The design of the Medium Mark D had its origins in the Central Tank Corps Workshops in France in 1918. When the Central Workshops were established in 1916 each tank battalion had its own workshop organisation. In January 1918 the workshops were reorganised and personnel were distributed between the Central Workshop and 5 advanced workshops. One of the advanced workshops was an experimental workshop with Lt.Col. Phillip Johnson as its head. Johnson was interested in improving the capability of existing tanks and conducted experiments with a modified Medium A Whippet with leaf spring suspension, different engine (believed to be a 300hp Rolls Royce Eagle engine) and a Wilson gearbox (possibly sourced from a Mark V tank). The modified Whippet was considerably faster than the standard Whippet, 30 mph vs 8 mph. Johnson also designed a new track system, called the "Snake track" based on a cable with track elements fixed to the cable but free to swing laterally. This was trialled on a Mark V tank, the top speed of the modified Mark V was increased to 20 mph compared to 4.6 mph of the standard tank. The Snake track permitted lateral movement of the track during manoeuvres with a lessened risk of throwing a track.
Johnson also invented a unique suspension system based on a sprung cable which was threaded through pulleys mounted on the roadwheels and the tank's chassis. Johnson's team began the design of a new medium tank based on the snake track and cable suspension. The initial design appeared quite promising and Col. J.F.C. Fuller saw fast medium tanks as being central to "Plan 1919" which he was working on. Plan 1919 required medium tanks to exploit a breakthrough opened by heavy tanks. Fuller lobbied the Minister of Munitions, Churchill, for the development and production of the Medium D. Churchill promoted the Medium D as essential to the development of Tank Corps and should be actively developed and produced. However, WW1 had ended, and expenditure for military hardware was rapidly reduced. After a protracted bureaucratic tussle between the Ministry of Munitions, the Mechanised Warfare Dept, the War Office and Treasury, funding was granted in December 1919 for 20 Medium D tanks. During the year or so before the final decision was made the quantities of Medium Mark D tanks requested were reduced from 500 in Dec 1918 to 75 in July 1919 and finally down to 20. Johnson had returned from France in 1918 to head the Department of Tank Design and Experiment, work continued on the Medium D design. A wooden mockup of the Medium Mark D was displayed at Woolwich in early 1919.
The first prototype Medium D was delivered in 1919 powered by a 240 hp Siddeley Puma engine. The fighting compartment was an oval version of the citadel on the Whippet moved to the front of the tank. Only machine guns were carried and the driver was located at the back of the citadel. The tank appeared to slope towards the front. This, in part, was a response to the criticism of the poor forward vision of the Mark A Whippet due to the long and high engine deck. The Medium Mark D has, by some authors, been said to be incapable of climbing over obstacles. However, the specification called for clearing a 4 foot (1.22m) obstacle when driving forwards and 6 feet (1.83m) driving in reverse. The performance over obstacles probably wasn't as good as rhomboid tanks but would have met specification. At a late stage in design the Medium D became an amphibious tank with the snake track elements acting as paddles in water. The requirement for a sealed hull meant that the Medium D could not be fitted with a reasonable sized gun since the gun port would have to have low in the hull to maintain stability and this would have meant the tank was unable to float.
11 mild steel pre-production machines were ordered from the following manufacturers:
The first of four Medium Ds delivered was burned out during trials, the remainder proved to be mechanically unreliable. The 13.5 ton Medium D, when it was running, was quite fast, 23 mph was recorded as the top speed on flat ground and up to 28 mph downhill. An analysis of the Medium D by Admiralty experts showed it was unlikely to be able to float stably so the hull was modified as the Medium D* and D**.
Johnson had been sent to India in 1920 to investigate the use of tanks in the colony. His report was positive and two Medium D tanks, Fowler and Vickers built examples, were sent to India for trials in 1922 for tropical trials. The Fowler tank was insulated with asbestos panels, among other trial measures, in an attempt to reduce the effects of hot weather on the crew. The tanks eventually were dispatched to Ahmednagar, Maharastra State but both broke down driving from the railway station to the Army camp. They were towed to the camp and abandoned.
One Medium D* was produced by Vickers at the end of 1919. The hull of the tanks was widened from 2.26m (7' 5") to 2.56m (8' 5") and a new track with the pitch increased from 7.5" (19cm) to 10" (25.4cm) was trialed. The original 3-speed transmission was replaced with a 4-speed but it retained the epicyclic steering gear of the Medium D. The top speed was slightly higher at 24 mph even though the tank's weight had increased to 14.5 tons. The improvement in floatation stability was only marginal so the hull was further modified.
One Medium D** was produced by Vickers in 1920. The hull width was increased to 2.7m (8' 10"), the tracks reverted to the 7.5inch pitch of the Medium D. The hull length was also increased from 30' (9.144m) to 31' 10" (9.7m) and additional internal bulkheads were built into the hull. A new flush plated rear bulkhead was introduced on the Medium D**. The Medium D** used different engines, initially it had a 300hp uprated Puma engine and later a 370hp Rolls Royce Eagle. The 15 ton tank achieved a top speed of 31 mph but it isn't known which engine this speed was achieved with. The epicyclic steering gear was replaced by cam-operated clutch/brake steering. In 1921 the Medium D** was fitted with an experimental hydraulic Williams-Janney transmission.
The Medium D** was used by Johnson as the basis of the definitive Medium DM or D (Modified). Two of these were produced in 1921 by the Woolwich Ordnance Factory powered by a Rolls Royce Eagle 12 cylinder engine. The fighting compartment had an additional cupola on top for the tank commander, which reduced the driver's visibility still further. The weight of the tank had increased to 18 tons and the top speed reduced to 20 mph. The DM was also amphibious, at least one sank in the Thames and had to be recovered. There is a 1921 film clip of the recovery at British Pathé. The Medium DM proved to be no more reliable than the original Medium Ds, the complexity of the tank and the difficulties in driving it made it unsuitable for service. It should also be noted that the cable suspension had no redundancy in that a failure of a cable would immobilise the tank through failure of the suspension on one side.
Johnson's design team reused some of the elements of the Medium D design in two further projects. An 8 ton amphibious light tank known as the "Light Infantry Tank", powered by 100 hp Hall-Scott engine, was similar in configuration to the Medium D but had some improvements in the track and suspension. The track no longer used a cable but was composed of ball jointed segments with the track shoes welded on the strut between the ball joints. The cable suspension also had a cam tensioning device to stop the cable going slack at the rear end of the tank. Only a single tank was produced in 1921 and although it could achieve a top speed of 30 mph, the design proceeded no further since it inherited many of the faults of the Medium D.
Johnson was also instructed to design a family of vehicles for colonial use. The tank of this family had twin MG turrets and the snake track was replaced by a conventional track although the cable suspension was retained. A single example was built at Woolwich as the "Tropical Tank" in 1922. It was tested briefly at Farnborough and then abandoned.
It's easy to dismiss the Medium Mark D as an oddball design that proved to be unreliable and flawed. However, the Medium D foreshadowed
the fast medium tanks which became so important in WW2. It's fairly clear that Churchill bulldozed the bureaucrats to not only
save the project from post-WW1 cutbacks but also to get continued funding. However, Churchill must have made enemies, British bureaucrats
are not noted for their easy-going pragmatism so its likely the Medium D was developed in an environment of continuing criticism and
covert subversion. Churchill had left the Ministry of Munitions by 1921 which deprived the project of its champion. The requirement
that the tank should be able to float in retrospect was a fatal flaw, not only did it limit the armament options but also introduced
needless distortions to the form of the hull. The relationship between Johnson, his design team and the Army seems to have been
a contributor to the failure of the project. Gen. Elles, the Tank Corps commander, noted that the design team lead by Johnson ignored
feedback on the Medium D from service personnel. This communication problem probably became worse after 1920 since Johnson and his team had left
the Army and became contract civilian employees. Johnson's department was closed down in 1923 which spelled the end of the Medium D
and its derivatives. No Medium D has survived.
The US Army Ordnance Dept created a specification for a new medium tank in 1919. This called for a tank with a max. weight of 18 tons, power to weight ratio of 10 hp/ton, max. speed of 12 mph and a range of 60 miles. The tank was to be armed with a light cannon and two machine guns and the armour to withstand 0.50 inch calibre AP rounds fired from close range. A wooden mock up was created and inspected by the Tank Corps Technical Committee in April 1920. With some minor modifications the Ordnance Dept was authorised to construct two pilot medium tanks. The first of these was fairly conventional with coil spring suspension and known as the M1921. The Ordnance Dept had heard of the Medium D and obtained drawings and specifications of the snake track and cable suspension. The second medium tank prototype was constructed with a snake track and cable suspension as the M1922.
The US Army was under severe financial restrictions at this time and although the production of new tanks was unlikely to be authorised the Ordnance Dept was allowed to build prototypes to investigate new tank designs so that the USA did not lose all expertise in tank design. The M1921 was constructed at the Rock Island Arsenal and delivered to the Aberdeen Proving Ground in February 1922. It was powered by Murray and Tregurtha engine which was rated at 220 hp but, in fact, produced 195 hp. The lack of power limited the M1921 to a top speed of 10 mph. The tank was armed with a 6 pounder (57mm) gun and a 0.3 inch MG in a turret. A further 0.3 inch MG could be mounted in the cupola on top of the turret. The M1922 was completed in 1923 and shipped to Aberdeen arriving in March 1923. Testing showed that the suspension cable wore out very quickly and this was replaced by a chain. The track elements were 18 inches wide and had a wooden inserts, similar to the Medium D track. The suspension worked well and although the tank was limited by the unreliable and underpowered Murray and Tregurtha engine it achieved a top speed of 16 mph. The T1 medium tank design was thought to be more promising, so the M1922 was used as test bed for various tank components and eventually retired to Ordnance Dept museum at Aberdeen. The M1922 still survives and is located at the Anniston Army Storage Depot, Bynum, Alabama.
Image(1) by Neil Baumgardner, taken at Aberdeen 2008, image(2) by user "ArmyJunk2" Comcentral forums, taken at Aberdeen 2008, images (3 - 5) by T. Larkum, taken at Aberdeen, 1983. Images (2 - 5) reproduced from the website preservedtanks.com with permission.
The Char de Bataille project was a French Army project to combine the roles of a medium tank and assault gun into a single vehicle. All of the manufacturers of WW1 French tanks were invited to build prototypes to meet a 1921 specification. Four prototypes were built with three of these using snake tracks, none of the prototypes used a sprung cable suspension. More details in the Char de Bataille Project article.
David Fletcher "Mechanised Force: British Tanks between the Wars", HMSO, 1991
David Fletcher "British Tanks 1915-19", Crowood, 2001
Peter Chamberlain and Chris Ellis "(AFV Weapons Profiles No.7) Medium Tanks Marks A-D", Profile Publications, 1970
R.P. Hunnicutt "Sherman: A History of the American Medium Tank", Presidio, 1977
Users "R Simmie" and "TinCanTadpole" of the Landships forum for their help with this article.
No models of this tank are known.