1500-ton Self-Propelled 80cm Gun

By Gary Zimmer

In a paperback titled Tanks of the Axis Powers published over 20 years ago there is a brief mention of some of Germany's armoured follies. It mentions a 1500 ton superheavy tank, cased in 250mm of armour, armed with an 80cm gun and two 15cm weapons, and powered by four U-boat diesels. Although there was no illustration I have always been curious as to what this 1500 tonner would look like.

However we do know something about the proposed main weapon, the 80cm. Although not the largest calibre gun ever made, or the longest ranged, the 80cm railway gun 'Dora' was the biggest. As far as we know it was used only sparingly, to shell Sevastopol in the Crimea, and later Warsaw. Too large to be transported whole, Dora required several trains to transport it. Before assembly could begin, and this took several weeks to acomplish, a second track had to be laid at the chosen firing site. Movable straddle cranes also had to be assembled, these were on their own additional rails. The two 20 axle halves of the chassis were shunted onto the double tracks side by side, and coupled together. Only then could the cranes start putting the really big bits on. Once assembled Dora must have been an awesome sight, all one thousand three hundred and fifty tons of it. The barrel alone weighed 100 tons, the breech was also another 100 tons. It could fling a 7 ton shell about 45 km. As a piece of static siege artillery there was no question of its effect, but even its creators, Krupp, admitted while it was a valuable research tool, as a practical weapon of war it was useless.

Which brings us to the 1500 tonner, aptly named 'Monster' by armaments minister Albert Speer. It may have been an attempt to make some use of Dora, or simply an extension of a policy to self-propell all heavy artillery, but someone got the idea of putting Dora on tracks. The wartime sketch (provided courtesy Karl Horvat, an Australian researcher) is all we have, but it allows us to deduce a few things.

One reason why you can't simply scale up an existing tank design is ground pressure. If you know the mass and dimensions (i.e. area of track in contact with the ground) of a vehicle, it is quite easy to work out ground pressure. Put simply, weight will be roughly proportional to the volume or the cube of the dimensions, while the area of track in contact with the ground will be proportional to the square of dimensions. If we double the size of a tank, we get eight times the weight but only four times the track area, thus twice the ground pressure. (There's also twice the stress in suspensions, axles and everything else, it's why elephants have thicker legs than flamingos.) A very light tracked vehicle, such as a Bren carrier, will have what appears to be rediculously narrow tracks. As a vehicle gets heavier, the proportion of its width covered by track increases. A Centurion has about 40% of its width as track, while for the 188 ton Maus tank, the figure was about 66% or two thirds. In fact the most striking thing about Maus is this proportion of track width to overall width.

Assuming a pressure of 1.2 kg per sq cm for this 1500 tonner, that's about midway between that of a Centurion and a Maus, and seems a realistic place to start. Working backwards, we can use ground pressure and weight (1500 tons, or thereabouts) to find how much contact area it needs. Track width appears to be around 80% of the width, giving tracks of 2.4m width (each) for an overall width of 6m. The illustration appears to be about 6m wide, as is the gun on its rail mount. If we stick to an assumed six metre width, close to an upper limit if we ever consider movement by road, this behemoth thus requires 27m of track on the ground. There's only one problem with this, it won't turn.

The shorter a tracked vehicle is, that is track length on the ground, the less resistance there is to turning. Also, the wider it is, the outside track is able to generate a greater turning moment, and overcome the resistance of both tracks to being pushed sideways. A governing aspect of tracked vehicle design is the ratio of the distance between track centres, and track contact length. Typically, this is about 2:1 for most vehicles. The 1500 tonner has a length/width ratio of about 7.5 to 1, and this is horrific. The way out of this is chassis articulation. By using four track units, each 14m long, and allowing each pair to be turned independently, it might just work.

Having four track units ties in nicely with the four U-boat diesels. All the Porsche heavy tanks were electric drive, and it seems hard to imagine anything else for a machine this size. In a U-boat, the diesels drove dual purpose electric motor-generators, but on the 1500 tonner these would function as generators only. It seems logical that each diesel would have its own generator. These four generators would each run an electric motor in each of the four track units. Of course the diesels and generators could be anywhere in the vehicle, as no mechanical drive to the tracks would be required. The other pieces of information are harder to fit into the picture. Just where the two turrets, each with a 15cm gun, would fit I have no idea. If the layout of Dora is preserved, as the illustration seems to indicate, there appears to be no place for them. Also, having these turrets side by side, as Axis suggests, implies a much greater width than 6m if these turrets are not to foul. More puzzling still is the 25cm of frontal armour. The illustration shows that the loading decks, and of course the crew doing the loading, had no protection at all, nor would they need any being many miles from whatever they were shooting at. Having this extent of armour is only required if the machine is going to be used as a direct fire weapon, in other words as a tank and not a piece of self-propelled artillery.

It also appears that the shell hoists are retained, as on the rail gun. While having no on-board stowage of 80cm rounds is not an issue for SP artillery, it would be an absolute must for a 'tank'. Dora was supplied with 80cm rounds from the rail lines it sat upon, but this would not be any use to an SP operating away from any railhead. I imagine that ammunition vehicles would be required to deliver one round at a time to the hoists, they could possibly be similar to the Panzer IV carriers used with the Karl morsers. Apart from these carriers there would probably be a whole retinue of vehicles accompanying this giant machine; fire control and signals vehicles, a flak unit, the cook's truck, and so on.

We can only speculate how this machine might be moved. As with Dora, it could conceivably be transported by rail in pieces, but once assembled and moving under its own power beyond the rail network the fun would really start. As with all oversize vehicles, the planned route would need to be carefully surveyed. It would occupy the entire width of a road on its own, and travelling through any town en route would no doubt lead to a fair bit of urban renewal. Rivers would be less of a problem, as the machine's great height would permit fairly deep fording. However the greatest problem would be the high centre of gravity due to the mass of barrel, breech, recoil system so high up, and sideslope of the ground would be the main restriction to travel, lest the vehicle keel over. As with other large land vehicles, there is a distinction between 'movable' and 'mobile'.

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