In October 1951, a heavy tank project was underway to mount an oscillating turret with an automatically loading 120mm Gun on the hull of the 120mm Gun Tank T43. (The T43 would later be serialized as the 120mm Gun Tank M103, America’s last heavy tank.). This was the T57, and the Rheem Manufacturing Company were granted a contract to design and build two pilot turrets and autoloading systems.
During the T57’s development, it became clear that it was feasible to mount a lighter armored version of the T57 turret on the hull of the 90mm Gun Tank T48 (The T48 later became the 90mm Gun Tank M48 Patton III). This combination granted the possibility of creating a ‘heavy gun tank’ that was lighter than any previously designed.
In May 1953, a development project was started to create such a tank. It would be designated the 120mm Gun Tank T77, and another contract was signed with Rheem to create two pilot tanks.
The hull chosen for the project was that of the 90mm Gun Tank T48. The tank weighed about 50 tons, with armor of up to 110mm thick.
The tank was powered by a 650 hp Continental AVSI-1790-6 V12, air-cooled twin-turbo gasoline engine. This would propel the tank to a speed of 30 mph (48 km/h). The tank was supported on a torsion bar suspension, attached to six road wheels. The drive sprocket was at the rear, while the idler was at the front. The idler wheel was of the compensating type, meaning it was attached to the closest roadwheel by an actuating arm. When the roadwheel reacts to terrain the idler is pushed out or pulled in, keeping constant track tension. The return of the track was supported by six rollers.
A small scale mockup of the T77. Photo: Presidio Press
The Oscillating type of turret consists of two actuating parts, consisting of a collar that is attached to the turret ring, allowing horizontal traverse, and a pivoting upper part that holds the gun, loading mechanism and crew. Both halves of the T57’s turret were cast in construction, utilizing cast homogeneous steel armor. Armor around the face was 127mm (5 inches) thick, angled at 60 degrees. This increased to 137mm (5.3 inches) of the sides of the turret and dropped to 51 mm (2 inches) on the bustle.*
*The T77’s turret was supposedly designed to be lighter by having thinner armor, however, Hunnicutt’s data shows it to be the same as the T57’s turret. Whether this is erroneous or not is unknown.
The sides of the collar were made to be round and bulbous in shape to protect the trunnions that the upper half pivoted on. The other half consisted of a long cylindrical ‘nose’ and a low profile flat bustle.
Cutaway views of the internal systems and layout of the turret. Photo: Presidio Press
Though it looks like two, there were actually three hatches in the roof of the turret. There was a small hatch on the left for the loader, and atop the turret, a commander’s cupola which featured five periscopes and a mount for a .50 caliber (12.7mm) machine gun. These hatches were placed on top of the third hatch, which was a large square which took up most of the middle of the roof. This large hatch was powered and allowed a larger escape route for the crew, but also allowed internal turret equipment to be removed easily. In front of the loaders, hatch was a periscope, there was another above the gunner’s position.
Behind the large hatch was the ejection port for spent cartridges. To the right of this was the armored housing for the ventilator. On each side of the turret were ‘frogs eyes’, the armoured covers for the stereoscopic rangefinder used to aim the main gun.
The initial Rheem concept had the gun rigidly mounted without a recoil system in a cast, low silhouette oscillating turret. The gun protruded from a long, narrow nose. The gun featured a quick change barrel, was basically identical to the 120mm Gun T123E1, the gun being trialed on the T43. However, for this turret, it was modified to accept single piece ammunition, unlike the T43 which used separately loading ammo. This new gun was attached to the turret via a conical adapter that surrounded the breech end of the gun. One end screwed directly into the breech, while the front half extended through the ‘nose’ and was secured in place by a large nut. The force created by the firing of the gun and the projectile traveling down the rifled barrel was resisted by rooting the adapter both the breech block and turret ring. As there was no inertia from recoil to automatically open the horizontally sliding breech block, a hydraulic cylinder was introduced. Upon firing the main gun this hydraulic cylinder was triggered via an electric switch.
This new variant of the T123 was designated the 120mm Gun T179. It was fitted with the same bore evacuator (fume extractor) and muzzle brake as the T123. The gun’s rigid mount was designated the T169, making the official nomenclature ‘120mm Gun T179 in Mount T169’
It was proposed that two .30 caliber (7.62mm) machine guns would be mounted coaxially. This was later reduced to a single machine gun placed on the right side of the gun.
In the oscillating turret, the gun could elevate to a maximum of 15 degrees, and depress 8 degrees. Projected rate of fire was 30 rounds per minute. The main gun had a limited ammunition supply due to the size of the 1-piece rounds. The T48 hull had to be modified to allow storage, but even then, only 18 rounds could be carried.
The automatic loader shared by the T77 and T57 consisted of a large 8-round cylinder located below the gun, and a ramming arm that actuated between positions relative to the breech and magazine. The loader was designed for one-piece ammunition but an alternate design was prepared for use with two-piece ammunition.
Operation: 1) The hydraulically operated ramming arm withdrew a round and aligned it with the breech. 2) The rammer then pushed the round into the breech, triggering it to close. 3) Gun fires. 4) Effect of gun firing trips the electric switch that opens the breech. 5) Rammer picks up a fresh round, at the same time ejecting the spent cartridge through a trap door in the roof of the turret bustle.
A diagram of the loading process. Photo: Presidio Press
Ammunition types such as High-Explosive (HE), High-Explosive Anti-Tank (HEAT), Armor Piercing (AP), or Armor-Piercing Ballistic-Capped (APBC) could be selected via a control panel by either the gunner or the tank commander (TC). The round could punch through a maximum of 330mm (13 inches) of Rolled Homogeneous Steel Armor.
The T77 had a crew of four men. The driver’s position was standard for T48/M48 hulls. He was located centrally in the bow at the front of the hull. Arrangements inside the turret were standard for American tanks. The loader was positioned at the left of the gun. The gunner was on the right with the commander behind him.
The T77 would share the same fate as other Rheem designed tanks such as the T69, T57 and T54. Like the T57, the T77’s development was arduously slow, and in 1957, the project was finally canceled by the US Ordnance Department. Both turrets were scrapped in February 1958.
|Dimensions (L-w-H)||20’10” (without gun) x 11’9″ x 10’10” ft.in
(9.3m x 3.63m x 3.08m)
|Total weight, battle ready||Around 48.5 tons (96 000 lbs)|
|Crew||4 (Commander, Driver, Loader, Gunner)|
|Propulsion||Continental AVDS-1790-5A V12, AC Twin-turbo gas. 810 hp.|
|Transmission||General Motors CD-850-3, 2-Fw/1-Rv speed GB|
|Maximum speed||30 mph (48 km/h) on road|
|Armament||Main: 120 Gun T179 Sec: 1 Browning M2HB 50. cal (12.7mm), 1 cal.30 (7.62 mm) Browning M1919A4|
Links & Resources
OCM (Ordnance Comittee Minutes) 36741
Presidio Press, Firepower: A History of the American Heavy Tank, R. P. Hunicutt
Illustration of the 120mm Gun Tank T77 by Tank Encyclopedia’s own David Bocquelet.