Tracklink Article - The A33 Excelsior “Assault Tank”

(This article was originally published in Issue 108 (Spring 2021) of Tracklink, the magazine of the Tank Museum.  The pictures originally used to illustrate it belong to the Tank Museum, so I have replaced them with public domain examples here)

British tank doctrine in the run-up to World War Two divided tanks into “Infantry Tanks” (slow but heavily armoured designs intended to support the infantry) and “Cruisers” (faster but more lightly armoured designs intended to act in a cavalry role).

By late 1942, there were serious concerns about the main Infantry Tank in British service, the A22 Churchill. This had been designed in great haste after the army lost most of its vehicles at Dunkirk in 1940, and the need for armoured vehicles to resist a potential German invasion was so urgent that it had been rushed into production “off the drawing board” without prototypes being built and tested.

Unsurprisingly, this led to severe teething troubles with the suspension and (especially) the engine. The manufacturer even included a note in the handbook frankly acknowledging these problems, but explaining they were a consequence of the vehicle being rushed into service and would be resolved at some future point.

Concerns about the Churchill worsened after the disastrous Dieppe raid of August 1942. In fact, this was unfair as though some Churchills landed were unable to cross the beach, the major problem was that engineer teams had not been able to breach the sea wall, which would have stopped any tank.

These factors combined to produce a lack of confidence in the Churchill in some quarters, and started a search for potential alternatives. There was no other viable infantry tank design in prospect, but while British Cruiser tank design in the early war years could be charitably described as “troubled”, the new A27M Cromwell cruiser was well regarded. It carried the same 75mm gun as the Churchill, and more importantly it had a powerful and reliable engine in the form of the Meteor, an adaptation of the famous Merlin aero engine.

This delivered 600 HP, and gave the Cromwell an excellent power-to-weight ratio and high top speed. The thinking was that if some speed could be sacrificed, the design could be up-armoured to infantry tank standards, gaining not only a more reliable replacement for Churchill but also the advantages of standardising spares and training on a “universal” tank chassis.

Rolls-Royce produced two upgrade proposals. The 32 ton A31 was essentially a base Cromwell with as much extra armour as could be carried on the standard Christie suspension with 5 road wheels per side, while the 35 ton A32 was a more thorough re-design with a stronger suspension and wider tracks (19in rather than 14in), allowing it to carry the same armour as contemporary marks of Churchill. However, Rolls-Royce' production capacity was fully occupied building Merlin engines, and would remain so for the foreseeable future.

Meanwhile, English Electric had been producing Cromwells (initially the Liberty-Engined A27L variant later designated as Centaur, then standard Cromwells) and proposed their own design.

This became the 40 ton A33, and two pilot models were built in 1943. The first used the Cromwell turret and hull with extra armour, combined with the suspension and tracks of the US M6 Heavy Tank. This was a horizontal volute spring system similar to that on the M4 Sherman series but reinforced and with much wider tracks.

The M6 design had four dual bogies (8 road wheels) per side to deal with its 50+ ton weight, but the lighter A33 required only three dual bogies (6 road wheels) per side. Like the M6, the suspension was protected by an armoured side skirt.

(The M6 had been intended as a heavy counterpart to the M4 Sherman, with thicker armour and the 76mm gun used on the M10 tank destroyer. It had been intended for use by both Britain and the US, but by the time it was ready for production the US Army had lost interest, as its advantages were offset by its weight, high silhouette and awkward layout, and the vehicle was cancelled in late 1943)

The second A33 prototype used a widened version of the Cromwell track and a reinforced helical spring suspension designed by the LMS railway company, who were also producing the Cromwell at that point. This prototype also had its suspension protected by side skirts, though of a different design.

A third prototype had been planned, using a lighter version of the LMS suspension, but was never built, since the similar suspension on the second prototype proved expensive and complex to produce and suffered reliability problems in use. As a result, the US suspension of the first prototype was preferred.

All the A33 prototypes were based around the standard British five man crew set up – driver and assistant driver / bow machine gunner in the hull, and commander, gunner and loader/operator in the turret. All were fitted with smaller versions of the side escape hatches fitted to the Churchill, increasing the chance of the crew being able to safely abandon a crippled vehicle under fire.

The files indicate the first prototype was to be armed armed with a 6 pdr (57mm) gun, though in reality both seemed to have been armed with the Ordnance QF 75mm. However, since this was essentially a 6 pdr bored out to fire US 75mm ammunition, the two weapons were interchangeable via conversion kits, as was done with Churchills in the run-up to D-day. Both weapons had particular advantages - while the larger HE round of the 75mm made it a better general purpose weapon, the 6 pdr had superior armour piercing performance due to its higher velocity.

Two 7.92mm BESAs were mounted as coaxial and hull machine guns, respectively. The usual Bren anti-aircraft machine gun which formed part of the equipment of most British tanks was replaced by a pair of Vickers K machine guns on a PLM mount on the turret roof. These had originally been developed as defensive guns for aircraft, and each had twice the rate of fire of the Bren. It is quite possible that the unusual AA fit was a test for installing it more generally, but in reality tank commanders preferred to close up under air attack rather than to return fire, and in any event allied air superiority soon made the question moot.

Ammunition stowage was 80rds x 75mm, 5,000rds 7.92mm for the BESAs and 2,000rds of .303 for the twin Vickers K, plus a 2in smoke mortar with 30 bombs.

Testing of the first prototype, with the preferred US style suspension, began in November 1943. It successfully completed two 1,000 mile cross country runs and though suffering a variety of minor issues, it proved surprisingly reliable for a prototype. Top speed was just under 25 mph, significantly better than the Churchill. Although it did not handle very muddy conditions as well as the Churchill, much of this was attributed to the US tracks, and it was thought that redesigning them with deeper “spuds” would fix this. After 799 miles, the reports noted the prototype had picked up more than two tons of mud during the tests, but suffered no apparent problems from this extra weight

However, by this point the reliability problems with the Churchill had largely been resolved, and it had proved itself in combat in Tunisia and Italy. With the requirement to replace the Churchill gone, the case for producing the A33 disappeared and the project was cancelled. The first prototype was worn out by the testing and was scrapped, but the second survives in the Vehicle Conservation Centre at the Tank Museum.

The files refer to the vehicles interchangeably as the “A33 heavy tank”, “A33 assault tank” and even the “A33 heavy assault tank”. Exactly why this terminology was used rather than the more familiar “Infantry Tank” or how its role might differ was never explained. It may be no more than a “flavour of the month” expression, given the UK and US were then developing their joint T14 assault tank This was essentially a similar project to create a more heavily armoured vehicle based on Sherman components, which also failed to enter production.

Interestingly, though the project documentation refers to the A33 as the “Commodore” for a brief period in late 1943, the name “Excelsior” doesn't appear in any of the original documents, and it does not follow the standard naming conventions for British tanks. It is thus tempting to think – though by no means certain – that it is a post-war coinage.

Interest in upgrading the Cromwell did not die with the A33 project, of course. Indeed, not one but four Cromwell derived vehicles served with the British Army. The first was the A30 Challenger, which shoe-horned the very effective 17 pdr anti-tank gun into a stretched Cromwell hull. This involved adding an extra road wheel station, deleting the hull machine gunner to make space for the larger ammunition and fitting a large but thinly armoured turret accommodating the commander, gunner and two loaders.

The Challenger entered service in July 1944, with one Challenger incorporated into each four-tank troops of Cromwells. These provided Cromwell regiments with an equivalent to the better known Sherman Firefly, which could not keep up with the faster Cromwells.

The Challenger spawned a variant itself, in the form of the A30 Avenger, essentially the same vehicle but with a much lower open topped turret. Neither was regarded as particularly successful, and they were produced only in small numbers as a stopgap.

The Cromwell itself was developed into the A34 Comet, effectively an improved “Super Cromwell” with an all new turret mounting the 77mm gun - essentially a shortened and slightly less powerful version of the 17 pdr. The Comet appeared in December 1944, to replace both Cromwell and Challenger. Only a small number saw service in the last days of WW2, but it remained in British service until 1958.

Finally, in the 1950s a number of later-model Cromwells were fitted with a new lightly armoured two man turret mounting the 20 pdr (84mm) gun used on early marks of the Centurion, to create the FV 4101 Charioteer. The idea was to create stop-gap vehicles capable of dealing with Soviet tanks until production of Centurions reached adequate numbers. It suffered from low ammunition capacity (only 25 rds even after replacing the hull gunner's position with additional ammunition stowage) and from lack of space for a commander in the turret; leading to a crew member being carried externally.

                                                        The first A33 prototype

The second A33 prototype, which still survives at the Tank Museum - note the different suspension)