Traditionally, cavalry occupied a very strong position in the American army. As soon as there was an opportunity to obtain its own tanks, the cavalry took it. Since, officially, the cavalry was not allowed to have tanks, the name «combat car» was used, even though these vehicles were actually tanks. The Combat Car M1 and several similar vehicles on its chassis are typical representatives of the small family of interbellum cavalry tanks.
Counterpart of Infantry Tanks
By the late 1920s, the American cavalry branch had many officers who understood what tanks were and why they were important, such as Adna Chaffee Jr. and George Patton. They made such a significant contribution to the development of American tank forces that they were immortalized in the names of tanks.
Patton fought in tanks during WWI. He was one of the first to understand the significant potential of the Christie tanks and promoted the American engineer's designs until the cavalry received the Combat Car T1. Sadly, the American army was starved for funding at the time, and despite his efforts, had to settle for only four of these tanks.
Meanwhile, the military made two tries to stick a tank with questionable potential into the cavalry. One of them was the Combat Car T2, Harry Knox's answer to Christie. He even patented this vehicle, but the design was very suspect. Despite some opinions, he didn't even come close to using Christie's patents. The frightening vehicle looked like a convertible drive armoured car with an engine in the rear. Attempts to make it suitable for service lasted for a year and a half, but nothing came out of it.
Attempts to sell the Light Tank T1E1 to the cavalry under the name Combat Car T3 also failed. This idea was killed off at the discussion stage.
The story of the Combat Car T4 went differently. This tank, built with help from Gladeon Barnes, was the continuation of Christie's ideas. Its modified version, named Combat Car T4E1, satisfied the cavalry branch, and they were ready to buy it.
However, by that time, Harry Knox designed a third tank, far better than its predecessors.
The starting point for the new tank design was the limit in 7.5 short tons (about 6.8 metric tons) set in the spring of 1933 by the Minister of War. Work on a new tank that met those requirements began on June 3rd, 1933. The first designs were presented on July 10th. Despite the fact that the designs were based on the Light Tank T1E6, this tank was noticeably different.
According to specifications, the combat car would weigh 14,000 pounds (6300 kg), reach a top speed of 30 mph (48 kph), and have a range of 100 miles (160 km). The armament would consist of one 12.7 and one 7.62 mm machinegun, and the armour would protect from rifle caliber bullets. Knox developed a new suspension with vertical volute springs for this tank. His patent shows the difference between the initial tank and suspension and what was finally built in metal.
The Ordnance Committee inspected and approved these designs and sent them to the Ordnance Department. There, the development of an experimental Combat Car T5 prototype was approved. The weight limit was also increased to 7.5 tons.
Work on the Light Tank T2, designed for the infantry branch, began around the same time. Both tanks were designed in parallel, but the final prototypes differed from one another. The Light Tank T2 kept the suspension from the Light Tank T1E6, which Knox copied from the Vickers Mk.E.
As for the cavalry's Combat Car T5, it used another design decision made on the famous British tank. In October of 1933, a decision was made to put its armament into two one-man turrets. It's doubtful that the designers wanted to make a «trench cleaner» like the Vickers Mk.E Type A. More likely, the potential problem of the highly placed driveshaft was already evident at this stage of the design.
The experimental Combat Car T5 was ready by April of 1934. The layout of the tank was the same as on the Light Tank T2. Same goes for the engine. Its mass was 5661 kg without armament and ammunition. However, the tank had a strange design decision of its own. For some reason, the turrets were open topped. This decreased the mass a little bit, but added many potential problems.
In addition to the turrets, the suspension was another difference from the Light Tank T2. The final configuration of the cavalry tank had four road wheels per side in two bogeys. The rear bogey also served as the foundation for the idler. The result was a compact and easy to service design.
The new suspension offered a very smooth ride and behaved well at high speeds. The first trials at the Aberdeen Proving Ground demonstrated the advantages of the cavalry tank's suspension. While the Light Tank T2 had a top speed of 43.4 kph, the Combat Car T5 had a top speed of 68.8 kph with the same engine. The real top speed was more than a third higher than calculated.
Trials continued in August of 1934 in Fort Benning, since the infantry was interested in the tank as well. Soon, the suspension of the cavalry tank migrated to the Light Tank T2, albeit with some changes. Changes were made to the cavalry tank as well: instead of fully metallic tracks, it now used rubber-metallic track links designed by Knox.
The two-turret layout also interested the infantry, and it was implemented on the Light Tank M2A2. The development of the cavalry tank went in a different direction. The experimental tank was altered to be more like the Combat Car T4E2. Instead of the turrets, the new Combat Car T5E1 received an immobile casemate with machineguns along the sides.
The idea was quickly pegged as questionable. The mobility of fire dropped radically, and the chassis was now overloaded. The casemate also partially overlapped with the engine compartment, which contributed little to creating comfortable working conditions for the crew.
Another design was later built, designated Combat Car T5E2. Its suspension was changed to that of the Light Tank T2E1. The hull was also redesigned, most noticeably around the engine compartment. The biggest difference was the new two-man turret, which had a lot in common with the turret of the Combat Car T4E1. A Browning M1919 in an AA mount was added to the turret in addition to the two existing machineguns.
The main characteristics, including mass and top speed, were identical to the Light Tank M2A1. Unlike the infantry, which discarded the idea of a two-man turret fairly quickly, the cavalry liked it. The tank was accepted into service as the Combat Car M1 in 1935.
Evolution Without Surprises
As with the Light Tank M2, the Rock Island Arsenal was tasked with the production of the Combat Car M1. Initially, the cavalry ordered more tanks than the infantry: 38 tanks in 1935. The Combat Car T5 prototype kept its number 1, and the first mass production tank received the serial number 2 and registration number U.S.A. W.40101. Only 19 tanks were built in 1936, and 32 in 1937. In total, 90 Combat Cars M1 were built, including the prototype, numbered 1-90 and with registration numbers U.S.A. W.40101-40189.
Tanks built in 1935-36 had turrets with curved sides. Tanks starting with U.S.A. W.40159 were equipped with a simplified turret. It was shaped like an irregular octagon. Around the same time, the engine compartment was also changed. The complicated rear plate was replaced with a simpler and more practical design.
Experiments with the Combat Car T5 continued. In 1935, it became the first American tank to use a diesel engine: an air-cooled 9 cylinder Guiberson T-1020, a tank version of the aircraft A-1020 engine, reduced to 220 hp. The main reason the Americans used a diesel engine was for fuel economy.
The first trials of this tank, indexed Combat Car T5E3, took place at Aberdeen in May of 1936. The idea of a diesel engine was a good one, but had one substantial drawback, namely that it was very difficult to start. Nevertheless, trials continued until September of 1936, and, as a result, production Combat Cars M1 were equipped with diesel engines. Three vehicles received the Guiberson T-1020 engine in total, indexed Combat Car M1E1.
Overall, the cavalry was satisfied with its new tanks. They were twice as cheap as the tanks built using Christie's inventions, but had similar mobility. The tanks were roomier than the Combat Car T1, and the two-man turret was much better. The absence of a cannon did not reduce effectiveness, since the penetration of the 37 mm gun was about as high as that of the Browning M2HB.
The cavalry tank had its drawbacks. The first of them was the raised driveshaft leading through the fighting compartment from the engine to the transmission. Lengthwise oscillations were also a big problem. The problem was corrected in 1937. One combat car, indexed Combat Car M1E2, had its idler pulled back by 28 cm, and the rear bogey followed. Trials at the Aberdeen Proving Grounds from August 3rd to October 5th showed a noticeable improvement in stability.
The experimental tank was converted back into a regular Combat Car M1, but the results of trials influenced subsequent orders. In 1938, the cavalry received 24 Combat Cars M1A1 with serial numbers 91-114 and WD numbers U.S.A. W.40190-40223. 7 of those vehicles were built with Guiberson T-1020 engines. They received the designation Combat Car M1A1E1. The Combat Car M1A1 had a slightly altered turret hatch shape, which became polygonal.
The last mass production variant of the tank, the Combat Car M2, appeared in 1940. Its design was similar to that of the Light Tank M2A4. Some solutions used in the tank later migrated to the Light Tank M3. The hull of the new cavalry tank was largely the same as the Light Tank M2A4.
The observation devices in the turret were redesigned, and later migrated to the Light Tank M3, as did the running gear changes. Since the mass of the tank grew to 11,450 kg, the idler was increased in size and lowered to the ground. As a result, the footprint of the tank increased, as did its longitudinal stability.
The first order for the new tank was for an impressive 292 Combat Cars M2. This is explained by the new mobilization program. WWII raged on in Europe, and the USA's involvement was only a matter of time. Modernizations of the Light Tank M2A1 and M2A2 were also planned, turning them into cavalry tanks.
All of these plans were rejected for obvious reasons. Machinegun tanks were completely outdated by 1940, and bulletproof armour left pre-war tanks little chance of survival. Combat in France and Poland was an effective demonstration. The most that the cavalry branch could do was push for the production of 34 Combat Cars M2. The military understood perfectly well that the tank was obsolete.
Some of the Combat Cars M2 were equipped with a Guiberson T-1020 diesel engine.
The cavalry performed fewer experiments with its tank than the infantry with the Light Tank M2. Nevertheless, they built a few interesting designs. In 1937, the long-suffering Combat Car T5 was converted once more. This time, the suspension was modified. Instead of springs, engineers tried out a rubber torsion bar. This design is different from the usual steel torsion bars, and takes up less space.
At the same time, enlarged idlers were installed. Trials of the tank, indexed Combat Car T5E4, began in early 1938 at Aberdeen Proving Grounds. Later, they continued at Fort Knox. The rubber torsion bar worked well, but its reliability was low. The 5 cylinder Guiberson T-570-1 engine was also tested on this tank. Neither novelty made it out of the experimental stage.
Arthur Harrington, one of the founders of the Marmon-Harrington company, promoted the idea of a running gear with rubber-metallic tracks since the mid-1930s. Aside from halftracks, this design was also used in export tanks. In 1938, one Combat Car M1A1 was equipped with such a design. Only the suspension remained on this tank, the running gear was borrowed from a halftrack, with some changes. The designers also tried to resolve the issue of the raised driveshaft on this tank, indexed Combat Car M1E3. Neither solution could be practically implemented.
A long and sad story of how the cavalry tried to get a convertible drive tank for the third time is the most interesting of the lot. Cancellation of the Combat Car T4E1 did not stop the cavalrymen. Work on a new tank, called Combat Car T6, began. Sadly, no graphical depictions of it remain. Its mass is estimated at 10,800 kg, and the crew consisted of 5 men. The driver was in the center, flanked by two machinegunners.
As with the Combat Car T5, the Combat Car T6 would have two turrets. The 400 hp Wright Whirlwind R-975 would propel the tank at 70.5 kph on wheels and 50.7 kph on tracks.
Work on this monster stopped at the design stage. In November of 1935, the Combat Car T6 was cancelled, since it went 1.5 times over the weight limit and had no advantages over the Combat Car M1.
A year later, the cavalry took a different route. A decision was made to take a Combat Car M1 and make it into a convertible drive tank. In 1937, the Combat Car T7 project began.
This tank could be considered a mix between the Combat Car T2 and Combat Car M1. Like Knox's fearsome design, it would have three large road wheels per side. Instead of rubber rims, inflatable tires were used. The engine powered the rear pair of wheels, and the front pair turned the tank.
Development dragged on, and the Combat Car T7 only saw trials in August of 1938. The last Combat Car M1A1, registration number U.S.A. W.40223, was used to build it. The first trials did not inspire confidence. Despite the engine power growing to 268 hp, the mobility did not improve greatly. Only improvements made during the trials increased the top speed to 85 kph on wheels and 56 kph on tracks. However, the convertible drive idea was not very interesting by that time. Work stopped in October of 1939.
Cavalry to Infantry
By 1940, the situation in the US Army changed. The Combat Car M2 became the last cavalry tank. The separation between infantry and cavalry tanks was senseless, especially once you consider their many similarities. The cavalry tanks were reclassified as light tanks. The Combat Car M1 became the Light Tank M1A1, and the Combat Car M2 became the Light Tank M1A2. At the same time, the tank units of cavalry and infantry began their transformation into armoured divisions.
The former cavalry tanks, now light tanks, were used as training vehicles. Much like the Light Tank M2 family, these tanks became desks for students of the American armoured forces. Due to the armour and armament unsuitable for modern war, these tanks were not meant for use outside the USA.
To be fair, one must remember that these tanks were some of the best in their class at the time of inception and grew obsolete alongside their foreign analogues. The former cavalry tanks were used for training until 1942. Unfortunately, none of them survived to this day.
The history of the Combat Car M1 would be incomplete without mentioning other vehicles on its chassis. Back in 1933, when work on the Combat Car T5 was just beginning, an artillery tractor was being designed in parallel. The suspension came from the tank, but the engine was different. The first T3 Tracked Light Tractor received the registration number W.9115. The vehicle was designed to accommodate a driver and a gun crew of six.
Due to the small chassis, the crew had to sit back-to-back. Initially, the artillerymen's feet dangled by the running gear, with a serious risk of being pulled in. Later, the designers added a railing. The driver sat in the front, flanked by fuel tanks, with the instrument panel mounted on the side of the right one. The 3.5 ton tractor was powered by a 95 hp Herculex WXTB engine, which gave it a top speed of 34 kph.
An improved variant of the tractor, indexed T3E1, registration number W.9116, was later sent to trials. The tractor was converted, as the designers realized that six men is a lot for such a small vehicle and reduced the number of passengers to three. Two sat on the rear mudflaps, and one more on top of the left fuel tank, which was reduced in size. A new engine was installed on the improved tractor, a 106 hp Hercules WXRT, as well as an improved gearbox. The T3E1 Tracked Light Tractor reached a top speed of 56 kph, although it would be an interesting question to see how the passengers felt at that speed. Judging by the layout, the odds of being catapulted out of the tractor during motion were high.
As an experiment, tractor with serial number W.9155 was equipped with a 100 hp Herculex WXC-2. The passenger seats were also removed. The T3E2 Tracked Light Tractor reached a speed of 48 kph.
In 1937, the T3E1 Tracked Light Tractor was convered for use by coastal defense. The vehicle, indexed T3E3 Tractor, received the engine from the T3E2 Tracked Light Tractor, protected electrical equipment, and a removable roof for the driver.
In July of 1937, the final version of the vehicle finally reached trials. After some changes, the Tracked Light Tractor finally became what the customers expected of it. The improved version received the index T3E4 Tracked Light Tractor, and earlier vehicles were improved to the same standard. Six vehicles were built in total.
Many think that the tractors spent their entire lives at Aberdeen Proving Ground, but that is not the case. In 1939, three T3E4 Tracked Light Tractors were sent to the Antarctic. They were used to carry cargo at Stonington Island (East Base) and the Ross ice shelf (West Base, Little America) along wit the Light Tank M2A2. After the mission was completed, the tractors were abandoned. Two of them remain at Stonington Island to this day.
Translated by Peter Samsonov. Read more interesting tank articles on his blog Tank Archives.
- Central Archives of the Russian Ministry of Defence
- Stuart History of the American Light Tank, Vol. 1, R.P. Hunnicutt, Presidio Press, 1992
- Author's photo archive