WW2 Australian Prototypes

AC IV 17-pdr Armed Sentinel Cruiser Tank

Commonwealth of Australia (1942-1943)
Cruiser Tank – 1 Prototype Built

The one with the big gun

Another offspring of the AC I Sentinel was the AC IV, which was to be equipped with the new British Ordnance QF 17-pounder anti-tank gun. After the July 1942 decision to proceed with increased armament for the Australian Cruisers, a plan for expedited local production of 17 pounders was initiated to replace the previously planned 6 pounder production.
Artists rendition of the AC IV
Artists rendition of the AC IV. Source: National Australian Archives MP730 10
A new prototype turret was fabricated to facilitate mounting the 17 pounder gun, and fitted to the E1 prototype hull. The new turret had increased dimensions over the previous AC I and AC III turrets, notably featuring a slightly taller roof and an extended rear bustle with an angled rear facing as opposed to the vertical rear face of the previous types. The turret ring diameter was also increased from 54 inches to 64 inches, with the E1 hull being suitably modified.
Testing began in October of 1942. The first test was a simulation of the recoil force of a 17 Pounder, and its effect on the tank. This was done by mounting two 25 Pounder howitzers side-by-side in the turret and firing them simultaneously, this gave an estimated recoil force 20% greater than that of the 17 pounder. The dual 25 pounder mount took up the majority of the turret space and had to be fired remotely via lanyard. In the same month, the first Australian produced 17 pounder guns were completed at the Maribyrnong Ordnance Factory.
The tank was fitted with a locally produced 17 pounder (allegedly one of the first guns produced) equipped with a modified recoil system based off of that developed for the 25 pounder tank mounting. The tank and gun mounting were first test fired on the 11th of November at Fort Gellibrand in Williamstown, Victoria, with tests continuing on until early 1943. These tests proved quite successful and made the AC IV one of the first Allied tanks to mount the 17 Pounder gun.

A constant improvement

This, however, was not the finalized design and work would be ongoing for the AC IV until the cancellation of the tank program in mid-1943. Concerns had been raised about the design which would further complicate the matter. The 54 inch turret ring had been considered cramped but workable with the 25 pounder, but there were doubts about the efficiency of loading a 17 pounder in a 64 inch turret ring. It was therefore decided to increase the turret ring diameter to 72 inches for production vehicles.
Additionally, the Army was not satisfied with the proposed quantity of 54 rounds of ammunition carried in the prototype, and insisted that a minimum of 74 rounds be met. It was also considered desirable to mount the 25 pounder in the new turret to take advantage of the potential benefits of the 25 pounder gun alongside the 17 pounder.
The DAFVP responded to these requests with a proposal for an AC IVA design. Documents, however, are unclear as to what the AC IVA design actually entailed. Some documents claim the AC IVA was to be a variation of the AC IV fitted with the 25 pounder and produced at a rate of one 25 pounder armed tank for every three 17 pounder armed tanks. Other sources list the AC IVA as a design with enlarged hull dimensions to allow for an increased turret diameter and increased ammunition stowage for either the 17 or 25 pounder gun.

New powerpacks

16 cylinder 410 horsepower gypsy major engine mock-up
gypsy major engine mock-up
16 cylinder 510 horsepower Gipsy Major engine mock-up. Source: National Australian Archives MP730 10
To cope with the added weight, two new engine designs were proposed. The first consisted of four Holden Gipsy Major engines, providing an estimated 510 horsepower, to be mounted together in a two layer opposed piston setup and utilizing air cooling as opposed to the water cooled engines used previously.A non-functional mock-up of the engine, utilizing as many genuine parts as were available was produced. However, funding to continue development was denied due to termination of the tank project.
The second design was a 600 horsepower Michell type crankless engine, the design of which had been extensively developed by respected Australian Inventor A.G Michell in the 1920s. Director of Armoured Fighting Vehicle Production, Alfred Reginald Code, had been the chief draughtsman for Michell’s Crankless Engine Company from 1925 until the company’s closure in Australia in 1928. The crankless tank engine would have had several advantages, such as a smaller size to horsepower output ratio as well as a higher fuel efficiency. The design did not proceed beyond the drawing board.
600 horsepower crankless tank engine
This cutaway drawing of the proposed 600 horsepower crankless tank engine. Source: National Australian Archives B6118 7

The AC IV prototype, based on the AC E1 – Illustrator: David Bocquelet
The AC E1 tank
The AC E1 – Fitted with the new turret and 17 pdr gun, Fort Gellibrand Victoria,  summer of 1943. Source:- Australian War Memorial PO3498.010

The AC E1 with the turret traversed to the right showing the elongated rear of the turret and higher turret roof, Fort Gellibrand Victoria,  summer of 1943. Source:- Australian War Memorial PO3498.009

Always changing requests

In the quest for a modern tank design the army added a veritable laundry list of new requirements from late 1942 through to 1943, largely focused on standardising with the latest developments in US tank design. By early 1943, it was not clear to Australian authorities if the M4 Sherman would continue to be the standard tank of the US forces or be superseded by the T20 series of medium tanks.
Based on information received from US sources and the assessment of Col G.A Green on behalf of the US Army, desired upgrades included: US style all round vision cupola with additional crew vision blocs to be added around the turret. Ford GAA 525 horsepower tank engine to replace locally designed engines. Oilgear hydraulic turret traverse mechanism and Westinghouse 24 volt gyroscopic gun stabiliser. Removal of the turret basket in favor of crew seats suspended from the turret ring. Torsion bar suspension, or, if not possible, the implementation of US M4 type road wheels and revision to US rubber tracks. Replacement of the Methyl bromide fire suppression system with a Carbon Dioxide system. Arguably the most ambitious and outlandish proposal was the suggestion of a mechanical ammunition rack to better facilitate loading the 17 pounder gun.
‘Accordingly, a magazine containing 18 shells has been designed and this is located across the rear of the turret. The noses of the shells point towards the gun and the magazine has been equipped with gear to traverse each shell in turn to the centre for loading’ – tank production programme Report, on behalf of Director AFVP A.R Code to Mr Pryke, July 21st 1943

Not in vain

The termination of the entire Australian Cruiser program in mid-1943 was dictated by a mixture of practical and budgetary reasons as well as an ongoing political rivalry between the Ministry of Munitions and the Army.
Despite the Australian tanks never seeing combat use, one notable benefit to the development of Allied tanks did occur as a result. In 1943, Colonel Watson returned to the UK after his secondment to the Australian tank program ended. Watson brought with him documents related to the Australian tank program, including photographs and drawings of the 17 pounder mounting on the AC IV prototype.

The experimental 17 pounder mounting complete with  mantlet dismounted from the E1 tank. The modified recuperator system can be seen extending out into the mantlet bulge above the gun barrel. Source: Ed Francis
Watson received a great amount of interest regarding the Australian work with the 17 pounder, notably from Sir Claude Gibb who was adamant that the 17 pounder could be mounted in an M4 Sherman turret, but faced severe opposition from the parties arguing otherwise. Subsequently, a series of meetings was convened where Watson was invited to provide detailed information about the Australian 17 pounder tank mounting, information which significantly expedited the decision to mount the 17 pounder gun on what would later become the Sherman Firefly.

Surviving vehicle

The Australian Armour and Artillery Museum displays a mock-up of the AC IV prototype, assembled from a 17 pounder gun barrel fitted to a fabricated replica of the AC IV mantlet and a salvaged AC III turret, mounted to a salvaged AC I hull. The remains of the AC I E1 prototype hull were held in the collection of the Melbourne tank museum until its closure in 2006. The E1 hull was not listed as an item in the auction of the Museum’s collection and its eventual fate is not known.
An article by Thomas Anderson

AC IV specifications

Dimensions 6.32 x 2.77 x >2.56 m
(20’9” x 9’7” x >8’4”)
Crew 4 (commander, loader, gunner, driver)
Propulsion 3 x V8 Cadillac ‘Cloverleaf’ 330 hp total, 12 hp/t
3 x V8 Cadillac ‘Perrier Cadillac’ 395 hp total
Ford GAA, 525 hp
16 cylinder Gypsy Major, 510 hp
Michell type crankless engine, 600 hp
Suspensions Horizontal volute springs (HVSS)
Armament : 17-Pounder (76.2 mm/3 in), 54 rounds
Vickers .303, (7.9 mm)
Armor From 45 to 65 mm (1.77-2.56 in)

Links and Resources

Australian War Memorial Archives
The AC Sentinel on Wikipedia
Tank Hunter
ACVI tank
Mock-up of the AC IV prototype on display at the Australian Armour and Artillery Museum

WW2 Australian Prototypes

AC III Thunderbolt Cruiser Tank

Commonwealth of Australia (1942-1943)
Cruiser Tank – 1 Prototype Built

The inadequate 2 Pounder

In 1941, The QF Vickers 2-Pounder had been recognised as likely to become obsolete by the time that the AC tanks were scheduled entered production. The armament of the Mark I “Sentinel” was seen as transitional since the beginning, and the tank had been designed and balanced with the intent of mounting the Ordnance QF 6 Pounder (57 mm/2.24 in) gun before said gun was even available, shadowing the evolution of the British cruisers such as the Crusader.
Artists rendition of the AC III tank
Artists rendition of the AC III Tank. Source: National Australian Archives MP730 10


Tanks equipped with the 6 pounder gun were to be designated AC IA and would have been in all other details identical to the AC I, with one minor exception. Tanks fitted with the 6 Pounder gun were to have the hull machine gun and accommodations for the hull gunner removed to make way for remodelled ammunition storage, of which 100 6-pounder rounds were to be carried. Due to production delays and a lack of supply of 6 pounder guns the AC IA never left the drawing board and was superseded by the AC III. Additionally, other armaments were investigated for the AC tanks.
As early as December 1941, Colonel Watson had proposed the possibility of mounting an ordnance QF 25 Pounder (87.6 mm/3.45 inch) field gun via means of an adapted gun cradle for the 3 inch (76.2mm) 20 cwt Anti-Aircraft gun. This proposal met with Army approval and in February of 1942 two 25 pounder guns were delivered to the DAFVP for experimental work.
The proposed 25 pounder tanks were to be designated AC IB and were in all aspects identical to the AC IA design bar the changes to mounting and fittings suitable for the 25 pounder gun. By June 1942, a 25 pounder gun, with a modified recoil system, was fitted to an enlarged turret and mounted on the E2 prototype chassis.
Test firing of the gun proved extremely positive results, with the Director of Artillery remarking in his 1942 report ‘Accuracy is superior to that of a field mounting. Noise and blast within the turret is minimal, similar to that of a big air rifle. It was discovered, from personal experience, that once layed successive shots could be fired on target without needing to re-lay the gun’
Like the AC IA, the AC IB would ultimately remain unproduced. Events in the North African theatre and changes in army policy at home would overtake both designs, leading to a more radical revision in the form of the AC III. The new AC I was intended to be sent to North Africa by late 1942, however concerns had been aired in regards to dealing with the latest 50 mm (1.97 in) armed Panzer III and up-gunned Panzer IV F2.

AC III, the howitzer version prototype

Tank number 8066, the AC III prototype fitted with the 25 pounder main gun . Source:- Australian War Memorial 101155

An interim solution

By mid-1942, the 2 pounder was widely derided by Australian officers as a ‘pea-shooter’ and ‘popgun’ and the 6 pounder was looking increasingly obsolete in the face of German gun advancements. To address these fears, Field Marshal Sir Thomas Blamey, Commander in Chief of the Australian Forces, tendered a proposal to the Australian war cabinet in July of 1942.
Blamey proposed that the Australian tanks needed to get ahead of the Germans in terms of firepower, and the only solution was the QF 17 pounder (76.2 mm/3 inch) anti-tank gun. However, the 17 pounder had barely entered production in the UK and surplus guns for shipment to Australia were not at all available in the UK. Hence the 25 pounder gun, mounted on the refined AC Mark III, received much consideration as a stopgap measure.

Frontal shot of tank 8066. Notice the absence of the AC I’s ‘Distinctive’ MG mounting and gunner’s hatch, as well as the large mantlet bulge housing the 25 pounder gun recuperator above the gun barrel. Source: Ed Francis

Three quarter rear shot of tank 8066. The spare track bracket on the rear of the hull covers the hull casting number which was ground flush to the hull. Unlike on AC I tanks the casting number on the rear of the turret is still visible as no turret stowage box is present. Source: Ed Francis

AC III turret and basket being assembled on a turret stand, the turret front, mantlet, and gun have not yet been fitted. The rear bustle of the turret is slightly longer than on an AC I turret, stowage boxes for 25 pounder propellant charges can also be seen on the turret basket. Source: Ed Francis

AC III Thunderbolt specifications

Dimensions 6.32 x 2.77 x 2.56 m
(20’9” x 9’7” x 8’4”)
Total weight, battle ready 28 tons
Crew 4 (commander, loader, gunner, driver)
Propulsion 3 x V8 Cadillac ‘Perrier Cadillac’ 395 hp total
Suspensions Horizontal volute springs (HVSS)
Max speed 48 km/h (30 mph)
Range (max) 240 km (150 mi)
Armament : 25-Pounder (87.6 mm/3.45 inch)
Vickers .303, (7.9 mm)
Armor From 45 to 65 mm (1.77-2.56 in)
Total production 1

Links and Resources

Australian War Memorial Archives
The AC Sentinel on Wikipedia
Tank Hunter

AC III Thunderbolt
The AC III “Thunderbolt”  was fitted with a 25-pdr (90 mm/3.54 in) howitzer. More than 100 hulls were in varying states of completion when the entire program was cancelled in July 1943.

The AC III Scorpion

The AC III designation was actually given to two tank designs, named ‘Scorpion’ and ‘Thunderbolt’ respectively. The Scorpion was a standard AC I, but modified to mount the desired Pratt and Whitney Wasp radial engine. Director Alfred Reginald Code and the DAFVP had struck a deal with the Commonwealth Aircraft Corporation (CAC) in October of 1941, CAC would deliver 200 engines for tank use by mid-1942 with the loss to aircraft production being taken up by engines ordered from the USA. The Cloverleaf Cadillac engines would be installed as an interim measure for the first 65 tanks, after which the new engine and other sundry modifications would be implemented from the 66th tank onward.
Production of the AC III Scorpion was planned to be conducted at a new tank assembly annex to be constructed in Port Melbourne, Victoria, in order to supplement the output of the Chullora Tank Assembly Workshops in Sydney. Hull castings were to be produced locally in a purpose built foundry operated by the Charles Ruwolts Company, however automotive components such as the gearbox and final drives were to be imported from the United States. The Australian type crash gearboxes were contracted to be produced by the US based Oliver Farm Equipment Company.
For tank use the Wasp was to be down rated to 400 horsepower and re-designated ‘Scorpion’ with tanks using the engine carrying the same name. However, experimental testing with the Scorpion engine revealed undesirable traits, such as poor torque output at low RPM and a high RPM ceiling required to reach maximum power output.
By Mid-1942, DAFVP engine expert Robert Perrier had proposed a new design of the triple Cadillac engine, named the Perrier Cadillac. The Perrier Cadillac took the V8 engines used in the AC I and rearranged them in a radial formation connected together on a common crank case, much like the Chrysler A57 multibank. This new engine took up less space than the previous cloverleaf configuration and also delivered an increased output of 396 hp with the army’s new higher octane petrol. The Perrier Cadillac was also shown to perform significantly better than the Scorpion engine in terms of torque and horsepower, particularly at lower RPM.
Perrier Cadillac 395 horsepower engine. Source: National Australian Archives MP730 10
Perrier Cadillac 395 horsepower engine. Source: National Australian Archives MP730 10

Perrier Cadillac 395 horsepower engine. Source: Ed Francis

The AC III Thunderbolt

The AC III ‘Thunderbolt’ was a distillation of many of the previous innovations mentioned. The 25 pounder gun was selected as the main armament, contrary to popular belief not for use as a close support weapon, but for the value of its 20 pound Armour Piercing shot as well as its obvious advantages in terms of High Explosive shells. The finalised version of the 25 pounder mounting was fitted with a shortened recoil system, later used in developing the QF 25 Pounder Short pack howitzer, mounted above the barrel in a distinctive bulge in the mantlet. Additionally, the barrel length was increased by 18 inches (457.2mm) yielding an estimated increase in muzzle velocity of 150 fps (45.72 m/s).
To make room for the more voluminous ammunition, the hull machine gun was eliminated and the crew reduced to four, although, again contrary to popular belief, the turret coaxial machinegun was not deleted from the design. The hull was remodelled with the deletion of the machinegun position, with an increased frontal slope and new design for the driver’s primary and escape hatches.

Interior photo of the driver’s position of the AC III, the driver’s seat backrest is folded down. The redesigned primary and escape hatches can be seen in the top right and the hull machinegun and gunner’s position of the AC I are absent. Notably the 25 pounder ammunition rack  normally located on the left side of the gearbox does not appear to be fitted in this photo. Source: Ed Francis

Closeup of the driver’s position of the AC III. The locally produced crash gearbox can be seen on the left, notably the driver’s instrument panels have been revised from the AC I, reflecting the changes to the engine. The similarity of the gearbox and driver’s controls to those of the M3 or M4 medium tanks can be clearly seen. Source: Ed Francis
Given its advantages, the new Perrier engine was chosen with the rear of the tank being remodelled to accommodate. Notably, the engine deck was distinctly flatter than on the AC 1 and the amount of access hatches was changed. To accommodate the new 25 pounder gun a new turret was produced, identical in layout and general fitting to the AC I, but with slightly increased dimensions, particularly at the rear. Turret ring diameter on the first production tanks was 54 inches but this was intended to be enlarged to 63 inches on later tanks (later suggested to be further increased to 70 inches) (1.37/1.6/1.78 m). Armor was intended to be raised up to 75 mm (2.95 inches) but this was never implemented.
The first production prototype (tank number 8066) AC III arrived in January of 1943 with a 1000 round test firing of the 25 pounder on the E2 tank occurring in the same month. Trials were delayed due to uncertainty on the future of the tank program and army prevarication on finalising the stowage requirements. Under testing a number of minor faults were revealed but overall the tank was received well, with the testing officer remarking that the production models would be ‘A good fighting vehicle with excellent armament’.
Interestingly, the prototype was not fitted with semi-automatic gear on the gun, and the test report recommended against the installation of semi-automatic gear or the use of one piece ammunition for the gun. The first prototype had just finished its trials and the first production batch of between 120-150 tanks was under construction in July 1943 when the entire program was terminated by the Australian Government.

An AC III hull on the production line at the Chullora Tank Assembly Workshop. The relocated driver’s escape hatch and increased angle of the glacis plate are clearly visible. Chullora, Sydney, New South Wales, 1943. Source: Ed Francis

Dead end

The termination of the entire Australian Cruiser program in mid-1943 was dictated by a mixture of practical and budgetary reasons as well as an ongoing political rivalry between the Ministry of Munitions and the Army.

Surviving vehicle

The prototype AC III (tank number 8066) is held in the collection of the Australian War Memorial in Canberra, formerly on display outside it has since been replaced by an RAAC Centurion 5/1, and is now housed in the Museum’s Treloar Technology Centre which is not open to public viewing.
Prior to closing in 2006 the Melbourne tank museum displayed an AC III tank assembled from castings salvaged from firing ranges, distinctly having a 2 pounder shell lodged in the side of the commander’s cupola. This composite tank was internally unfurnished and missing several parts, such as the main gun,  turret front & mantlet , and the driver’s hatch. The gun was substituted by a salvaged 25 pounder barrel mounted in the turret with the turret front covered over by a shaped tarpaulin. The driver’s hatch was substituted by  a driver’s view hatch taken from an M3 medium tank. After the closure of the Museum and auction of the collection the location of the tank is not currently known.

An article by Thomas Anderson

WW2 Australian Prototypes

AC II Cruiser Tank

Commonwealth of Australia (1941)
Cruiser Tank – None Built

The Sentinel that never was

Contrary to a popular misconception the AC II was not a design for the installation of an Ordnance 6 pounder gun in the AC I Sentinel cruiser . Such a design did exist, entitled AC IA, but this is covered more appropriately with the AC III Thunderbolt. In reality the AC II was a simplified design intended to expedite production of the tanks Australia so desperately needed.
Due to the lack of progress by mid-1941, doubts were raised about the practicality of Australia attempting to manufacture such a complex tank design as the AC I. As a result, Australian AFV engineer Alan H Chamberlain proposed a competing design in June of 1941. The proposed design, entitled AC II, was intended to overcome the limitations of Australian industry by substituting the complex M3 drivetrain and gearbox for a commercially available Mack truck gearbox and drive, imported from the USA.
Due to the Mack components not being required for US munitions production it was estimated that deliveries could begin in October of 1941 with series production of the AC II beginning in January of 1942 at a rate of 8 tanks per week. Compared to 5 tanks per week in mid-1942 for the AC I. The Mack components however required a reduction in the weight of the vehicle and were limited in the horsepower that could be used to power the tank. It was alternatively mooted that the Mack gearbox could simply be substituted into the AC I design. However concerns that in a 28 tonne tank the Mack gearbox would be overloaded, risking the gear teeth stripping at low gears, put an end to this idea. As proposed, the AC II weighed 19.5 tonnes with a hull armor basis of 2 ¼ inch (57.15 mm) frontal and 1 inch (25.4 mm) side and rear, and 2 ½ inches (63.5 mm) of all around turret armor.
The intended engine was to either be a twin mounting of the same Cadillac V8 engines used in the AC I or a 225 horsepower GM 6-71 diesel engine. A Curtiss aircraft engine was also investigated but found to be overly powerful for the Mack gearbox and thus unsuitable without substantial modification . Despite the decreased weight, the corresponding decrease in engine power resulted in an estimated top speed of 19 mph (30 km/h). The armament was the same as on the AC I.
Line drawing and armour specifications of AC II
Line drawing and armour specifications of AC II. Source: National Archives of Australia MP730/13 14

Unable to meet the Requirement

The Army were concerned by the armor of Chamberlain’s AC II design, arguing that 1 inch (25.4 mm) of side and rear armor was too thin to protect against light anti-tank weapons such as anti-tank rifles and 20 mm (0.79 in) cannon shells. Subsequently, the Army tendered a modified 22 tonne version of the AC II with an increased armor basis of 2 ½ inches (63.5 mm) frontal armor and 1 ¾ inch (44 mm) side and rear armor. However, the increased weight of the modified design was considered to be the absolute limit of the Mack components and the speed had been further reduced to 16 mph (25 km/h).

Artist's rendition of a production AC II
Artist’s rendition of a production AC II

The AC II design was favourably reviewed by Mr Michael Dewar of the British Purchasing Commission, claiming that the design was likely to be the equal of and in some respects superior to the British Valentine Infantry tank. This was small consolation however, as the Army had already rejected the Valentine as too slow and inconsistently armored. The Army would not accept the AC II due to it not meeting the listed top speed requirement of 35 mph (56 km/h). Some consideration was given to increasing the front and side armour to a basis of 3 and 2 1/16 inches (76.2-55mm) respectively and implementing the AC II as an infantry tank with a top speed of roughly 10mph (16 km/h), however this was not further pursued due to lack of army interest in infantry tanks.

For emergency use only

The AC II also caused considerable problems with relations between Australia and the US Lend Lease authorities. From the perspective of US authorities Australia seemed to be wasting resources on pursuing two almost identical designs simultaneously, which made the US reluctant to invest support in the project. Such was the level of hesitance and confusion associated with the AC II that it required Director of AFV Production , Alfred Reginald Code, to travel to the US in late October of 1941 in order to personally expedite deliveries of US parts to Australia and reassure US authorities that Australia only wanted one design of tank.
By October of 1941, no shipments of parts for the AC II had been delivered and initial production was estimated for mid-1942, the same time as the preferred AC I. Consequently consideration was given to producing the AC II as a lead up to AC I production, after which excess production would be exported to allied nations in need of tanks, but this plan was not pursued. After October of 1941, the AC II was only ever considered as a stopgap measure for emergency defence and ultimately did not progress beyond the drawing board.

An article by Thomas Anderson

AC II specifications

Dimensions 6.32 x 2.77 x 2.56 m
(20’9” x 9’7” x 8’4”)
Total weight, battle ready 22 tons
Crew 5 (commander, loader, gunner, driver, machine gunner)
Propulsion 2 x V8 Cadillac, 220 hp total
GM 71 diesel, 225 hp
Suspensions Horizontal volute springs (HVSS)
Max speed 16 mph (25 km/h)
Armament 2-Pounder QF (40 mm/1.57 in), 130 rounds
2x Vickers .303, (7.9 mm) 4250 rounds
Armor 57.16 mm frontal (2.25 in)

Links and Resources

Australian War Memorial Archives
The AC Sentinel on Wikipedia
Tank Hunter