Monday 21 June 2021

Intermediate Experiment

The American program to create a replacement for the Medium Tank M4 was a mess by the end of 1943.  Three types of medium tanks were being tested in parallel. These included the Medium Tanks T20 and T22. Two prototypes of each were built, and proven to be ultimately disappointing. There was also the Medium Tank T23. It turned out to be better than its competitors and even put into production, but it turned out that there were issues with using it. Finally, development of two more tanks was launched in May of 1943. This made for five medium tanks in development at the same time. This article will describe the fate of the Medium Tank T25, an intermediate vehicle.

Same thing with a bigger gun

The Medium Tank T23 had the highest priority as of the spring of 1943. A prototype was available before its competitors, and the electromechanical transmission was easy to handle and reliable, which could not be said about the planetary and mechanical gearboxes of the Medium Tanks T20 and T22. Because of this, a proposal to put the T23 into production was already tabled by May 1943. It seemed that this would be the best way forward.

90 mm gun M2. Like the German, the Americans used their AA guns against ground targets.

Meanwhile, the Americans began thinking about replacing the 76 mm M1A1 gun back in the fall of 1942. At first this applied only to the Heavy Tank T1, as if the 76 mm gun fit into a medium tank, then a heavy tank should have something more powerful. A candidate was soon found among AA guns. The 90 mm M1A1 AA gun, the main American medium AA gun, surpassed the German 88 mm Flak 18 in every way. An order was given to develop a gun indexed the M2 in September of 1942. This gun could also engage ground targets. There were no foreseeable issues with turning the M1A1 into a tank gun as well.

90 mm gun T7 developed on the basis of the 90 mm AA gun.

The tank gun was indexed T7. The first prototype was finished in late November of 1942, but it was installed in the Heavy Tank T1E1 much later, in March of 1943. The trials were successful, but the Heavy Tank M6's star was already setting. The tanks were rejected in December of 1942, and on March 25th, 1943, it was decided that only 40 units would be produced. The 90 mm gun was without a chassis. There was already a worthy target for it by this point: the heavy Tiger tank. Fighting in March of 1943 showed that the 76 mm gun could penetrate it, but not at a long distance. The Americans turned to the Medium Tank T23, the most promising tank on hand by the spring of 1943.

The first experimental Medium Tank T25 soon after assembly at the Detroit Tank Arsenal.

The Ordnance Committee decided to build a series of 250 Medium Tanks T23 on May 6th, 1943. Another decision was made on that day. 50 more tanks armed with 90 mm guns would be built. Of those 40 would have the same armour thickness as the Medium Tank T23. These tanks were initially called T23 equipped with 90 mm gun, but later renamed to Medium Tank T25. 10 more tanks would have their front armour thickened to the level of the Tiger. This tank was later named Medium Tank T26. This tank deserves a separate article, and so we will not discuss it. As for the T25, work started in July of 1943. A full sized model was built. It was not built from scratch, but converted from the Medium Tank T22.

The gun is locked in travel position.

Initial requirements called for a weight of 32,659 kg. These requirements were unrealistic since the start, since the Medium Tank T23 already weighed 32,885 kg. The Medium Tank T25 also had to have a torsion bar suspension, which was heavier than the VVSS. The turret also needed changes. Theoretically the 90 mm gun fit into the stock turret, but it was a nightmare to service. The Ordnance Committee began to suspect something by the fall of 1943. By that point a production Medium Tank T23 was found to weigh 36,741 kg. This meant that the transmission's weight limit was already surpassed by a long shot.

The hull of the Medium Tank T25 was not so different from the hull of the T23.

The Ordnance Committee had to adjust their plans. The order for 40 tanks remained, but now they were equipped with the Alisson Torqmatic planetary gearbox and a torque converter. A decision was made to build two prototypes converted from the Medium Tank T23. The tanks with a torque converter were renamed Medium Tank T25E1.

The biggest difference was the new HVSS suspension as well as wide T66 tracks.

There were several reasons to build the two additional tanks. First of all, the Torqmatic gearbox was struggling not just on the Medium Tank T20, but also on the GMC T70. The latter was already in mass production, and issues with the transmission were being resolved on the fly. The process only finished by the spring of 1944. The Ordnance Department wanted to have a backup plan in case the transmission was a dead end. Second, the torsion bar suspension was also not doing well at the time. The Medium Tank T20E3 with this suspension was not only heavier than the ordinary T20, but also less reliable. A backup plan was needed here as well.

Inside the Medium Tank T25's fighting compartment.

The Medium Tank T23's stock suspension was also far from a universal solution. The suspension came from the Medium Tank M4, which was not designed for such a mass, and so an experiment was performed. Work on the HVSS suspension began in the spring of 1943. This suspension was a variant on Harry Knox's design with horizontal springs. Experiments were also underway on an improved track link. The Medium Tank M4 only got heavier and ground pressure increased as a result. One of the solutions was the 584 mm wide T66 track. This combination (the HVSS suspension and T66 tracks) was used on the Medium Tanks M4 and T25.

The same tank during trials at the Aberdeen Proving Grounds. Here it was seen by Soviet specialists. According to them, the Americans evaluated this suspension as superior to the torsion bar suspension.

Hulls for Medium Tank T25 pilots were built at the Midland Steel Corporation. The welded hull of the pilot tanks was identical to the T23 hull with the exception of mountings for the HVSS suspension. The driver and assistant driver hatches as well as some internal equipment also changed. General Steel Casting Company produced the turrets that were then equipped at Chevrolet. Unlike the chassis, the turret was radically altered. It became larger, with both the height and the size of the turret bustle increasing. The gun mount was also altered to accept the 90 mm Gun T7, receiving the index T99. The mantlet became more rounded. This decision is contestable, as there was a risk of ricochet if a shell hit the lower part of the mantlet. 

The second Medium Tank T25 pilot on trials at Fort Knox.

The first Medium Tank T25 with registration number U.S.A. 30103053 was completed at the Detroit Tank Arsenal in January of 1944. After a 400 km trial run the vehicle was sent to Aberdeen Proving Grounds on January 21st. It turned out to be heavier than calculated: 37,426 kg. Trials showed that it was almost as mobile as the Medium Tank T23. The top speed was measured at 55 kph. Interestingly enough, this vehicle was shown to the Soviet commission in the summer of 1944. It was mistakenly reported to be the T25E1. The Soviet report mentioned that American officers considered the HVSS suspension to be superior to torsion bars. They said that this suspension was more reliable and should be used on new generation medium tanks.

The second tank incorporated feedback from the first vehicle and thus looked somewhat different.

Despite such statements, there were also some issues with the HVSS suspension. Assembly of the second Medium Tank T25 was delayed by several months because of it. The tank with the registration number U.S.A. 30103054  arrived at the Aberdeen Proving Grounds much later, on April 29th, 1944. This tank was even heavier: 38,197 kg. 416 kg of the additional 771 was gained in the running gear. The tracks got heavier and the road wheel tires were thickened. There were some changes introduced into the turret as well. A rack was added on the right side and a port for discarding spent casings was added to the left. This turret was also approved for installation on the Medium Tank T25E1, although a portion of the tanks had the same turret as the first T25.

The biggest difference was the turret. It had a slightly different configuration.

The second experimental tank was sent to Fort Knox where it underwent additional trials. The second prototype underwent additional changes during the trials. T66 tracks were replaced with T80 tracks. These trials were academic at best, as the torsion bar suspension had priority by the fall of 1944. Nevertheless, several Medium Tanks T23 received the HVSS suspension with T80 tracks after these trials.

Experimental series

As mentioned above, the decision to produce two Medium Tanks T25 coincided with the order for 40 Medium Tanks T25E1. Fisher Body, now renamed Fisher Tank Division, was given the contract. This choice was no accident, as the Medium Tanks T20 and T20E3 with a torque converter were built here as well. The factory already had experience building a tank with a torsion bar suspension. This experience was mixed. The GMC T70 and Light Tank T24 more or less worked, but the medium tank's suspension had a lot of issues that needed to be ironed out.

Overall layout of the T25E1. This was identical to that of the Medium Tank T26E1.

Work on the Medium Tank T25E1 continued alongside development of the Medium Tank T25. The first production Medium Tank T25E1 was shown to Ordnance Department representatives on January 13th, 1944. The demonstration was done in Grand Blanc, Michigan, home of the Fisher Tank Division. The T25E1 was closer to the T20 than the T25, but even this relationship was distant. The hull of the T25E1 differed from those of its predecessors. The T20, T22, T23, and T25 had welded hulls, but the T25E1 had a cast hull. This was the first production tank that had an almost entirely cast hull (the Medium Tanks M3A1 and M4A1 had welded superstructures, but the bottom of the hull was still welded). The driver's compartment also changed, especially the driver and assistant driver's hatches. The hull grew a little in length, the transmission compartment and exhaust system changed.

The first pilot Medium Tank T25E1, Aberdeen Proving Grounds, March 1944.

The running gear also changed. The road wheels and idlers were modernized, the drive sprockets changed significantly. The biggest change was the new all-metal tracks. The T81 track was very different from the tracks used on the T23E3 and was closer to that of German tanks. The width increased to 483 mm. The tracks had to be widened, as the mass increased to 34,745 kg and experience with the Medium Tank T23 urged caution. The ground pressure increased compared to the T20E3, but not by much. As for the transmission, it was largely the same as on the Medium Tank T20. Small changes were made due to the increase in mass.

Vehicles of the pilot batch had noticeable differences.

The increased mass compared to the Medium Tank T23 had to do with more powerful armament in a new turret, but also with increased protection. The front hull was now 76 mm thick, although keep in mind that cast armour is 10-15% weaker than rolled. The top speed was limited to 48 kph, which is still quite high. As for the turret, it was identical to the one used on the Medium Tank T25. The ammunition capacity was reduced to 42 rounds.

The fourth pilot tank went to General Motors for trials.

Production of the pilot batch commenced without a prototype. The tanks received registration numbers U.S.A. 30103252-30103291. Officially Fisher Tank Division finished production in May of 1944, but whether or not this was actually the case is up to interpretation. Soviet specialists who visited the Fisher Tank Division left a report that gives food for thought. The factory planned to deliver 14 tanks in April  of 1944 and 10 in May, but in reality only 22 tanks were delivered by June 1st. Soviet specialists saw 15 T25E1 hulls on the assembly line, but almost no work was performed on them. There were a number of reasons for this. First, subcontractors were failing to meet deadlines. Second, the tanks were improved as a result of trials. Pilot T25E1 tanks visually differed from one another depending on when they were built. The fenders and turret changed during production. The biggest difference was the appearance of a port for discarding spent casings on the left side of the turret, just like on the second Medium Tank T25.

The tank was very different from its predecessors. This applied to the chassis as well; the hull was now fully cast.

There was a suspicion from the start that the vehicle would be quite picky. For this reason, the first pilot T25E1 was sent to the Aberdeen Proving grounds by February 10th, 1944. Two more tanks joined it a short time later. Four more tanks were sent to various proving grounds, including the GM proving grounds in Milford and the Buick proving grounds. Five tanks were sent to Fort Knox for army trials.

The driver's station.

The Ordnance Department's hunch was not wrong. A number of issues cropped up, some of which were already seen on the Medium Tank T20E3: broken shock absorbers and return rollers. Transmission elements failed routinely and the brakes did not last long. The T20/22's trademark overheating of the engine and differential also persisted. There were also issues with the electric system. Desert tests in Phoenix revealed issues with engine cooling as well as breakdowns of the front road wheels. These tests also noted that the ground pressure was high.

The third pilot tank tested at the Fisher Tank Division proving grounds.

Other proving grounds, including General Motors and Fort Knox, reached the same conclusions. Trials showed that the lifespan of the T81 tracks did not exceed 1400 km, which was very small by American metrics. Various improvements were introduced, but it was clear that the tank needs more work. Trials of the Medium Tank T26E1 took place in parallel. This tank had a reinforced suspension to deal with the increased mass. It turned out that this suspension suffered fewer breakdowns. It's not surprising that the Americans preferred the better protected if less mobile variant.

This tank has a full set of dust shields and mudguards.

Soviet specialists also observed the testing of the T25E1 at the Aberdeen Proving Grounds. They knew nothing about issues with the suspension, but they noticed the heavily worn rubber on the road wheels that had travelled 3200 km and the new set of shock absorbers. They came to the conclusion that these components did not last long.

Five tanks were tested in Fort Knox.

Despite difficulties with the suspension, remarks were made on its advantages, chiefly the smooth travel. The tank drove across an obstacle course at a speed of 40 kph and its driver felt nothing. Observers wrote down that the tank's hull did not oscillate during this test.

The results of the trials were poor. The T25E1 was plagued by various technical issued, and the Heavy Tank T26E1 ended up winning in the end.

The fate of the Medium Tank T25E1 was sealed in the summer of 1944. It was clear that the tank is not suitable to replace the Medium Tank M4, as it had too many drawbacks and appeared to be already obsolete, chiefly from the point of view of protection. The Medium Tank T26E1 was much more promising, although it was not ready for production either. As a result, the Medium Tank T25E1 became a test lab. Components later used on the T26E3 were tested on them. The T26E1 and T26E3 were reclassified as heavy tanks as their mass crossed the 40 ton mark.

The fifth Medium Tank T25E1 pilot improved to the level of the T26E3. This applied both to the armament and equipment of the tank.

The tanks went through a modernization process to improve their reliability. There was no longer any hope of sending them to fight, as their use would have been far too troublesome. There was also the idea of installing the Heavy Tank T29 turret on this chassis. This was not the turret you would expect, but really the one used on the Heavy Tank M6A2E1. Work did not progress past drafts, as there were far too many issues with this proposal, including the chassis and cooling system.

Trials of the 90 mm T14 gun with an original recoil system and two Browning M2HB coaxial machine guns.

The Medium Tanks T25E1 were used to test various systems, including armament. For example, the fifth pilot tank was modernized, with its equipment (including armament) upgraded to the level of the T26E3. The first pilot tank received the experimental T14 gun with an original compact recoil system. This tank also had two coaxial .50 caliber machine guns.

The 13th Medium Tank T25E1 was converted for installation of a rangefinder in 1948. The tank received characteristic "ears" as a result.

The use of the Medium Tank T25E1 as a test lab continued after the war. The last such event took place in 1948. The 13th tank was modified for installation of a new fire control system. It included a rangefinder, which required the turret to be converted. This system was tested as a part of the same program as the Heavy Tank T29E3. This work was not done in vain, as a similar rangefinder was later used on the Medium Tank T42 accepted into service as the Medium Tank M47.

Draft of a Heavy Tank T29 turret on the Medium Tank T25E1 hull. This was the turret tested on the Heavy Tank M6A2E1.

Even though the Medium Tank T25E1 was less than one sixth as numerous as the T23, the same amount of tanks survives to this day: two vehicles numbered 18 and 40. This tank was not destined for the front lines, but it was the second to last step towards the tank that would finally replace the Medium Tank M4.

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