The Ball Turret – a Vertigo Sufferer’s Nightmare

A bomber’s ball turret position is often looked at with fear. Suspended underneath a bomber, it’s the only thing separating you from a 30,000 feet drop. The turret was a small, cramped place to be, often operated by the smallest men available.

From this position a gunner could cover virtually the entire underside of the aircraft with his machine guns.

It was named the Sperry turret, after the company that made them; the Sperry Corporation.

They were most famously used on the Boeing B-17 Flying Fortress, but also saw use on the B-24 Liberator bomber.

The ball turret on a B-17G. Image by JustSomePics CC BY-SA 4.0

The Ball Turret’s Origin

Despite the strong association with the aircraft, the B-17 was not originally designed with a ball turret. Early models had very different defensive setup.

B-17 models A and B had blister canopies on the waist section of the fuselage armed with machine guns. The underside was protected by another blister with a single machine gun.

The waist blisters were a poor design that caused considerable drag, disturbed airflow, and limited the gunners’ traverse. These were removed in the B-17C and replaced with flush windows.

An early B-17C
An early B-17C. Note the ‘bathtub’ gunner position under the aircraft.

The B-17C also changed the belly position to a ‘bath tub’ style, similar to the German Heinkel 111. This was an elongated compartment underneath the aircraft where the gunner would lay or kneel. From here, the gunner could protect the rear section below the plane.

In the 1941 B-17E model, the tub was replaced with the famous ball turret. It was able to turn on two axis; 360 degrees horizontally and 90 degrees vertically. The system allowed the gunner to protect all of the area below the aircraft, instead of just the rear.

How it Worked

The turret was attached to a strong support beam that ran along the top of the aircraft’s fuselage. Its movement was controlled by a self-contained, electrically-powered hydraulic system, and could rotate at high speeds – important for tracking fast-moving enemy aircraft.

It moved horizontally by a ring gear on the turret. A trunnion system provided vertical movement. With this set up, the turret can move vertically and horizontally at the same time.

A turret on display
The turret outside the aircraft showing the ring gear.

Entering the Turret

The ball’s door was located directly behind the gunner, so with the guns at 0 degrees of elevation the turret could technically be entered from the outside while the B-17 was on the ground.

However, since there was only 40 cm between the ball and the runway, gunners would wait until the aircraft was in flight before entering. This was the same for the return flight.

B-24 Liberators were too low to the ground for the ball to fit, so they were fitted with a retractable turret that was stowed away for take off and landing.

Ball turret access hatch.
The door of the ball turret can be seen from above, inside the aircraft. Image by The PIPE CC BY-SA 4.0

To enter in flight, the gunner had to manually traverse the turret so the guns were pointing directly down, which provided access to the door at the rear of the turret.

Once in, the door was closed shut, and he placed his feet into stirrups in front of him, holding the triggers above his head. His position resembled the fetal position. On the ends of the grips were the buttons to fire the guns.

Operating the Turret

The gunner looked through a central window between the guns to aim. He was shielded by an armour plate in front of him. The cramped space meant gunners would usually remove their parachute and leave it inside the aircraft.

Ball turret training simulator.
Ball turret training simulator mounted on the back of a truck.

For defence, two Browning AN/M2 .50 calibre machine guns were squeezed into the turret either side of the gunner. This provided some incredible firepower in a small space. The AN/M2 guns were lightened versions of the ground based M2 machine gun, with a rate of fire of 600-800 rounds per minute each.

To aim the guns, the Sperry turrets were equipped with the advanced K-4 gun sight. It was mounted above and in front of the gunner’s head, so its reticule would be in line with his eyes. The K-4 sight worked by using data inputs from the gunner to calculate the required deflection (lead).

Ball turret on the ground.
The small size of the turret is apparent here.

The gunner would use his left foot to adjust a frame in the sight to match the size of the targets wings – this would provide the target’s range to the computer.

Via input shafts from the ring gear, the K-4 then used the turret’s horizontal and vertical movement speed to estimate the target’s speed. With this information, the site’s reticule would automatically adjust to the lead required to hit the target.

For 1941 this was cutting edge technology.

Interior of the turret
The cramped space and K-4 sight can be seen from this view inside. Image by Mr.Z-man CC BY-SA 3.0

Bomber Crews

As with all roles in the B-17, the ball turret gunner needed to be fearless. He can look out of his porthole straight down for almost six miles, while battling incoming aircraft, sealed inside an aluminium ball without a parachute.

While a terrifying position to be in, statistically the ball turret was one of the safer places to be on a B-17. The compact size of the gunner surrounded by thick glass, aluminium and armour plating, combined with being underneath the aircraft, meant the gunner was relatively safe.

Sperry Gyro 151 turret.
An experimental US ball turret that contains four .50 caliber machine guns, rather than the typical two. It is the Sperry Gyro 151, and very little is known about it.

This however quickly flips when the aircraft is shot down, as the gunner has to turn to the turret to the exit position, climb out and put on his parachute, all while the aircraft is falling out the sky.

Ball turrets were eventually abandoned in later aircraft in favour of remote controlled systems, and then radar aimed turrets in aircraft like the B-52. The last aircraft shot down by a defensive gunner was in 1972, where a B-52 tail gunner destroyed a MIG-21 over Vietnam.

Who Invented the Ball Turret?

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Though both Sperry Corporation and Emerson Electric designed ball turrets, the latter stopped before they could fully develop them. Sperry designed the turret to fit under bombers. From there, a gunner could defend the plane 360° directions with its two machine guns. To protect the gunner, the turret was covered in armor plate.