Sunday, 17 July 2016

Light Tank M3: America's First Thousand

American tank building fell behind those of other nations during the interbellum period, but rapidly closed the gap. In May of 1940, mass production of the Light Tank M2A4 began, a tank that caught up to other members of its class, and surpassed them in speed and armament. At the same time, the Americans realized that the war in Europe will last a long time, and tanks and guns grow obsolete quickly. This was the trigger that resulted in the Light Tank M3, the first American tank to result in more than a thousand mass produced vehicles.

Logical Modernization

By the time the M2A4 entered production, the war in Europe was raging on for six months. Naturally, the Americans were watching closely and making their own conclusions. For example, it was clear that armour which can only protect from large caliber machineguns and autocannons was not enough. In Poland, the main killer of tanks was already anti-tank cannons. The campaign in France only highlighted the fact that tank armour needs to grow. In addition, tanks armed with 20-25 mm cannons were the minority in the French campaign. Thick armour was becoming more and more important. Recall that Gladeon M. Barnes proposed 1.5" (38 mm) thick armour back in 1938.

The Light Tank M2A4, despite its outstanding characteristics, had many drawbacks. The length to width ratio was small, and if it was not a problem with 8-9 ton tanks, then the 12 ton M2A4 had a tendency to rock back and forth. The solution was simple: lengthen the contact surface between the tracks and the ground. The design of the observation devices was also not the best, and the massive gun mantlet had its weak spots.

Unlike its predecessor, the Light Tank M3 made no attempt to hide its rivets.

On June 3rd, 1940, the OCM signed statement #15864, according to which, tanks planned for production in 1941 must have 1.5 inches of armour. They didn't stop at thickening armour, and on July 5th, statement #15932 was signed, designating the new production tank Light Tank M3. This was a reasonable decision, as little remained from the Light Tank T2E1, which was the basis of the M2A1. The new tank also had many differences from its predecessor, more than just its armour.

Unlike the Light Tank M2A4, which was tested for nearly a year, the Light Tank M3 hit the assembly lines immediately. The American Car & Foundry Company launched production in March of 1941, and the first sample arrived at Aberdeen only in April. This also seems like a reasonable decision: even though the M3 was a new tank, there were no radical differences. The engine and transmission remained the same, the suspension, aside form the idler, was also the same, and the mass grew only slightly (from 11.6 tons to 12.7 tons).

The transmission cover became cast, and the gun mantlet lots its long bump protecting the recoil brake.

At first glance, the M3 looks like a step back. The hull and turret are covered in rivets, while the M2A4 had fewer visible rivets. The first impression is wrong: the M2A4 used no fewer rivets than the M3 did, but some of them had flat heads. If the design of the hull changed, it was for the better. For starters, the thickness of the front armour grew from 25 mm to 38 mm, reliably protecting from the German 37 mm gun at a range of half a kilometer.

The thickness of the transmission cover was even more impressive: 44 mm. Keep in mind that cast parts have poorer shell resistance and 44 mm of cast armour was more or less equivalent to 38 mm of rolled armour. However, the new transmission cover was made from one piece, which made it both more resistant to shells and easier to make.

The designers also removed the side ports for the driver and the radio operator. Even though the visibility on the march was reduced, the tank lost parts that could have fallen off from being shot at by even a high caliber machinegun. The thickness of the sides and rear remained the same: 25 mm.

The rear had many more noticeable changes. The tank received a small "tail", which allowed simplification of the cooling system. Instead of a grate vulnerable to grenades and Molotov cocktails, the engine now had a well protected air intake. Only the air filters remained outside, and they were also changed. The new idlers were also a noticeable change. They were larger and moved further back. Thanks to this solution, the length of the contact surface increased, which reduced the tank's oscillations and ground pressure.

The amount of ports on the turret decreased, and the rear changed noticeably.

The turret and armament of the tank were mostly the same, but there were many changes here too. The shape of the turret remained, but the number of ports was reduced from seven to three. The remaining ones opened outwards, which helped with shell resistance. The thickness of the turret armour was increased to 38 mm, but only of the front plate.

The gun mantlet was also thickened, and it became a lot more compact due to the gun losing its long recoil brake. The new gin mount was indexed M22 Mount. One often forgotten feature is worth mentioning. Both the M20 and M22 mounts had the ability to precisely adjust the aim of the gun without moving the whole turret. As a rule, this features is only mentioned for Japanese tanks, but it was already present on the Renault FT. The feature persisted on the M1917 light tank, the T1 light tanks, the medium M1921/T1, T2, and T3. American tanks only lost this feature completely in 1942.

In Search of an Ideal Turret

Even before the first Light Tank M3 left the assembly line, the Ordnance Committee began looking into improving its design. This seems unusual, but it's fairly normal practice. It's worth noting that no radical changes were planned, but there was room for improvement.

Light Tank M3 with the D38976 welded turret.

The main complaint of the Ordnance Committee was the tank production technology, especially in the turret. A riveted design has one drawback for the crew: if a shell hits, even if it does not penetrate, the rivers on the inside fly off. As a result, several hits could disable the crew without penetrating the armour. A logical decision was to replace the riveted D37182 turret with a welded one, signed by the Committee on December 27th, 1940.

Tank with D38976 turret, view from the right.

The new turret was named D38976. It was almost the same as its predecessor, the changes only touched the assembly process. The turret was tested on the tank with serial number 279, and soon after launched into production. As a result, tanks with the initial D37182 turret were few in number. Of course, it was not possible to use welds entirely. The M22 Mount came off with the front plate of the turret, which was held on by bolts. Due to this peculiarity, all Light Tank M3 modifications retained this feature.

Tank with the D39273 turret with curved sides and a splash protection ring.

The next round of changes to the turret was on March 24th, 1931. This time, Committee decree #16583 added a special ring that would protect the turret ring from bullets and shells. This was a sound decision, but it caused a change to the whole turret.

Tank #1946 and its new D39273 turret arrived at Aberdeen in November of 1941. As a result of these changes, the only things left from the old turret were the gun mount, AA machinegun mount, and turret ring. The turret became not only welded, but curved, with a ring of armour welded to the bottom. The commander's cupola also changed its shape, becoming a lot smaller. The observation devices also changed shape, and became a lot more comfortable to use. The new turret entered production in late 1941.

The slot for a periscope in the hatch can be seen clearly, but it was not often installed.

The final steps in the evolution of the M3's turret was the Ordnance Committee's decision to replace the gun with the M6 37 mm tank gun. While the M5 tank gun differed little from the M3 anti-tank gun, the new gun had a different barrel and breech. The gun, standardized on November 14th, 1940, was to be installed in the new M23 Mount, equipped with a vertical stabilizer. The stabilizer left no room for a "precise aiming" system. This new gun mount was tested in May of 1942, and it was installed in a new turret where the commander's cupola was replaced by two hatches.

It's hard to say that the crew lamented the loss of the cupola, as it was rarely used in combat. However, the stabilizer was met with mixed feelings. In the M2A4, the horizontal traverse was controlled by the commander (also loader). This was not a problem until now, as the "precise aiming" system allowed additional traverse of 20 degrees, but in the stabilized variant there was no longer any ability to aim horizontally. The gunner was left with only the travel lock. Seeing how their crews suffered, the Americans revised this fun layout, but only for their army. The portion of tanks built with new turrets were generously shifted to the British, plus the Soviet Union received some amount.

One of the new tanks with an M23 gun mount.

Unlike the turret, there was no similar experiment with the chassis. Like the Light Tank M2A4, most tanks were equipped with the Continental W-670–9A engine. Its power was enough to accelerate it to 50 kph (the speed that is set in the TM9-726 field manual). Of course it was short of the M2A4's 58 kph, but it was enough to solve the tasks the tank faced. 4525 tanks were built with this engine.

In June of 1941, tanks with the Guiberson T-1020–4 diesel engine entered production. Externally, the tanks with diesel engines differ in their piping and air filters. 1281 of these tanks, designated Light Tank M3 (Diesel) were produced. Officially, production sopped in August of 1942, but the last 4 tanks were delivered in January of 1943.

Extra fuel tanks during trials.

The only significant change to the chassis was made in 1942. Its cause was British complaints that the tank could only travel for 120 km with a gasoline engine and 144 km with a diesel one. The fix was simple: an addition of two 95 L fuel tanks. The tanks were connected to the main fuel system. Since they presented a tempting target, they were made from self-sealing material like aircraft fuel tanks. Unfortunately, this limited the traverse of the gun to 90 degrees in each direction. It was easy to deal with this drawback in the desert, especially since the tanks could easily be removed.

Light Tank M3E1 equipped with the Cummins HBS-600 engine.

There was a point in time when the M3 could have had an alternative engine. The aircraft engine that it used was far from economical, and had problems with power loss at low RPM. Because of this, a program was launched in March of 1941 to test truck engines in the M3 tank. The Cummins HBS-600 engine, a turbocharged version of an engine used in heavy trucks, was proposed. The engine produced 230 hp, comparable with the Guiberson T-1020–4. 

In November of 1941, the Aberdeen Proving Grounds received the converted tank, indexed Light Tank M3E1. Trials showed that the Cummins HBS-600 greatly increased fuel efficiency, plus it was more robust. However, there was one small problem. The engine was noticeably longer than the Guiberson T-1020–4, which made it necessary to lengthen the engine compartment. In May of 1942, after trials and deliberations, the change was declined.

Debut at the Front

The appearance of the Light Tank M3 coincided with the mobilization of America's industry for war. In the spring of 1941, the United States was not yet involved, but all signs suggested that it wouldn't be able to stay that way for long. Shipments of tanks to the British already began, and relations with Japan grew more and more tense. It is not surprising that the tank's production surpassed the M2A4 by an order of magnitude.

A lack of armament was not a rarity on tanks with the D37182 turret.

The first tanks entered service as training vehicles, some without armament. Tanks with the D37182 turret were retained for the training role in the US Army. Knowing the issues that arose when shells hit riveted armour, the tanks were all sent to training centers. These tanks were almost not used by the Allies, just the British received a small amount. They are seen infrequently on film, only acting as training tanks, often without armament. However, even in this condition, they were more useful than the previously used training tanks.

Light Tanks M3 with the D38976 turret had a different fate. These were tanks sent to the British forces, and these were tanks that Americans themselves first tasted battle in. In September of 1941, the 194th Tank Battalion commanded by Lieutenant Colonel Miller was sent to Luzon (Philippines) to strengthen the forces stationed there. The battalion was placed in Manila, the capital. In November of 1941, it was joined by the 192nd Tank Battalion commanded by Major Wickord. Their task was the protection of Clark Air Base.

One of the tanks that fought at Luzon.

The Japanese landing at Luzon began on December 8th, the day after the attack on Pearl Harbour. Five days later, the Japanese reached Clark Air Base. The American tanks were no longer there. In late November, the 192nd and 194th battalions were united into one tank group commanded by Colonel Weaver. Understanding that the tanks were an important reserve that shouldn't be wasted, the Americans withdrew them to the Bataan peninsula.

The American tanks went into battle on December 22nd. 5 tanks under the command of Lieutenant Morin supported the cavalry. The tanks were on the march when they suddenly encountered the Japanese 4th Tank Regiment commanded by Lieutenant Colonel Kumagaia, consisting of 38 Ha-Go light tanks. Qualitatively, the opponents were equal, but the Japanese had the quantity advantage. Morin gave the order to retreat, but it was too late. The Lieutenant's vehicle was hit and caught fire, the wounded crew was taken prisoner. The remaining tanks managed to get away after taking some hits, but were finished off by Japanese aircraft, which dominated the skies.

Later, the Japanese 7th Tank Regiment commanded by Colonel Sonoda, which was armed with light tanks including two Shinhoto Chi-Ha, also fought in the Philippines. However, the American tanks mostly fought with Japanese infantry that was moving onto the Bataan peninsula. During the retreat, 24 tanks out of the 194th battalion's 54 were lost. The Americans fought until April 8th, 1942, when they received the order to destroy their own tanks. Nevertheless, the Japanese captured about 20 intact Light Tanks M3. These trophies filled out the ranks of the 7th Tank Regiment. The fate of the officers and soldiers of the 192nd and 194th battalions was not an enviable one: most of them died during the Death March and in camps.

Light Tank M3 knocked out during battle at Guadalcanal.

The next application of the M3s by the Americans was at Guadalcanal. Here they were used by the Marines, and used alongside the Light Tank M2A4 and Light Tank M3A1. Tanks from the 1st Tank Battalion fought until January of 1943. These battles were the last for the Light Tank M3, as superior tanks replaced them in the US Army by that point.

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