TOG Citadel

United Kingdom (1939)
Heavy Tank – Concept Only

Background

The battlefields of the Western Front during WW1 were characterized by thick belts of barbed wire covered by machine-gun fire often from concrete bunkers, creating an area which was all but impassable to infantry. The ground, shattered by years of war and millions of rounds of artillery fire, was often a quagmire of mud into which men, beasts, and machines would drown. Even if they managed to cross all of that, they would be faced with having to cross enemy trenches, anti-tank ditches, minefields, and other obstacles.

The British tanks of WW1 were specifically designed to overcome much of these problems, adopting a characteristic quasi-rhomboidal shape in which the tracks would run over the top of the hull, producing a high leading point for the track and carefully shaped to maximize the ability to both climb a step and cross ditches.

The early designs were relatively crude affairs, with inadequate armor, quickly falling prey to German anti-tank rifles and slow enough to be hit by enemy artillery. As the war progressed, the British progressively improved the armor and layout to the pinnacle of the whole design evolution, in the form of the Anglo-American Mk.VIII heavy tank. It had improved armor, improved mobility, in a larger tank with more firepower, and still retained the ability to extract itself from the terrible ground conditions. However, that tank did not get the opportunity to show its true power during the war and the mass production of it, which was being put in place, was canceled with the end of the war. Nonetheless, the principles had been established and if only thinking in WW1 terms, then this layout of the tank was clearly going to be ideal.

In 1939, many people could see the clouds of war gathering over Europe as an expansionist Nazi Germany under Adolf Hitler became more and more assertive, dominant, and militaristic. With the 1938 invasion of Czechoslovakia by Germany, any doubts about the future aspirations of Hitler to become the preeminent military power in Europe were over. Despite the appeasement of men such as the British Prime Minister, Neville Chamberlain, Germany was not going to cease in its growth and there was little time to prepare for a new major land war.

Thus, on the eve of war, and apparently with little comprehension that the nature of this forthcoming conflict was going to be different from those conditions experienced 20 years prior, the Army was in a rush to find a heavy tank. They simply resorted to exactly what they knew had worked before, a Mk.VIII shaped vehicle, albeit with more armor and firepower than before and the additional task of smashing reinforced bunkers.

It is not without a substantial degree of irony that the man substantially responsible for the Mk.VIII design, Sir Albert Stern, had, unlike the Army, moved on in thought. Along with his colleagues on the not yet named design committee, he was proposing a much more modern design in the form of the 300G. That tank featured a high leading track to help climb, a longer hull to help cross obstacles, and firepower concentrated in the turret to better deliver its firepower.

Origins

Lt. Col. Sir Albert Stern, a man who was very much ‘made’ by his experiences in helping to shape armored warfare in WW1, was in a powerful position, with a title, wealth, experience, and contacts in government. He also foresaw what he thought was to come and, in June 1939, was asked to visit the Minister of Supply, Mr. Leslie Burgin, to discuss the issue of heavy tanks.

The outlook was dire. British tank development had, since the end of WW1, stood almost stationary. There were few tanks, and what there was a mixed bag of various types and quality, with the best armored of the bunch being the A.11 and A.12 Matildas. Both carried substantial armor for the time, 60 mm or more, enough to protect from most infantry weapons available short of artillery, yet both were under-armed and not long enough to perform the sort of assault role envisaged by some British military planners.

Despite the protestations of men such as General Sir Maurice Taylor, Senior Military Advisor to the Ministry of Supply, who was dead-set against heavy tanks, Stern had gathered supporters in the form of men such as Sir Maurice Grove Taylor and Major General Alexander Elliot Davidson, Director of Mechanisation at the Ministry of Supply, to his side.

If there was to be a new and heavy tank, these men decided it should be along the lines of what was already proven and with which they were familiar. Given the parlous state of interwar British tank design and the utter lack of a viable alternative, the outcome was as obvious as it was inevitable, the new heavy tank should be along the lines of the Mk.VIII tank of WW1. Whilst Stern and his team gathered together their expertise and came up with their idea in the form of 300G, the Army would busy itself with its own ideas and generate a list of specifications identified as RBM-17. Then, they made their own outline of a tank to meet their own specifications – the Citadel.

RBM-17

With the 300G in hand, Sir Albert Stern had an outline for what he and the members of his as yet unofficial committee felt would meet the sort of need they saw coming. This was not a full-scale reversion to the conditions of WW1, but an improved vehicle with more trench and obstacle crossing ability than existed before.

The philosophy of a new special tank was therefore already tacitly in place and set roughly even before war was declared on 3rd September. At that point, Britain was suddenly at war with a major and aggressive European power and had no heavy tanks at all. Although this initial idea that a new tank was needed was already in place, it is inextricable that it still took until 29th September for the results to travel all the way from the General Staff to the Adelphi Hotel, where rooms had been prepared for Stern and his team to work.

On 28th September 1939, however, when the Army brought with them their list of requirements for a new tank, it immediately meant that the 300G design, on which the team had been working, was redundant. The Army was absolutely insistent that the vehicle had to have certain features, including the firepower concentrated in sponsons and a large gun in the front to smash bunkers, two features impossible to accommodate into 300G. Further, they wanted a fundamental shift in design from a turreted machine back to an ‘all-round’ track machine, as this would facilitate heavy unditching equipment.

The specifications themselves clearly show exactly what the Army felt it needed in terms of a short-range special-purpose tank, but they also show the naivety on engineering matters and of tank design in general, especially as at one point, some felt this could be achieved for a vehicle under 40 tonnes in weight.

Criteria Set

Design

The tank to meet the requirements of RBM-17 was going to have to meet a set of criteria like no other tank had ever been asked to fulfill. This set, in September 1939, may well have seemed impossible to achieve to the General Staff. If one were to assume that they had deliberately set Sir Albert an impossible task to keep him busy and quiet, they were to find that even these extreme criteria were met and exceeded.

Requirement one, and the most important, was that it had to be able to cross a 16’ (4.9 m) wide trench and climb a parapet or other obstacle 7’ (2.1 m high). This was basically the widest anti-tank ditch and a high wall. Both of these obstacles were uncrossable by any British tank then in existence and the RBM-17 was to cross these without the aid of a fascine (a large bundle of sticks to fill in a ditch) or bridge.

Length Requirements

This first criterion, right from the start, guaranteed more than any other that the final size of the machine would have to be at least twice the length of the trench simply to avoid falling into it. A 16’ (4.9 m) wide trench, therefore, meant a tank 32’ (9.8 m) or so long. To climb a 7’ (2.1 m) step meant a very high track at the front in order to get purchase (grip) high on the wall or parapet. It is no surprise that these match the general ‘all-over’ track shape of the Mk.VIII tank.

Shape Requirements

The other reasons the RBM-17 was to follow the Mk.VIII’s shape were equally practical. The tank needed the maximum bearing surface on the ground, meaning the widest track possible, so it would not get stuck in soft ground. It also had to carry heavy unditching gear. In other words, it had to be able to get itself out of a hole or soft ground using a method like that used in WW1, a large spar of timber carried over the top of the tank, on which the tracks could get purchase to pull itself out. This ‘log extraction’ is still in use today and the carriage of an unditching beam or log is now most famously associated with Russian/Soviet tanks, which are often still seen with a log on the back. The method of use is identical in principle except that, in the RBM-17’s case, no crew would need to get out. In order for that spar to be carried over the tank by the tracks and underneath, it also determined that an ‘all-round’ track machine and one without a turret, which would get in the way of the spar, was needed.

Armor Requirements

The tank would have to be immune to both 37 mm and 47 mm anti-tank fire at 100 yards (91 m) and against the impact of a German 105 mm howitzer shell at normal impact. Given that the ‘rival’ A.20 was being considered around a 60 mm basis at this time due to a similar need to be immune to the 37 mm gun and yet could not meet the demand, the RBM-17 would have had to have not less than this thickness of armor at any point. More armor, of course, meant more weight.

The preeminent 37 mm anti-tank gun of the era was the German Pak 36, which could achieve around 64 mm of anti-armor performance at 100 m. For a 47 mm gun, weapons such as the French 47 mm SA 37 could deliver an anti-armor performance up to around 90 mm at just over 500 m and around 100 mm at 100 m.

A 105 mm shell, such as that from the German 10.5 cm leFH 18, was nearly 15 kg in weight with nearly 2 kg of explosives inside. Bearing in mind the often wafer-thin armor on the roof of tanks, being hit directly by such a shell would be devastating. Even a close ‘hit’ landing and bursting nearby was perfectly capable of crippling a vehicle, stripping off wheels or tracks or topping it over.

The Army was demanding immunity at 100 yards (just under 100 m), so clearly anything less than 80 mm of armor was going to be unacceptable, although the attention to protection from artillery would wane a little in emphasis as time went on.

Firepower Requirements

The gun was still not yet decided but had to be in the front and capable of defeating the heavy German bunkers (7’/2.1 m thick concrete) which were so worrisome to the General Staff. As such a gun would, by its very nature, be restricted to only a limited range of fire to the front, the tank would also need side armament to rake German positions as it passed them. Here, the General Staff wanted something simple, just a 2 pdr. and Besa machine gun combination in a sponson on each side. On top of this was to be a separate Besa pointing forwards and another to the rear. Eight smoke dischargers completed the required armament, as these would provide cover for the tank and infantry to follow.

All of this equipment and armament meant a crew complement of 7 to 8 men. The tank was to be powered by a diesel engine to reduce fire risk, fitted with a No.9 radio to speak with other tanks and troops, had to be able to go 50 miles (161 km) on its own and, on top of this, be able to be transportable by rail with little or no disassembly.

It must also be considered that Sir Albert and the soon-to-be-named ‘Old Gang’ clearly thought little of the ‘no turret’ and all-over track idea. Their first design was, in fact, far more similar in shape to the A.12 and A.20 than the Mk.VIII. When the requirements for the length of trench to be crossed were decided, the design grew longer, and when the turret was abandoned and all-round track selected by the General Staff, the Mk.VIII shape was inevitable. Those other TOG designs are known only by drawing number 300G in both a long and ‘compact’ form. Both were shelved in favor of the Mk.VIII approach, although the longer version would later be resurrected when a modicum of sanity returned to the General Staff.

In these early days, the selection of armament was a key consideration and a variety of armament and mounting options were considered across Sir Albert’s work and the A.20, including a 2 pdr./Besa 7.92 mm machine gun combination, as found in the turret of the A.12 Matilda, a 3” howitzer, a 3.7” howitzer, naval 6 pounder, and the French 75 mm gun, as used on the Char B1.

The 2 pdr./Besa option would only work if a turret was going to be selected for the tank, which meant a hull-mounted gun. With the 3” and 3.7” guns being low-velocity weapons, they were abandoned. This was because the work of Sir Albert had been given a very strict and very specific set of requirements, one of which would require a particularly powerful gun firing a high-velocity shell capable of breaching 7’ (2.1 m) of reinforced (ferro) concrete.

The requirements were specifically listed under the heading “Super-Heavy Tank (Land Battleship)” under the code ‘RBM-17’. The exact meaning of those code letters has never been adequately explained but, given that Sir Albert’s committee was already being labeled in a sort of British public-school humor kind of way as ‘The Old Gang’, it could be speculated that such a boyish kind of name was being thought of here for this ‘Really Big Machine’. The committee designing this vehicle would later (October 1939) receive a formal acknowledgment as the Special Vehicle Development Committee (S.V.D.C.), but they were equally happy using the ‘TOG’ term themselves as a badge of honor rather than as a mark of scorn, as has been happily assumed by some authors in the decades since the war.

The Citadel Design

Brigadier Kenchington from the War Office and Colonel Watson were the men who brought the RBM-17 specification ‘wish-list’ to the meeting with Sir Albert Stern. With them too was an interpretation of what this would look like for the committee to work on. It is not clear if the vehicle outline that they brought with them as a ‘Citadel’ tank was directly from these individual officers themselves or from the War Office or General Staff or a mix of the bunch, but the design was clear in realizing the needs of RBM-17.

Layout

The presented vehicle was a long, low, lozenge-shaped tank, roughly along the lines of the Mk.VIII, but with a large field gun mounted in the front of the hull with heavy unditching gear. It was drawn showing large round, presumably cast sponsons for the 2 pdr. / coaxial machine gun combination. One important note on this design is the issue of crew access. No doors are shown and, in correspondence over the next month or so, the only comment on this topic was on the removal of side doors behind the sponsons. This too was presumably similar in intent to the Mk.VIII, although the shape or style of such a door is unlikely to have been the same given the heavy armor requirement.

On top of the tank and projecting above the level of the tracks was a raised superstructure with a small cupola. This lookout allowed the commander to see where he was going and communicate to the driver in the front left. Whilst it may or may not have been rotatable, it was not an armed turret. The more notable issue on this raised section in the fighting chamber was that, just like the Mk.VIII, it would prevent the whole unditching beam over the top of the tank idea as well. Here, then, there seems to have been a disconnect in the minds of the military planners for the General Staff, who seemed to be confusing the earlier marks of British tanks, which used rails over a small raised structure for an unditching beam to travel over, with the rear-mounted beam on the Mk.VIII. Photographs of the Mk.VIII clearly show that the rails on its roof only extended over the rear section of the tail and thus that the beam would then not be able to be carried forward to help unditch the vehicle. The Mk.VIII therefore would only be able to deploy this beam backward to reverse out of a particularly boggy hole, whereas the early tanks, such as the Mk.IV, could deploy the beam forwards to get out of a hole forwards or in reverse.

Armor

No details on the armor for the Citadel idea were noted, other than the immunity requirement. Given that 37 mm and 47 mm anti-tank guns could respectively perforate between around 60 and 80 mm of armor at 100 yards (91 m), the requirements guaranteed armor not less than that already in use of the A.12 Matilda, with 3” (76 mm) of armor. Importantly, the immunity requirement did not specify that the armor had to be that thick per se, just that it needed to provide that level of protection. Whilst the vertical sides would need to be at least that thick, the front may not have needed to be, given the slope, but even so it would seem unlikely that the front, even sloped, would be thinner. The same is true of the sponsons, with their distinctive curved shape, projecting from the sides. Given the size of them, each would likely weigh roughly the same as the turret of the A.12 as well. It is not hard, therefore, to see why the desire to replace two of them with just one turret would finally win out later.

Armament

RBM-17 made it clear that the Army wanted a field gun in the front which could breach enemy heavy bunkers up to 7’ (2.1 m) thick and various options would be discussed with the S.V.D.C. as they tried to meet this demand. Of the options considered, there was little to choose from.

The biggest gun which could potentially be made to fit in the front was the venerable 60 pounder. The B.L. 60 pounder was over 30 years old and had seen extensive service in WW1, firing a 60 lb. (27 kg) shell containing 8 lbs. (3.6 kg) of high explosive at 650 m/s out to a range of 9 km. The gun itself was massive, employing a wheeled carriage and usually serviced by a crew of 10 men when used as a field gun. Even so, it produced a rate of fire of just 2 rounds per minute. The gun was also very long, with the barrel alone measuring nearly 5 m from breech to muzzle. This produced a problem for the front of the vehicle, as the barrel would potentially impale itself into an obstacle, such as the opposite face of a ditch when the vehicle was crossing it. Thus, the option of shortening the barrel was considered, even though this would reduce the muzzle velocity of the gun. However, as the long version firing HE could not defeat the 7’ (2.1 m) of concrete, the Army demanded that the gun would not be shortened.

Little discussion seems to have focussed on the two other huge problems of using such a gun in the front. Firstly, the fact that just one man was supposed to operate it and, no matter how much bully beef he might get, this would be an enormous task for one man who was also at some point supposed to use the front machine gun too. The second issue was how to mount such a heavy gun in the hull. Perhaps thankfully, this gun was discounted as a realistic option before any precious design resources were expended on trying to create a mounting that could take both the weight and the recoil.

The reality was that, in September 1939, there was no gun that could be mounted in the front which could achieve that 7’ (2.1 m) requirement. Whilst the demand for concrete destruction would be kept, it would end up as an ‘as much as possible’ requirement going forward through the end of 1939, rather than an absolute figure to be achieved.

Speed

The tank did not need to be fast in any way. There was simply no need. This vehicle would primarily be used for smashing enemy positions. Further, a slower vehicle emphasizing protection would resist the deleterious effect of enemy fire which it would attract, provide a more stable firing platform for firing back, and also clear a path for further tanks and troops to follow.

The low speed was also a reflection of reality. Whilst 5 mph (8 km/h) is certainly not by any means fast, it is surprisingly quick across the sort of terrible terrain which might be encountered in a Flanders-type shattered battlefield, with heavy mud and waterlogged ground. In fact, this speed would not only be faster than its forebears a generation earlier over such ground but also faster than any other tank in such conditions as well.

Crew

RBM-17 called for a commander, a driver, and a separate radio operator. Separating the radio operator from the commander, which was usually his dual job in a British tank, would at least take away one burden from him, but the job of commanding the vehicle was not going to be simplified much, as he would have to now control the crew operating the gun in the front hull and both sponsons. Two men, one for the 2 pdr. and one for the machine gun, would crew the sponsons on each side and just one man was supposed to operate the front hull gun on his own.

This herculean task for the front hull gunner/loader would certainly have been more than a little burdensome if a gun like the 60-pounder was adopted, having to haul the shells on his own, load them, aim the gun and fire, and then repeat. This would have been exhausting and slow work in the confines of the tank with all of the other activities going on, especially if it was moving at the time.

Power

The powerplant for this machine was not mentioned, described, or suggested. The fuel type was clearly spelled out as ‘diesel’. Although high speeds were not called for in the design brief, there was still going to be an issue over the availability of high-power diesel engines and how to transmit that power from the engine/s to the tracks. An eventual solution would be found to meet the need for power from a diesel thanks to Harry Ricardo, the engine designer on the S.V.D.C.

However, in September 1939, it was not that clear cut and engine options were severely limited by not only the power output needed, but also by the fuel type, as few diesel engines were available which could deliver the power of more than 500 hp which would be required and options to be considered included more than one engine, various domestic and foreign engines, and different types of transmissions to maintain efficiency.

Conclusion

The Army’s design for the Citadel was odd, harkening back to the worst days of the slaughter of WW1 and no doubt that conflict served up generous helpings of concerns of a repeat of it. Seemingly in haste, the Army had leapt on Stern’s idea that, quite rightly, the Army needed a new special tank to prevent that type of warfare from taking place. Equally, the high command appeared to be panicking. This rush to get ‘something’ is seemingly made clear by the disconnect over the general outline of a tank. The Army were insistent on a turretless tank, so as not to interfere with heavy unditching gear, yet this very requirement was gone even by the end of WW1 with the change from all-over rails to just rails at the back for the unditching beam. Indeed, it is unclear how the High Command seemingly lacked knowledge on the topic, as even in WW1, vehicles had gone away from this type of complete top rail, as seen on the Mk.VIII, Medium Mk.B Whippet, and Medium Mk.C Hornet. Why the Army seemed so insistent on no turret because it would interfere with this equipment makes no sense when a turret would make no more interference than the raised casemate. This, perhaps more than anything else, shows that the Army was rushing to get ‘something’ to fill a need rather than relying on experts like Stern’s committee to develop a new vehicle.

The proposed design had too many crewmen and was too hard to control. Reducing the crew meant fewer men would be needed, more space in the fighting chamber for air to circulate, more space to move around, and more space for storage of ammunition, etcetera. Fewer men could be achieved by the adoption of a turret which would concentrate the firepower equivalent to both sponsons in one place as, afterall, both sponsons could not fire on the same target at the same time in anything other than the very limited circumstances of the target being directly ahead of the tank a distance away.

Removing sponsons would not only eliminate the need for so many crew and improve the interior volume, but would also remove two other significant burdens. One was the problem of transshipment, as no sponsons, or just small machine gun sponsons were far easier to move, fold in, or remove than these huge sponsons demanded. Secondly, their removal would save a lot of unnecessary weight from men, armor, and guns.

Adopting a turret would become the logical conclusion as the S.V.D.C. got to work on the Army’s idea, as saving crew and weight, and improving the distribution of firepower issues altered the general shape of the eventual vehicle into the ‘TOG-1’. Even with a turret, it was not the vehicle that the S.V.D.C. would design to meet the needs of RMB-17. That vehicle would have to wait, as the committee formed under Stern got to work in October 1939.

The size of the machine was inevitably going to be big given the size of the trench that needed to be crossed and the same is true of the front contact, with a 7’ step requiring a high front end. The Army’s demand for an all-road track likewise demanded a machine shaped like the Mk.VIII.

What the struggle to find a suitable powerplant would show was just how unrealistic RBM-17 was as a demand. At one point, the Army’s goal was to make this monster of a tank under 40 tonnes, a completely ludicrous idea that any engineer or designer would have laughed at when the Army was literally demanding a gallon in a pint pot.

In the end, the Army would not be able to get what they wanted. The requirements, although they could be met, could also be improved upon. The S.V.D.C. under Sir Albert Stern would develop a vehicle along the lines wanted by the Army and eventually persuade them of the value of a turret over large sponsons, and that vehicle would be TOG-1. The performance of TOG-1 would also exceed the Army’s requirements for mobility and obstacle crossing and, in fact, exceed their extreme requirements for a vehicle for all but the ability to smash a 7’ thick reinforced concrete structure. That requirement would be practically impossible at the time anyway, regardless of what design they might have come up with, and would remain out of reach for a couple of years until the advent of the 17 pdr.

The Citadel, however, was a starting point for the SVDC, as limited and relatively crude as it was. With a team of experts and mandate for work, the restrictions of RBM-17 would fade a little as the war developed, but the special tank concept would continue and the Citadel became just a footnote in the history of the committee.

Specifications TOG Citadel
Dimensions	>32’ (9.75 m) long
Weight	>40 tons
Crew	7 – 8 (commander, driver, wireless operator, 4 – 5 gunners)
Speed (road)	5 mph required (min. off road of very bad ground)
Range	50 miles (80 km)
Fuel	Diesel
Primary Armament	heavy field gun such as 60 pdr. or shortened 60 pdr.
Sponson Armament	2 pounder gun and 7.92 mm BESA MG on each side
Other Armament	front and rear 7.92 mm BESA MGs
Armor	Sufficient to protect against 37 and 47 mm anti-tank guns and 105 mm howitzer
For information about abbreviations check the Lexical Index

Sources

Hills, A. (2017). The Tanks of TOG. FWD Publishing, USA.