Oscillating turret

An oscillating turret is a form of turret for armoured fighting vehicles, both tanks and armoured cars. The turret is unusual in being made of two hinged parts. Both elevation and recoil rely on the upper part of the turret moving relative to the lower part.

Oscillating turrets have only rarely been used. Their only widespread use was on two French designs, the AMX-13 light tank and the Panhard EBR armoured car. Both of these used the advantages of the oscillating turret to mount a relatively heavy gun on a lightweight chassis.

Design
The turret is formed from upper and lower parts, joined by a swinging hinge. In appearance, the gap between these two parts is covered by a distinctively visible rubber or canvas bellows.

The gun itself is fixed rigidly to the upper part of the turret. Elevation of the gun, i.e. aiming for range, is achieved by tilting the entire upper part of the turret. Traverse is achieved conventionally, by rotating the turret.

Since the gun barrel is attached rigidly to the turret, the entire upper part of the turret recoils when fired. There is no mantlet as such and the barrel and breech retain their position in the upper turret. Recoil energy is absorbed by the usual oleo-pneumatic dampers, but these are mounted between the turret halves, rather than linearly alongside the gun.

Advantages
There are two major advantages, smaller turret size and simpler fitment of an autoloader, although these benefits were outweighed in practice by the disadvantages, which included auto-loaders not living up to their promised performance.

Heavy tanks and the AMX-50
The initial claimed advantage of oscillating turrets was that of reducing the turret size for a large main battle tank gun. In the 1950s, tanks were rapidly growing more heavily armed, larger and heavier. Western armed forces were trying to catch up with the increasingly formidable Soviet tanks, such as the T-55. Weight was the main problem, particularly where this then required extra engine power or a stronger transmission. As the thickest armour is generally on the turret, reducing turret size appeared to be a worthwhile goal.

Size may be reduced because the non-elevating gun breech does not need to move up and down inside the upper turret. Working space thus does not need to be allowed for it above or below the breech, space that is normally wasted in conventional turret designs. In particular, the oscillating turret design is particularly shallow above the breech, allowing for a low turret silhouette, a considerable advantage.

This was the justification for the first oscillating turret, that of the French AMX-50 medium or heavy tank in the 50 tonnes class. This used first a 90 mm, then 100 mm, gun in an oscilllating turret, primarily to save weight. The final 120 mm version first reverted to a conventional turret, but then used another oscillating design, the Tourelle D. However the need to elevate the gun still requires room for the breech to be lowered into the lower turret. This has tended to produce oscillating turret designs with a high gun axis relative to a conventional turret, even where the turret height is otherwise shallow.

One problem was that the armour of a turret is primarily in the front face of the turret and this was not made any smaller in the AMX-50 design, the turret of the 120 mm version being so tall as to be reminiscent of the WW2 Challenger, the turret being a whole foot taller than the contemporary and comparably armoured Conqueror. The AMX-50 grew progressively heavier and although it might have proved a capable heavy tank by 1950s standards, this whole class of slow-moving AFV was becoming outdated by the development of lightweight anti-tank guided missiles in the 1960s and so the project was abandoned.

Light tanks
Whilst the oscillating turret was unsuccessful for the heavy tank, it proved more successful in allowing light tanks and armoured cars to carry an unusually heavy main gun of 90 mm. In French doctrine, light reconnaissance vehicles were heavily armed and expected to also fulfil a role in defending the flanks of a main force. They were not expected to act as tank destroyers though, and so a heavy-calibre but relatively low velocity gun with high-explosive shells was effective in their role.

To give an additional anti-tank capability to the AMX-13, it was later fitted with four SS.11 missiles on a turret-front rack.

Auto-loading
As the gun remains fixed relative to the upper turret, even during recoil, it is easier to install an ammunition autoloader than for a conventional turret, where the gun must return to a fixed elevation for reloading. The French design used two six-round rotating magazines, allowing a high rate of fire and also a selection of two ammunition types. The disadvantage was that once the magazine ready capacity was used, reloading of the magazines was a slow process that could not be carried out under fire. For the AMX-13, this reloading had to be carried out from outside the vehicle.

As was so often the case with autoloaders though, their complexity was their downfall. The 120mm for the AMX-50 was simply unreliable, due to the weight of the ammunition round.

AFVs fitted with oscillating turrets

 * French
 * AMX-13 – Widely used light tank
 * AMX-50 – Heavy tank, prototypes only
 * Panhard EBR – Armoured car


 * Austrian
 * SK-105 Kürassier – Used the 105 mm FL-12 turret of the AMX-13


 * United States
 * T69


 * T54E1 Tank, Heavy, 105mm Gun – Two prototype vehicles with 105mm guns and autoloaders were constructed on the M48 tank chassis. They were constructed around 1952 by United Shoe Manufacturing. One T54 had a conventional turret, the other T54E1 an oscillating turret.
 * T57 Tank, Heavy, 120mm Gun – A single prototype was constructed in the 1950s on a T32 heavy tank chassis, with a 120mm gun in an oscillating turret.