Friday, 28 July 2017

M24 Chaffee: Test Drive at the End of Lend Lease

Starting in the second half of 1943, the approach to sending British and American Lend Lease armoured vehicles to the USSR changed. Instead of immediate large scale shipments, the Western Allies sent a few samples of new vehicles. If the tank or SPG was satisfactory for the Soviet side, full scale shipments followed.

The first vehicle to arrive on this trial basis was the Light Tank M5A1. By that point, production of light tanks in the USSR was wrapping up, so the American novelty never made it into service. Nevertheless, the USSR received another foreign light tank. This was the Light Tank M24, the best American light tank of WWII.

Not in a rush

In many ways, the arrival of the Light Tank M24 was triggered by American attempts to increase the firepower of a light tank. It was clear that the 37 mm gun was insufficient in 1942. Trials of the M3 75 mm gun in the turret of the HMC M8 showed that the light tank platform was suitable for firing a gun of this caliber. However, the turret of the Light Tank M5 was too small for such a gun.

Because of this, the Ordnance Committee composed requirements for a new tank in March of 1943: the Light Tank T24. It carried the T13E1 gun, a lightened aircraft version of the M3, installed on B-25G/H bombers. Cadillac acted as the subcontractor in charge of development.

The tried and tested Cadillac Series 42 engines from the Light Tank M5A1 were used as the powerplant. This also improved parts compatibility with earlier models. Another logical solution was to reuse the concepts designed for the Gun Motor Carriage T70, designed by another division of General Motors: Buick. This was especially true of the torsion bar suspension.

American Light Tank M24, Kubinka, NIBT proving grounds, April 1945.

The first sample of the Light Tank M24 arrived at the Aberdeen Proving Grounds on October 15th, 1943. Interestingly enough, Soviet specialists saw the prototype before the American testers. In the late summer of 1943, a group of Soviet specialists visited the proving grounds of the General Motors company in Milford, Michigan. It just so happened that they piqued the interest of Major Berg. The reason was simple: the officer was responsible for the development of the GMC T70's suspension, and wanted to receive as much information as possible about torsion bar suspensions of Soviet tanks. As it turned out, the Soviet side received a lot more information. They did not see the GMC T70 in detail, but their unexpected visit to Detroit revealed a large number of experimental vehicles. Among them was the "T-24 light tank".

Since the tank was only seen from afar, the data on it was inexact. According to engineer Sorvin's report, dated 1944, the tank was designed on the chassis of the "T-70 self propelled gun". The gun caliber was estimated as 37-57 mm. The armour thickness was estimated at 38 mm, same as other American light tanks.

The tank differed noticeably from its predecessors, both in hull shape and suspension.

Reliable information about this tank began to arrive significantly later, in the summer of 1944. Precise information was received in August, not just about the tank, but about the rate of production. Our specialists also knew the exact date of standardization under the name Light Tank M24: July 25th, 1944. On August 22nd, the GBTU received a precise report on the new tank, which included a technical description spanning 17 pages.

Soviet specialists were also invited to the Tank-automotive Center in Detroit (modern day Tank-automotive and Armaments Command, or TACOM). From September 11th to 15th, Guards Major B. Afonin and Engineer-Captain G. Bogolubov studied transmissions of American tanks. Among others, that included the Hydra-Matic 255-T, used on the Light Tank M24. One important feature of the 255-T was that it had no planetary reverse gear. Instead, a geared reductor was used.

Conclusions regarding the gearboxes for light tanks were against their use in domestic tanks due to both difficulty in production, and the forced nature of their appearance. Soviet specialists considered the fact that these gearboxes worked well in the hands of poorly trained drivers a definite plus.

The three-man turret was another difference. The first time American engineers made a three-man light tank turret was for the Light (later Medium) Tank M7.

Afonin judged the M24 highly when composing his report. According to his conclusions, the M24 was the best armed light tank at the time. The overall layout was also good, as it allowed for a compact and highly maneuverable vehicle. However, the armour was poor. Another significant drawback was the need to provide the tank with highly trained service personnel.

In his conclusions, Afonin proposed that samples of the American tank should be purchased. However, there was no hurry to do so. The USSR had not produced light tanks for a year at that point. Renewal of light tank shipments was not planned, so the purchase of the M24 was postponed.

Progress in the light weight class

The issue of buying M24 tanks was raised in early 1945. By that point, the tanks were actively used in Western Europe, and were no secret. Five tanks (the number of Light Tanks M5A1 ordered) already seemed too much, so the number of tanks purchased was reduced to two.

On March 11th, 1945, convoy JW 65, composed of 26 transports, left for Murmansk. Out of those 26, one transport, SS Horace Bushnell, was seriously damaged by the U-995 submarine, and another, Thomas Donaldson, was sunk by U-968. This happened close to Murmansk. On the next day, the convoy arrived at its destination, bringing the two M24 light tanks along with its other cargo. On March 25th, they were loaded onto a railroad and sent to Kubinka.

The large hatch for removal of the gearbox in the upper front plate was a distinguishing feature of the new American tank.

The tanks arrived at the NIBT Proving Grounds closer to April 10th. Out of the two, the tank with registration number U.S.A. 30120376 was chosen to undergo the full trials program. In addition, the new tank was thoroughly studied from April 15th to April 20th. Overall, the opinion of the examiners was largely the same as the conclusions made by Afonin in August of 1944. According to the verdict, the M24 was a composition of the best ideas taken from General Motors' vehicles: the Light Tank M5A1 and GMC M18 (T70).

The engine compartment became lower, thanks to its rearrangement.

Like with the GCM T70, the attached technical manuals made studying the tank much easier. However, a serious mistake was made in judging the tank due to this documentation. According to the manual, the output of the Cadillac Series 44T24 engine was 110 hp. This is true, but with a small clarification: this was the nominal power. The maximum power was 148 hp, meaning that the real maximum power of the pair of engines was 296 hp. The same mistake was made here as with the Light Tank M5A1, which was equipped with the same engines.

The Browning M2HB AA machinegun can be seen on this photograph.

Unlike the M5A1, the M24 joined the output of the two engines using a demultiplexor in the engine compartment, reducing the number of driveshafts to one. In addition, it was now possible to disable one of the engines. Another novelty was the addition of a neutral pedal, and dual driver controls. The first time this solution was implemented by American engineers was on the Medium Tank T20. Now, the assistant driver finally earned his title. 

Cutaway of the tank. The overall layout is the same as that of its predecessors, but with many changes.

The differential was the same as the one used on the GMC T70. Its connection with the final drives was made with knuckle joints. This made the design and installation of the differential simpler. The installation was done through the large hatch in the upper front plate,

Light Tank M24 transmission diagram.

The suspension of the American tank was close to that of the GMC T70, which is not surprising. However, there were enough differences. For starters, instead of horizontal bidirectional shock absorbers used on the GMC T70, the Light Tank M24 used slanted telescoping shock absorbers. Another difference noticed (and sketched) by Afonin was the design of the idler carrier. It was connected to the balancer of the rear road wheel. This way, the tracks were always taut.

As for the track links, they were very similar to those used on the SPG. The size of the connector pins and pin stopper were changed. The latter change was deemed unfortunate, as the pins and pin channels bent during usage, and removing the tracks became a complicated task.

Diagram of the idler carrier made by Soviet specialists in 1944.

The observation devices were also lauded. For starters, the commander had a cupola with six observation devices, which provided good visibility. There was also a periscope in the hatch. The gunner, driver, and his assistant also had observation devices with 60 degrees of horizontal and 50 degrees of vertical view range.

The sights were also good, but the markings were only made for the M61 armour piercing shell. A lack of an HE shell scale was a clear drawback. In addition for a direct fire sight, an angle measurement device for indirect fire was added.

Visibility diagram of the American Light Tank M24.

The crew stations were also highly rated. The loader's position drew the most criticism, as he had no observation device, and his seat was uncomfortable. However, the fact that the tank could be crewed by four men must be kept in mind, in which case the loader's duties were performed by the assistant driver. Overall, the tank was rather comfortable, especially considering its small size.

The M24 was not only comfortable to sit in, but easy to drive, with 9-12 kg of effort required to move its levers. The aiming flywheels were also easy to turn, requiring only 1.5 kg of effort. The addition of a hydraulic turret traverse was also a plus. Without it, it took 95 seconds to fully rotate the turret, and 15 seconds with it.

The M24 could climb a grade of up to 30 degrees.

Mobility trials of the American light tank took place between April 21st and May 24th, 1945. The tank drove for 859 km, 145 of which were on a paved highway. The Light Tank M5A1 and SU-76M SPG were compared with the M24. The incorrect information about the engine power introduced confusion. With it, it seemed that the SU-76M  had a higher power to weight ratio at 12.38 hp/ton. The power to weight ratio of the M5A1 was deemed to be 15.7 hp/ton, and the M24's to be 12.2. In reality, the power to weight ratio of the M5A1 was 19.5 hp/ton and the M24's was 16.08.

The top speed of the M24 was lower than that of its predecessor, 55 kph. However, the SU-76M's was even lower: 41 kph. Soviet specialists explained the difference in top speed with the different gear ratios, missing the fact that the M24 had more than two times the engine power of the GAZ-203. The average speed on a highway was also high, 30.05 kph, with a fuel consumption of 203 L per 100 k, or 1.5 times more than the M5A1 (135 L). Driving on a highway was performed in 4th gear.

The tank during a descent.

On a dirt road, the tank's average speed was only 17.45 kph. This was caused by a thick layer of mud covering the road. The tank could mostly drive only in 3rd gear, using 300 L of fuel per 100 km. In the same conditions, the SU-76M used up 215 L per 100 km and had an average speed of 16.2 kph. To be fair, recall that the M24 was 1.5 as heavy as the Soviet SPG, which explains the higher fuel consumption. Another reason for higher fuel consumption was the mechanically switched demultiplexer. To compare, the 15 ton Light Tank M5A1 spent 197 L per 100 km, but it drove on a dry road.

Another factor that influenced fuel consumption was engine problems. The liner on the left engine was slowly being burned through due to loosening of the cylinder head hold-down stud. In addition, the fuel filter was dirty, and the control rods of the demultiplexer were miscalibrated. There were also issues with the suspension. On the 396th kilometer, the rubber rim of a road wheel on the right side peeled off. Overall, a week was spent repairing various breakdowns.

Driving on a tilt.

Obstacle trials showed that the maximum climbable grade on a hill without topsoil was 30 degrees. The slipping hydraulic clutch did not allow the tank to take a steeper hill even in first gear. The maximum tilt was 32 degrees, after which the tank started to slip. Trials showed that the minimum turning radius was 5-5.35 meters.

Acceleration trials were very interesting. The mobility of the M24 and GMC T70 was compared. It turned out the the light tank could accelerate to 16 kph in 23 meters, and to 32 kph in 80 meters. With a significantly more powerful engine, the distance needed for the T70 was 180 and 360 meters respectively.

One of the discovered defects was the peeling off of a road wheel rim.

Trials showed that the tank was stable while firing. This meant that its precision was also high. The aimed rate of fire was 9-10 RPM, which dropped to 7.2-7.7 RPM when using the ammunition underneath the commander's seat. While firing on the move, the tank could hit 30-40% of its targets while driving at 11-13 kph, and 10-20% at 24-26 kph. When the stabilizer was enabled, this number increased to 70-80%.

As for the 75 mm M5 gun, the recoil brake was the most interesting thing in it. However, the penetration was deemed insufficient. The M61 shell could penetrate the side of a Tiger Ausf. E tank at 500 meters, and the side of a Panther tank at 1500 meters. Measurement of gas concentration inside the fighting compartment led to interesting conclusions. It turned out that the fan didn't help with gases, but the opposite, kept them inside the tank due to the gusts it created. The best method of ventilation turned out the be the tank's engine.

Undervalued tank

The final verdict regarding the new American tank was unexpected. The NIBT Proving Grounds staff deemed the Light Tank M24 as not meeting modern requirements. This was mostly due to the firepower and thin armour (only 25-38 mm). This conclusions is debatable, to say the least. However, it can be explained by a very interesting opinion of the Soviet military regarding what a light tank should look like. Its enough to say that the military demanded a light tank with 90 mm of front armour and an 85 mm cannon in March of 1944. The resulting light tank with heavy armour was a fearsome sight by the summer of 1945, especially its armour. The idea that a light tank is meant for slightly different tasks than a heavy or medium one was not clear until the late 1940s.

As for the "not meeting requirements" M24, its career continued for several decades. The tank was used by over twenty nations and actively participated in Cold War conflicts.

Despite such a negative verdict, the M24 did influence Soviet tank development, or rather the SPAAG on its chassis did. In late January of 1945, the GBTU learned of the 40 mm GMC M19. This vehicle was designed in 1943-44 on the converted M24 chassis. The distinctive feature of this SPAAG was the pair of 40 mm Bofors autocannons. Thanks to this pair, the density of fire increased.

Preparations for launching the Soviet ZSU-37 into production were underway, and someone had the bright idea to change it to be the same as the M19. The ZSU-37 had a long and unfortunate fate: factory #40 barely produced 75 of them in 1945-46. The idea of modernizing it to be similar to the American SPAAG was buried. Nevertheless, the M19 seriously influenced development of domestic SPAAGs.

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