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BOEING B-17 FLYING FORTRESS
Sperry Ball Turret

Sperry and Emerson Electric each developed a ball turret, and the designs were similar in the nose turret version. Development of the spherical Emerson was halted. The Sperry nose turret was tested and preferred, but it's use was limited due to poor availability of suitable aircraft designs. The Sperry-designed ventral system saw widespread use and production, including much sub-contraction. The design was mainly deployed on the B-17 Flying Fortress and the B-24 Liberator, as well as the United States Navy's Liberator, the PB4Y-1. The ventral turret was used in tandem in the Convair B-32, successor to the B-24. Ball turrets appeared in the nose and tail as well as the nose of the final series B-24.

Sperry Ball Turret Illustration
The Sperry ball turret was very small in order to reduce drag, and was typically operated by the smallest man of the crew. To enter the turret, the turret was moved until the guns were pointed straight down. The gunner placed his feet in the heel rests and occupied his cramped station. He would put on a safety strap and close and lock the turret door. There was no room inside for a parachute, which was left in the cabin above the turret. A few gunners wore a chest parachute.

The gunner was forced to assume a fetal position within the turret with his back and head against the rear wall, his hips at the bottom, and his legs held in mid-air by two footrests on the front wall. This left him positioned with his eyes roughly level with the pair of light-barrel Browning AN/M2 .50 caliber machine guns which extended through the entire turret, located to either side of the gunner. The cocking handles were located too close to the gunner to be operated easily, so a cable was attached to the handle through pulleys to a handle near the front of the turret. Another factor was that not all stoppages could be corrected by charging (cocking) the guns. In many cases, when a stoppage occurred, it was necessary for the gunner to "reload" the gun, which required access to the firing chamber of the guns. Access was severely restricted by the guns location in the small turret. Normally, the gunner accessed the firing chamber by releasing a latch and raising the cover to a position perpendicular to the gun but this was not possible in the ball turret. To remedy that, the front end of the cover was "slotted". The gunner released the latch and removed the cover which allowed space to clear the action. Small ammunition boxes rested on the top of the turret and additional ammunition belts fed the turret by means of a chute system. A reflector sight was hung from the top of the turret, positioned roughly between the gunner's feet.

The directional control was by two hand control grips with firing buttons. The left foot controlled the reflector sight range reticule. The right foot operated a push-to-talk intercom switch. The turret was electrically powered in azimuth and altitude. An emergency hand crank could be attached to reposition the turret from inside the aircraft fuselage. In the event of a power failure another crewman would use this to crank the turret into the vertical position to allow the gunner to exit.

On the B-17, the A-2 turret was close to the ground, but had enough clearance for takeoff and landing. However, the gunner did not enter the turret until well into the air, in case of landing gear failure. During take-off and landing, the turret had to be positioned with its guns horizontal, pointing aft. As the guns had to be vertical before the gunner could enter or leave the turret, a set of external controls were fitted so the turret could be repositioned while unoccupied.

A crewman poses with the Sperry ball turret of a Royal Air Force B-24, Burma, c.1943-1945
A crewman poses with the Sperry ball turret of a Royal Air Force B-24, Burma, c.1943-1945
[Source: RAF Photo]

In the case of the B-24, the Liberator's tricycle landing gear design mandated that its A-13 model Sperry ball turret have a fully retractable mount, so that the ball turret would always be retracted upwards into the lower fuselage while the aircraft was on the ground, providing ground clearance with it in the stowed position.

Sources:
Gunston, Bill - The Encyclodepia of the Worlds Combat aircraft, 1976, Chartwell Books, Inc., New York
Wikipedia
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