XM-3E1: the modernized stuart anti-aircraft version

VEIO_da_HAVAN · November 30, 2023, 3:25pm

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XM-3E1

xm-3e1proto_1

The vehicle’s history:

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The SPAAG stemming from the aforementioned tank projects was designated VBC-AAe (anti-aircraft armored combat vehicle in Portuguese) and later labeled XM-3E1. Notably, this wasn’t the initial project of its kind. In 1969, a Brazilian M3 Stuart underwent a unique transformation, with its turret replaced by an M45 quadmount armed with Browning M2 .50cal machine guns. Each gun was fed by a 200-round tombstone magazine, maintaining the M3’s WWII baseline features. Although marginally successful, this project wasn’t pursued further.

The late 1970s introduced the XM-3E1, aiming to convert basic M3 Stuarts using the lower section of the X1A tank conversion as its base. Instead of a turret, it featured an M45 quadmount, repurposed from M55 towed AA trailers acquired by the Brazilian army from the USA. The M45, proven successful in WWII and the Korean War, could be fine-tuned for convergence, allowing all four guns to fire simultaneously or alternating the top/bottom pairs to prevent overheating.

The versatile M45 quadmount, known for its ground-to-ground capabilities, delivered a total of 1,800rpm when all four .50cal guns fired, making it a formidable option. On the XM-3E1, the lower portions were derived from the X1A tank, and the M45 quadmount rotated on a newly implemented electrically-operated ring.

However, the XM-3E1 didn’t receive approval for production. As a SPAAG, it lacked onboard radar, limiting engagements to daytime below cloud level. Economically, each XM-3E1 would be costly due to the X1A’s restricted production.

Another complication arose as Bernardini originally envisioned the SPAAG variant equipped with a remotely-operated twin Brazilian 20mm autocannon (still under development). The M45 was intended as an interim solution, but the 20mm weapon was ultimately canceled.

Additionally, there was a separate proposal for a second SPAAG, considering a X1A lower hull conversion paired with a naval Bofors L/60 40mm AA gun. This proposal was terminated during the planning phase.

Hull:

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The chassis of the XM-3E1, a modified and elongated version of the M3 or M3A1 Stuart, maintained comparable overall protection to that of the M3 model.

Front (Upper Glacis): 38 mm at a 17-degree angle
Front (Middle Glacis): 16 mm at a 69-degree angle
Front (Lower Glacis): 44 mm at a 23-degree angle
Sides (estimated): 25 mm
Rear (estimated): 25 mm
Top: 13 mm
Floor: 13 to 10 mm

Mobility:

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Powered by the Scania-Vabis DS-11 A05 CC1 6-cylinder in-line 256 hp diesel engine, the XM-3E1 demonstrated robust mobility. Generating 256 hp at 2,200 rpm, the vehicle achieved an impressive horsepower per ton ratio of 15.1. This power allowed the XM-3E1 to reach a top speed of 55 km/h on roads, boasting an operational range of 520 kilometers.

Suspension:

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Equipped with a modified VVS suspension system derived from the 18-Ton High Speed Tractor M4, the XM-3E1 featured a dual-bogie configuration on each side. Each bogie comprised two rubber-tired steel wheels, oscillating on hinge pins within brackets bolted to the hull. This suspension design granted the vehicle a ground pressure of 0.59 kg/cm2, an on-ground track length of approximately 3.22 meters, and the capability to traverse a trench of 1.2 meters.

Modernised 12.7mm Anti-aircraft Machine Gun M55:

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The M-55 Machine Gun Trailer Mount system gained a new application in 1969 when the Brazilian Army considered converting part of its fleet of light tanks, M-3 and M-3A1 Stuarts, into self-propelled anti-aircraft vehicles. The project involved replacing the tank turret with a quadruple .50 caliber anti-aircraft machine gun mount, the M-45 Quadmount, provided on loan by the 5th Anti-Aircraft Artillery Group 90mm (Gcan90 AAe). Despite successful tests, the project was canceled, and the vehicle was restored to its original condition.

In the 1970s, Lysam, a company based in Rio de Janeiro, proposed a repowering process for the M-55 system, aiming to extend its operational life. The proposal included a complete overhaul and the replacement of the original electrical system with a more modern model powered by a Montgomery M-226 or M-252 gasoline engine. A prototype was produced, and while maintaining the four Browning M-2 .50 caliber machine guns, it improved its traverse speed and elevation capacity to over 90° per second, enabling effective engagement of fast-moving targets. This project exemplifies the Brazilian Army’s ongoing commitment to modernize its anti-aircraft capabilities to address emerging threats

Drawing depicting the turret with twin 20 mm cannons that would be developed by Lysan Indústria de Máquinas e Equipamentos Ltda for the M-3A1 Light Tank Stuart.