At the beginning of the 1970s, the Brazilian Army started developing armored vehicles, starting with wheeled vehicles. After having successfully developed the prototype concepts which would become the EE-9 Cascavel and the EE-11 Urutu, the Brazilians looked to tracked vehicles. Like the previous wheeled vehicle projects, the engineers started small. They began by remotorizing readily available M3 Stuarts, and then started developing the vehicle that is known as the X1 light tank. The X1 was a modernization of the Stuart, which was armed with a low-pressure 90 mm gun, and would be developed into an entire family of vehicles.
One of these vehicles was a planned improvement of the X1 tank. The X1 had some limitations because of the components it used and some basic concepts. To fix these issues, the Brazilian Army engineers started the development of the X1A1, which was effectively a lengthened X1 with improved components.
The X1 Project
In 1973, the first X1 vehicle was developed and presented at the Brazilian Independence Day Parade on September 7 of the same year. The X1 was a modernization project of the M3 Stuart, carried out by the Parque Regional de Motomecanização da 2a Região Militar (PqRMM/2) (English: Regional Motomecanization Park of the 2nd Military Region), together with Bernardini and Biselli, two Brazilian companies. The PqRMM/2 was responsible for the development of the wheeled vehicles, but also for the tracked vehicles of the Brazilian Army at the time, and were under the supervision of the Diretoria de Pesquisa e Ensino Técnico (DPET) (English: Army Research and Technical Educational Board), which coordinated the projects.
The tracked vehicles were researched and developed by a team of engineers within the Army and PqRMM/2, which were part of the Centro de Pesquisa e Desenvolvimento de Blindados (CPDB) (English: Centre for the Research and Development of Tanks). The CPDB was a study group of Army engineers which analyzed the possibilities of locally produced tanks. The first goal was to develop a new family of light tanks using the M3 Stuart as its basis.
The reasons for the M3 Stuart modernization were the lack of new and cheap materiel from the United States (then involved in the Vietnam War), the fact that they were the most numerous vehicles to be converted, they were cheap to run and maintain, and their lightweight made them perfect for fighting on the difficult terrains of Brazil and their neighboring countries if needed. But the most important reason was that they were relatively easy and low risk to convert in order to gain experience to eventually build a national Brazilian tank. The M41s which Brazil had at the time were their best vehicles and much more risky to improve with the lack of experience.
After having successfully developed the first X1, a pre-series of 17 vehicles was ordered. These vehicles would, due to extensive delays, finally be delivered in 1976. At this point, various flaws of the X1 were already found and the X1A1 already developed.
Bernardini and Biselli
For the construction of the X1A1, multiple parties and companies were involved. The most important two companies which built the X1A1 were Bernardini and Biselli. Both companies manufactured truck bodies and value transport vehicles at the time, and came in contact with the Brazilian Armed Forces by manufacturing trucks for the Brazilian Marine Corps and the Army. Since both companies had some experience in the manufacture of armored vehicles, and with Bernardini being a manufacturer of safes and armored doors, they were requested by the Brazilian Army to help build the X1. After the X1 was successfully developed, Bernardini and Biselli started developing the X1A1 together with the PqRMM/2 and the CPDB engineers. Although Biselli would never fully commit to the project, Bernardini would commit and eventually become the tank counterpart to Engesa’s wheeled vehicles.
|United States||The M3A1 Stuart|
|Biselli||Most likely: Hull extension, engine installation, equipment installation, and track mounting|
|Bernardini||Most likely: Turret and suspension|
The exact starting point for the development of the X1A1 is unknown. Considering the first X1 was still extensively tested in 1974, and the X1A1 was first presented at the Independence Day Parade of 1976, it can be estimated that the vehicle might have been developed anywhere between 1975 and September 1976. Considering that the first X1 was developed and built in a span of just 2 months, it is not unlikely that the X1A1 started its development in early 1976.
The development of the X1A1 was meant to fix some issues identified on the X1. The most notable were the replacement of the massive idler wheel from the 18-ton M4 Artillery Tractor suspension, which had replaced the original M3 Stuart suspension on the X1, and the replacement of the M3 Stuart transmission with the transmission from the 18-ton M4.
The large ground touching idler wheel of the 18-ton M4, was a perfect solution to provide more track length while not needing a particularly large hull. The downside of these types of idlers is that they are made for relatively slow-moving vehicles, which the X1, at 55 km/h, was not. The high speed and weight of the X1 caused premature wear on the swing arms of the idler, which would eventually start to crack.
To fix this issue, it was decided to replace the ground-touching idler of the 18-ton M4 with the idler of the M4 Sherman. This meant that the idler would not provide the on-ground track length which was needed for the vehicle. As a result, an additional bogie was added, totaling three sets of bogies per side. In essence, a sort of 18-ton M4 Tractor/M4 Sherman hybrid suspension was created.
Because of the way a bogie has to be installed, the hull of the X1A1 had to be significantly lengthened compared to the M3 Stuart. The hull was lengthened by about 0.8 meters to accommodate the newly designed suspension and thus the required on-ground track length. The on-ground track length of the X1 was about 3.22 meters (10.6 foot), while the X1A1 was about 3.66 meters (12 foot) in comparison.
The lengthened hull brought a few extra advantages. A larger fuel tank was installed and more ammunition could be stored in the hull compared to the X1, increasing from 10 to 34 90 mm rounds.
The lengthened hull also enabled the engineers to design a larger turret bustle for the original BT-90A1 turret. In contrast to the initial turret of the X1, which was supposedly covered with 25 mm (1 inch) thick armor, the X1A1 turret would have 25 mm armor (1 inch) on the front and 12.5 (0.5 inch) mm armor on the rear parts of the turret. The extension of the turret bustle meant that an extra radio could be installed in addition to 24 90 mm rounds, compared to 10 rounds in the X1 turret.
All these changes caused the vehicle to increase in weight, which meant that larger brakes had to be installed to compensate for this increase. The X1A1 was finished somewhere before the Independence Day Parade of September 7th 1976. The vehicle would gain its nickname during a promotional film, when General Pedro Cordeiro de Mello, head of the PqRMM/2 and also the one who named the X1, named it Carcará, after an indigenous crested bird. It would be officially designated as Viatura Blindada de Combate – Carro de Combate MB-1A (VBC-CC MB-1A), (English: Armored Fighting Vehicle – Combat Car MB-1A, with MB meaning Medio Bernardini or Medium Bernardini). Considering the X1’s similar designations, it would most likely have also been referred to as Carro de Combate Leve X1A1 Carcará (CCL X1A1 Carcará), (English: Light Combat Car X1 Carcará), but this is more of an educated guess that cannot be actually confirmed. The X1A1 used EB11-376 as its vehicle number in the Brazilian Army, most likely being converted from an M3A1 Stuart which bore the same number and was owned by the CPDB.
Somewhere around this time, Biselli left the X1 family projects. Although the exact reasons are unclear, there are some statements that Biselli had some internal struggles, and Bernardini demanded more recognition for their efforts in the X1 project. In addition, it is also suggested that Biselli saw limitations in the defense industry and decided that focussing on the civilian industry was more profitable, while taking on more of a support role in the defense industry. With Biselli quitting the project, all tank development would be taken on by Bernardini, which would one day build the MB-3 Tamoyo main battle tanks.
The X1A1 in Detail
Multiple characteristics are presented incorrectly in the sources. As a result, all the length values were calculated with ratios and are reasonable estimates. Most of the other values are rough estimates. The X1A1 weighed somewhere between 17 and 19 tonnes (18.7 to 21 US tons) and was 7.04 meters (23.1 feet) long including the gun, had a hull of about 6.04 meters (19.8 feet), 2.4 meters (7.9 feet) wide, and 2.45 meters (8 feet) tall. It had a crew of four, with the driver located on the front left of the hull, the co-driver on the front right of the hull, the commander/loader on the left side of the turret, and the gunner on the right side of the turret.
Hull and Armor
The hull of the X1A1 was a lengthened and modified M3A1 Stuart hull, recognizable by the curved rear plate. As such, the overall protection for most of the X1A1’s hull remained the same as the M3. The thickness of the plates which were used to lengthen the hull is unknown. The upper front plate of the X1A1 had an armor thickness of 38 mm (1.5 inch) at 17º vertical, a middle front plate of 16 mm (0.6 inch) at 69º, and a lower front plate of 44 mm (1.7 inch) at 23º. Its sides were most likely about 25 mm (1 inch) thick. The rear armor and the lengthened parts of the side are unknown. Considering the original Stuart had 25 mm (1 inch) thickness on the sides and rear, it would not be unreasonable to assume that the lengthened structure is about 25 mm (1 inch) thick as well. The top plate would have been 13 mm (0.5 inch) thick and the floor plate would have gradually decreased in thickness from 13 mm at the front to 10 mm (0.5 to 0.4 inch) in the rear (although the thickness for the lengthened structure is unknown).
The rest of the X1A1 had a very similar layout as the Stuart. It had two headlights, one on each side of the front mudguards, two towing hooks on the front hull, and a .30 caliber hull machine gun on the right side. The driver had a two-piece hatch, while the co-driver had a single-piece hatch.
The X1A1 was powered by a Scania-Vabis DS-11 A05 CC1 6-cylinder in-line 256 hp diesel engine. A difference with the X1 is that the X1A1 uses more drive components from the 18-ton M4 instead of those of the M3 Stuart. This meant that the X1A1 had the same 3-speed transmission as the 18-ton M4. Another interesting detail is that the steering levers of the vehicle were mounted on the top of the hull instead of the bottom. The X1A1 had a top speed of potentially 50 to 55 km/h (31 to 34 mph) on roads and an operational range of 520 km (323 miles).
The X1A1 used a copied and altered VVS suspension system of the 18-ton M4 artillery tractor. It had 6 road wheels divided over three bogies, with 3 bogies per track, 3 return rollers on each side, a drive sprocket in the front and an M4 Sherman idler wheel on the rear. The newly designed 18-ton M4 Tractor/M4 Sherman hybrid suspension gave the X1A1 a ground pressure of 0.55 kg/cm2 (7.8 psi). The X1A1 had an on ground track length of about 3.66 meters (12 foot) and could cross a trench of 6.1 meters (4.9 foot).
The X1A1 turrets were practically the exact same turrets as the X1’s BT-90A1 turrets, except for the extended turret bustle. The front turret was armored with 25 mm (1 inch) thick steel plates at various angles to protect it from .50 caliber machine gun fire at 200 m (218 yards). The rear parts of the turret were armored with 12.5 mm (0.5 inch) thick steel plates. It is suggested that the overall turret layout and the internal turret construction and components were more or less copied from the French H-90 turret. It had the exact same turret ring and its overall shape seems to match the H-90. In addition, in the first BT-90 turret of the X1, a lot of equipment was carried over from the H-90, like the periscopes.
The X1A1 turret had a mount for a .50 cal machine gun on the left side of the turret, in front of the commander’s cupola. The commander’s cupola’s structure was slightly raised from the turret top to provide the commander with a 360º view. The antenna of the radio sets was located behind the gunner’s cupola on the right side of the turret. Placement-wise, there were a few differences with the original X1 turret. Spare tracks were mounted on the turret bustle sides, which might hint that the turret bustle sides are 12.5 mm thick as it uses additional tracks as armor. This placement of the spare tracks meant that the smoke dischargers were moved to the front of the turret, in a set of 3 dischargers on each side. A small light was also installed on the turret side of the commander’s cupola.
The X1A1 was armed with the 90 mm D-921 low-pressure gun. The low-pressure gun allowed vehicles like the X1, but also the 5 tonne AML-90, to mount a gun on light platforms with significant armor penetration capabilities.
The trade-off with these types of guns is that Kinetic AP or APFSDS rounds are not really worth it from a penetration point of view compared to the HEAT rounds these guns fired. A 90 mm APFSDS round for the later Cockerill guns would penetrate 100 mm (3.9 inch) of armor at 60º from vertical at a range of 1,000 m (1,090 yards), compared to 130 mm (5.1 inch) at 60º for HEAT at any range. The D-921 did not even have AP rounds available for this reason.
The X1A1 had access to HEAT, High Explosive Squash Head (HESH), and High Explosive rounds. The HEAT round was meant for anti-armor purposes and was the X1A1’s anti-tank round. The HESH round was mainly meant for bunkers, walls, and light vehicles, and not as ‘anti-armor’ ammunition. The HE round was used as a general-purpose support round. Another downside of these low-pressure guns was their limited combat range and decreased velocity. This meant that the gun became much less accurate at longer ranges compared to high-velocity guns, which could also outrange the low-pressure 90 mm guns.
|HEAT (High Explosive Anti Tank)||320 mm flat at any range.||1,500 meters (1,640 yards)||750 m/s|
|HE (High Explosive)||Lethal radius of 15 meters (16 yards)||–||650 m/s|
The X1A1 stowed 24 rounds in the turret, and an additional 34 rounds in the hull, for a total of 58 rounds of 90 mm ammunition. In addition to the 90 mm, the X1A1 mounted a turret top .50 caliber machine gun for the commander, a .30 coaxial machine gun, and a .30 machine gun for the co-driver in the hull.
In the end, after all the improvements done on the X1A1 in an attempt to fix the mistakes of the X1, the Brazilian engineers accidentally ‘broke’ the vehicle even more. Although most of the issues with the individual components were solved, some issues were unfixable, as they resulted from the simple fact that the basis of the X1A1 was a 30-year-old M3A1 Stuart. The torque converter also provided problems throughout the X1 family, as the quality of diesel used by the Brazilians was quite poor. This resulted in premature wear of the components.
The biggest issue though, and what really caused the engineers to break the vehicle more than they fixed the X1, was that the length to width ratio was off. The X1A1 was too long and too narrow. This made the X1A1 a very sluggish and hard to steer vehicle, which worsened the longer the vehicle was operated. An anecdote describes the steering issues of the X1A1. The X1A1 was the only vehicle of the X1 family to have its steering levers attached to the roof of the hull instead of the floor. In an instance, a driver had to apply so much force in order to turn, that he pulled out the lever from its fixation point. This problem was only worsened because the X1A1 also suffered from an increasingly problematic differential, on top of the length to width ratio of the vehicle.
The engineers analyzed how they could potentially fix these issues, but they quickly discovered it was not worth the resources. They would have had to widen each M3 Stuart to achieve the needed length to width ratio, and also adjust most of the components to fit in this new configuration. In addition, the fixed X1A1s would still suffer from the same issues as the other X1s, because the same old Stuart was still the base vehicle. As a result, it was decided that it was easier to start developing Brazil’s first and practically only serially produced tank in Brazilian service, which was completely designed in Brazil. This new vehicle became the X1A2 and would see limited service in the Brazilian Army, as the project was fairly quickly canceled for the much more promising M41 modernization projects.
The only produced prototype now serves as a gate guardian at the PqRMnt/1 in Rio de Janeiro.
For all intents and purposes, the X1A1 itself was a failure. The Brazilian engineers literally found the limits to what one could do with an M3 Stuart. Even though the X1 program was a sort of trial and error program to gain experience, the X1A1 was a step too far in the modernization of obsolete vehicles. The significant mobility issues and the continued usage and wear of old M3 Stuart components would cause the X1A1 to be a very immobile vehicle.
It was not a complete failure though. The 18-ton M4 Tractor/M4 Sherman hybrid suspension would be carried over from the X1A1 to the X1A2, and the lengthened turret would also be carried over for at least the first X1A2 prototype. The X1A1 would, due to the significant problems, be the only vehicle in the X1 family that was lengthened this way with the M3/M3A1 Stuart as its basis. The X1A1 functioned more as a technology bridge between the X1 and the X1A2 than anything else. It was a useful unsuccessful project.
Specifications CCL X1A2
|Dimensions (L-W-H)||7.06 meters (23.2 feet) long including the gun x 2.4 meters (7.9 feet) x 2.45 meters (8 feet) tall|
|Total weight||17 to 20 tonnes (18.7 to 22 US tons)|
|Crew||4 (Driver, Co-driver, Commander-Loader, Gunner)|
|Propulsion||Scania-Vabis DS-11 A05 CC1 6-cylinder in-line 256 hp diesel engine|
|Speed (road)||50-55 kph (31-34 mph)|
|Operational range||520 km (323 miles)|
|Armament||90 mm D-921 low-pressure gun
.50 machine gun
.30 coaxial machine gun
.30 hull machine gun
Front (Upper Glacis) 38 mm (1.5 inch) at 17º
25 mm (1 inch) frontal part
Special thanks to Expedito Carlos Stephani Bastos, the leading expert in Brazilian vehicles, please visit his website for further reading on Brazilian vehicles: https://ecsbdefesa.com.br/, Jose Antonio Valls, an Ex-Engesa employee and expert in Engesa vehicles, Paulo Bastos, another leading expert of Brazilian Armored vehicles and the author of the book on Brazilian Stuarts and the website https://tecnodefesa.com.br, Adriano Santiago Garcia, a Captain in the Brazilian Army and ex-company commander on the Leopard 1 and ex-lecturer on the Brazilian Armored School, and Guilherme Travassus Silva, a Brazilian with whom I was able to endlessly discuss Brazilian Vehicles and who was always willing to listen to my near endless ability to talk about them.
Brazilian Stuart – M3, M3A1, X1, X1A2 and their Derivatives – Hélio Higuchi, Paulo Roberto Bastos Jr., Reginaldo Bacchi
Blindados no Brasil – Expedito Carlos Stephani Bastos
Personal correspondence with Expedito Carlos Stephani Bastos
Personal correspondence with Paulo Roberto Bastos Jr.
Engesa brochures and manuals
TM 9-785 18-Ton High Speed Tractors M4, M4A1, M4C, and M4A1C – US Army April 1952.
Stuart: A history of the American Light Tank, Volume 1 – R.P. Hunnicutt
Tecnologia Militar Brasileira magazine