Saturday 21 March 2020

In Search of an Ideal Weapon

The poor results of the Heavy Tank M6 program led to a lethargy in American heavy tank development by the first half of 1944. The success of American medium tanks in North Africa led the American military to believe that they will not need heavy tanks. The summer of 1944 in Normandy proved sobering. It turned out that the Germans had new types of heavy tanks that were a tough nut to crack for American tanks and tank destroyers. As a result, development of several new heavy tanks was expedited in late July of 1944. The Heavy Tank T30 was one of them.

A reserve option

Talk about a brand new tank cropped up in early August of 1944. At that point, the discussion revolved around the Heavy Tank M6A2E1, a thorough modernization of the Heavy Tank M6. General Lucius Clay spoke out against this idea, and he had his reasons. The M6 was already far from the most mobile tank, and the M6A2E1 had an estimated mass of 77 tons. The idea of developing a brand new tank called Heavy Tank T29 was first voiced on August 14th. The chassis was inspired by the Heavy Tank T26, the turret and armament were similar to those of the M6A2E1.

A sketch of the Heavy Tank T30 with the initial turret.

However, there were questions about the T29 from the very beginning. The primary enemy of the future T29 would have been the Tiger II, which had a 150 mm thick upper glacis plate. Doubts that the 105 mm T5E1 gun could penetrate this much armour were raised early on. Because of this, one more prospective tank was announced on August 14th: the Heavy Tank T30. Its hull and turret were identical to the T30, but the gun was different.

The 155 mm M1A1 gun, known as the Long Tom, was chosen as the starting point for this weapon. This gun was developed in the late 1930s to replace the 155 mm M1918 gun, a copy of the French GPF. The characteristics of this new gun were close to that of the Soviet 152 mm special power Br-2 gun. The muzzle velocity was slightly lower, the shell was slightly lighter, but generally the performance was the same. While only 39 Br-2 guns were built, American industry put out 1882 guns from the M1 family between October 1940 and June 1945. Few operations in North Africa and Europe were performed without the Long Tom taking part.

155 mm T7 gun. The first samples were ready in 1946.

Officially, work on the Heavy Tank T30 began on September 14th, 1944. Plans were made to produce two prototypes, the same number as for the T29. Unlike its brother, which was scheduled to be produced in a series of 1200 tanks, the T30 was never going to be ordered in bulk. This indicates that the T30 was always going to be a backup plan and was chiefly meant to test the 155 mm gun, which was given the index T7. The length of the barrel was reduced to 6414 mm (41.4 calibers). The muzzle velocity decreased to 717 m/s. Nevertheless, this was the most powerful tank gun built in metal to that day. However, the M1A1 did not yet have an armour piercing shell. It was designed later.

AV-1790 Continental engine that was used on the T30 and T34 heavy tanks. This engine ended up on top in competitive trials.

The first changes to the Heavy Tank T30 program were made towards the end of 1944. It was clear that the heavy tanks would be late to the war, and not only was the volume of the orders reduced, but the chassis was revised. The engine was the first to change. Initially, the plan was to use the 27 L Ford GAC, which put out 770 hp. However, even initial estimates showed that the power to weight ratio of the T29 and T30 would be low, and a new engine was suggested: the air cooled 29 L Continental AV-1790. This engine could produce up to 810 hp. The transmission also changed. Like the T29, the final selection was made in favour of the Alisson CD-850-1.

T30 with serial number 1 at the Aberdeen Proving Grounds, May 1948. This tank survived to this day.

Changes were also made to the turret. Like the T29, the turret of the T30 was initially the same as on the M6A2E1. This turret was referred to as the T29 turret: the M6A2E1 was nothing but a test platform. A number of issues were discovered, such as the possibility that fragments and bullets could enter the turret through slits, and the turret had to be redesigned. It ended up changing significantly, but the T29 and T30 still had identical turrets. The only difference was the number of coaxial machine guns: one or two. The new gun mount was indexed T124.

In addition to a different gun, the Heavy Tank T30 only had one machine gun.

Even though the gun was much larger than on the T29, the difference in mass was not great. The T30 ended up weighing 64,682 kg, while the T29 weighed 64,183 kg. The Heavy Tank T29E3 was even heavier: 65,317 kg. The more powerful engine gave the T30 a higher power to weight ratio: 12.52 hp/ton, compared to the Heavy Tank T29E3's 10.74 hp/ton. The T30 did have one drawback, and it was linked to the gun. The tank only carried 32 rounds of ammunition compared to the T29's 63. The rate of fire was low, which could be expected from a gun of such caliber.

In travel position.

Work on experimental heavy tanks proceeded slowly. The T30 was only built three and a half years after development began. The tanks received the Continental AV-1790-3 engine, an improved variant that was developed in parallel with the tank. The first prototype T30 tank was completed in March of 1948 and received the registration number USA 30162842. It was sent to the Aberdeen Proving Grounds at the end of March. Meanwhile, the number of T30 tanks that were ordered went up. Often only two tanks are mentioned, but documents indicate otherwise. By May 1948 there were already six T30s with registration numbers USA 30162842-USA 30162847. Of those, tanks with serial numbers 1, 2, 4, 5, and 6 were sent to the Aberdeen Proving Grounds. Tank #3 remained at the Detroit arsenal and became a test lab. A number of the tanks did not remain at the proving grounds for long. On June 8th, 1948, tank #5 departed for Fort Knox, on July 23rd tank #6 was sent to Yuma, and on January 20th, 1949, tank #4 was sent to Fort Churchill in Canada. Tanks #1 and #2 remained at the Aberdeen Proving Grounds and went through the most testing.

Cutaway diagram of the Heavy Tank T30.

Even though some of the tanks spent only a few months or even weeks at the proving grounds, they were all broken in. For instance, tank #4 travelled for 213 km before being sent to Canada for winter trials. Some defects were discovered in the transmission. Tank #6 that was sent to Yuma had some engine components replaced before it was sent out. Additional trials took place in October-December of 1948, and the transmission had to be replaced. In 1949 the transmission was replaced with the improved CD-850-3.

As for the other tanks, they drove significantly further during trials. T30 #1 drove for 800 km, of which 142.5 km was using a CD-850-2 transmission. T30 #2 set the record among all the heavy tanks. The tank drove for 3766 km in total, 1492 of which were with the CD-850-2 transmission. The engine and transmission had to be replaced during trials, but nevertheless the AV-1790 showed itself to be much more reliable than the Ford GAC. Since this engine was also noticeably more powerful, it was considered the winner of the comparative trials. Production of the Medium Tank M46 with the Continental AV-1790-5A engine and CD-850-3 transmission began in 1949. This combination of engine and transmission was also used in the Heavy Tank T43, which turned into the M103.

T30 #5 during mobility trials, Fort Knox, 1948. This tank also survived to this day.

Unlike the mobility trials, testing of the gun was not going so well. The large dimensions and mass of the T7's ammunition made it difficult to work with in the fighting compartment of the tank. The maximum rate of fire was less than 2 RPM, three times lower than the 105 mm gun. Such a low rate of fire put the idea of using such a large gun in a tank under question. 

The Heavy Tank T30E1 had a raised commander's cupola and a hatch in the back of the turret.

An attempt was made to improve the situation with the Heavy Tank T30E1 conversion. This tank has a small hatch in the back of the turret and a raised commander's cupola. These changes were introduced due to the automatic loader in the fighting compartment. The system automatically moved the gun to the required elevation, extracted the spent casing, then loaded a new shell and propellant. Trials showed that this system worked and the rate of fire increased. However, the idea of a 155 mm tank gun proved a dead end. A winner among the weapons was already decided.

The automatic loading mechanism installed in the Heavy Tank T30E1. As you can see, the casing was extracted between the commander's legs. We will never find out what the commander thought of this feature.

Surprisingly, the T30 was the longest lasting among the experimental American heavy tanks. 4 of the 6 prototypes survive to this day. Of the two tanks that were tested at the Aberdeen Proving Grounds, #1 is clearly still around. Tank #3 also survived, the only heavy tank at the Detroit Tank Arsenal that was not scrapped. Tank #6 is currently in Fort Jackson, and tank #5 is on display at the National Armor & Cavalry Museum. 

Intermediate caliber

An interesting trend can be observed in American tank and SPG development. Starting with the Medium Tank M3, many of the best and most popular American tank guns were based on AA guns. Of course, other nations often put the ballistics of an AA gun to work in a tank, but the Americans did this most often. The same thing ended up happening to the armament of heavy tanks. Several towed AA guns were used by the American military. The most powerful of them all was the 120 mm M1. Work on it lasted for 20 years and it saw very little use in battle. Nevertheless, its appearance in 1944 was not overlooked by the Ordnance Department. The gun had exceptional characteristics, surpassing its closest analogue, the 128 mm Flak 40, in muzzle velocity, while having a shorter barrel.

The 120 mm T53 gun used in the Heavy Tank T34.

The idea to build a tank gun from the 120 mm M1 did not come immediately. Initially, design work focused on longer 90 mm guns and new 105 and 155 mm guns. It was clear by early 1945 that this development was not going as planned. A study of the effectiveness of tank weapons showed that the 120 mm AA gun was an ideal candidate. The gun was long, 7391 mm without a muzzle brake, but the T15E2 gun used on the Heavy Tank T32 was only 10 inches shorter. Even the 105 mm T5E1 gun was not much shorter, only by 13 inches. The high muzzle velocity, and therefore high penetration, was worth it. The projectile was also comparatively heavy. The characteristics of the 120 mm tank gun were similar to those of the Soviet OBM-51 gun, later renamed BL-13. The difference was that the Americans actually finished their gun.

Cutaway of the Heavy Tank T34.

Work on the 120 mm tank gun began in January of 1945. The gun was indexed T53. On May 17th, 1945, a decision was made to build two Heavy Tanks T30 equipped with T53 guns. These were indexed Heavy Tank T34. Like other tanks of the family, they were initially supposed to use Ford GAC engines and CD-850-1 transmissions. Another idea came up later: the 870 hp Alisson V-1710-E32. This engine was never installed on the Heavy Tank T34. The final variant had the Continental AV-1790, the same engine as on the T30.

The first experimental prototype with the registration number USA 30162832 remained at the Detroit Tank Arsenal.

The turret was also the same initially. However, changes were introduced at the very beginning of development. The T53 gun was not only longer, but half a ton heavier than the T5E1. As a result, the balance of the turret shifted. A 102 mm thick plate had to be welded onto the back of the turret as a counterweight. The tank's gun mount received the index T125, even though it was identical to that of the T123 on the Heavy Tank T29. Like the T29, this tank had two coaxial machine guns.

From this angle the counterweight welded onto the back of the turret is visible.

Like the T29 and T30, the Heavy Tank T34 was built at the Detroit Tank Arsenal. However, work was not urgent, and these vehicles are only mentioned in the documents of the Aberdeen Proving Grounds starting with April 1948. According to plans, tank #2 and registration number USA 30162833 was going to be sent for trials. The other tank remained in Detroit as a test lab. The final tank weighed 65,136 kg, more than the T30 but less than the T29E3. As further trials showed, its mobility was analogous to that of the T30.

The Heavy Tank T34 did not remain in Aberdeen for long. On August 18th, 1948, it was sent to Fort Knox. Considering that the tank only arrived on July 27th, it drove for only 80 km. Firing trials were also performed. The tank returned to Aberdeen in December of 1948 with the same number on its odometer, meaning that the tank was not used at all. Only 136 km was driven by mid-1949. These negligible numbers were due to the bulk of the effort being spent on the Heavy Tank T30. The T34 was mainly built to test the gun.

The second T34 prototype at the Aberdeen Proving Grounds.

The major changes made to both Heavy Tanks T34 were linked to gunnery trials. Due to the bulky ammunition, the ammunition capacity was the same as the Heavy Tank T30: 34 rounds. On the other hand, the lighter ammunition made loading the gun faster, especially with two loaders. The maximum rate of fire was 5 RPM, impressive for such a caliber.

The gunnery trials were much more important. The gun penetrated a 198 mm thick plate at 30 degrees from 1000 yards. This meant that the T34 could penetrate the side armour of the Maus from medium distances, and at close range could even penetrate the front. A HVAP shot was also available, with a penetration of 381 mm at 1000 yards and 318 mm at 2000. This meant that the gun was powerful enough to defeat any German tank, even prospective ones. However, the penetration dropped drastically if the angle of the plate was increased: at 60 degrees the AP shell penetrated 102 mm and the HVAP shell 112 mm.

The converted gun on the first Heavy Tank T34. A new muzzle brake and a fume extractor were added.

The biggest problem encountered during firing was the amount of fumes that collected in the turret. Gunnery trials at Fort Knox ended up with the loaders hospitalized, and the Aberdeen Proving Grounds had similar issues. Several solutions were tested to resolve this problem. The most effective method was the installation of a fume extractor on the barrel. The design of the fume extractor was much simpler than the complex solution with locking the breech and clearing it with compressed air that were tried before that. Additional trials were performed after the fume extractor was added, which showed its effectiveness. A new muzzle brake was also tested on the first prototype.

The second prototype also received a fume extractor. It survives to this day.

Of the two Heavy Tanks T34 only the second survived. Today it is on display at the National Armor & Cavalry Museum. Even though the T29, T30, and T34 tank family was not put into production, they played an important role in American tank building. Production of the Heavy Tank M103 began in May of 1953. Its design used many solutions tested in the late 1940s in these tanks.

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