There are a few tank designs that just look sort of… odd. For example the French
AMX-13. That’s because the AMX-13 uses what is known as an oscillating turret.
The turret itself is split into two parts, one for rotation and one for elevation.
In this video I’ll walk you through the design, how it works, the advantages, the disadvantages,
and the fatal flaw that meant this type of turret has no real place on the modern battlefield.
Conventional tank turrets work as follows. The turret is mounted on a turret ring which
is set into the hull of the vehicle. The turret can rotate 360 degrees in this ring.
The gun is mounted onto the front of the turret in such
a way that it can pivot up and down, rotating around the mounting point.
This means that the gun can be pointed in any direction by rotating the turret, and can be
elevated or depressed in order to aim at something above or below the tank. This has been the system
used on the vast majority of armoured fighting vehicles, from the FT-17 to the M1 Abrams.
An Oscillating turret achieves rotation and elevation in a very different way. Firstly,
the turret itself is split into two parts. The lower turret, sometimes called the collar,
works the same as a conventional one. It engages with the turret ring and can spin 360 degrees,
providing the rotation. The upper turret contains the entire gun, and, importantly, the gun is fixed
to this upper turret and cannot elevate or depress on it’s own. Instead, to aim the gun, the entire
upper turret pivots up and down, rotating around trunnions set into the lower turret.
But this seems quite complicated, right? Right. But it does have some pretty huge advantages.
Firstly, it means the turret ring can be smaller. You have to remember that
the visible gun poking out of the tank is only part of the assembly.
Inside the turret you have the large, blocky gun breech that the loader puts the shells into. So
when the gun is elevated, i.e pointing up, the breech drops down, and in a conventional turret,
the turret ring has to be large enough to allow the breech to drop through it.
On an oscillating turret this is no issue. When the upper turret pivots to elevate the gun the
pivot point is much farther back, meaning the breech drops a much smaller distance
and so you can get away with a smaller turret ring, and, by extension, a smaller turret. The
turret ring diameter is also a huge driver in how wide a vehicle has to be, so using an
oscillating turret means you can mount a big gun on a small tank, as is the case with the AMX-13.
When a conventional turret aims upward, or elevates the gun, the breech sinks
down into the turret ring. If the gunner aims down, or depresses the gun, the breech rises,
and the gun depression of the vehicle is usually limited by how far the breech can rise before it
hits the roof of the turret. Gun depression is an incredibly important feature, as vehicles with
more gun depression can expose a lot less of their tank when shooting over hills and other terrain.
Again, by mounting the gun rigid inside the upper turret, oscillating turret vehicles
don’t have this issue, and usually have quite good gun depression as a result,
limited more by physical features such as the shape of the lower part of the turret
or the gun hitting the hull of the tank itself. In terms of gun depression and elevation though
these vehicles do have a disadvantage – the breech may not hit the turret ring but the rear of the
turret can hit the engine deck when elevating the gun. You can see this clearly on the AMX-13, the
gun can’t actually elevate that much because the turret bustle would strike the rear of the hull.
To combat this, and maximise elevation and depression, oscillating turrets try
to mount the gun as high as possible in the upper turret. This comes with its own pros
and cons – it means the vehicle can peek over obstacles and expose very little of the turret,
but it means oscillating turrets tend to be quite tall, like you can see on the AMX-50.
The other major advantage of the oscillating turret design concerns loading the gun. The
loader is inside this upper turret and so he is moving along with the gun, meaning the breech is
in the same relative position to him at all times. But the real kicker is when you want to fit your
vehicle with an autoloader. This is a device that automatically takes rounds from the ammunition
stowage and rams them into the gun breech. And this works fine in conventional turrets as
long as the gun is completely level at 0 degrees elevation, or what is known as the index position.
But when the gun is elevated away from this index position, the autoloader can struggle.
Conventional turrets can solve this in two ways – either use a more complex autoloader
that can load the gun at any elevation, or simply bring the gun back down to the index
position between every shot. The first option is expensive and the second isn’t ideal – aiming
your gun away from your target after firing is a huge disadvantage for obvious reasons.
An oscillating turret provides a neat solution, as the autoloader mounted in the upper turret
will be in line with the breech at all times.
So, a vehicle with an oscillating
turret and an autoloader can be small, fast, and pack a big punch – but there is a catch.
Splitting the turret into two, as we’ve discussed,
comes with a lot of advantages. But there is one major disadvantage – there’s a gap between these
two parts. This means that vehicles with oscillating turrets are almost impossible
to seal – against the elements, against water, and, critically, against NBC threats.
NBC stands for Nuclear, Biological, Chemical – and almost every tank today is fully protected
against these threats through excellent sealing and state of the art air conditioning and
filtration systems. Oscillating turrets just cannot be sealed to the same level,
as some sort of gap is required between these pivoting parts for them to function properly.
This means that if subjected to a gas attack or exposed to nuclear fallout, the vehicle’s
crews would be incredibly vulnerable. And at the end of the day it doesn’t matter how good your
vehicle is if your entire crew is incapacitated. The oscillating turret had it’s golden age in the
50s, 60s and 70s. Spearheaded by French designs like the AMX-13, Panhard EBR and AMX-50, many
countries created their own oscillating turret vehicles, with few actually entering service.
However an oscillating turret vehicle did enter service with the US Army in 2007 – the M1128
Stryker Mobile Gun System, or MGS. The MGS features an unmanned, 105mm armed oscillating
turret, with its own autoloader. In theory this design would have all the benefits of an
oscillating turret with none of the drawbacks, as all the crew is in the fully NBC protected hull.
In this case it was not to be – the vehicle’s autoloader had a large
number of issues and the entire system was just too expensive to be worth it.
So while oscillating turrets may not be cutting edge or even that relevant anymore,
it was an interesting idea that certainly had it’s advantages at
the time. And the look of those 60s and 70s oscillators is just so iconic and space age.
If you’ve an idea for what I should “explain” next please do leave it
down in the comments or feel free to put it into the discord! As always,
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