Friday 19 May 2017

Hellcat: Highway Tank Destroyer

The 76 mm Gun Motor Carriage M18, more commonly known as the Hellcat, is one of the best known American wartime tank destroyers. With a powerful 76 mm gun and high mobility, the vehicle was an effective anti-tank measure. A number of sources mention that several Hellcats ended up in the USSR, but that is usually all information that is offered. This is not surprising, since, unlike the GMC M10, which was accepted into service, the M18 never made it past trials. More precisely, the USSR received the pre-production prototypes, indexed Gun Motor Carriage T70. What was the fate of these vehicles?

Loose lips leak tanks

Work on the Gun Motor Carriage T70 began in January of 1943. The idea of a light tank destroyer that resulted in the GMC T49 and T67 was kept, but the implementation was seriously reworked. The first GMC T70, built in April of 1943, was significantly different from its predecessor, the GCM T67. The coil springs were replaced with torsion bars, one Continental R-975-C1 aircraft engine was used instead of a pair of Buick motors, and the transmission was moved from the rear to the front. The hull and turret also took a different shape. The only thing left from the previous vehicle was the overall concept and armament.

Pilot vehicles entered trials in July of 1943. It's not surprising that the GMC T70 was absent from lists of foreign SPGs that were discussed by the Soviet military in June of that year: the tank destroyer was still a secret. This makes the situation in which the USSR found out about the GMC T70 that much more interesting.

In the summer of 1943, a group of Soviet specialists visited the General Motors proving grounds in Milford, Michigan. The purpose of that visit was to inspect the T1E2 and M6 heavy tanks. The inspection was very shallow, as the Americans were not inclined to show too much. Nevertheless, engineer Sorvin, the head of the group, received a lot of information.

Sketch of the "T-70 destroyer-tank", drawn from memory.

During an inspection of the heavy tanks, a "T70 destroyer-tank" was mentioned by the American by accident. According to Captain Karlson, the "destroyer-tank" was designed according to specification of the Tank Destroyer Command. One of the key features of the vehicle was high mobility. This was achieved as a result of thinning out the armour.

The T70 had to "be able to conduct quick and sudden attacks on enemy tank units, fire from short range, and disappear just as quickly". Sorvin immediately criticized the concept, mentioning that this kind of speed could only be reached on flat terrain.

Shipping record of tank destroyers that arrived at Molotovsk and the equipment that accompanied them.

Major Berg turned out to be m ore talkative. According to him, the hull and turret were designed by the same man who designed the GMC M10. There is a certain resemblance between the two, so the Major was likely telling the truth. The American officer also described the characteristics of the vehicle in detail. This kind of detailed information was known to him because Berg himself was one of the vehicle's creators, responsible for the suspension.

Berg's tale sheds a new light on the history of trials of the T-34 and KV-1 tanks at Aberdeen. The suspension was designed using the experience from detailed examination of the two tanks. Berg used the T-34's design for the road wheels, and the suspension from the KV. However, he modified it. The diameter of the front and rear torsion bars was increased. The torsion bars were also rotated by 2 degrees and installed on ball bearings. There were also additional shock absorbers, which made the ride a lot smoother.

The "T-70 destroyer-tank" at the NIIBT proving grounds, spring of 1944.

The reason why Berg was so eager to share information quickly became clear. Work on the GMC T70 was going at full speed, and the presence of Soviet engineers gave hope that they would share some information about how the suspension was designed.

Another confirmation of this was Berg's behaviour once he found out that the group that arrived consisted of Soviet tank engineers. 15 minutes after the inspection of heavy tanks was done, Berg caught up to the group by car and invited them to Detroit to view the experimental tank destroyer. Berg asked many questions about torsion bars on the way: about their design, the composition of their metal, etc. Sorvin concluded that the Americans must be having problems with their torsion bars.

The mudguards on the tank destroyers were quickly lost.

In an attempt to get information, Berg said too much. The Soviet engineers also learned of the Medium Tanks T20 and T23. It looks like the Major was directly involved with the development of the T20E3, a variant of the T20 with a torsion bar suspension.

As for the "destroyer-tank", the introduction was brief. The Soviet engineers saw the vehicle in motion when it was coming back from trials. Even though they received permission to inspect it, the Buick representative interjected, saying that the vehicle was secret. Nevertheless, Sorvin received important information, and was later able to reproduce a sketch of the secret tank destroyer from memory.

Light, not not easy

The Gun Motor Carriage T70 did not remain secret for long. In July of 1943, the tank destroyer was accepted into service by the American army. Buick's design had its problems, as the limited standardization status indicates. The tank destroyer debuted under the index T70 (mass produced vehicles usually received an M letter code). The index was changed to Gun Motor Carriage M18 only in March of 1944. All vehicles built before that were recalled to the Buick factory to be modernized. The vehicle's path into production was not an easy one.

This placement of ammunition was risky, but made reloading quicker.

It is uncertain when the agreement was made to import the new American tank destroyers. Most likely, this happened in late 1943. What is known for sure is that on January 12th, 1944, convoy JW-56A departed from Loch Ewe, Scotland. SS Thorstein Veblen, a famous Liberty type steamboat, was a part of the convoy. On February 1st, 1944, the ship successfully arrived in Murmansk, delivering 11 M4A2 tanks and spare parts. The SS Thorstein Veblen also carried three "T-70 self propelled guns" and two sets of parts for each.

On January 22nd, 1944, convoy JW-56B departed from Loch Ewe. It arrived at Molotovsk (Severodvinsk) on February 3rd. Another Liberty ship, the SS Albert C. Ritchie, brought 12 M4A2 tanks and two "76 mm self propelled guns".

The American tank destroyer looked more like a light tank.

A tank destroyer with registration number U.S.A. 40108549 was chosen for trials. The vehicle came from batch 446, built at Buick in late 1943. The M1A1 76 mm gun #1272 was built at Oldsmobile, another General Motors factory. unlike the GMC M10, which came with no documents at all, this time the paperwork was in order. A large number of illustrations in the technical description composed by the proving grounds specialists were taken from the American manual. The overall photographs were also taken from there.

Component layout of the GMC T70.

The proving grounds specialists took great interest in the transmission. The engine was connected to the transfer case, which was connected to a three stage torque converter with a driveshaft. The torque converter passed on the torque to a planetary gearbox, then a differential, and then to the final drives. The torque converter, gearbox, and differential were all in one unit. A large hatch on the front was used to remove it.

Diagram of the transmission on the American tank destroyer.

The hatch posed certain questions in regards to shell resistance. However, these questions disappeared when the thickness of the armour was measured. The gun mantlet (51 mm), turret rear (40 mm) and turret front (35 mm) were the thickest parts. This strange distribution of armour was used because it acted as a counterweight. The rest of the armour was no thicker than 12.7 mm. Such was the price for speed and power to weight ratio of 23.5 hp/ton.

Armour diagram. As you can see, there isn't much of it. The armour was sacrificed for high mobility.

Since this time the testers had a manual, there were no mistakes when it came to placement of the crew. Unlike the GMC M10, the crew had to work while sitting down, which raised some eyebrows, especially when it came to the loader's duties. His seat was positioned very low and did not fold down, which made his job difficult. If he stood, then it became hard to take shells out of the ammunition rack. This ergonomics decision made the rate of fire of this tank destroyer limited by the speed of loading instead of speed of aiming.

Placement of crew members within the fighting compartment. Soviet testers had many issues with it.

There were questions about the commander's station too. Unlike the loader, he had to stand, also serving as a machinegunner. The commander's station was deemed insufficient and dangerous. There were no observation devices except for the commander's own eyes, and the chances of catching a bullet were high. The only turret crewman who was comfortable was the gunner. No complaints were recorded about his position.

Visibility from the GCM T70.

There were also complaints about visibility. The gunner fared well, with telescopic and a periscopic sights. The commander, as mentioned above, had nothing aside from his eyes, and his placement in the turret did not let him see the terrain directly in front of his vehicle. The visibility of the driver and his assistant was also far from good. Even though the full range of vision was 180-200 degrees, the periscopes were shifted to the side, which made them difficult to use.

Speed and hunger

As with the GCM M10, gunnery trials could not happen because there were no shells sent. After the vehicle was studied, it could only undergo mobility trials. The trials program scheduled 1000 km of driving. In reality, the tank destroyed traveled 1022 km: 172 over a stone highway, 214 over a paved highway, 530 on a dirt road, and 132 off-road, over snow.

The vehicle never reached its calculated top speed of 55 mph (88 kph). The top speed achieved over four tries was only 75.6 kph. The vehicle could reach a speed of 70 kph in 64 seconds, travelling 700 meters. When trying to brake at a speed of over 50 kph, the tank destroyer would skid. If the brakes were applied gradually, then the vehicle would not skid, but the stopping distance was 70-80 meters.

The average speed was 38.4 kph on a stone highway and 47.6 kph on a paved road. The highway speed was limited by the stability of the vehicle. This was most obvious when driving on an icy road. At 60-70 kph, the vehicle began to slip.

This driving consumed a lot of fuel: 203 L for 100 km. The double differential increased the turning radius to 12.3 meters. On the other hand, the suspension behaved well. Thanks to additional suspension elements (hydraulic shock absorbers and vertical springs), the tank destroyer drove smoothly on good roads.

Fuel consumption on dirt roads increased drastically.

When driving on a dirt road, things did not go as well. Sorvin was right when he judged that the high speed tank destroyer concept would only work on flat ground. On a dirt road, the average speed dropped to 23.8 kph. To compare, the Light Tank M3 reached an average speed of 29 kph, and the German PzIII Ausf. H had an average speed of 25 kph. The tank destroyer could only drive in second gear. Movement in third gear was limited by low torque supplied by the torque converter. The torque converter also meant that the vehicle could not drive over a road with deep potholes. Insufficient torque also prevented the vehicle from crossing 0.75 meter deep roadside ditches. The tank destroyer also consumed excessive amounts of fuel. It took 357 L of gasoline and 11.3 L of oil to travel 100 km. The tracks also had poor traction.

When driving off-road, the light tank destroyer consumed more fuel than some heavy tanks.

The biggest problems arose then trying to drive off-road. The torque converted would not allow any gear higher than the first to be used. An attempt to shift into second gear led to slipping of the torque converter and loss of power. The average speed was 19.4 kph. Snow more than half a meter deep was impassable for the tank destroyer. The torque converter was at fault once again. The fuel expenditure became unbelievable: 708 L of gasoline and 20.2 L of oil were consumed to drive 100 km. Some heavy tanks needed less fuel than this.

The cause of such an appetite was that the engine was working at constant RPM for maximum power. The torque converter automatically changed the gear ratio, but the engine RPM remained the same. A mechanical gearbox would have been much more suitable for these conditions.

Hill climbing trials.

Natural obstacles were also poorly handled. A 16 degree slope could be climbed, but anything higher was impossible due to insufficient traction. With a running start in second gear, it was possible to climb up an 18 degree slope. After that, the torque converter was the limiting factor.

Driving at a tilt.

The maximum angle of descent before the tracks began to slip was 24 degrees.The maximum tilt before the vehicle began to slide off was 18 degrees. If the driver actively worked the levers to keep level, it was possible to drive at a tilt of 18-22 degrees.

The last step of the trials was fording. The maximum depth that could be forded was 1.55 meters. After that, water would enter the engine compartment through the engine deck. This was a good figure. For example, the German PzIII could not for a 1.4 meter deep river.

The tank destroyer is fording a river.

Gunnery trials came last. Since the characteristics of the M1A1 gun were the same as those of the M7 gun on the M10, penetration was not tested. The trials were mostly concerned with precision. It was compared to the Soviet SU-76M. When using the telescopic sight, the American gun was more precise than the Soviet one. When using the periscopic sight, the dispersion of the shells grew by a factor of 2.

Firing from short stops gave good results: 10 hits out of 10. Firing on the move was not as good. At a range of 700-1000 meters, not a single shot out of 8 hit the mark. The same thing happened on the second attempt. Finally, the rate of fire trials showed that the practical rate of fire was 7.7-9.7 RPM and the maximum rate of fire was 11.4 RPM

Overall, the armament of the GMC T70 was good for a tank destroyer, but the chassis was rejected.

"The American T70 SPG could not be recommended for import for the following reasons:
  1. Poor armour, which protects only from small arms fire and small shell fragments.
  2. High fuel expenditure, which is several times higher than that of SPGs of the same class in the same conditions.
  3. Use of high octane gasoline, which makes the SPG a fire hazard.
  4. Poor visibility from the SPG."
As mentioned above, the old GMC T70s were modernized after standardization into the M18. Looks like the Americans encountered the same issues. However, a minor modernization could not radically improve the vehicle, especially its off-road performance. Trials of the American T26E3 tank, which also had a torque converter, showed similar problems and a similar hunger for fuel.

The penetration of the GMC T70's gun had no issues.
Photo #31: penetration of the left side with a 76 mm shell from the American M1A1 gun from a range of 2000 meters.
Photo #32: Diagram of the penetration of the left side with a 76 mm shell from the American M1A1 gun from a range of 2000 meters.

Despite such a crushing verdict, the SPG found a place at Kubinka. It served in the the penetration trials of the Tiger Ausf. B, where it turned out that it could penetrate the side of the heavy tank from two kilometers away. The idea of a "highway tank destroyer" turned out to be reasonable.

However, such tank destroyers could only be effective in an area with a developed network of paved roads. These vehicles were unsuitable for the Soviet-German front, where battles were often fought in an unpaved field.

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