If you like the Canadian Skink AA tank, then you will love the T77, which was basically a miniature version of that vehicle. The T77 was a small, fast, self-propelled anti-aircraft gun based on the M24 Chaffee that had a group of six .50 caliber machine guns in a two-man turret.
This little machine could have kept up with armored columns and provided continuous close air defense. It was a very promising and interesting design, but it didn’t enter service. As a result, the T77 has been mostly forgotten about.
Contents
Development
The increased importance and involvement of airpower in the ground-attack role during the Second World War naturally led to an increased need for ground-based air defense systems to shoot them down.
However, while static defenses were useful around stationary positions, they weren’t any use for mobile units, like convoys or armored divisions. For this, anti-aircraft guns would need to be made mobile so they could move with them, providing continuous air protection.
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The idea of a self-propelled anti-aircraft gun (SPAAG) was nothing new, with designs dating as far back as the First World War, but in the 1940s they would receive significant attention.
The US ramped up its work on SPAAGs early in the war, before they themselves had even entered it. These efforts used half-tracks as the basis of SPAAGs, producing famous machines like the Multiple Gun Motor Carriage M16, which had four .50 caliber machine guns in a power-operated turret.
Vehicles like the M16 were very effective, but the US ideally wanted a fully-tracked SPAAG. By giving the anti-aircraft vehicle the mobility of a tank, it could keep up with them in battle, greatly improving their practicality.
What is the easiest way of providing tank-level mobility? You use a tank chassis, of course!
Early attempts saw a .50 caliber quad-mount turret and a Mark 17 twin machine gun mount (the type used by Navy PT boats) fitted to the Stuart light tank, but neither were pursued.
In July 1943, work began on an SPAAG based on the M24 light tank, which, at the time, was still in development and known as the T24.
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The M24 was a modern, promising design that incorporated sloped armor, torsion bar suspension, and a good power to weight ratio.
Its solid foundations were very adaptable, so the chassis leant itself well to becoming the basis of a SPAAG. The vehicle was designated Carriage, Motor, Multiple, Cal. .50 Gun, T77, and showed the signs of a very good design.
Initially it was to be armed with four .50 caliber machine guns. These would be contained within the T89 quadruple mount, which had been developed by the United Shoe Machinery Corporation with help from the Army Air Force.
The mount was fully powered, and could be operated remotely to stop muzzle flashes and smoke obscuring the gunner’s aim.
By the end of 1943 the design was approved for a mock-up, which was finished by December.
After inspecting the mock-up, officials requested that an additional two .50 caliber machine guns be added, bringing the total to six.
Two prototypes were ordered in April 1944, following these changes. Construction of the first prototype began in November 1944, and was completed in July 1945.
Firing trials of the guns in the T89 mount had started back in October 1944.
The T77’s Design
The T77 was based on a mostly unmodified M24 Chaffee light tank chassis. As with the standard tank, in the rear was two 345 cu in (5.6 litre) Cadillac 44T24 V8 engines, each producing 110 horsepower and 240 ft lbs of torque.
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The engines were connected to two Cadillac Hydramatic automatic gearboxes.
At the front of the hull sat two crew; the driver on the left, and a co-driver on the right. Like the Chaffee, the co-driver had duplicated controls of the driver, and operated a .30 caliber machine gun in the bow.
Virtually all of the T77’s changes came with the turret, which was an entirely new unit that contained six .50 caliber machine guns in a elliptical arrangement at the front.
The turret only contained two crewmembers – a commander and a gunner – who were both located at its rear in circular hatches. It appears that on the second version, the T77E1, these hatches were covered with clear acrylic domes.
They aimed the guns with either two Mark IX reflector gun sights, or T5E1 computing sights.
While we don’t have exact numbers for the turret’s rotation speed, as it was intended to track low aircraft, it was certainly faster than the 15 seconds for a full rotation of the standard Chaffee.
It was probably closer to the Skink, a Canadian AA tank based on the Grizzly, which could complete a full rotation in just six seconds.
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The T77’s arrangement of six .50 caliber machine guns gave it considerable firepower. Each gun could fire 500 rounds per minute, for a combined output of 3,000 rounds per minute. They could move vertically between -10 degrees, and +85 degrees.
The guns were fed by 6,700 rounds of ammunition, 1,200 of which were stored in the hull. There was an additional 2,500 .30 caliber rounds in the hull for the bow machine gun.
Armor protection was the same as the M24 Chaffee on the hull, but the turret only had half an inch (12.7 mm) of armor on all sides. However, as the T77 would be unlikely to encounter any anti-tank weapons,, and half an inch was enough to stop small arms fire, this was acceptable.
Total weight of the T77 came in at 19 tons, the same as a standard Chaffee. Top speed was 35 mph.
Fate
After the first T77 was completed in July 1945, it was transported to the Aberdeen Proving Ground for evaluation. A second prototype was finished shortly after, and this version came with an advanced fire control system. As a result of these differences, the second prototype was designated T77E1.
The T77 appears have been quite a good design that likely would have entered service, but this never happened as the Second World War ended soon after the prototypes were built.
There was now a reduced need for new vehicles, and the T77 would have been less-effective against faster jet aircraft.
Still, the project seems to have continued for a few years after the war, with one document from 1951 stating “A definite requirement exists in the armored division for a vehicle of this type. This vehicle is undergoing field tests at present and will probably fill the requirement.“
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But in the end it wouldn’t enter service, and the two prototypes were presumably scrapped.
