38(t).
The designs based on this new chassis would all be in the 10 to 25 tonnes weight class, and using only four Tiger II-style but larger all-steel road wheels per side in an overlapping layout for its main "slack-track" suspension with no return rollers and a rear drive sprocket. Much simplified was the suspension, which was intended to be based on the concept of using conical-spring Belleville washers for the E-series AFVs as a whole. These were simply bolted on to the chassis and could be easily removed for repair or replacement. This suspension system was later used on the Swiss Panzer 61. A most interesting aspect was the ability to lower the hull by placing the pivot points of the suspension units higher in respect to the hull bottom; by not having torsion-bars occupying the entire hull planform in the lowermost interior areas of the hull, but via cranks driven by hydraulic actuators on the sides of the hull. This reduced the height of the vehicles from 176 cm to 140 cm.
The intention was to create several new light tank destroyers as a replacement for the Jagdpanzer 38(t), as well as a new family of gun platforms (Waffenträger) armed with heavy anti-tank guns. However, from September 1944 the design was abandoned in favour of a redesigned and enlarged Jagdpanzer 38(t), using German parts as opposed to the Czech parts for the Jagdpanzer 38(t). This new design was called Jagdpanzer 38D, and was to use a Tatra 12-cylinder air-cooled diesel engine similar to the one fitted to the Sd.Kfz. 234 'Puma'. None of these vehicles reached series production.
Jagdpanzer E-25
The E-25 designs, in the 25-50 tonnes weight class, were to be the replacements of all Panzer III and Panzer IV based designs, with involvement of the Alkett, Argus, Adler and Porsche firms. This family would include medium reconnaissance vehicles, medium Jagdpanzer vehicles and heavy Waffenträger vehicles. All would use five Tiger II style road wheels but larger per side in a similar overlapping layout to the lighter E-10 suspension, as well as "slack-track" design and a rear drive sprocket. As main armament, a 75 mm Pak 42 L/70 was planned, with a possible MG in a small turret. Overall, this vehicle would have been similar to the Hetzer tank destroyer. The tank would have been very maneuverable, and due to its low profile, hard to spot on the battlefield.
E-50 Standardpanzer
The E-50 Standardpanzer was intended as a standard medium tank, replacing the Panther and Tiger I and the conversions based on these tanks. The E-50 hull was to be longer than the Panther – in fact it was practically identical to the Königstiger (Tiger II) in overall dimensions except for the upper and lower glacis plate layout. Compared to these earlier designs however, the amount of drilling and machining involved in producing these Standardpanzer was reduced drastically, which would have made them quicker, easier and cheaper to produce, as would the proposed conical spring system, replacing their predecessors' torsion bar system which required a special steel alloy.