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The Medium D Tank was a fast tank designed for the planned campaign of 1919, and which was eventually cancelled after the war in favour of the more advanced Vickers Medium Tank Mark I.
The Medium D was designed to take part in Plan 1919, a plan developed by J.F.C. Fuller to use heavy tanks to pin the Germans in place and fast tanks to get into the German rear area and cause chaos. The plan itself was completed in May 1918, but it relied on having tanks capable of reaching 20mph, almost three times quicker than the fastest British tank of the time.
In order to design the new tank Major Philip Johnson was recalled from France and placed in charge of the Mechanical Warfare Department’s experiment ground at Dollis Hill. Johnson was brilliant and inventive, but not always practical. He was later said to have been the only person capable of driving the Medium D!
The specification for the Medium D called for a tank that weighted under 20 tones, had a range of 200 miles and a speed of 20mph. Work on this began in October 1918, and a full size wooden mockup had been completed by the Armistice (suggesting Johnson already had many of the ideas in mind).
The layout of the Medium D was rather unusual. Like the Medium B and Medium C it had the engine and transmission at the rear and the fighting compartment at the front. On the Medium D the fighting compartment was a fixed circular turret, armed with three ball mounted machine guns. The driver sat under a raised cupola at the rear of the fighting compartment. If the tank had been level his view would have been poor, so the tank was raised at the rear, and sloped down towards the front, so the driver could look over the top of the fighting compartment. However this meant that the tank had a limited ability to cross vertical obstacles when moving forwards, so the plan was for it to turn around and reverse over any obstacles it found, leaving the guns facing backwards.
The Medium D used flexible steel wire rope tracks and suspension. This system was first tested out on a Mark V tank which Johnson passed on to Fowlers of Lincoln, who used steel wire ropes on ploughing machines. The rope was used in two ways on the Mark V. First one piece of rope was used to provide suspension, by attaching it to springs at either end of the track, then threading it over and under a series of rollers to create a single long spring. The track rollers would move against this long spring, providing all of them with suspension without having to include a whole series of springs. The second use was as the core of the track itself. A complete circuit of steel cable was used at the core of the track, with long narrow track shoes threaded onto it. They were free to pivot on the cable, adapting themselves to rough ground. Photographs of the modified Mark V show it with the track shoes at all sorts of different angles, a very unusual look for a tank track. The modified Mark V was completed in May 1919 and powered by a 225hp Ricardo engine was able to reach 20mph, a great improvement over the standard Mark V.
An order for ten prototype Medium D tanks was placed before the end of the First World War. A report from August 1919 splits this into four ordered from Fowlers of Leeds in August 1918 and six from Vickers in September 1918.
The first Medium D was completed in June 1919. One aim had been to make it amphibious, but the Admiralty dismissed it as being unstable in the water. It was powered by a Siddeley Puma aero engine, reached 23mph on level ground and 28mph down hill and the wire rope suspension performed well.
Two of the tanks on the Vickers order were completed to modified designs. Late in 1919 one was completed as the Medium D*. This version was 1ft 2in wider than the normal Medium D, in an attempt to make it more stable in the water. This wasn’t enough for the Admiralty, so a second vehicle was completed in 1920 as the Medium D**. This was 9ft wide, adding another 7in. This tank performed perfectly well during a test in the river at Christchurch.
Soon after the first prototype had been completed the Army Council had provided £1 million to build 75 production tanks. It was soon realised that this money would only actually fund 45 tanks. This order was later reduced to twenty, of which three appear to have been built.
These three tanks were built as the Medium DM (D Modified). They were built by the Royal Ordnance Factory at Woolwich, and were powered by a Rolls-Royce Eagle VIII engine. An extra cupola was added in the middle of the top of the turret for the commander, blocking the driver’s view directly forward. It also got side escape hatches.
Plans were in place for a Male version with a short 6-pounder
In 1923 the first prototype of the Vickers Medium Tank Mark I were sent for trials. This was a more advanced tank than the Medium D, with a rotating turret armed with a 3-pounder gun and ball mounts for four machine guns, and was shorter in height and length than the Medium D. It was also lighter, at only 11.7 tons, and used a less demanding engine – the 90hp Armstrong Siddeley V-8. The Medium D’s only advantage was its higher top speed but this was balanced by its lower reliability. In March 1923 Johnson’s department was shut down and work on the Medium D ended.
Variants
Light Infantry Tank
Johnson produced several more designs based on the Medium D. In 1921 he produced the Light Infantry Tank, which used the same layout but was slightly smaller at 22ft 3in long and 17.5 tons. It was powered by a 100hp Hall Scott aero engine, and used the Johnson Snake Track. Each track plate was fitting to a hollow tubular backbone, which was joined to the next track plate by a spherical joint, allowing the track to be flexible in all directions. It could reach over 30mph and was amphibious.
Light Tropical Tank
A visit to India inspired the Light Tropical Tank. This had a major change of layout. The driver and engine were at the front, side by side. Two machine gun turrets were installed, diagonally across from each other at the rear of the tank. It used Johnson Snake Tracks, weighted 5.5 tons, was powered by a 45hp Taylor engine and could reach 15mph.
Supply Tank
The Supply Tank was based on the Topical Tank, and replaced the machine gun turrets with a large cargo compartment. It wasn’t produced, but the prototypes were used in army trials and exercises in the early 1920s.
Names
Stats
Production:
Hull Length: 30ft
Hull Width: 7ft 5in
Height: 9ft 2.5in
Crew: 3
Weight: 20 tons
Engine: 240hp Armstrong Siddeley Puma
Max Speed: 23mph
Max Range:
Armament: Three machine guns
Armour: 8-10mm