After the first British Mark I tanks went into action in 1916 there followed a period of comparatively rapid development ending with the Mark IX and the Medium D tanks. As well as those cases where tanks were built, trialled and, in most cases, entered service there were others in which a projected tank existed only on the drawing board, in mock up form or as a single prototype that did not exist long enough to leave any photographic record of its existence. It is possible to use surviving diagrams, photos of mock ups and descriptions to attempt the production of drawings illustrating what these tanks might have looked like.
The tadpole tail was an attempt in 1917 by Tritton to solve the problem created by the Germans digging wider trenches and anti tank ditches. The idea was to replace the rear horns of Mk IV and Mk V tanks with longer tails thus increasing the length of the tank by over 11 feet. A large number of conversion kits were delivered to the Central Workshops in France. A number of Mk IV and Mk V Tadpole tanks were built. However in practice this proved to be an ineffective solution as the new structure suffered from flexing (the strain imposed when turning must have been considerable). It is also probable that the extra weight of the tadpole extension was insufficient to move the centre of gravity sufficiently rearward to extend the trench crossing capability as far as the extra length would suggest (the tank would tip forward when too little of its length had crossed the lip of the trench).
A better solution was found in November 1917 in the shape of the Mk V* tank which effectively increased the length by six feet by adding extra sections in the middle of the main body of the tank. This was structurally more rigid and also ensured that the heavy fuel and water tanks were still to the rear of the extended vehicle. This would significantly move the centre of gravity rearward. Some 700 Mk V* tanks were built by mid March 1919. They equipped British, American and French tank units (serving with the French throughout the 1920s) and played an important role in the battles that broke the German army at the end of 1918. However the Mk V* proved to be difficult to turn due to the "drag" of the extra length of flat track in ground contact.
The tadpole conversion units were equally compatible with the rear of the Mk V and Mark V*. As an experimental project sometime during 1918 the staff of Central Workshops converted a Mk V* to a tadpole configuration. This produced a tank of over 44 feet long, not only the longest tank completed in World War 1 but also the longest tank ever built.
The resulting tank proved to be almost impossible to drive in anything other than a straight line! The "drag" of the track in ground contact was too much to allow turning. There was also no rail vehicle extant that was capable of transporting the beast to the front. The Mk V* Tadpole was clearly unworkable, what happened to it is unclear. It is quite possible that the tadpole tail was removed and the original horns replaced so that it reverted back to an ordinary Mk V* and was shown as such in the Central Workshop’s records.
In order to overcome the turning problems of the Mk V* the Mk V** was produced. This remodelled the shape of the hull to reintroduce the flattened curve so characteristic of the rhomboid tank. Improvements were also made in the siting of the commander’s cupola and in the cooling/ventilation system. A shortage of armour plate delayed the introduction of this tank until after the end of the war. In the meantime there was yet a third revision of the original Mk V design, this being the Mk V***. This appears to have been insurance against a failure to reach agreement on the construction of the Mk VIII International.
The MK V*** introduced so many changes that it could effectively be classed as an entirely new tank and it was eventually renumbered as the Mk X. It appears that this vehicle was to have been produced, in male form only, with a new design of sponson. An advanced filter and fan system was to be used for the radiator cooling and air inside the fighting compartment would have been maintained at a positive pressure to prevent the ingress of gas. As it happened the Mk VIII was approved and the MV***/Mk X never made it past the full-sized mock-up stage.
This was a British project intended to produce a tank to meet the expressed needs of the American Army. Although a "rhomboid" design, the large sponsons of both male and female tanks were eliminated in favour of a nose mounted heavy gun firing from between the horns and much smaller machine gun sponsons in the hull sides. The field of fire for the heavy gun would have been very limited.
The engine was to be in a separate compartment on one side of the tank. It would be interesting to know if this implied an asymmetric weight distribution and what impact this might have had on the tank’s steering. The US Tank Board ordered the Mk VI in some numbers but this was later cancelled in favour of the MK VIII (possibly on the advice of George Patton). The Mk VI only existed in the form of a full sized mock up.
Images: IWM Q14567, Q14525
In 1918 the Mark VIII was intended to be a joint effort between Britain, France and the USA to produce a breakthrough heavy tank for the planned Allied 1919 offensive. Most of the design work was done in the UK which would also undertake some construction. France and the USA would, between them, build the majority in 1919. This was enshrined in an international treaty. However France did not abide by her commitment under the treaty, and never produced the tank manufacturing facilities required (deciding instead that the home grown Char 2c should be the 1919 breakthrough tank for France).
In Britain there were concerns that the Germans might produce even wider trenches and anti tank ditches. Accordingly a Mk VIII* was designed, this was an extended tank (much as was the Mk V*) having no less than five additional hull panels added, two before and three after the sponsons. The result was the design of a tank that would have been 44 feet long (10 ft longer than the standard VIII and 2ft longer than the German K Wagen). If built it would have equalled the Mk V* Tadpole’s record of being the longest tank ever constructed.
The Mk VIII* would have weighed about 42½ tons empty and probably 50 tons with equipment. As with the Mk V* Tadpole there was no railway vehicle capable of transporting one intact to the front. Like the German K Wagen it would have had to be moved in sections and re assembled in the battle area. The end of the War in November 1918 effectively removed the need for extended production of the Mk VIII much less the development of the Mk VIII* so it remained a "paper" tank.
The Medium B was designed by Wilson (whilst the Mediums A and C were Tritton designs). Missing WW1 proper it did see action in Russia. All production Medium Bs were machine gun armed but there was a design for a Male version mounting a long 6 pounder (57mm) gun in a fixed superstructure.
It has been reported that one Medium B male was actually built but no photos are known to exist. The Medium B Male would probably have made an effective tank killer but in 1919 there were no potential enemies of Britain with an effective tank force. It was probably killed itself by a cost conscious "Whitehall warrior".