Australian Military Vehicles Research

Australian Sentinel Tank by Michael Koudstaal (updated 2020)

The AC1 Sentinel at the RAAC Tank Museum.

Australian Cruiser Tank Mk I "Sentinel"
With the rapid advance of Germany in Europe placing Britain under pressure and the possibility of Japan going to war to seize territory in the Pacific it became clear that Australia could not rely on supplies from elsewhere and would have to manufacture most of its own military equipment, including its own tanks.

The rebuilt AC1 Sentinel at the Melbourne Tank Museum. Note this vehicle is in fact fitted with a 37mm barrel and not the 2 pounder it should carry.

The Australian Cruiser Tank Mark I (or AC1) began as a General Staff specification issued in November 1940. The Army needed a tank capable of a top speed of 56km/h (35 miles per hour) with a range of 240 km (150 miles), armaments consisting of a 2 pounder tank gun and a Vickers machine gun, a crew of 4, and armoured to provide protection equivalent to 50mm (2 inch) of armour plate. Following the British practice 1 in 8 were to be armed with a weapon capable of firing smoke rounds and if possible high-explosive, this weapon to be fitted in place of the 2 pounder. A second machine gun and gunner to operate it were to be included in the hull if practical.

The Directorate of Armoured Fighting Vehicle Production was established within the Ministry of Munitions. Abbreviated as DAFVP, the Directorate would oversee the design, and production, of carriers for machine guns, antitank guns, and mortars, as well as wheeled vehicles such as scout and armoured cars, and most important of all tanks, as well as associated equipment such as recovery tanks and tank transporters.

Initially a locally produced tank based on the United States M3 medium tank was considered as a possible solution. This tank would have the lower hull, suspension, engine, and transmission of the M3 combined with a new upper hull and cast turret. This would have resulted in a tank broadly similar to the Canadian "Ram" which was also based on the M3 design. However this raised several problems not least of which was that the M3s engine was not available in Australia, and all machinery capable of producing rolled armour plate of the required thickness were already occupied with other projects.

Realising that the M3 design could not be produced within Australia design work then began on what would become the AC1. This design was based around what the available industry in Australia could produce while maintaining where possible as much in common as possible with other allied tanks. The power unit of the M3 was replaced by three petrol V8 Cadillac automobile engines coupled together. The M3s synchromesh gear box was one of the few original components retained in the new design and it proved impossible for Australian industry to produce.

Deciding to try and avoid such apparently insurmountable problems, during May 1941 the AC1 design was suspended and work began on a simpler, lighter design largely based on imported parts and assemblies, including power and transmission units designed for heavy trucks. Given the title Australian Cruiser Tank Mark II (or AC2) this design was not liked by the Army who considered it to be too light for most purposes, but accepted it on the basis of rapid production, and that it would be better than nothing. The AC2 design quickly ran into problems with the very thing that had made it an attractive option in the first place, the imported components. These were continuously delayed. As the design emerged the projected weight was calculated and found to be well above the maximum limit of the transmission units. The only way to reduce the weight was by removing armour, which was totally unacceptable and so in September 1941 the AC1 design was revived and the AC2 was abandoned. Work on the M3 gearbox and final drive that continued during the AC2 project had by this time arrived at a design that did away with roller bearings where possible, used straight cut teeth throughout, and did away with the synchronising mechanisms reverting to a "crash" type instead. The gear ratios were altered to accept the Cadillac engine's output shaft speed of 3600 rpm versus that of the M3 Medium's 2400. Such a unit was within the capacity of the machinery then in Australia and coupled with the imported engines formed the complete drive train for the tank.

The armour of the AC1 provided no less of a challenge, not being able to use rolled armour left only one option, and that was cast armour. Taking advantage of the prior work done on rolled bullet proof steel plate developed for local pattern carriers and other light armoured vehicles, a new castable armour steel was developed. Initially the design called for multiple castings bolted together to form the hull, but the firm of Bradford Kendall Ltd. who were responsible for the main armour castings believed that it should be possible to cast the entire hull in one piece. This would make the hull seamless, improve rigidity and eliminate the need to machine many surfaces, simplifying production and saving both time and manpower. Seven gas fired furnaces were constructed to heat treat the armour sets, the hull, turret, axle housing, and other armour components need to make a complete tank. Five of these furnaces were for high temperature work, the normalising, annealing, and hardening, and two for low temperature work to be used for tempering. Production would eventually reach 5 tank sets per week. The steel was be poured into sand moulds and allowed to freeze, then straight from the mould, the still hot casting would be placed in one of the furnaces to be normalised, at this point the risers, gates, and vents could be cut off and any defects welded up, the casting would then be hardened and tempered before being cleaned and sent to be machined. The steel used was found to be susceptible to temper brittleness, that is as the casting was allowed to cool slowly from tempering and passed through a particular temperature range alloying elements and contaminants migrated to the grain boundaries in the steel and weakened it, the solution was to simply quench the castings from final tempering. The amour castings for the first 13 tanks, serial numbers 8001 to 8013, were produced prior to this change in the process and so were classed as unarmoured, making them suitable for training only.

The AC1's middle HVS suspension bogie unit and emergency escape hatch.

The suspension system was originally identical to that of the M3 medium however as the M3 suspension units were regarded as unnecessarily complicated they were changed to a suitably modified version of the suspension units designed for the AC2, a horizontal volute spring similar to the Hotchkiss type suspension. The AC1 road wheels were of a plain dished design for the E series, with four, six, or eight holes drilled in the side for production vehicles, with the most common pattern having ten holes, four in the outer face and six in in the inner face. They were otherwise the same dimensions as the M3 as were the drive sprocket, tensioning idler and track return rollers, allowing the use of any track type that could be fitted to the M3 and most Sentinels were fitted with imported American T41 tracks. These tracks were a dual pin rubber bushed design which had a pitch of 152mm (6 inches) and were driven by a 13 tooth drive sprocket. A series of all-steel track Mark V through to Mark XI, was designed for the tanks, these were all cast Manganese Steel links and had a 102mm (4 inch) pitch, were 40.6cm wide (16 inches) and used a 20 tooth drive sprocket. The pins were retained by lead plugs, the same method as used for the Local Pattern Carrier tracks. The track links all had two outer guide horns so as to be interchangeable with M3 tracks. The end result was similar in appearance to CDP (Canadian Dry Pin) tracks and offered better traction and control than the rubber tracks.

By 1942 the General Staff, still with an expectation of fighting German tanks, had changed the specification increasing the armour basis to 65mm (2 inch). Three pilot models the "E series" were assembled to the revised specification. The first "E1" was ready in January 1942 for automotive tests, the second "E2" for gunnery trials, and the third "E3" as a pilot model for mass production, with E3 seemingly counted as AC1 8001. The automotive model performed well achieving a top speed of 64 km/h (40 mph) on roads and 52 km/h (32 mph) cross country.

By August 1942 the first production AC1s were completed and given the name "Sentinel". The hull front varied in thickness from 65mm (2 inch) to 45mm (1 inch) depending on the angle. The sides and rear specified as 45mm (1 inch) thick with an armoured overhang covering air vents that ran along the top sides of the tank. An oval escape hatch was placed between the first and second bogies on both sides. The one piece axle housing and final drive cover is bolted onto the front of the hull. Armoured to a maximum thickness of 65mm (2 inch) thinning to around 50mm (2 inch) as it curved away from the vertical. The turret armour was 65mm (2 inch) thick thinning to around 50mm (2 inch) on the sloped parts and around 35mm on the top. Like most countries tolerances were made for the armour castings to be thicker than specified in the design. For ballistic traceability on every major cast component in raised lettering is the manufacturer code, part type and the number of the casting. The clearest example is found on the front of the axle housing. An example would be BK AH 31, indicating it was cast by the firm of Bradford Kendall Ltd. and is Axle Housing number 31. The numbers on the rear of the hull were ground off to allow the spare track length to sit flush on the rear of the hull, and on the AC1 the turret number is usually concealed by the turret bin. The only rolled plates used in the tank were the 16mm (5/8 inch) floor plates and the 23mm (7/8 inch) engine cover plate.

The three Cadillac engines were placed two abreast with the third located behind and between the others. Each engine was connected via a clutch to a three-in one-out transfer box mounted under the turret. The output shaft ran though the main clutch to the gearbox and the Cletrac type controlled differential similar to that used in the US M3/M4 medium tanks. This unusual arrangement allowed the AC1 to be driven on only two or even just one of its engines, which could be started independently or simultaneously. Using 70 Octane fuel the Cadillacs were limited to a 6.2:1 compression ratio and the combined output of the engines was around 276kW (370 hp) gross, and 246 kW (330 hp) net. The radiators and fuel tanks were fitted around and behind the centrally mounted rear engine. Internal fuel tank capacity was around 590 litres (130 imperial gallons), with an jettisonable external fuel tank capacity of around 200 litres (44 gallons), a valve allowed fuel for the engines to be drawn from either, and the external tank could be released from inside the tank. Maximum speed was limited to around 48 km/h (30 mph), average road speed 45 km/h (28 mph), average cross country speed 34 km/h (21 mph).

The steering system operated in the same manner as that of the M3 Medium, by engaging one of the brakes the differential could be forced to turn the sprockets at a fixed ratio with respect to each other and engaging both brakes would slow or stop the tank completely. A power assist system was added to the steering controls fed by a compressed air reservoir. Pulling either of the steering levers back first allowed air into a chamber containing a diaphragm, pressure acting on the diaphragm engaged the brake it was attached to, further pulling back the steering lever would manually close the brake. An air compressor, driven by the transfer case, recharged the air reservoir to 414kPa (60psi) when its pressure dropped below 276kPa (40psi), a safety valve prevented the reservoir being pressurised above 483kPa (70psi).

The turret could be traversed either by hand, or power traversed by a 40 volt motor at up to 20 degrees per second in either direction. The hand traverse was a simple hand crank permanently geared to the turret traverse gear box, where drive from the crank was combined through a worm driven differential with the drive from the turret traverse motor in such a manner that while any motion of the crank would turn the turret, no movement of the turret or power traverse would rotate the crank. Power traverse could be engaged by pulling a lever on the manual traverse crank, this closed a clutch so that any twist of the crank handle would now be transferred via a roller chain and sprockets to the turret traverse controller. Simplified, the turret traverse controller consisted of a 650 rpm 40 volt motor driving an eccentric contact brush, as the traverse controller input shaft was rotated the circle described by the eccentric brush would pass for an increasing percentage of the time over a contact plate resulting in variable width pulses of 40 volts being sent to the turret traverse motor, and therefore both the turret direction of rotation and speed could be effectively controlled. The second means of controlling the power traverse was a hand wheel directly mounted to the input shaft of the traverse controller, this bypassed the roller chain linkage and was intended to allowed finer control of the system to allow smooth tracking of moving targets. Electrical current was supplied to the traverse system by a generator attached to the three in one out transfer case. Operating any of the engines immediately provided power to the turret traverse. As the generator would be provided with a variable input shaft speed two voltage regulators reduced the generator's field current as the output voltage increased past 40, this effectively limited the output of the generator to 40 volts over the full operational range of 1250 to 4000 rpm. An interlock switch on the turret lock prevented the operation of the power traverse while the turret lock was engaged.

Stowage was provided for 130 rounds of 2 pounder ammunition, the bulk of this in four racks around the turret basket bolted to the sides of the hull above the battery boxes, a 25 round ready rack was located in the right rear of the turret by the loader's left shoulder, and additional rounds stored in an ammunition box that formed part of the loader's seat. Like British tanks of the era only AP shot was to be carried although high explosive and case shot were a possibility as used by Australian Matilda tanks and 2 pounder anti-tank guns.

The driver sat on the right hand side and the hull machine gunner on the left separated from the driver by the water cooled Vickers machine gun and the gearbox. There were no sighting arrangements for the hull machine gun other than the gunner's periscope and the gunner was expected to direct fire by means of tracer. The loader/radio operator was in the right hand side of the turret, the radio, a Wireless Set No. 19, was located in the turret bustle and provided both radio for external communications and intercom for the tank crew. The 2 pounder was centrally mounted with the water cooled Vickers machine gun on the right and the gunner to the left. Elevation was, like most British tanks armed with 2 pounders, manually controlled directly by a shoulder brace. The tank commander's station was located behind the gunner and equipped with a fully rotating cupola with two periscopes, and a Bren LMG for anti-aircraft fire.

The AC1, like all new tanks, was not without problems. The original liquid cooling system was found to be inadequate and to allow an unequal flow to the three engines, resulting in overheating to the point of cracking some engine blocks and seizing others. The tyre bonding was poor and heat build up caused tyre delamination at low mileage. Testing revealed problems with the turret traverse system, this however was found to be the combined result of being operated on an incline, the turret being unbalanced, spacers rubbing against the moving parts, and the use of a 32 volt system in place of the 40 volt production equipment. All problems were rectifiable by various means. The tyres and road wheels were interchangeable with M3 tyres and all steel road wheels were trialled as another solution. A redesigned cooling system and new fan solved the cooling problems on one tank. But the programme was cancelled before these could be fitted to all AC1 tanks.

The Sentinels were painted with a two tone camouflage scheme consisting of a base colour of Australian Standard Camouflage (ASC) J Khaki Green with a disruptive pattern of ASC W Light Earth. The only markings carried was the serial number in white on both sides of the drivers compartment and on the back of the hull on the right hand side under of the rear deck overhang. A second three colour scheme consisting of Vehicle Dark Green, Vehicle Medium Green, and Vehicle Grey was also drawn up but not used. The only AC3 was finished in the two tone Khaki Green, Light Earth camouflage scheme, but only carried its number only on the left hand side of the drivers compartment and on the centre of the rear engine deck overhang. Internally all the tanks were painted white.

Even before the first AC1 left the production line it was recognised that the 2 pounder was inadequate as a tanks main armament. Work on fitting a 25 pounder in to the Australian tank had been underway for some time and when it was successfully tested in June of 1942 the Army requested that the 25 pounder, be fitted to the production vehicles, but as a number of 2 pounder mounts had already been manufactured any changes in design at that time would entail a delay of several months. It was decided to limit the production of the AC1 to 65 vehicles and change over to a new tank design with a heavier main armament as soon as possible. Drawings were prepared for conversion of 2 pounder armed AC Mk1 tanks to either a 6 pounder armed AC Mk1A, or a 25 pounder armed AC Mk1B. It was also suggested that to improve the fire power of the 2 pounder in the AC1 tanks, the gun could simply be fitted with a Littlejohn adaptor.

No AC1 ever saw combat, although with a few superficial modifications some were used as "German Panzers" in the 1944 film "The Rats of Tobruk".

Australian Cruiser Tank Mk III "Scorpion"
Recognising the Clover-leaf Cadillac's limitations substitutes were considered for tanks produced beyond the 65th vehicle from the production line. This new design took the form of an AC1 with the only major change being the tank would be powered by a single row Wasp radial, the Pratt and Whitney R-1340-AN-1, suitably modified for tank use and given the name "Scorpion", a name which also came to be applied to the tank as a whole. This would be a naturally aspirated engine fitted with an overdrive unit with a 3:2 gearing ratio. As the AC1 gear box was geared to accept an input shaft speed of 3600rpm it could not be used with an engine capable of only 2400rpm without incurring a reduction in speed, and with the limited gear cutting capacity in Australia it was not desirable to produce a second type of gear box. The overdrive unit solved this problem and also allowed the drive shaft height to be lowered to pass under the turret basket while keeping the engine upright. Originally the tank engines were to be produced in Australia by the Commonwealth Aircraft Corporation with the loss of aircraft engines to be made up through importation from the US, it proved more practical to simply import the required engines for the tanks from the US. Orders for a total of 1000 engines were placed and 573 were delivered prior to a decision being made not to proceed with the design. 400 Scorpion tanks were ordered, but it was overtaken by circumstances including the need for more firepower than the 2 pounder could deliver, and undesirable torque characteristics of the Scorpion engine as compared to the Cadillacs.

Australian Cruiser Tank Mk III "Thunderbolt"
The Australian Cruiser Tank mark III (or AC3) was a redesign and greatly improved over the AC1. Heavy tanks being fielded by Germany at his time were immune to all but the luckiest of shots from the 2 pounder anti-tank gun. The AC1 from the beginning was designed to accommodate the larger 6 pounder anti-tank gun, however while an order for 6 pounder tank guns was placed it had somehow been overlooked and not acted on, by the time this fact was discovered the resulting wait for the guns to be produced would have meant an unacceptable delay in production and so it was decided to skip over the 6 pounder and go directly to the 25 pounder.

A variety of alternative armaments had been considered to increase the fire power of the Australian cruisers, including the 6 pounder anti-tank gun, the obsolete 18 pounder field gun, the 25 pounder field gun/howitzer, the 3 inch 20 cwt anti-aircraft gun, the 3.7 inch anti-aircraft gun, and the 17 pounder anti-tank gun. The 87.6mm (3.45 inch) 25 pounder was selected for the main armament, as it had excellent high explosive capability while maintaining adequate armour penetration performance. To test whether the Australian Cruiser could cope with the recoil one of the development vehicles, the E2, was fitted first with a 25 pounder field gun/howitzer, then with a 25 pounder tank gun in a new turret equipped with a redesigned overhead short recoil system. The tests were successful and a decision was made to mount this weapon on a production basis.

Other major changes included the removal of the hull machine gun and gunner to allow for storage of the larger ammunition, the front hull being changed from the AC1s quite complex front to a smooth flat plate approximately 50mm (2 inch) thick sloped back to 24 degrees from horizontal. The axle housing was reshaped to present a sharper front profile and the top raised the meet the slope of the hull front. The top of the armoured cowl that runs along the sides and back of the hull for air to be drawn in and exhaust to be expelled was raised so the turret ring was protected from all sides. The engine access hatches were changed for the Perrier-Cadillac and the cast tail of the AC1 rear engine deck overhang was made integral to the hull casting. The AC3 had the same provision for an external fuel tank as the AC1.

The 25 pounder was mounted in the turret with a coaxial .303 Vickers water cooled machine gun. The AC3 was of similar dimensions to the AC1 and used the same 137cm (54 inch) turret ring, this in particular meant that the AC3 turret was somewhat cramped particularly for the loader who had to be careful to keep out of the recoil path of the gun. To swing the larger turret the 40 volt electrical traverse system of the AC1 was replaced by a 110 volt equivalent. Stowage was provided for 120 rounds of ammunition, 60 High Explosive or Smoke shells and 60 of the shorter 9kg (20 lb) Armour Piercing shot, and 2500 rounds for the coaxial Vickers, stored in 250 round boxes. As the 25 pounder used 2 part ammunition 120 cartridges were to be carried consisting of charge 3 and charge Super. Despite the two part ammunition and the cramped turret the rate of fire showed that the tank was far from unusable, in tests with the commander assisting the loader by ramming the projectile the crew were able to fire 8 rounds in 73 seconds. The tank mounting of the 25 pounder proved to be excellent, unlike the towed version multiple rounds could be fired without having to relay the gun on the target.

The "Perrier-Cadillac" power unit was completely new, it was a design that had been started in parallel with the Clover-leaf power pack but as a longer term development. This engine used three of the same type V8 Cadillac motors as the AC1 but instead of the two abreast and one behind Clover-leaf layout, the three engines were mounted around a common crank case to form a single 24 cylinder unit. The bell-housing, clutch, and flywheel were removed and a crankshaft extension and gear fitted to each engine. Geared permanently to a central gear and output shaft the three engines were timed at assembly to give the smoothest possible firing order. As the individual starter motors had been removed, a heavy duty aircraft stater motor was installed on the output shaft. This design saved space and eliminated the three in one out transfer box, resulting in reduced losses through the drive train. The use of 80 octane fuel allowed the compression ratio to be increased to 6.75:1 boosting the output of the power unit to 296kW (397hp) gross. This engine had been tested in the E1 and found to be entirely satisfactory. The transmission was largely the same as that of the AC1, and the steering and braking system was also carried over with one additional feature, the brake's compressed air power assistance system would be extended to also assist the operation of the clutch.

Only one AC3 was fully assembled for tests before the programme was shutdown, serial number 8066. During May '43 the hulls of serial number 8066 and one other Mk.3 had completed road trials without their turrets. Work had begun on the assembly of 25 others and various parts and assemblies were being produce in large numbers, including around 150 cast hulls and turrets. Overall the AC3 was shorter, faster, more heavily armoured with the exception of the turret front, and carried a larger gun than the basic M4 Sherman.

Australian Cruiser Tank Mk IV
The AC4 design was a further refinement of the AC3, with a 178cm (70 inch) turret ring providing more room for gun and crew. The General Staff specification called for armament of a 17 pounder anti-tank gun and coaxial Vickers machine gun with elevation of 20 degrees and depression of 10 degrees. Stowage for 50-60 rounds of 17 pounder ammunition comprised of a mix of AP and HE and 2500-4000 rounds of machine gun ammunition. The 17 pounder gun was then in local production along with Armour Piercing Capped (APC) ammunition. The armour basis was still the same as the AC1 and AC3 at a maximum of 65mm.

The weight of the tank had increased to around 32 tonnes but nominal ground pressure was lower than the AC3 due to increased track contact area. Top speed was expected to be 56km/h (35mph) as more power was available from the Perrier-Cadillac. The reintroduction of synchromesh gears was requested if it had not already been included in the AC3. The Perrier-Cadillac was to have been used but a power plant of four Gipsy Major air-cooled inline aircraft engines were considered as an alternative if further Cadillac engines could not be obtained from the US. The four engines were turned on their side and placed in two layers with their crankshafts geared together in the same manner as the Perrier-Cadillac. The combined output of the Quad Gipsy multibank engine was estimated at 375kW (510 hp).

The hull design was broadly similar to the AC3s with the driver's door moved to the side of the hull, where on the AC1 and AC3 the hull roof dropped behind the driving compartment the AC4 hull roof remained level. To make room for the larger turret ring the engine bulkhead was shifted to the rear slightly lengthening the hull, and the air intake located across the top of the hull behind the turret was removed and the air vents on the sides and rear of the fighting and engine compartments enlarged to compensate. The lengthen hull resulted in the suspension bogies being spaced slightly further apart.

Consideration was given to using US turret traverse equipment and M4 suspension units instead of the AC1 and AC3 HVSS units in order to move to a more standardised tank.

By this time British experience had shown that tanks faced two major threats on the battlefield, Anti-tank guns mounted in tanks, and the more numerous towed Anti-tank guns. To combat these threats tanks needed either a powerful anti-tank gun or a powerful high explosive weapon respectively. As a result two interchangeable turret fronts were designed to enable the AC4 to mount either the 17 pounder anti-tank gun, or for close support, a 25 pounder tank gun as fitted to the AC3. The 25 pounder armed variant was to replace part of the AC3 order but not all, as a large number of parts and assemblies had already been manufactured for the AC3.

The design for the AC4 was not finalised before the Australian Cruiser Tank Programme was terminated in July 1943, and as such did not receive a formal name. It was desired to give the tank a suitable name in an aboriginal language and the Director of the DAFVP, Alfred Code, had "Woomera" in mind for the AC4.

The end of the Australian Cruisers
By 1943 for Australia the war had changed no longer fighting in North Africa or Europe against Germany and Italy, but in the jungles and on the islands of the Pacific against the Japanese. With the USA now fully geared up for war and likely to be able to supply large numbers of tanks should they be required and American M3 Lees and Grants, M3 Stuarts and British Matildas already in Australia to equip all armoured formations, the pressing need for tanks of any description had been filled. Although the requirement for a tank like the AC3 or AC4 with heavy armour, and fully traversable armament never really went away nor was it filled.

Lend Lease authorities in the United States had been exerting pressure on Australia for some time to standardise its tank production on the M4 or T20 in order to streamline supply and maintenance. Unless Australia agreed to switch over to a US design it would receive no more assistance in this programme, or others, in the way of components or machine tools from the US.

Great Britain had shown greater interest and support for the Australian programme, first lending one of its own tank designers, Colonel W.D. Watson, to the project and then supplying first a Crusader, purchased in 1941 as an example of a modern cruiser tank, and later a Cromwell, purchased in September 1942 to assist with the up gunning of the ACs, although the Cromwell didn't arrive until after the program had already been terminated. Impressed with the reports brought back by Watson in late '42, the War Office even asked that three ACs be sent at the earliest possible time, the first to mount a 17 pounder the other two with 25 pounders. Consideration was given to sending pre-production examples but this request ultimately went unfulfilled with the cancellation of the project.

The Australian Government had agreed in late '42 in principle, like its allies, to the production of a standard tank but when faced with the fact that both the M4 and T20 were significantly more complex than any of the Australian Cruisers, would require twice the man-hours to manufacture, used armaments and ammunition that were not in production in Australia and engines that would have to be imported, all problems the Australian cruiser was designed to avoid, the programme was instead terminated, AC3 production was halted and workforce diverted to other tasks. The DAFVP was closed down and its remaining functions transferred to the Ordnance Production Directorate. To replace the Australian Cruisers, 310 M4 tanks were budgeted for at A35,000 each, these were never supplied and subsequently 510 Churchill tanks were ordered instead. After 51 Churchills were delivered the remainder was cancelled in favour of a future order of Centurion tanks.

The 65 completed AC1s, the two prototypes and the only complete AC3 were handed over to the army at the end of the programme and spent the rest of the war in storage. They were too different to the M3s and Matildas to be used for training and would require modifications and manufacture of spares if they were to be used as special purpose vehicles. In 1945 at the end of the war of the 68 ACs three were selected to provide "a physical record" for preservation in war museums in Australia and Britain, the rest were dismantled, the hulls sold off for conversion into bulldozers, or other heavy machinery, their turrets and other small parts for scrap metal. Some or possibly all the AC3 hulls and turrets produced but never used were setup as hard targets on live fire ranges. The three complete vehicles saved for war museums were the two most complete AC1s, serial numbers 8030 and 8049, and the only AC3 serial number 8066. AC1 8049 was shipped to Britain in 1946 were it was placed in storage and exists today in the Bovington Tank Museum in Dorset and is probably the most complete example, although neither it nor any of the others have an original or correct camouflage scheme. AC1 8030 survives in a less complete but restored to running condition at the Royal Australian Armoured Corps Tank Museum in Victoria. AC3 8066 survives in an unrestored state at the Australian War Memorial in Canberra.

A composite vehicle at the Melbourne Tank Museum's junk yard, built from an ex-dozer AC1 hull, an AC3 turret recovered from a firing range, and 2 pounder anti-tank gun. Despite being roughly the same weight as the M3 medium to its left, the Sentinel presents a frontal target area only about the same size as the M3 Light to its far left.

Trials, Tests and Experiments


  25 pounder field gun/howitzer armed test vehicle
The 25 pounder armed test vehicle was used to evaluate the 25 pounder as a tank armament for the AC3 design. A field gun was suitably modified and fitted as the main armament. Proofing tests were held on the 29th of June 1942 in a prototype turret mounted on development hull E2 by a 137cm (54 inch) turret ring. Gunnery trials were held shortly after on the 27th of July.

25 pounder tank gun armed test vehicle
Following the field gun test the 25 pounder was then redesigned as a tank gun, primarily by shortening the recoil system and shifting it to above the barrel. The new 25 pounder tank gun was then fitted in place of the modified field gun in the E2 and proofing tests were held on the 15th of October 1942. A 1000 round endurance trial was later conducted from March to June 1943. The tests were successful and the AC3 was ordered to be produced immediately after AC1 production had finished.

Double 25 pounder tank gun armed test vehicle
The twin 25 pounder armed tank was a test vehicle only. As a 17 pounder was not available at the time two 25 pounders were fitted into a turret mounted on the development hull E1. The intent was to test the recoil system and turret ring by significantly exceeding the recoil force of a single 17 pounder tank gun. The pair of 25 pounders fired together produced approximately 120% of the recoil force of a single 17 pounder anti-tank gun. The rather unusual armament was tested on the 2nd of November 1942. The tank survived and was subsequently armed with a 17 pounder.

17 pounder anti-tank gun armed test vehicle
The 17 pounder anti-tank gun armed test vehicle used to test the armament for the AC4 design. The design for an experimental 17 pounder armed version of the AC had started back in February 1942. Development hull E1 was fitted with a new turret mounting a 17 pounder tank gun. Firing trials on the 17th of November 1942 were successful and the weapon was selected for the AC4 design.

Suspension test vehicle
One hull was modified to test a different suspension system of five large road wheels with three track return rollers. Each road wheel had a vertical volute spring semi-recessed into the hull, similar to "Christie" suspension. Probably inspired by the suspension of the British "Crusader" Cruiser tank that had been imported to assist with the design and development of the Australian Cruisers. The concept was not adopted for production.


Armour Basis - Armour Basis for AC1, AC3 and AC4. The armour basis here is defined as providing protection equivalent to a vertical plate of the specified thickness. So if the armour is sloped so as to present an angled face to an incoming projectile it can be of a reduced thickness and still provide the same level of protection.

  65mm (2 1/2 inch) Hull Front
45mm (1 3/4 inch) Hull sides and rear
23mm (7/8 inch) Hull top
16mm (5/8 inch) Hull floor
65mm (2 1/2 inch) Turret all round
25mm (1 inch) Turret roof

Initially the specification called for 50mm (2 inches) of armour which was considered proof at close range against anti-tank guns of performance similar to that of the 2 pounder. Experience against German tank and anti-tank guns proved that this was inadequate and the specification was raised to 65mm (2 1/2 inches).

Australian Zirconium Alloy Steel Cast Armour specification

  Composition: Carbon:0.25%-0.35%, Silicon: 0.7%-0.9%, Manganese: 1.6%-1.9% Chromium: 0.7%-0.9%, Zirconium: 0.1%-0.2%, later reduced to 0.05%.
Brinell Hardness: 230-240.
Izod Impact Strength: 20ft/lb rising through production to 52ft/lb.
Ultimate Tensile Strength: 756MPa (49 T/sq inch). Purposely kept low to reduce spalling.


This armour was specially developed for the Australian Cruiser Tanks. Australian Bulletproof Plate type 3 (ABP3), was used for Australian built Local Pattern Carriers, the Dingo scout car, and other AFV projects. ABP4 was a later, experimental type of rolled armour plate developed for the for the Rhino amoured car. The cast armour for tanks was an off shoot of ABP3. Unlike armour produced in other countries the Australian cast armour contained no nickel as there was no source for this in Australia. Similar to British IT90 cast armour it was slightly softer but much tougher, resulting in less spalling from hits. When an AC1 turret was compared against a specially imported M4 turret the armour proved at least as good the American armour if not better.


Australian Cruiser Tank designs and projects

  Australian Cruiser Tank Mark I "Sentinel"
Armament: one 40mm 2 pounder anti-tank gun, two Vickers machine guns, one anti aircraft Bren LMG, one Thompson SMG.
Ammunition: 130 rounds 2pdr, 4250 rounds Vickers machine guns, 900 rounds for Bren, 300 rounds for Thompson, 6 hand grenades, 1 Very signal pistol and 12 flares.
Armour: Lower hull front 65mm, upper hull front 65-45mm at varied angles, top 25mm, remainder 45mm. Turret all round 65-50mm at varied angles, top 25mm.
Turret ring: 137cm (54 inch)
Traverse Rate: 20 degrees/second.
Crew: 5
Weight: 26.4 tonnes unladen, 27 tonnes combat ready.
Height: 2.56m (8ft 5 inches)
Length: 6.35m (20ft 10 inches)
Width: 2.8m (9ft 2 inches)
Ground Clearance:0.4m (1ft 4 inches)
Ground Pressure: 92 kPa (13.4 psi), at 76mm (3 inch) sinkage 79kPa (11.5 psi)
Engine: 3 V8 Cadillac engines.
Power output: 246 kW (330 bhp) at 3050 rpm.
Gearbox: Simplified M3, 5 forward gears, 1 reverse.
Steering: Cletrac type controlled differential.
Turning circle diameter: 17.7m (58ft)
Tracks: 13 tooth sprocket driving American T41, WE210, or Mark IV, Steel track, Australian Pattern. Alternatively 20 tooth sprocket for Marks V-XI Steel track, Australian Pattern
Vertical Step: 1m (3ft 6inch)
Fording depth: 1m (3ft 6inch)
Trench crossing: 2.4m (8ft)
Maximum grade: 35 degrees
Fuel: Internal 590L (130 imp. gallons), external tank 200L (44 imp. gallons).
Range: 175 km (110 miles) without external tank.
Maximum permissible speeds: 1st 5km/h (3 mph), 2nd 10km/h (6 mph), 3rd 20km/h (12.5 mph), 4th 35km/h (22 mph), 5th 48km/h (30 mph), Reverse 6km/h (3.5 mph).
Speed: maximum 48 km/h (30 mph), average road 45 km/h (28 mph), average cross country 34 km/h (21 mph).
Serial numbers: 8001 to 8065
Unit cost: A15,000, A17,250 including spares.
Status: 65 units produced.

Australian Cruiser Tank Mark IA
Armament: one 6 pounder tank gun and one Vickers machine gun.
Turret ring: 137cm (54 inch)
Engine: 3 V8 Cadillac engines.
Status: Designs completed, not intended for production.

Australian Cruiser Tank Mark IB
Armament: one 25 pounder tank gun and one Vickers machine gun.
Turret ring: 137cm (54 inch)
Engine: 3 V8 Cadillac engines.
Status: Designs completed, not intended for production.

Australian Cruiser Tank Mark II
Armament: one 6 pounder tank gun and two Vickers Machine guns.
Engine: two diesel engines.
Status: Abandoned Design Project.

Australian Cruiser Tank Mark III "Scorpion"
Armament: one 2 pounder tank gun and two Vickers machine guns.
Turret ring: 137cm (54 inch)
Engine: 400HP Single row Pratt and Whitney Radial
Status: Planned variant of the AC1 with a different power plant. None produced.

Australian Cruiser Tank Mark III "Thunderbolt"
Armament: one 25 pounder tank gun and one Vickers machine.
Ammunition: 120 rounds 25pdr (60 AP, 60 HE and Smoke), 2500 rounds Vickers machine gun.
Armour: Lower hull front 65mm, upper hull front 50mm at 24 degrees, top 25mm, remainder 45mm. Turret all round 65-50mm at varied angles, top 25mm.
Turret ring: 137cm (54 inch)
Traverse Rate: 12 degrees/second.
Crew: 4
Weight: 27.4 tonnes unladen, 29 tonnes combat ready.
Height: 2.56m (8ft 5 inches)
Length: 6.3m (20ft 8 inches)
Width: 2.8m (9ft 2 inches)
Ground Pressure: 99 kPa (14.4 psi)
Engine: Perrier-Cadillac
Power output: 296 kW (397 bhp)
Gearbox: Simplified M3. 5 forward gears, 1 reverse.
Steering: Cletrac type controlled differential.
Turning circle diameter: 17.7m (58ft)
Tracks: American T41 or Mark VI, Steel track, Australian Pattern.
Vertical Step: 1m (3ft 6inch)
Fording depth: 1m (3ft 6inch)
Trench crossing: 2.4m (8ft)
Maximum grade: 35 degrees
Fuel: Internal 840L (185 imp. gallons), external tank 200L (44 imp. gallons)
Range: 300 km (180 miles) without external tank.
Maximum permissible speeds: 1st 5km/h (3 mph), 2nd 10km/h (6 mph), 3rd 20km/h (12.5 mph), 4th 35km/h (22 mph), 5th 48km/h (30 mph), Reverse 6km/h (3.5 mph)
Serial numbers: 8066 to 8265
Status: At least one unit produced. 200 ordered.

Australian Cruiser Tank Mark IIIA
Armament: one 25 pounder tank gun and one Vickers machine gun.
Turret ring: 178cm (70 inch)
Engine: Perrier-Cadillac
Status: Proposal only.

Australian Cruiser Tank Mark IV
Armament: one 17 pounder tank gun and one Vickers machine gun.
Ammunition: 50-60 rounds 17pdr, 2500 rounds Vickers machine gun.
Armour: Lower hull front 65mm, upper hull front 50mm at 24 degrees, top 25mm, remainder 45mm. Turret all round 65-50mm at varied angles, top 25mm.
Turret ring: 178cm (70 inch)
Traverse Rate: 12 degrees/second
Crew: 4
Weight: 32 tonnes
Height: 2.77m (9ft 1 inches)
Length: 6.53m (21ft 5 inches) w/o gun.
Width: 2.8m (9ft 2 inches)
Ground Pressure: 96 kPa (14 psi)
Engine: Perrier-Cadillac
Power output: 296 kW (397 bhp)
Steering: Cletrac type controlled differential
Turning circle diameter: 17.7m (58ft)
Tracks: American T41 or Australian Steel track
Vertical Step: 1m (3ft 6inch)
Fording depth: 1m (3ft 6inch)
Trench crossing: 2.4m (8ft)
Fuel: Internal 840L (185 imp. gallons), external tank 200L (44 imp. gallons).
Range: 300 km (180 miles) without external tank.
Maximum permissible speeds: 1st 6km/h (3.5 mph), 2nd 12km/h (7.3 mph), 3rd 23km/h (14.3 mph), 4th 42km/h (26.3 mph), 5th 56km/h (35 mph), Reverse 6km/h (4 mph).
Serial numbers: 8266 to 8775 (Including Mark IVA).
Status: Intended for production. Design incomplete. 400 ordered.

Australian Cruiser Tank Mark IVA
Armament: one 25 pounder tank gun and one Vickers machine gun.
Turret ring: 178cm (70 inch)
Engine: Perrier-Cadillac
Status: Intended for production. Design incomplete. 110 ordered.


  Clover-Leaf Cadillac
Power: 276kW (370HP) gross, 246kW (330HP) net, at 3200rpm
Torque: 987Nm (728ft/lbs) at 1850rpm
Displacement: 3x 5.7L (346in)
Bore: 90mm (3.5in)
Stroke: 114mm (4.5in)
Compression ratio: 6.2:1
Weight: 1497kg (3300lb) including engine subframe.


Power: 296kW (397hp) gross at 3440rpm
Torque: 1091Nm (805ft/lbs) at 1850rpm
Displacement 17L (1038in)
Bore: 90mm (3.5in)
Stroke: 114mm (4.5in)
Compression ratio: 6.75:1
Weight: 1043kg (2320lb) without radiators and fan.

Power: 316kW (430HP) at 2300rpm
Torque: 1220Nm (900ft/lbs) at 1000rpm
Displacement: 22L (1344in)
Bore:146mm (5.75in)
Stroke: 146mm (5.75in)

Quad Gipsy
Power: 375kW (510HP) at 2500rpm
Displacement: 24.5L (1495in)
Bore: 118mm (4.646in)
Stroke: 140mm (5.512in)
Compression ratio: 5.25:1



  Vickers Mk.XXI fixed type
Calibre: 7.7mm (.303inch)
Muzzle velocity: 744m/s (2440fps)
Rate of fire: 450-500 rounds per minute.
Magazine: 250 round belt
Notes: A purpose made AFV gun with no sights, water inlet and outlet at the rear of the jacket, water fed by a 12V pump, and remotely fired by cable. Used in both the coaxial and hull mounts.


2 pounder tank gun
Calibre: 40mm (1.57inch)
Barrel length: 2.1m (82 inches) 52cal
High Explosive: 843g (1lb 13oz 12dr) Shell Q.F. HE 2-pdr. Mk.1T with No.243 Fuze, Muzzle velocity: 863m/s (2830fps). Bursting charge: 64g (2oz 4dr) TNT.
Canister: Q.F. 2-pdr. Case Shot. 2 pounder cartridge fitted with US 37mm Canister M2, filled with 122 lead balls, 9.5mm diameter (3/8in), set in resin. Effective range of 140m (150y).
Armour Penetration at 30 degrees:

2 pound AP Shot penetration at 30 degrees


Muzzle Velocity


457m (500y)

914m (1000y)

1372m (1500y)

1829m (2000y)

A.P. Mk. I T

853m/s (2800fps)






A.P.S.V. Mk.II T

1189m/s (3900fps)






Notes: The high explosive round was created by fitting the shell from the Vickers 2pdr "Pom Pom" High Velocity naval anti-aircraft gun to the 2 pounder tank gun case with the self-destruct igniter portion of the tracer removed. The canister round was similar but used a US 37mm canister as the projectile until such time as a 40mm canister could be produced.


25 pounder tank gun
Calibre: 87.6mm (3.45 inches)
Barrel length: 2.5m (97 inches) 28cal
High Explosive: 11.34kg (25lb) HE Shell Mk.1.D with No.117 or No.119 Fuze, Charge 3, Muzzle velocity: 454m/s (1490fps). Bursting charge: 827g (1lb 13oz 3dr) TNT.

20 pound AP Shot penetration at 30 degrees


Muzzle Velocity


457m (500y)

914m (1000y)

1372m (1500y)

1829m (2000y)

A.P. Mk.I T Charge 3

488m/s (1600fps)






A.P. Mk.I T Charge Super

579m/s (1900fps)






Notes: An otherwise standard 25 pounder Mk II gun, fitted with an overhead shortened recoil system for tank use.


17 pounder tank gun
Calibre: 76.2mm (3 inches)
Barrel length: 4.2m (165 inches) 55cal
High Explosive: 6.08kg (13lb 6oz 6dr) Shell HE Mk.II.T with No.244 or No.257 Fuze. Muzzle velocity: 899m/s (2950fps) Bursting charge: 491g (1lb 1oz 5dr) TNT.

17 pound APC Shot penetration at 30 degrees


Muzzle Velocity


457m (500y)

914m (1000y)

1372m (1500y)

1829m (2000y)


884m/s (2900fps)









Bellona Military Vehicle Prints Series Twelve, UK, Merberlen, 1967.
AFV Weapons Profile Series Two, Number 31, Profile Publications Limited, England.
The Role of Science and Industry, D.P. Mellor.
Records held by The National Archives of Australia.
Paul D. Handel's Crusader Down Under and Cromwell Down Under

Mr Richard Simmie, without whom this would have been impossible.

2nd revision 2020. This article was first written way back around 2000, and while it has held up reasonably well earlier versions of this article contained several errors, some partly of my own making, such as relying on a museum vehicle for identification of the T41 tracks as T51, others down to the sources used. Still, errors are errors and need to be corrected as much as they can be.

1st revision August 2005, and it's still not done.

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