The tank – development gathers pace

by Chris Chant on 18/04/2011

Tank Mk IV, deploying its unditching beamAlthough the Tank Mk I had made its debut in the Battle of Flers-Courcelette in September 1916, this could not be regarded as a real technical or tactical success. However, the combat debut of the tank did finally persuade sceptical British army officers in France that here was a potentially decisive weapon. Thus orders for the tank were increased as the fledgling tank arm was increased in personnel size to 9,000 by February 1917 and 20,000 by the time of the Armistice in November 1918. After Flers-Courcelette Field Marshal Sir Douglas Haig, commanding the British forces in France, requested the production of another 1,000 tanks. Stern moved swiftly to order the required armour and powerplants, though the limited power of the Foster-Daimler petrol engine led this astute pioneer to consider alternatives to this weakest feature of the Tank Mk I. That the army as a whole was still uncommitted to the tank found expression in the Army Council’s 10 October cancellation of Haig’s order, which was reinstated when Stern went straight to Lloyd George, who was now a keen advocate of any device that could reduce the horrific toll of head-on infantry battles. Lloyd George was to become prime minister on 6 December, and Stern persuaded him of the need not only for more tanks but for better tanks.

An improved machine was now under development as the Tank Mk IV, and to keep the production line open until this was ready Lloyd George allowed production of 100 examples of the Tank Mk II and Tank Mk III, essentially the Tank Mk I with detail modifications: the Tank Mk II (50 built) had a revised roof hatch with raised coaming and wider track shoes at every sixth link for greater traction, while the Tank Mk III (50 built) was the Tank Mk II up-armoured to the standard of the Tank Mk IV. The Tank Mk II and Tank Mk III complemented the Tank Mk I in the trench battles of 1917 at Arras (9/15 April), Messines (7 June) and 3rd Ypres (31 July/10 November), but these first three models were rapidly superseded by the Tank Mk IV during the second half of 1917. Once discarded at first-line tanks, the Mks I, II and III were used as training tanks or as wireless tanks (one sponson fitted out as a ‘wireless office’ and the other carrying the wireless equipment) or, with their sponsons removed and the resultant openings plated over, as supply tanks, known at the time as tank tenders fitted with sponson-like side panniers each 3 ft (0.91 m) wide and made of mild steel: like the standard sponsons, these panniers were rectangular and all too prone to embedding themselves in the mud when the tank tipped sideways or forward. The Tank Mk IV introduced sponsons with upward-swept lower sides, but this more practical design was not carried forward to the supply tenders’ panniers. Each supply tank could also tow three sleds each carrying 22,400 lb (10160 kg) of stores, and in the case of Tank Mk IV conversions an uprated 125-hp (93.2-kW) engine was often fitted to provide greater tractive power. These supply tanks were generally used to ferry forward ammunition and fuel, and to carry back the most seriously wounded.

By February 1917 the Tank Mk IV was ready for production. The type still relied on the indifferent Foster-Daimler engine and its associated gear system, but was otherwise a much improved vehicle incorporating the lessons of Flers-Courcelette and, as they were fought, the 1917 battles. Several of the earlier tanks had suffered because of the gravity feed from their 50-Imp gal (227-litre) internal fuel tanks, which also leaked highly inflammable petrol over the inside of the tank if it was punctured, and failed to work if the tank was trying to climb up to or down from a steep parapet: in the Tank Mk IV, therefore, the tank was moved to the outside of the vehicle between the rear horns, increased in capacity to 70 Imp gal (318 litres), provided with armour and fitted with a pump to ensure an uninterrupted flow of petrol. Other major improvements were the use of thicker armour (16 mm/0.63 in at the front and 12 mm/0.47 in declining to 8 mm/0.315 in elsewhere) to defeat the Germans’ new anti-tank rifle bullets; steel ‘spuds’ bolted to every third, fifth or ninth track shoe to increase traction in heavy going; smaller sponsons that could be shifted inboard rather than removed for transport; an exhaust and external silencer for the engine; improved internal stowage, ventilation and cooling; shorter 6-pdr (57-mm) L/23 guns to prevent the muzzles digging into the ground; an unditching beam carried above the vehicle on special rails; and 0.303-in (7.7-mm) Lewis machine-guns in place of the original 8-mm (0.312-in) Hotchkiss weapons in ball-and-socket mountings that allowed 90° movement in elevation and traverse. Experience soon confirmed that the Lewis gun was unsuitable for tank use as its air-cooling system filled the tank with fumes, the gun itself was vulnerable to enemy fire, and the need for a large opening in the sponson to accommodate the Lewis gun’s air-cooling jacket increased the problem of splash: a revised Hotchkiss was then substituted, though without the original form of very limited-movement trunnion mounting. After development from October 1916, the Tank Mk IV was delivered from April 1917, and the production of 1,015 vehicles was undertaken in the ratio of three female to two male tanks. The Tank Mk IV Female was somewhat different to the earlier females, for the sponsons were shallower (allowing the incorporation of a pair of hatches on each side in the area previously covered by the sponson) and narrower at only 1 ft 3 in (0.381 m). The reduced weight of these sponsons contributed significantly to the reduction in combat weight to 58,240 lb (26418 kg).

The growing sophistication of the tank is attested by the development of several Tank Mk IV variants as dictated by the nature of combat experience. There had been some criticism of the November 1916 decision to eliminate the rear steering wheels as this also reduced trench-crossing capability, and in an effort to improve this capability once more, Tritton in 1917 suggested the lengthening of the tank by 9 ft (2.74 m) through the extension of the rear horns. These extensions were of mild steel, strapped and riveted to the original structure, and braced diagonally by stays to provide a rigid frame for tracks each lengthened by the insertion of 28 extra shoes and driven by a lengthened drive. Trials were also conducted with a 6-in (152-mm) Newton and Stokes mortars on a platform inserted between the rear horns to fire forward over the hull. Some interesting results were achieved with the ‘tadpole tail’ and mortars, but the lengthened horns lacked rigidity and were not put into production.

British Tank Mk IV with tadpole tail, standing in a churned-up muddy field, with trees and a house in the backgroundEven the tadpole tail could not offer much hope for the crossing of the extra wide trenches built by the Germans in their so-called ‘Hindenburg Line’ defences, and to overcome this problem there was evolved the Tank Mk IV Fascine. In its original form this could carry wooden fascines on its unditching beam rails, these chain-bound bundles of brushwood, each 10 ft (3.05 m) in length and 4 ft 6 in (1.37 m) in diameter, being dropped over the nose under control of the driver; later versions carried hexagonal wood or steel cribs for the same purpose of filling the trench and providing a roadway for the tank’s further progress.

Ditching was always a problem with the British tanks of World War I, and several experimental unditching systems were evaluated as alternatives to the standard beam. Some of these were promising, but none got past the evaluation phase. There were also salvage tanks of several patterns, developed in the field for recovery and maintenance work: these were generally fitted with a manually-operated jib crane for tasks such as engine changes.

When the Germans first began to put tanks into action in the field during April 1918, the British realised that their blend of male and female tanks was perhaps not the optimum solution, for the female tank was distinctly vulnerable to the efforts of the German tanks. The result was the Tank Mk IV Hermaphrodite, essentially a standard Tank Mk IV Female with one of its sponsons replaced by the gun-armed sponson of a Tank Mk IV Male to provide a blend of male and female firepower.

The key moment for the Tank Mk IV, and indeed for the tank in general, came on 20 November 1917 with the beginning of the Battle of Cambrai. This was the first occasion on which tanks were used in a homogeneous mass, and the practice nearly proved decisive. The British 3rd Army was entrusted with a surprise offensive against the German 2nd Army: in ideal terrain conditions to the south of Cambrai, the army was to attack without the protracted artillery bombardment that had previously been standard, this novel concept being designed to give the Germans no forewarning of the offensive, and to prevent the ground from being churned up. This latter ensured excellent going for the 400 tanks that spearheaded the offensive behind a sharp creeping barrage that forced the Germans to keep their heads down. Tactical surprise was achieved by the new tactics, and the tanks were instrumental in opening a 6-mile (9.7-km) gap in the German line, through which the British advanced to a depth of 5 miles (8 km). Two cavalry divisions were poised to exploit any advantage, but the extent of this success caught the British so unexpectedly that there were insufficient infantry and tanks in reserve to allow any rational exploitation. The Germans recovered with remarkable speed, and their counterattacks of 30 November forced the British to fall back most of the way to their start line by 3 December. The Battle of Cambrai was thus a draw, but this fact cannot disguise the fact that the first mass use of tanks had in general secured unprecedented results.

As usual, however, the tanks had suffered large numbers of mechanical breakdowns, and many of these non-runners were captured by the Germans as they pushed back the British. Impressed with the capabilities of the tanks, and lacking their own counterparts, the Germans rushed the captured machines to their depot at Charleroi for refurbishment and rearming: in this latter aspect the 6-pdr (57-mm) guns were replaced by Sokol 57-mm guns captured from the Russians, and the 0.303-in (7.7-mm) machine-guns by 7.92-mm (0.31-in) MG08 weapons. The vehicles were then issued to the Germans’ fledgling tank arm with the designation Beutepanzerwagen IV, the comparative extent of British and German tank production at this time being indicated by the fact that captured tanks equipped four of the Germans’ seven tank companies in December 1917.

As noted above, Stern appreciated from the early days of the Tank Mk I that the weakest point of the tank was its transmission/gearing and associated Foster-Daimler petrol engine. To meet operational requirements the Tank Mk IV was rushed into production and service even as Stern was investigating alternative powerplants, so from October 1917 the Tank Supply Department had a modest breathing space in which to consider other tank automotive systems. These included a Westinghouse petrol-electric drive with one-man control via a separate petrol-electric generator on each track for infinitely variable speed control; a Daimler petrol-electric drive with similar capabilities; a Williams-Janney petrol-hydraulic drive similar in concept to the petrol-electric drives though using hydraulic rather than electric motors; a Wilkins multiple clutch drive of extraordinary complexity; and a Wilson mechanical system using epicyclic gears and brakes instead of the standard change-speed gearing. The petrol-electric drives offered superior capabilities, but were sufficiently complex and expensive to urge on Stern the advantages of the Wilson system, which offered the possibility of one-man control without the potential problems of the petrol-electric systems.

Wilson was entrusted with the overall design of the tank to use his epicyclic gearbox, and this emerged as the Tank Mk V with hull and armament based on those of the Tank Mk IV but fitted with the Wilson gearbox and a new 150-hp (112-kW) Ricardo petrol engine commissioned by Stern. The Tank Mk V Male weighed 64,960 lb (29466 kg) and the Tank Mk V Female 62,720 lb (28450 kg), but the use of a more powerful engine boosted maximum speed to 4.6 mph (7.4 km/h) and combat radius was 45 miles (72 km) on 93 Imp gal (423 litres) of fuel in armoured external tanks. The Tank Mk V entered production at the Birmingham works of the Metropolitan Carriage, Wagon and Finance Company during December 1917, and began to reach service units in May 1918 in equal numbers of male and female tanks, of which a proportion were converted in the field to Tank Mk V Hermaphrodite standard. The initial 200 machines had tracks 20.5 in (521 mm) wide, but later examples had tracks 26.5 in (673 mm) wide for better performance in poor going. One-man control of the Mk V’s automotive system made for considerable improvement in control and manoeuvrability, and the more powerful engine made useful contribution to performance, but the Mk V was also a considerable advance over its predecessors in its better engine cooling and ventilation system; the provision of a cupola above the roof at the rear for the commander, who thus had far better fields of vision than in earlier tanks; and facility for the unditching beam to be connected and disconnected from inside the vehicle, thereby obviating the need for at least two crew members to leave the vehicle in the fashion that had at times cost earlier tanks considerable casualties.

British Tank Mk V (male), spattered with mud, muddy field meeting the sky in the backgroundThe Tank Mk V first went into action at Hamel in July 1918, and thereafter partnered the more numerous Tank Mk IV for the rest of World War I. The new type was dimensionally similar to the Tank Mk IV, and thus suffered the same limitations when faced with a wide trench. Not surprisingly, therefore, the Tank Mk V was evaluated with a ‘tadpole tail’, but a better expedient was adopted after development by the Tank Corps Central Workshops in France from February 1918. This resulted in the Tank Mk V*, in which the vehicle was cut in half to allow the insertion of a 6-ft (1.83-m) armour section, comprising three 2-ft (0.61-m) panels, between the rear of the sponsons and the epicyclic gearbox, increasing ground length and so providing a 13-ft (3.96-m) trench-crossing capability at an increase in weight of 8,960 lb (4064 kg). The additional section carried two machine-gun positions to complement the standard fit based on that of the Mk IV but already boosted by the provision of two positions in the new commander’s cupola. The extra weight inevitably reduced performance slightly and manoeuvrability considerably, but the additional length had the incidental advantage of increasing internal volume to the extent that the Mk V* could be used to carry 25 troops, who suffered badly from the heat and the poor ventilation, or more usefully a substantial load of supplies. Production was undertaken by Metropolitan from May 1918, production reaching 579 examples by the time of the Armistice.

The Tank Mk V** was similar to the Tank Mk V*, but designed as such and built in small numbers by Foster for post-war service. The forward rise was increased to provide better parapet-climbing capability, an uprated 225-hp (168-kW) Ricardo engine was located farther to the rear, and the commander’s cupola was therefore moved forward to a position just behind the driver’s cupola. It is interesting to note that post-war development gave evidence of the tank’s multiple uses: the Tank Mk V** was developed as a Royal Engineer Tank in two forms as a carrier and launch vehicle for a 20-ft (6.1-m) bridge capable of supporting a tank, and as a mineclearing vehicle fitted with a forward roller to detonate pressure-activated mines.

The Tank Mk V* was the ultimate version of British mainstream battle tank development to see service in World War I, no fewer than 324 Tank Mk V and Tank Mk V* machines spearheading the decisive breakthrough offensive of 8 August 1918 in the Battle of Amiens, which the German commander, General Erich Ludendorff, characterised as ‘the black day of the German army’. In this offensive the Tank Mk IV and Tank Mk V variants were partnered by a number of light tanks, but before turning to these it is perhaps sensible to complete a summary of British battle tank development in World War I. Next in sequence came the Tank Mk VI, which was designed in from December 1916 and reached mock-up form in February 1917. The type was designed to provide better cross-country performance than the Tank Mk IV and Tank Mk V, and the armament was centred on a 6-pdr (57-mm) gun located in a limited-traverse mounting between the front horns, backed by six machine-guns carried in a substantial cupola above the hull roof (four guns) and in two small hull-side sponsons (one gun each). Work was cancelled in favour of addition Tank Mk V production to meet the German offensives in France from 21 March 1918.

The Tank Mk VII was built by Brown Bros of Edinburgh, the sole prototype completing successful trials between October and November 1917. The Tank Mk VII was based on the Tank Mk V but with a tail lengthened by 3 ft (0.91 m) for improved trench-crossing capability, and the electrically started 150-hp (112-kW) Ricardo petrol engine was used to power twin Williams-Janney hydraulic motors. The cross-country performance and agility of the type were most encouraging, and early in 1918 an order was placed for 75 production vehicles. Production of the Williams-Janney hydraulic motors proved troublesome, however, and only one production machine was completed before the Armistice and the wholesale cancellation of production orders.

Up to the Tank Mk VII, British tank design followed a linear line of descent from the Tank Mk I. But with the Tank Mk VIII all the lessons of earlier development and combat use were melded into a completely new design by the army’s Mechanical Warfare Department for joint Anglo-US manufacture of an initial 1,500 machines whose hulls and armament were to have been provided from the UK to be assembled with American-supplied engines, transmission systems and controls on a special assembly line in France. The tank was variously known as the Anglo-American Tank, the Liberty, the International and the Allied, and the hopes vested in the machine are evidenced by the fact that the 1,500 joint-venture tanks were to be supplemented by 1,450 purely British examples and 1,500 purely American examples for a grand total of 4,450 Mk VIII vehicles.

The design was modelled on the standard rhomboidal shape that had proved generally successful in previous British tanks, but provided with lower contours that allowed 7 ft 6 in (2.29 m) of contact length with the ground by comparison to the Mk V’s 4 ft 6 in (1.37 m), and ground clearance of 1ft 9 in (0.533 m) by comparison to the earlier tanks’ 1 ft 4 in (0.406 m). The result was a considerably heavier machine, with a combat weight of 82,880 lb (37594 kg). The Tank Mk VIII was also a generally larger machine, with an overall length of 34 ft 2.5 in (10.43 m) to the Mk V’s 26 ft 5 in (8.05 m), width of 12 ft 4 in (3.76 m) to the Mk V Male’s 12 ft 10 in (3.91 m), and height of 10 ft 3 in (3.12 m) to the Mk V’s 8 ft 8 in (2.64 m). In terms of battlefield capabilities, the additional length reduced manoeuvrability, but provided an impressive trench-crossing capability of 15 ft (4.57 m). The crew remained the standard eight men, and the armour thicknesses were the same as those on the Tank Mk V. The larger dimensions of the Tank Mk VIII allowed a useful compartmentalisation of the interior, with the engine and transmission in the rear section cut off from the rest of the tank by a bulkhead for considerable improvement in terms of noise and fumes. The main armament comprised two 6-pdr (57-mm) Hotchkiss L/23 guns in the side sponsons with 104 rounds each, and the secondary armament was seven machine-guns with a total of 13,484 rounds: the machine-guns were located as one in the bow, one in each of the doors behind the sponsons, and four in the centrally-mounted main fighting turret (one in each side, one on the front left and one on the rear right). The sponsons were hinged and fitted on roller bearings so that they could be hand-pulled into the main fighting compartment for transport, and each machine-gun was carried in a ball-and-socket mounting (itself carried in a spherical mounting) to allow 130° movement. The main fighting turret was also fitted with a commander’s rotating sub-turret offering a 360° field of vision.

Grey graphic representation of a British Tank Mk VIII standing on grassConsiderable difficulties were encountered with the design and final development of the Tank Mk VIII, the most troublesome individual component being the 300-hp (224-kW) engine. In the joint and American versions this was a Liberty unit adapted from the standard V-12 aeroplane engine by lowering the compression ratio and using cast iron rather than steel cylinders. In the British version the engine was a V-12 Ricardo, produced by mounting two 150-hp (112-kW) Ricardo V-6 engines on a common crankcase. Under good conditions the Ricardo unit provided a maximum speed of 7 mph (11.25 km/h) in a lightly-loaded Mk VIII. Some 200 Imp gal (909 litres) of fuel were carried in three armoured tanks at the rear, and pump-fed to a gravity tank above the end, surplus fuel being vented back into the main tankage by an overflow pipe. This fuel tankage provided a 55-mile (88.5-km) combat radius. Considerable thought was given to engine cooling, and the location of the engine in its own compartment greatly aided the designers, who opted for a system in which air was drawn in through a roof louver to pass round the engine and through the rear-mounted radiator before being expelled by a fan through a rear louver. Compartmentalization also allowed proper ventilation of the fighting section by an electrically powered fan that created a slight internal overpressure that was vented through any available opening to take with it any gun fumes.

Mild steel prototypes had been successfully trialled by the end of the war, and a large number of components had been produced for assembly in the unfinished French factory at Nervy-Pailloux. The programme was cancelled at the end of the war, but in 1919 and 1920 the Americans used existing components to build 100 tanks at the Rock Island Arsenal: these differed from the wartime standard only in having 0.3-in (7.62-mm) Browning machine-guns instead of Hotchkiss machine-guns, and formed the mainstay of the US tank arm up to 1932, when they were placed in storage and finally used by Canada as training machines in the early days of World War II.

The final designations in the mainstream of British battle tanks during World War I were the Tank Mk IX and Tank Mk X. The Tank Mk IX was really a supply tank based mechanically on the Tank Mk V, and at a combat weight of 60,480 lb (27434 kg) could carry 50 troops or 22,400 lb (10161 kg) of supplies with access through four large side hatches; only three prototypes were completed. In 1919 one of these was modified for amphibious trials, being fitted with a raised cab, high exhausts and ex-naval ‘camel’ flotation chambers. The Tank Mk X was schemed as a revised Tank Mk V with modifications intended to increase habitability, reliability and manoeuvrability. If the war had continued into 1919 some 2,000 Tank Mk X vehicles would have been ordered as the backbone of the tank army round which the assault into Germany was being planned.

The Allied concept of military operations after November 1914 had been posited without significant deviation on a breakthrough of the German line and exploitation into the enemy rear. Even before the battle tank had begun to prove itself as a weapon for the breakthrough phase, tank advocates had begun to work on a lighter and more mobile tank suitable for the exploitation phase, for the battle tank was clearly too slow and too short-ranged for any but the most direct battlefield tasks. In 1916 Tritton designed a high speed tank with light armour for the task of co-operating with the cavalry, and this initial ‘Whippet’ scheme was revised from December 1916 as the Tritton Chaser or, more prosaically, the Tritton No. 2 Light Machine. The type first ran in February 1917, and trials were generally successful. Various changes were required before a firm order was placed in June 1917. The definitive version became the Medium Tank Mk A, generally named the Whippet, and deliveries from the Foster works in Lincoln began in October 1917 to meet the initial requirement for 200 machines. This order was subsequently increased to 385 machines, and then reduced once more to 200 when it became clear that the double automotive system of the Medium Tank Mk A was both expensive to produce and difficult to maintain at a reasonable standard of reliability. The overall design of the Whippet was totally different from that of the battle tanks, with long low-set unsprung tracks whose shoes were based on those of battle tanks but of lighter construction and fitted with provision for spuds: the tracks were long enough to provide a 7-ft (2.13-m) trench-crossing capability. The track units were fitted on each side with four chutes along much of the length of the top run to keep the tracks clear of mud and thus lighter. The long forward section of the hull above the upper run of the tracks accommodated two 45-hp (33.6-kW) Tyler four-cylinder inline engines (as developed for truck use), located side by side and each provided with its own clutch and gearbox to drive one track. Twin throttles were located on the steering wheel, their movement together controlling acceleration to a maximum speed of 8.3 mph (13.4 km/h) in this 31,360-lb (14225-kg) vehicle. Steering was effected by the driver’s steering wheel, whose movement worked on the throttles to increase the power of one engine and decrease that of the other (to a maximum variation of 12 hp/8.95 kW) and so provide additional power to one track or the other: the system was complex and extremely demanding on the driver, who often stalled one engine in a tight turn and then shed a track, thus immobilizing his vehicle. An armoured tank in the extreme nose held 70 Imp gal (318 litres) of petrol fed to the engines by a pump, and this capacity was sufficient for a combat radius of 80 miles (129 km).

The crew of three or four was located in the fighting compartment at the rear of the vehicle. This compartment was essentially a fixed barbette (the original notion of a rotating turret having been abandoned to simplify production), and in addition to the driver was occupied by the commander and one or two gunners, with between them three or four Hotchkiss machine-guns (with 5,400 rounds) for all-round fire. The second gunner and the Hotchkiss in the rear of the barbette were generally omitted by operational units to mitigate the appalling conditions inside the barbette. Armour varied from a minimum of 6 mm (0.24 in) to a maximum of 12 mm (0.47 in).

The type first saw action in March 1918 near Herbertune in northern France, and was used up to the end of the war. The type’s greatest moment came in the Battle of Amiens on 8 August 1918, when the 3rd Tank Brigade’s two battalions were fully equipped with 96 Medium Tank Mk A machines. The brigade was tasked with support of the Cavalry Corps, and though liaison was poor some useful results were gained. The major tactical problem was that in good conditions the cavalry was faster than the tank brigade, but had to wait for the tanks to catch up and deal with opposition armed with anything but small arms. The result was that the tanks were not employed in a homogeneous mass that might have completely destroyed the German rear areas to a depth of 10 miles (16 km) or more. The Medium Tank Mk A machines were used in small numbers attached to specific cavalry units, but nonetheless achieved the successes that fully vindicated their overall capabilities.

Obviously the type was limited by its use of two low-powered engines and unsprung tracks, and in an effort to overcome this limitation Major Philip Johnson, an Army Service Corps officer serving at the Tank Corps Central Workshops in France, reworked the design with sprung tracks and a 360-hp (268-kW) Rolls-Royce Eagle aero engine working through the transmission of a Mk V battle tank: speeds well over 20 mph (32 km/h) were achieved, but this important advance failed to find favour.

As noted above, production of the Medium Tank Mk A was limited to just 200 machines, a fact ensured by the development of the Medium Tank Mk B, whose arrival in service was anticipated in good time for the grand tank offensive designed to finish off Germany in 1919. The Medium Tank Mk B was also called Whippet, but in this instance was designed by Wilson and bore closer conceptual affinities to current battle tanks than to the Medium Tank Mk A, despite being intended for the same cavalry or exploitation role.

The Medium Tank Mk B was based on a new automotive system using a 100-hp (76-kW) four-cylinder version of Harry Ricardo’s 150-hp (112-kW) six-cylinder battle tank engine located at the rear of the hull in the first instance of a tank engine in its own compartment. The fan-cooled engine drove the twin tracks via epicyclic gearing, and provided greater levels of manoeuvrability with much improved reliability and reduced production cost. The rest of the vehicle was also different from the Medium Tank Mk A, being based on the rhomboidal shape of contemporary battle tanks with the tracks running right round the hull and its projecting front and rear horns. On each side of the Mk B were three large chutes to clear mud from the upper run of the tracks.

At 7 ft 0 in and 2 ft 6 in (2.13 and 0.76 m) respectively, the Medium Tank Mk A’s trench-crossing and parapet-climbing capabilities were notably poor: the shape of the Medium Tank Mk B improved these figures considerably, the trench-crossing figure to 8 ft 6 in (2.59 m). Inevitably the Medium Tank Mk B was a larger and, at a combat weight of 40,320 lb (18289 kg), a heavier tank. The crew of four comprised the driver, commander and two gunners, all accommodated in a large barbette at the front of the tank. The Medium Tank Mk B was proposed in two forms, a female with the standard armament of four Hotchkiss machine-guns with 7,500 rounds, and a male with a revolving turret carrying a 2-pdr (40-mm) gun: only the female type was produced. Armour was comparable to that of the Medium Tank Mk A, though the thinnest plates were to a 6- rather than 5-mm (0.24- rather than 0.2-in) basis.

Other features of the Medium Tank Mk B were removable rear decking to provide access to the engine compartment, a crude smoke-generation system whereby sulphonic acid was dripped onto the hot exhaust from a special tank, and provision for electric or hand starting. Design work was completed late in 1917, but it was mid-1918 before the first production order was placed with the Metropolitan Carriage, Wagon and Finance Company for 450 vehicles: this delay was occasioned in part by the need for modifications to the original design, and in part to the caution of the army, which wished to assess the capabilities of the first medium tank (the Medium Tank Mk A) before committing scarce production facilities to a successor. The first vehicle appeared in September 1918, and only 45 had been completed by the time of the Armistice, when all further production was cancelled. The Medium Tank Mk B displayed a maximum speed of only 6.1 mph (9.8 km/h) and a combat radius of 65 miles (105 km) on 85 Imp gal (386 litres) of fuel, so outright performance was well down on that of the Medium Tank Mk A. Other criticism of the Medium Tank Mk B centred on the engine installation: all appreciated the fact that its separation from the rest of the tank by a bulkhead reduced the problem of engine noise and fumes in the fighting compartment, but the compact nature of the engine compartment meant that the entire engine had often to be lifted out for even the most minor of maintenance tasks.

As Wilson worked on development of the Medium Tank Mk B, Tritton was involved on his own successor to the Mk A in the form of the Medium Tank Mk C, generally known as the Hornet and judged to have been the best medium tank design evolved in World War I. The Medium Tank Mk C bore a strong conceptual similarity to the Medium Tank Mk B (a rear engine, all-round tracks and a forward fighting compartment complete with a rotating cupola for the commander), and the basic design was completed at about the same time in December 1917. Again like the Medium Tank Mk B, the Medium Tank Mk C was offered in male and female forms, the male with a 6-pdr (57-mm) gun located in the hull front and the female with four Hotchkiss machine-guns and 7,200 rounds. The engine was the 150-hp (112-kW) six-cylinder version of the Ricardo tank engine, driving the tracks by means of epicyclic gears. Tritton gave considerable attention to the engine and transmission installation to ensure easy access for maintenance, and planned the whole design for rapid manufacture making maximum use of subassemblies.

The Medium Tank Mk C prototype was completed in the summer of 1918 and underwent a highly successful evaluation programme before a production order for 200 Medium Tank Mk C Female machines was placed at the beginning of October. Faith in the type was considerable, and longer-term plans called for production of 6,000 Medium Tank Mk C machines (4,000 females and 2,000 males) for the proposed ‘Plan 1919’. Building of the Medium Tank Mk C was drastically curtailed at the time of the Armistice, and only 48 such tanks were completed, serving with considerable distinction up to 1925.

Weighing 44,800 lb (20321 kg), the Medium Tank Mk C had a length of 26 ft (7.92 m) in comparison with the Medium Tank Mk B’s 22 ft 9 in (6.93 m), width of 8 ft 4 in (2.54 m) in comparison with 8 ft 10 in (2.69 m) and height of 9 ft 6 in (2.90 m) in comparison with 8.5 ft (2.59 m). The Medium Tank Mk C’s additional length contributed to a trench-crossing capability of 10 ft (3.05 m), and the type’s other main performance figures included a maximum speed of 7.9 mph (12.7 km/h) and a combat radius of 140 miles (225 km) on 150 Imp gal (682 litres) of fuel.

Mock up of a British Medium Tank Mk D, standing in an empty warehouse, bright light coming in through the windows in the backgroundThe ultimate British tank design of World War I was the Medium Tank Mk D, which was designed by the same Johnson responsible for development of the Medium Tank Mk A with leaf-spring suspension and (from February 1918) a modified Eagle aero engine. It was not thought practical to modify in-service Medium Tank Mk A machines in this fashion, but the capabilities of the single experimental vehicle did not go unremarked. With the support of the corps’ chief engineer, the Tank Corps’ chief-of-staff, Colonel J. F. C. Fuller, in May 1918 produced his now-classic Tactics of the Attack as Affected by the Speed and Circuit of the Medium D Tank. This was the direct inspiration for ‘Plan 1919’, which envisaged a breakthrough on a 90-mile (145-km) front by concentrated masses of battle tanks (mostly Tank Mk V** and Tank Mk VIII types) to allow an exploitation by Medium Tank Mk C and Medium Tank Mk D machines supported by lorried infantry and close-support aircraft.

The core of the concept was the Medium Tank Mk D whose development was entrusted to Johnson, who capitalised on his Whippet experimental programme to suggest a machine capable of high cross-country speed as a result of its automotive system, which was to comprise a converted aero engine and sprung tracks that had two-axis freedom of movement. In basic shape the Medium Tank Mk D was similar to the Medium Tank Mk A, but convention was flouted in the fact that Johnson’s layout was exactly opposite to the Medium Tank Mk A, with the lower end of the track at the front to provide good fields of vision and fire for the four men in the front-mounted barbette, which was well shaped and fitted for an armament of perhaps three Hotchkiss machine-guns in the female version that was later to be supplemented by a male version with a short 6-pdr (57-mm) gun. Development was protracted, and four prototypes (the first appearing in May 1919) were followed by only two production machines before the programme was cancelled in 1921. Powered by a 240-hp (179-kW) Armstrong Siddeley Puma aero engine driving through epicyclic gears, the Medium Tank Mk D weighed 44,800 lb (20321 kg) and was 30 ft (9.14 m) long. Maximum speed was 25 mph (40 km/h), and the tank’s combat radius was about 200 km (322 km).

Specification

Tank Mk IV (Male)

Type: battle tank

Crew: 8 (commander, driver/brakesman, two gearsmen, and four gunners)

Combat weight: 62,720 lb (28450 kg)

Dimensions: length overall 26 ft 5 in (8.05 m); width 13 ft 6 in (4.115 m); height 8 ft 2 in (2.49 m)

Armament: two 6-pdr (57-mm) Hotchkiss L/23 guns with 332 rounds and four 0.3303-in (7.7-mm) Lewis machine-guns with 6,272 rounds; the main guns were stabilised in neither elevation nor azimuth (120º out from the forward centreline), and simple optical sights were fitted

Armour: riveted steel between 6.35 and 12.7 mm (0.25 and 0.5 in)

Powerplant: one 105-hp (78.3-kW) Foster-Daimler water-cooled six-cylinder inline petrol engine with 70 Imp gal (318 litres) of fuel

Performance: road speed 3.7 mph (5.95 km/h); road range 35 miles (56.3 km); fording 4 ft 6 in (1.37 m) without preparation; gradient 47%; vertical obstacle 4 ft 6 in (1.37 m); trench 10 ft 0 in (3.05 m); ground clearance 16 in (0.406 m)

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