Monday, 10 July 2023

A Medium Tank with a Heavy Burden

The heaviest American tank at the start of the Second World War was the Medium Tank M2. It looked like an anachronism compared to other tanks in the same class, and so it was quickly replaced by the Medium Tank M3. The M3 was also a temporary measure, and even having completed the Medium Tank M4 the American tank designers were not resting on their laurels. Work on the Sherman's successor began as the tank was just being put into production. The Medium Tank T26E1 was meant to replace the Sherman, but after a number of changes in its development cycle it entered production in a completely different weight class.

Origin of species

The concept of a new generation of tanks formed in May of 1942. The basic tank had a 76 mm gun and was lower than the M4, which allowed the designers to add more armour without exceeding the weight of its predecessor. The tank also used an automatic gearbox. The Ordnance Committee gave permission to build two prototypes indexed Medium Tank T20

The number of experimental tanks multiplied. Since it wasn't clear how well the idea of an automatic gearbox is going to work out, the army decided to play it safe and also build the Medium Tank T22 using components already tried and tested in the Sherman tank as well as the Medium Tank T23 with an electric transmission that showed itself well in the Heavy Tank T1E1. Each tank had three types of armament. The basic tank would get a 76 mm M1 gun, E1 variants were equipped with a 75 mm gun and an autoloader, E2 variants received the 3" M7 gun from the GMC M10. There was also an E3 variant. These tanks had the 76 mm gun but also a torsion bar suspension.

The Medium Tank T23 surpassed the Sherman in both armament and armour, but the army's appetites had grown beyond what it could offer.

Work on prototypes continued until early 1943, at which point the T23 was recommended for production. The recommendation called for a sizeable pilot batch of 250 vehicles. However, the army's appetites had grown. The Allies found out about the Tiger tank. Even the new 76 mm gun didn't penetrate this newcomer at a range considered adequate. The Americans had no heavy tank that could fight a Tiger as its equal.

The idea to arm a tank with a 90 mm gun came up in October of 1942, before anyone knew about Tigers. This idea came in handy in 1943 and the concept of the new tanks took shape on April 24th, 1943. In addition to the basic T23, a Medium Tank T25 with a 90 mm T7 gun and a T26 with the same gun and "armour equivalent to that of the German Mk.VI tank or surpassing it". Preliminary estimates showed that if the production Medium Tank T23 weighed 30 tons, the T25 would weigh 33 tons and the T26 36 tons. 50 vehicles were reallocated from the pilot order: 40 T25 and 10 T26.

90 mm M1 AA gun. The Americans wanted a gun with the same ballistics in their tank.

The plans had to be corrected almost immediately. The T23 ended up weighing 34 tons instead of 30 and the designers started looking for ways to slim down the heavier variants. Since the electric transmission added a lot of weight, it was done away with. The variants with a torque converted instead of an electric transmission were called T25E1 and T26E1 respectively. The tanks had a similar design. Both had hulls welded together from rolled armour, and only the V-shaped front was cast. The T25E1 had 76 mm of armour in the front and the T26E1 had 102 mm. In both cases the armour was sloped at 46 degrees. The T26E1's turret had 76 mm thick side and rear armour. The gun mantlet that spanned the whole width of the turret front was 114 mm thick.

The improved armour came at a cost. Even the lighter T26E1 weighed in at an impressive 81,300 lbs (37.8 tons). To compensate, the tracks were widened to 24 inches (61 cm). The road wheels were also widened. The transmission gear ratios changed to allow the 500 hp Ford GAF engine to deal with such a heavy tank.

First one out

Production of prototypes began in February of 1944 and ended in May. Two of the ten experimental T26E1 tanks ended up at the Aberdeen Proving Grounds and one was sent to Fort Knox. Trials began before the whole batch was finished.

One of the experimental tanks from the Aberdeen Proving Grounds, August 3rd, 1944.

The first T26E1 arrived at the Aberdeen Proving Grounds on March 6th, 1944, and drove for 820 miles (1320 km) by March 24th. Even though the tank was built largely with pre-existing components, there were still problems. The bolts that held the transmission and the engine flywheel together loosened while the tank was driving. The drive sprocket wore out twice as fast as the one on the Sherman. The tester blamed the rear transmission, since a drive sprocket in the back of the tank was more exposed to abrasive elements.

The crew's conditions were also far from ideal. The gunner's station was badly laid out. The loader found it very difficult to retrieve ammunition, particularly ammunition stowed under the fighting compartment floor. There was also very little ammunition: the tank only fit 42 rounds while crews demanded at least 70. The extractor fan was very weak and the tank turret was full of fumes after shooting. The gun was derisively called "one-shot", partially because the first shot kicked up a huge cloud of dust, making follow up shots impossible. Additionally, the rubber lining around the turret ring let in water.

The same tank from the rear.

The tank had travelled 1200 miles (1930 km) by May 18th. Trials revealed many additional problems. For instance, the brakes overheated and had to be adjusted every 25 miles (40 km). The heavy tracks tore off the return rollers during driving. The second prototype arrived in late March and went through special trials 351 miles (565 km) long. It turned out that the tank did not fit on any pontoon bridge with the exception of the 25 ton model. The brakes on this tank also overheated.

Meanwhile, the tanks at Fort Knox were undergoing gunnery trials. The testers here also noted that it was hard to retrieve ammunition and impossible to correct fire due to the fumes inside the turret and dust outside. The turret also turned by about a mil every time the tank fired. On the other hand, the torsion bar suspension received a good evaluation.

View from above. The T26E1 was wider than the Sherman, which led to problems.

The location of the ammunition led to a whole separate trial. The testers pulled out all of the ammunition racks and the turntable from the floor of the fighting compartment. The racks were replaced with crates that held eight rounds each. The inserts folded automatically after ammunition was retrieved. In addition, an 11-shot ready rack was added to the wall next to the loader. The 2" bomb thrower and rack of smoke bombs for it had to be sacrificed to fit the ready rack. The tank's ammunition capacity grew to 59 rounds. The testers noted that if the wet ammo rack walls were removed, 12 additional rounds could be carried. 

This tank also had transmission troubles. The torque converter broke after 184 miles (296 km) and 25 hours of operation. The width of the track was judged to be insufficient. Even though the tank was already wider than any acceptable limit, the question of track extensions was raised. The rubber tires were also an issue. On one tank, eight tires were replaced after an average service of just 8.6 miles (14 km). The suspension arms loosened after short drives as well. This issue was not caused by excess weight as the T25E1 had the same problem. Track links also broke often.

Improved ammunition stowage in the T26E1.

Final ammunition stowage layout. By discarding the wet ammo rack, the tank could carry 12 additional rounds of ammunition.

The new tanks were tested in the summer and fall of 1944. In addition to experiments with the engine and transmission, various improvements of the gun and turret were tested. A muzzle brake was requested for the 90 mm gun, and the gun mount was altered to remain in balance both with a muzzle brake and without. The elevation mechanism, cradle, and travel lock were reinforced to accommodate the muzzle brake. Since the British were also interested in the new tank, the turret bustle was modified to accept either an American or a British radio. A 10 round ammo rack was added into the fighting compartment and the turntable was removed. The ammunition racks on the floor were different than the ones used in Fort Knox. Now the ammo was held in two crates of 12 rounds each plus six on the sides in the places where the water reservoirs used to be.

The troubles with the 90 mm gun were gradually resolved, but soon a problem was discovered with the tank's other special feature: the armour. Shooting 76 and 90 mm rounds at the cast front hull showed that the armour had poor toughness and was very brittle. The bulges for the machine gun and driver's compartment exhaust fan as well as the edges near the running gear were particularly vulnerable.

The tank could fit on the 25 ton Trestle Bridge, but the bridge would not be able to hold it. Note that the word Medium is crossed out and Heavy is written in.

These problems had to be resolved as quickly as possible. New components were sent straight to proving grounds for installation into the experimental tanks. The Armor Board stirred up a storm of correspondence. Fisher and Chrysler were supposed to begin producing 800 of these tanks a month by the end of the year, but the design was so raw that it was not yet clear which components would go into mass production. As it often happened, the tank gained weight along with all these improvements. By the end of the summer of 1944 the tank weighed 41.7 tons. Even before that, on June 29th, it was upgraded from the medium class to heavy.

It became clear that the T26E1 would not be battleworthy without radical changes. The issue of fire correction was not resolved. Neither the gunner nor the commander could observe a falling shell. The ventilation and splash protection of the tank were inadequate. Every member of the crew as well as the gun could be disabled by a lucky hit from bullets or shell splinters. The new ammunition rack layout was approved, but not yet tested. Almost all of the tank's components suffered from some kind of design or manufacturing defect. The suspension was the most trouble prone part. Out of 3706 man hours spent on repairs, repairs of tracks and suspension took up 1112 hours. The engine came second at 846 hours. In total, 100 hours of operation required 530 hours of work by mechanics.

A crane to extract the power plant could be attached to the turret.

The critical situation with the Heavy Tank T26E1 did not mean that production would not go forward. Requirements composed in mid-1944 for tanks that were in demand for 1945 covered two classes of vehicles: a fast one for breakthrough exploitation and a heavy one for supporting infantry during a breakthrough. The Medium Tank T25 died here. Even though it had a 90 mm gun, no one was considering cutting production of the Sherman or even the Chaffee for it. On the other hand, there were no other contenders for the role of a heavy tank. The improved T26E1 was the only one that could fill it. This tank was designated Heavy Tank T26E3, but that's a story for another time.

Experimental Electrification

The T26 didn't disappear entirely. The Ordnance Department authorized the completion of two experimental tanks, although the order was cut down to one on May 6th. The tank with an electric transmission was not built anew. The running gear and tracks came from one of the T26E1 tanks sent to the Aberdeen Proving Grounds. The engine and transmission were the same as those used on the T23. The hull was new. The 102 mm thick front armour was now rolled instead of cast. The T26E1 turret incorporated improvements. For example, the turret basket was eliminated. The new tank weighed just over 43 tons.

The tank arrived at Fort Knox on October 28th, 1944. Trials began on November 30th. The ground pressure was acceptable, 0.84 kg/cm². The tank reached a speed of 28 mph (45 kph), a decent speed for a heavy tank. The T26 was quite agile in general. It was much easier to drive it than the tank equipped with a torque converter. The T26 accelerated better than the T26E1 on flat terrain and slopes less than 11.6 degrees. On steeper slopes, the situation was the opposite.

Electric transmission on the Heavy Tank T1E1. The transmission had no luck either here or in the T26E1.

The Heavy Tank T26 had travelled for 993 miles and driven for 172 hours by February 3rd, 1945, but interest waned afterwards. By March 24th the tank had worked for 226 hours and driven 1179 miles, by May 5th - 280 hours and 1500 miles. Work on the tank ceased almost completely after VE Day. By June 9th, it had driven for 290 hours and travelled 1558 miles (2500 km). By this time it had completed 75% of the planned trials, but sat idle due to a broken generator. By July of 1945 the tank had driven for 302 hours, after which the engine was replaced. The new engine worked for only 11 hours before the project was closed. There was no point in testing the tank any further since neither it not any other tank with an electric transmission was going into production. The cause was the same as for the Medium Tank T23: the electric transmission was considered too complicated for American mechanics.

The 30 ton medium tank intended to replace the Sherman gained more than 10 tons in mass by the end of its development. These cases are not uncommon in the history of tank building. Typically, such a growth in mass leads to catastrophic consequences. Reliability takes the biggest hit, as seen with the T26E1. However, the Americans did not give up. Even though the T26E1 was far from perfection, the general concept proved correct. Even such unrefined prototypes paved the way to the most powerful mass produced American tank of the Second World War: the M26 Pershing.

  • R.P. Hunnicutt. Pershing: A History of the Medium Tank T20 Series — Echo Point Books & Media, 2015
  • NARA
  • Archive of the Canadian Military Headquarters, London (1939–1947), RG 24 C 2
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