The Australian Sentinel

AC I turret and hull components. This tank was the first in the world to have a fully cast hull. The French tank hulls were still assembled from two halves.

As in the First World War, the Australian Imperial Forces began forming in September of 1939 on a purely voluntary basis. At that time the main theatre of war for Australia was North Africa. Meanwhile, Australia cautiously watched the increasing military activity of Japan. Japan's main front was still in China, but only a blind man could miss Japan's aims at total hegemony in the Pacific. The biggest military in the region at the time was the Royal Netherlands East Indies Army (KNIL) and even their tank forces were not large. These were mostly export Vickers-Carden-Loyd tanks and light Marmon-Herrington tanks, both armed with machine guns only. Compared to them, even Japanese tanks looked modern. After the defeat in France and start of the fighting in North Africa, Australia was no longer a priority for Britain. Australians had to fare for themselves. In these difficult conditions, the idea of creating their own tank was born.

The idea of building a new light cruiser tank was first voiced at a meeting on June 12th, 1940, in which Australian Prime Minister Robert Menzies took part. During the meeting, the Chief of the General Staff Sir Bradenell White announced the plan to order 859 10-ton class tanks. They were due in the end of 1941. The issue was that the General Staff did not even have a set of requirements for a tank that they wanted to order in such large amounts.

Another feature of the AC I was the triple Cadillac engine. This engine was the Achilles heel of the Sentinel.

The Australian Cruiser began gaining weight almost immediately. According to calculations performed by a group of designers led by Major Alan Milner, the mass of the tank would have to be 12 tons to allow for 28 mm of armour all-around. This did not last for long: as the French campaign showed, 28 mm was not enough to stand up to the German 37 mm gun. The armour thickness grew, and with it the mass: the tank would now weigh 15 tons. The armament remained the same: a 2-pounder (40 mm) gun, 1 7.7 mm machine gun, a 2" bomb thrower.

Menzies understood that it would not be possible to build a tank alone and asked the British War Department for help on August 20th, 1940. The British sent Colonel Watson, who would later become the main developer of the tank. In October of 1940 Watson departed for the USA. This kind of detour was no accident. The colonel worked in the technical section of Royal Artillery and no doubt was well informed of the issues that plagued British tank building. The flow of British designers to the USA drastically increased as of the fall of 1940. They saw the country as a mega-factory, capable of meeting British needs.

The British were in for a cold shower. The Americans studied the Matilda and Valentine infantry tanks and refused to produce them, stating that they are obsolete. These conclusions were correct. Tanks that were being built and designed in the USA were superior to British ones. On the other hand, British engineers that arrived in the USA were a great help to their American colleagues. For instance, the British helped the Americans create dual engines for their tanks. The USA was a form of incubator for armoured vehicle development by the end of 1940. Australia was also much closer to the USA than Britain, which was an important consideration for shipment of engines.

The gearbox was supplied by Sonnerdale & Co.

Watson remained in the USA until the end of 1940, studying American tank building alongside Australian engineer Alan Chamberlain. In addition to receiving important information about tank building, Watson accumulated data on tank building. One of the most important conclusions was that the thickness of the armour had to keep increasing to withstand enemy weapons. In addition, Watson gained access to materials on the Medium Tank M3. This tank had a significant influence on the Australian cruiser.

New requirements were announced at a meeting on February 18th, 1941. The mass of the tank was now 20 tons. The armament consisted of a 2-pounder gun, 2" bomb thrower, and two Vickers machine guns. The Medium Tank M3 was set as a baseline. One of the most important topics discussed at the meeting was the engine. The tank needed a 400 hp engine to accelerate it to 40 kph. The first candidate was a Guiberson radial diesel engine, but it was quickly rejected as there were issues with delivering them. The second option was a pair of General Motors 6-71 (GM 6046) engines. Finally, a third option cropped up at the end of February 1941. This was the 5.7 L Cadillac engine used on the luxury 75 series of cars. This engine could develop up to 150 hp. The most important consideration was that the engine was readily available. However, it was clear that one engine was not enough, and so several had to be used.

Assembly was performed by the New South Wales Government Railways company.

The AC I (Australian Cruiser Mk.I) began to take its final shape by the spring of 1941. As mentioned before, the Medium Tank M3 had a significant effect on it. This mostly affected the layout (engine in the back, transmission and drive sprockets in the front). The tank was not a copy of the American tank. The Canadian Ram was built on the chassis of the Medium Tank M3, but the chassis of the AC I was original. Watson tried to combine the best solutions from worldwide tank building practices, adapting them to the abilities of Australian industry. This gave the tank an unusual look. Recall that the tank had to be built in large numbers. As of January 1941, the army needed 660 tanks.

9th production AC I.

Strangely enough, Watson's brainchild had not only American, but French roots. The layout with a transmission in the front was not an American solution exclusively. It could also be found on French light tanks. The French influence on the hull and suspension can also be seen. Both were the creations of Henry Einsworth, the chief designer of the Hotchkiss company. He was the ideologue behind light tanks with front transmissions and fully cast hulls and turrets. Einsworth's ideas migrated to the Renault R 35 and SOMUA S 35 tanks. He also designed the suspension that was used on the Hotchkiss H 35 cavalry tank. Einsworth managed to escape from France in 1940 and end up in England. It is not known if he met with Watson, but the influence of the Hotchkiss H 35/H 39 is evident. The French influence on the suspension can be seen: it became a hybrid of the American VVSS and Hothckiss H 35's horizontal spring suspension.

AC I compared to the Matilda IV and Lee I.

The influence of the light Hotchkiss tank can be seen in the design of the hull and turret. On one hand, Watson could have seen the production of large cast components in the USA. For instance, the turret of the Medium Tank M3 was cast, and the M3A1 had a cast upper hull. A large amount of casting (turret, front hull) can also be seen on the British infantry tank. On the other hand, only the French used fully cast hulls at the time. The AC I used casting whenever possible. This was caused by the ability of Australian companies whose involvement was planned in the production of the cruiser tank. In the case of the hulls, they were produced by the Bradford & Kendall company in Alexandria, Sydney. 

The concept was the same as the French tanks. Cast components were assembled with bolts. This allowed manufacturers to avoid rivets and welding almost completely. However, the casting ability of Bradford & Kendall were much higher than those of the French. The AC I hull was one large casting with a transmission housing that attached at the front and engine compartment components from the rear and back. Only the floor was welded on. No other country could do this at the time. The turret was also fully cast.

The tank was still fairly modern even in the second half of 1942, when it entered production.

Other subcontractors were selected in April of 1941. Elements of the running gear and the tracks were produced by H.V. McKay Massey Harris Pty Ltd, which built tractors. Unlike the suspension, the tracks had purely American origins. The gearbox (a copy of the American tank gearbox) was supplied by Sonnerdale & Co. of Sydney. Coote & Jorgensen, another factory that was responsible for production of transmission elements, was also located in Sydney. Meanwhile, the appetites of the military increased. The mass of the tank was estimated at 25 tons by the spring of 1941. The armour was now 60 mm thick, with a possible increase to 80 mm. The armament was the same: a 40 mm gun and two water cooled Vickers machine guns. It was the machine guns that gave the tank its distinctive look. An armoured casting protected the hull gun's cooling jacket.

Layout of the AC I. Mobility requirements and a deficit of engines led to a triple engine configuration.

Despite the gradual increase in weight, the Australian army expected it to have high mobility. The bar was raised in the spring: now the tank had to reach a speed of 55 kph and have a range of 240 km. Meanwhile, optimism regarding engine selection waned. Requirements listed a 350 hp aircraft engine, but the real choice was the same V-8 from Cadillac. The necessary power could only be provided by a triple combo of engines. This was not three engines in one assembly, but three separate engines that combined their output in a transmission under the turret. Of course, these three engines could not be set up in a row, and the layout was no less original than the armoured cover for the hull machine gun. Two engines were placed in parallel, with one behind them. The first trials were held in April of 1941. Surprisingly, this strange combination worked fine. The engine power was reduced, and the combined output was 330 hp.

Some transmission elements were located under the turret. They did not get in the way as there was a turret basket.

Even though work on the AC I was underway by the spring of 1941, it was clear that it was behind schedule. In addition to technical issues, the military can be blamed for not being able to decide on requirements. The date for the start of production remained the same: November 1st, 1941. 340 tanks were expected by August 1st, 1942. In practice, only the first test hulls were completed in September and the first prototype was completed much later, in January of 1942. By this time the Battle of the Pacific was raging on. Australia was not left without tanks. Deliveries of Light Tanks M3 began in September of 1941. These tanks were perfectly adequate for the "Australian Cruiser" concept and were the first to be used in battle by the Australians. The country received 400 of these tanks, which filled up the armed forces. However, Australia had no intention of abandoning its domestic program. The AC I received another designation in February of 1941: Sentinel.

The first experimental hulls, including the armour for the first prototype, was produced in Newcastle. As for Bradford & Kendall, the first set of hull and turret components was assembled in early 1942, when assembly of an AC I prototype was already underway. The first issues cropped up with the hulls. During the design phase it turned out that the steel composition had to change, as Australia had trouble with nickel supplies. Armour steel indexed APB4 was developed where nickel was replaced with zirconium, which Australia had in droves. During production, it turned out that the armour had serious issues with hardening. As a result, the first dozen AC I tanks produced were made from mild steel.  New South Wales Government Railways (NSWGR) was chosen as the hull manufacturer, specifically Chullora Railway Workshops. The choice was simple: railway factories already had the necessary equipment (most importantly, for lifting).

AC I assembly process.

The second experimental AC I began trials in March of 1942. The first production tank was delivered in July. Despite all the delays, the AC I was still a modern tank even in the summer of 1942. The tank weighed 26 tons, full battle mass was estimated at about 28 tons. Even though the Australian Cruiser surpassed the Matilda in mass, it was quite mobile. The top speed was 38.4 kph in normal operation, and when the governor was modified to increase the engine RPM from 3000 to 3600 it could accelerate to 46.4 kph. The AC I was the fastest medium tank of the Australian army in WWII.

The first tanks of this type entered service in November of 1942.

The armour of the tank was also modern. According to requirements, the armour varied from 45 to 65 mm. Of course, one must consider issues that were encountered during hull production, but this was honest shellproof armour. The dimensions of the Australian tank are no less important. The overall height was 2534 mm, 35 mm taller than the PzIII Ausf.L, half a meter lower than the Medium Tank M3, and slightly lower than the Matilda III. The tank had the same turret ring diameter as the Matilda: 1372 mm. This was enough to fit three crewmen in the fighting compartment. The transmission was partially under their feet, but they did not trip over it as there was a turret basket. The ammunition capacity was higher than expected: instead of 120 rounds that were required the tank stored 130 as well as 4250 rounds for the Vickers guns.

The AC I was not used in battle, but only to train crews.

When the AC I was put into production, it was clear that the army's appetites do not match the realities of Australian industry. Limiting factors cropped up even at the hull production stage. No matter how they tried, Bradford & Kendall could not deliver more than five sets of armour per week. The aforementioned issues with hardening also impacted the rate of production. As a result, supplies of the AC I began only in November of 1942. The rate of production was much lower than what was required by the Australian army. However, it is hard to blame the Australian industry. It's impossible to make a modern tank, especially an original one, with no experience. There is no such thing as miracles: it takes several years to set up a proper design and production flow, especially if the tank produced is purely original and not a licensed copy.

The characteristics of the AC I were modern, but the army only ordered 65 tanks.

However, the issue was not just with production. Military trials showed a number of issues. One of the main ones was overheating. Considering the hellish system that the British developed, this drawback was to be expected. Do not forget that the engines in question were not even used on trucks, but on light cars. Another issue was the low lifespan of road wheel rubber rims. This was the cost for the constantly increasing protection requirements. The same problems were experienced by American tanks, especially after their mass grew to over 30 tons. Finally, the turret traverse mechanism worked poorly.

AC I tanks were used until the end of WWII and then were scrapped.

The biggest issue for the Australian tank industry was not mechanical in nature. In mid-1942 shipments of Matilda tanks began to arrive (200 were shipped in total) and Lee I, Grant I, Lee II, and Grant V tanks began arriving in December of 1942, with a total of 737 American tanks received. This was more than enough to equip Australian units. By the summer of 1943 there was no reason to continue domestic production. The tanks turned out more expensive, had serious issues with their use, and there was no hope of increasing the rate of production. There was no point in producing more of them past mid-1943, anyway. As of June 1943 65 tanks were built with serial number 8001-8065. No registration numbers were assigned. Production stopped here.

Instead of a trip to North Africa, the tanks were retained locally and used for training. Almost all were scrapped, only three survive to this day. Despite the issues that the AC I suffered from, it cannot be called completely unsuccessful. For the first try this was not a bad result, especially when compared not to German tanks, as the Australian army liked to do, but with the Japanese ones that they were likely to face. Even the 2-pounder gun was enough to defeat any Japanese tank, which in turn had limited means of penetrating the Australian Cruiser. The only medium tank that the Australians used in battle was the Matilda (American medium tanks remained in reserve). If the cruiser tank program was not closed, it could have received continued development.

The AC I was not the only tank developed in Australia. In the summer of 1941 Alan Chamberlain developed an alternative called AC II. This was a simpler and lighter AC I. The initial specifications called for a 19 ton tank with up to 57 tons of armour. Instead of three engines, it would have two. Later, the mass of the AC II grew to 22 tons. The result was closer to an infantry tank similar to the Valentine. This was the biggest impediment for the AC II. The Australian army was sceptical of the Valentine and did not want an equivalent tank. The AC II was not even built as a prototype.

The second AC I received a new turret with a 25 pounder gun.

Another tank, also a direct successor to the AC I, had a different fate. Everything started with the armament. Even as the AC I was put into production the armament seemed insufficient. In addition to North Africa, where the 2-pounder was not very effective by mid-1941, information began to arrive from the UK. Preparation for a 6-pounder (57 mm) gun was underway. It had superior penetration characteristics. With information about the new gun, changes were made to the AC I turret that would later allow for it to be installed. This tank, called AC IA, remained on paper, but the turret was put into production. This ended poorly: 6-pounder guns never arrived, but the designers ended up with turret traverse issues. Expecting a larger gun, the turret was unbalanced, which had an effect on the traverse mechanisms.

AC III hull. The hull machine gun and gunner were removed. The upper front plate is sloped.

As it was obvious by late 1941 that the 6-pounder is delayed, Watson began to seek an alternative. The search was short, as there was only one other gun: a 25-pounder (88 mm) field gun. Its penetration characteristics were somewhere between the 2-pounder and 6-pounder. These guns were often used as anti-tank guns in North Africa. The field gun was much larger than an anti-tank gun, but with some changes it could fit into the turret. The first designs of a 25-pounder tank gun were produced in December of 1941, and two such guns were allotted for the AC IB in February. Unlike 6-pounders, there were no problems with obtaining these guns.

Perrier-Cadillac 41-75. In addition to greater power, this engine had no issues with overheating.

The second production AC I was used as a test bench. A turret with an enlarged bustle was produced, the gun mount was also changed. The turret was delivered in June of 1942 and trials were held. It turned out that the 25-pounder gun worked well. Even separate loading did not result in significant issues. However, the Australian commanders had differing opinions. On one hand, the AC I was earmarked for North Africa, and either a 6-pounder or a 76 mm 17-pounder were ideal. On the other hand, both guns were yet to be delivered. A decision was made to build an improved AC I with a 25-pounder gun.

Full sized model of the AC III.

The modified vehicle was indexed AC III and called Thunderbolt. It was desirable to put it into production immediately, but the chassis also needed improvements. Experience in North Africa showed that the hull gun was a vulnerable spot, and removing it from the upper front plate was the correct decision. The hull gunner was removed as well, as he was no longer needed. The slope of the front plate was increased. As a result, the AC III was better protected than any American or British medium tank of the time.

AC III prototype. As the Australian tank program was closed, production was cancelled.

The next item to be changes was the power plant. When the AC III was being designed, results of the first AC I trials had already come in. The overheating issue was known. A solution was obtained from the Americans. The Chrysler A 57 Multibank engine was produced out of 5 smaller engines for the Medium Tank M4A4. The Australians could not build a similar beast for their tank, but engineer Robert Perrier managed to make an analogue. He combined three Cadillac engines in one frame, giving the Perrier-Cadillac 41-75. It was more compact than the Chrysler Multibank, so the engine compartment did not need to be changed. The Perrier-Cadillac 41-75 solved the issue with cooling and was also more powerful. It gave 397 hp, which accelerated the tank to 48 kph and gave it a higher speed off-road.

A stand-in for the 17-pounder. A turret with two 25-pounders was installed in the second AC I tank.

The first AC III prototype was ready in January of 1943. Even though the vehicle did not look very different from the AC I, it was a big step forward. In addition to improved front armour, it had a better engine and more powerful armament. The tank with serial number 8066 was not the last step. It was proposed to increase the thickness of the front hull to 75 mm and the turret ring diameter to 1600 mm. Even in its current state, the tank did not look too bad. Production of 150-200 tanks was planned, but preparations were stopped in the summer of 1943. The North African campaign ended and Australia had enough British and American tanks. It was senseless to spend money on making another tank and only one AC III was ever built. This tank got lucky and survives to this day.

A 17-pounder gun mount for the AC IV.

The last tank developed by Watson was the AC IV. This was not a whole new vehicle, but an alternative development of the AC III. The idea of installing a 17-pounder gun was not abandoned. However, it was clear that it would not fit easily. A new turret with a 1626 mm turret ring was developed. The gun was altered to move the recoil elements inside the fighting compartment. Shipments of the gun were delayed, and so a trick was proposed. The turret was installed on the converted first AC I prototype, and the turret was equipped with a pair of 25-pounders to imitate the recoil of a more powerful gun. The result was a strange volley-firing hybrid.

The tank with a 17-pounder gun installed.

The first trials with the dual 25-pounders began in October of 1942. A mount for the 17-pounder was produced in late October. This was the first tank with this gun (the A30 doesn't count, as it was a tank destroyer). The converted tank began trials on November 11th and passed them successfully. This was cause for raising the issue of mass production. It would be different from the AC III. The turret ring diameter would be increased to 1823 mm (even bigger than the Tiger's). Ammunition capacity was to be increased to at least 74 rounds.

Model of a 510 hp Quad-Gypsy engine. This engine was planned for installation into the AC IV.

The new engine was another important change. It would be a radically converted variant of the Havilland Gypsy Major, which was produced at the Holden factory as of 1941. An opposite engine designated Quad-Gypsy was developed there. It put put 510 hp. This engine would have increased the mobility of the AC IV even further. There was not enough time to install the engine into a tank, only a demonstration unit was produced. As work on Australian tanks ceased, it was never put into production. As for the AC I with a 17-pounder, it survives to this day.

Strangely enough, the decision to cease production of its own tanks was correct for Australia. There were no longer any reasons to continue production in the summer of  1943. Even with all of its changes the Australian Cruiser would not have been a widespread tank. If nothing else, war is a detriment to the economy. The Matilda and Lee/Grant tanks were enough for the Australian army. In the future, the Australians kept to this model, first buying Centurion tanks, then Leopard 1, the M1 Abrams. It was easier to buy tanks abroad than set up production domestically. As for the Australian Cruiser, it was a perfect example that an original modern tank could be created quickly, given the desire.