Part of the schwerer kleiner Panzerkampfwagen drawing in the version dated 7 April 1944 – with the PAW 1000 cannon in the hull and the MK 108 in the rotating cupola; the drawing states a total vehicle weight of 26,000 kg, source: Flickr.com with permission of the publishing user, edited

Origins of the Project

The first mention of collaboration between Rheinmetall and designer Ferdinand Porsche on this new tank dates from their joint meeting on 13 May 1943. The new combat vehicle is referred to there simply as "Panzerkampfwagen Porsche" – literally "Porsche tank." But what prompted the development of this highly unconventional vehicle in the first place? The literature offers essentially two very different answers. The first theory holds that Porsche designed it as a competitor to the tank destroyer project E-25, being developed by Argus Motoren GmbH within the Entwicklungreihe programme. In some respects this theory holds together; in others it is rather less convincing.

In terms of timing everything fits fairly well. The Entwicklungreihe programme was officially launched in April 1943, so Porsche's response in mid-May of the same year would certainly make sense. The weight category also corresponds, since the E-25 was to weigh 25 to 30 tonnes and Porsche's design fell within a similar range. There are, however, certain question marks. The E-25 project was intended to produce a tank destroyer, whereas Porsche referred to his design from the outset as a tank, and the armament he chose did not particularly fit that of a tank destroyer either. And why, within the Entwicklungreihe programme as a whole, would Porsche have entered into competitive rivalry only against the E-25 and not also offered his own proposals for the E-10, E-50, or E-75?

According to the second theory, Porsche's project was a response to a proposal by the so-called Panzerkommission that development of an entirely new generation of tanks, resistant to air attack, should be initiated. Clearly supporting this theory is the fact that the "Panzerkampfwagen Porsche" was to have, according to its initial specifications, a roof armour of an extraordinary 80 mm. What sits slightly less comfortably with this theory is the timing. The Panzerkommission formulated this proposal on 27 May 1943 – a full two weeks after Porsche was already busily negotiating with Rheinmetall. This relatively minor discrepancy can, however, be explained quite readily. Ferdinand Porsche was himself a member of the Panzerkommission, and therefore undoubtedly knew in advance what was going to be discussed on 24 May 1943, allowing him to act early and steal a march on any potential competitors.

This cross-section of the schwerer kleiner Panzerkampfwagen shows, among other things, the mechanism linking the gun sight and controls to the cannon – which had to be routed to the opposite side from where the weapon's own controls were normally located, source: Flickr.com with permission of the publishing user, edited

First Version

In any case, the first version of the "Panzerkampfwagen Porsche," described on 13 May 1943, was to have a low cast hull with no overhang beyond the tracks. The front armour was to be an extraordinary 120 mm thick, while the sides, rear, and – remarkably – the roof were to be 80 mm. Despite this very heavy protection, the smaller hull dimensions allowed the weight to be kept at a reasonable level of around 25 tonnes, at least according to Porsche's calculations. The tank's armament consisted of a 105 mm howitzer mounted in the front hull wall and protected by a massive cast mantlet. The vehicle was therefore turretless – more of an assault gun than a tank, though Porsche insisted it was a tank. The howitzer's firing height above the ground was a mere 137 cm, and the barrel could be elevated over a range of –7 to +15 degrees and traversed up to 8 degrees to either side. A total of 60 rounds were to be carried on board, with hollow-charge shells intended for use against armoured targets.

The crew was to consist of four men: a driver, a commander, a gunner, and a loader. In the hull roof was a large commander's cupola with a two-piece hatch and ten periscopes arranged around its circumference. The vehicle had no secondary armament, and the crew was to defend against close infantry attack by firing personal weapons through embrasures in the armour. The gunner's position was to the right of the gun – a very unusual arrangement, since the weapon's controls were on the left-hand side. The mechanism for adjusting the barrel's position therefore had to be intricately re-routed to new controls in front of the gunner's seat on the right. The engine and gearbox were located in the rear of the hull, so the drive sprockets were logically placed at the rear as well, with the idler wheels at the front and six road wheels on each side between them.

Second Version

The first concept of the new tank did not enjoy a long life. In mid-May 1943, Hitler endorsed Guderian's proposal that every future German tank should have integrated anti-aircraft armament (just how faithfully this requirement was actually met in the years that followed is another matter). Porsche and Rheinmetall were therefore forced to revise their design. On 29 July 1943 a new proposal emerged bearing the lengthy official designation schwerer kleiner Panzerkampfwagen mit 10,5 cm leFH 43 und 3 cm automatische Fliegerabwehrkanone MK 108 – translated literally as "heavy small tank with 105 mm light field howitzer leFH 43 and 30 mm automatic anti-aircraft cannon MK 108."

This cross-section even more clearly shows the complex linkage between the gun sight and controls and the cannon; also worthy of note is the ammunition shown – at a glance it is evident that the rounds for the PAW 1000 were derived from mortar bombs, source: Flickr.com with permission of the publishing user, edited

The narrow hull without overhang beyond the tracks was retained. However, instead of the original commander's cupola, a somewhat larger cupola was now envisaged, mounting the aforementioned MK 108 automatic cannon of 30 mm calibre (MK = Maschinenkanone). The running gear continued to feature six road wheels on each side, now 780 mm in diameter, with 600 mm wide tracks. New calculations revealed that the original weight estimate had most likely been optimistic. The second design therefore incorporated a reduction in side, rear, and roof armour from 80 to 70 mm. Even with this weight-saving measure, the expected total weight of the vehicle grew to approximately 27 tonnes.

Secondary Armament

In the MK 108 cannon the tank received a truly formidable secondary weapon. It is almost surprising that Porsche and Rheinmetall opted for such a "heavyweight" rather than simply making do with the far more conventional choice of a machine gun. The MK 108 was developed by Rheinmetall – naturally – for the Luftwaffe, which in the second half of the war was searching for an aircraft-mounted weapon capable of shooting down a B-17 Flying Fortress bomber with as few hits as possible. While the MG 151/20 machine gun of 20 mm calibre required 15 to 20 hits to bring down a "B-17," the MK 108 needed only 4 or 5 well-placed explosive shells – and against an enemy fighter, a single hit was often enough.

The MK 108 was reasonably compact given its performance, but installing it in the tank's small fighting cupola was a tight fit. The cannon was just over a metre long and weighed 58 kg. It fired relatively large rounds with an overall length of approximately 180 mm (of which 90 mm was the cartridge case length) and a total weight of 480 g (the projectile alone weighed 330 g). The ammunition belt for such a weapon thus took up considerable space. The cupola allowed the cannon unlimited lateral traverse and vertical elevation from –5 to +90 degrees. Operating the secondary weapon was evidently assigned to the commander. Standing with his head inside the cupola right next to the cannon, squeezing the trigger, and experiencing the 650-rounds-per-minute rate of fire at such close quarters must have been quite an experience. To fit 700 rounds of MK 108 ammunition on board, Porsche had to reduce the howitzer shell stowage from the originally planned 60 rounds to just 44.

How it might have looked had it actually reached production... American soldiers walk past an abandoned schwerer kleiner Panzerkampfwagen somewhere in Germany near the end of the war. Background: internet (public domain), plastic model: Thomas Hartwig (www.panzer-modell.de), composite: Panzernet

Third and Fourth Versions

The sources do not record how the Army actually responded to the schwerer kleiner Panzerkampfwagen proposal, but Porsche evidently continued to develop it. On 3 February 1944, drawings for what was by now the third variant of the vehicle were produced. The hull was widened so that it overhung the tracks on either side, though the roof plate remained relatively narrow, giving the side walls a sharper inward slope. Hull height also increased. Both changes meant substantially more steel, and Porsche was therefore forced to reduce the armour thickness in order to keep the vehicle's weight within reasonable limits. The upper front hull plate was now to be 80 mm thick, the lower plate a "mere" 60 mm. Sides, rear, and roof were now only 40 mm thick – still an above-average figure for the roof.

The road wheels were reduced to 600 mm in diameter and the tracks narrowed to 550 mm. For the tank's powerplant, the lead designer specified an air-cooled engine of his own manufacture producing 345 hp. The wider hull overhanging the tracks created additional space which was used to stow more ammunition. In its third variant the tank was therefore to carry a total of 77 howitzer rounds and 600 rounds for the automatic cannon. It is worth noting that the standard leFH 43 howitzer was not of course to be installed as-is; a modified version chambered for fixed (non-separate) ammunition was required, since the standard leFH 43 used semi-fixed ammunition.

The PAW 1000 Cannon

On 7 April 1944, Porsche and Rheinmetall prepared another version of their design. The main change was the replacement of the leFH 43 howitzer of 105 mm calibre with the anti-tank PAW 1000 cannon, also of 105 mm calibre (later designated PWK 10H64, where PWK stood for Panzerwurfkanone). Curiously, on this occasion the chosen weapon came not from Rheinmetall but from Krupp. The PAW 1000 operated on the principle of differing pressures in the cartridge chamber and in the barrel – a system the Germans called the Hoch-Niederdruck System, meaning high-low pressure system. The high pressure generated by the ignition of the propellant charge was contained together with the cartridge case in the chamber. Only a portion of this pressure was allowed into the barrel, which contained the projectile itself. The chamber was therefore the only part of the weapon that had to withstand high pressure and required suitably thick walls. The barrel wall, exposed to only a fraction of the pressure, could be comparatively thin, making the resulting weapon relatively light – which was precisely the aim of the system's designers.

An abandoned schwerer kleiner Panzerkampfwagen standing in a street, most likely after running out of fuel. Background: internet (public domain), plastic model: Thomas Hartwig (www.panzer-modell.de), composite: Panzernet

The projectiles fired by the PAW 1000 were in fact originally mortar bombs, converted to hollow-charge rounds. The modified round weighed approximately 6.6 kg and could penetrate sloped armour up to 200 mm thick at a range of 1,000 metres. The cartridge was designed such that it itself served as the partition dividing the interior of the cannon into the high-pressure and low-pressure zones. The top of the cartridge case was sealed by a metal plate with drilled holes. In the centre of this plate was a short spigot, on which the mortar bomb itself – fitted with a hollow-charge warhead – was seated. The enormous pressure that built up in the cartridge case (and thus in the chamber) after the round fired seeped "slowly" – everything of course happened in fractions of a second – through the holes in the partition into the barrel, where it accumulated in the space between the partition and the head of the projectile (the area where the bomb's narrow section left a gap around the full diameter of the barrel). The pressure in this space gradually built until it was sufficient to break the bomb free from the spigot, whereupon the projectile began its travel towards the muzzle. The pressure in the barrel was thus limited by the strength of the joint between the bomb and the spigot.

As already noted, the pressure-division system allowed the use of a thin-walled barrel. The PAW 1000 was therefore a comparatively light weapon with a substantially lower recoil force than a conventional gun of equivalent calibre – which in turn meant that the mounting in the armoured vehicle did not need to be as robust (or as heavy) either. Porsche's design provided for horizontal cannon traverse of up to 10 degrees to either side. On-board ammunition stowage was to be 56 rounds. The PAW 1000 did not progress beyond the prototype evaluation stage before the end of the war.

Fifth Version and End of the Project

The last known milestone of the schwerer kleiner Panzerkampfwagen project was the production of a fifth design variant in June 1944. The drawings now incorporated a 128 cm coincidence rangefinder and night vision equipment. The rangefinder was to be housed in the rotating cupola behind the MK 108 cannon, and as a result the cupola now featured cylindrical protrusions on either side concealing the rangefinder optics. The rangefinder could presumably be used to determine the range to a target on the ground (for the PAW 1000) as well as in the air (for the MK 108). It is not immediately obvious from the drawings how the crew would actually look through the rangefinder, given that it was positioned very close to the rear wall of the cupola – though Porsche undoubtedly had some solution in mind. The night vision equipment was presumably intended for the main gun operator, the MK 108 gunner, and the driver alike.

This image clearly shows the protrusions on either side of the cupola's rear wall that housed the optics of the coincidence rangefinder, background: internet (public domain), plastic model: Thomas Hartwig (www.panzer-modell.de), composite: Panzernet

No detailed information is available about the project's subsequent fate. What is certain is that the schwerer kleiner Panzerkampfwagen remained on paper until the end of the war. Whether it was formally cancelled before Germany's capitulation or simply disappeared along with the Third Reich is unknown. The literature is generally thin on information about how the Army actually regarded the project as a whole. On the one hand, it might seem that the Army was interested – after all, the project remained "alive" for perhaps two years and went through five variants in total (and some drawings suggest further sub-variants were also produced). On the other hand, while Porsche kept bringing new drawings, there was no sign of any effort on the Army's part to advance to the next stage and actually build a prototype. Is it possible that the Army's representatives simply did not want to offend Hitler's personal friend by throwing his proposal in the bin, and so preferred to play a delaying game? Letting him work on something he clearly enjoyed, keeping him out of their hair, while knowing from the start that a vehicle like the schwerer kleiner Panzerkampfwagen would never actually be built? That, we shall probably never know.

ARTICLE
CONTENTS:

ORIGINS OF THE PROJECT

FIRST VERSION

SECOND VERSION

SECONDARY ARMAMENT

THIRD AND FOURTH VERSIONS

THE PAW 1000 CANNON

FINAL VERSION