The B-17 Ball Turret, also known as the
Sperry Ball Turret, was a spherical gun emplacement mounted on the underside of
the Boeing B-17 Flying Fortress. It was famous for its iconic
design and its claustrophobic firing position. It was one of the most
vulnerable positions on the aircraft, designed to provide defensive firepower
against enemy fighters attacking from below. For this film, we'll focus
on the A-2 variant of the turret, which was fitted into the B-17F aircraft. Similar turrets were also
fitted to the B-24 and B-32 bombers, where the turret could be
retracted for takeoff and landing. However, on the B-17, contrary to popular
belief, the turret couldn't retract into the fuselage. My name is David Webb, and this is Blue Paw Print. The A-2 turret was 44 inches
in diameter and weighed 850 pounds. There were plexiglass domes fitted over
support brackets on each side of the turret. This gave the turret a completely spherical
appearance, increasing its aerodynamic properties and protecting the vertical
running gear from externally low temperatures. The curved front centre
section assembly was made from steel. It housed the forward-facing main
sighting window and four plexiglass panels. The entry hatch assembly was cast aluminum, and the rear half of the assembly,
which formed the bottom curvature of the turret, was also made of armour-plated
steel, and it doubled as the gunner's backrest. There was a 46.5-inch hole in the fuselage of the
aircraft to take the slightly larger turret trunnion ring. Attached to the turret and sat on the
trunnion ring was the trunnion ring support assembly. This rotated freely as the turret turned. Then came a harness that was
bolted to the trunnion ring support assembly, which also rotated with the turret. The harness provided structural
rigidity, joining the turret to the airframe. At the top of the harness was a swivel
bracket that allowed the harness to spin freely. In addition, the harness acted as a conduit for the
aircraft's electrical power to reach the rotating turret. All wiring from the turret ran into this housing, where it connected to the aircraft
wiring through a series of brush connectors, which rotated round one another. This ensured that there was no
twisting of wiring as the turret rotated. It was strictly forbidden for the gunner to be
inside the turret, either on takeoff or on landing. The guns protruded 36 inches from
the turret, more than the ground clearance, so it was vital that they be elevated
to their maximum 85 degrees from the vertical, and near flush with the fuselage and
locked in position for takeoff or landing. When the B-17 had reached an altitude of between
9,000 and 10,000 feet, the gunner would enter the turret. before the aircraft went into enemy
territory and before oxygen was necessary. To enter in flight the gunner unlocked the
turret by first releasing the handbrake next to the hand crank on the turret ring trunnion assembly. He then released the elevation clutch control which disengaged the elevation gearing allowing the hand crank to move the turret. He would then hand crank the turret
into a vertical position so that the guns pointed straight down and the entry hatch
would be fully exposed inside the fuselage. The hatch could then be
opened and the gunner would reach in and re-engage the elevation
gear using the internal clutch control. He would then lower himself
straight down into the turret through the hatch, putting his feet into the footrests
either side of the main sighting window. The gunner lay in a fetal position,
with his knees drawn up towards his chest. His feet in footrests were on either side of the main sighting window, in line and shielded by the two machine guns. His back would lay on the
curved bottom surface of the turret, and his head rested on the
closed hatch, which formed the turret rear. After entering a turret, the gunner made sure
the hatch was locked before powering up the turret by activating the switch on the switch box. He would then switch the gun sight on. The gunner would plug the heated
suit into a receptacle beneath the seat. He would plug his oxygen
hose into the regulator under the seat and made sure the oxygen gauge situated
just to the right of the turret hand control unit read 400 psi and that the oxygen
flow indicator adjacent to it was working. Because the gunner's hands were full when
in combat, he had a microphone pushed a talk button next to his right heel rest operated by his right foot. There were two other switches on the switch box which were gun selectors for the left and right machine guns. Because it was very easy to accidentally brush the gun firing buttons on top of the hand grips, especially when wearing thick mittens or gloves, the gun selector switches were left to the off position until the gunner went into combat. After powering up the turret, the gunner charged the guns twice. Normally for a 50 caliber machine gun this
was a straightforward operation but in the cramped turret the slide assemblies were near the
ears of the prone gunner and he simply couldn't apply enough physical power to pull the cocking handle from this angle. The solution to this problem was a pulley system. Two cables were attached to the slide assembly and they ran via two sets of pulleys to two handles near the gunner's feet. To charge the guns he would lean forward grasp the handles cross-armed and pull them towards him
exerting tension through the pulleys and sliding back the bolts in the slide assemblies and charging the guns. The interior was so cramped that gunner could not wear body armor nor a parachute, but he did wear a parachute harness. The parachute itself would have been attached to the
inner fuselage wall in the radio-operator's position of the B-17. The A2 had a separate oxygen system for the gunner. This was necessary because if the
turret was attached to the main crew oxygen system, the continual rotating turret would tangle up and rupture, as oxygen hoses could not twist over
360 degrees multiple times during flight. The oxygen was fed to the gunner from a
pressurized bottle attached to the A2 harness inside the fuselage. The bottle and hose rotated with the turret. The oxygen system gave just over two hours of supply. It was a job of the waste gunner
to replace the bottle every two hours, or upon request from the ball turret gunner. The A2 was not pressurised or insulated and internal temperatures in flight often
dropped as low as -50 degrees Fahrenheit. This meant that if an ungloved hand or any other body part touched the interior surface it could freeze to it, so a gunner always wore a leather flying
suit, fur-lined boots and thick gloves or mittens. Under the outer layer would have
been an electrically heated flight suit. The gunner moved the turret using
the hand control unit which sat above the K4 gunsight in front of and above the gunner's head. If he wanted to turn the turret to the right he pushed the hand grips to the left and vice versa if he wanted to turn the turret to the left. Pushing them away made the turret go down. Pulling the hand grips towards him elevated the turret. The turret could be both rotated and elevated or
depressed at the same time. So, for example, putting the handles towards you while pushing them to the left would elevate the turret while at the same time slewing it to the right. The movement speed reflected the force from which the gunner
pushed or pulled the hand grips. The harder he pushed them the faster the turret moved. This movement could operate even if only one hand grip was moved. When operating under power it could rotate 360 degrees in eight seconds and move up or down 90 degrees in three seconds. It had a maximum elevation of 85 degrees
from the vertical. The turret ring had a Ford facing cam fitted. This projected down from the ring and was contoured to mirror the arcs of the propellers. When the turret rotated to face forwards a pin on the turret front situated above the main sighting window touched the cam and cut power to the solenoid firing circuits for the guns so that the gunner could not accidentally fire
into the aircraft propellers. However the cam shape did not take into account open bomb bay doors. If the aircraft was engaged in combat during its bombing run it was possible for the gunner to shoot the doors. There was no provision at all for the gunner to be able to relieve their bladder while locked in the ball turret. Many gunners took to drilling a hole in one of the smaller
plexiglass windows and feeding a rubber tube through it with a siphon on the end just in case. But given the near impossibility of
moving around inside the small enclosed space, let alone removing any parts of his multi-layered clothing, it can be left to the imagination as to
what most gunners did when they were caught short. The gunner's seat was made of 0.6 inch thick steel and
the curved bottom panel was made from 0.26 inch thick steel. This formed the gunner's backrest. The hatch which formed the plate behind the
gunner's head, when in position, was cast of thick aluminum. When in combat, the gunner looked out
of the main sighting window between his legs, and therefore his seat base would be
between much of his body and any incoming enemy fire. The armor plate was designed to give full protection against rounds of up to 30 caliber, and would also help by deflecting flak fragments. The windows in the ball turret were all plexiglass, except for the main sighting window which
was double thickness, tempered, laminated glass. Although not completely bulletproof, it
could resist flak fragments quite effectively. In addition, the position of the guns, ammunition, and gun sight within the turret gave
the gunner further protection for the areas of his body not protected by the armour plate. Because his legs sat between the guns and below the ammunition bins, they gained protection from them and the gun sight further shielded the gunner's upper torso for many hits from above. When the 8th Air Force surveyed the
wound ratios of returning air crew, they found that ball turret gunners received only 6% of
crew wounds, the lowest of any crew position. Although this was only on planes that made it home to base, so there is some survivors bias in those numbers. Other statistics from the 95th Bomber Group suggested that the ball turret gunner was on par with the tail
gunner in terms of injuries and survivability. When entering combat, the two gun selector switches on the switch box would be turned on, thus
powering the solenoid gun triggers. The turret's two M2 .50 caliber machine
guns fired at a rate of 750 rounds per minute. It was standard practice in 1943 to make every fifth bullet a tracer round to assist the gunner with correcting his aim. But the brightness of the tracer glare through the gun's sight optics often meant the gunner couldn't see his sighting reticule
properly, which made his aim worse, not better. So the practice was soon discontinued. The ammunition for the A2 turret was carried completely
within the unit itself, in two ammunition bins, one for each gun, sitting in the roof, forward of the K4 gun sight and the hydraulic power unit. The smaller ammunition can carried 445 rounds
for the right hand gun, while the inner ammunition can carried 571 rounds for the left hand gun. Near the gunners right foot was an indicator which showed the turret facing relative to the direction of flight of the aircraft. This was incredibly useful as it was very
easy to become disoriented in a rapidly moving turret, especially if it was pointing
below the horizontal when the gunner could not see any of the airframe his turret was attached to. The gunner used his Sperry K-4 automatic
computing gun sight when targeting the enemy in combat. This hung from the roof of the turret below the hand control unit. The optical sighting head sat at the bottom of the gun sight in the eye line of the gunner, who looked through it and then out through the turret's main sighting window. By framing his target using the reticule, the gunner's optical sight head, and then using the pedal next to his
left foot to zoom in and out with the reticule, maintaining its framing of the target in
flight, he automatically activated the mechanical computer in the gun sight unit, which calculated the
effects of gravity, wind speed, direction of flight, and speed of both the firer and the
target, and even the rate of rotation of the turret to come up with a fire control solution which
automatically adjusted the gun's direction of fire. The gunner didn't even need to know the range
to target. That too was calculated by the computer. The guns were fired by depressing a button situated on top of both hand grips on the hand control unit. Pressing either button would fire both guns, so one hand could be left free for
other tasks in the turret if necessary. The gunner exited the turret when the aircraft was returning from the mission, if it had descended below 10,000 feet and is within fighter escort range, or if he
had to bail out because of battle damage to the B-17. Prior to leaving the turret, if returning to base,
he made sure that both guns were cleared of ammunition. Before the hatch would fully open, the turret
had to be rotated so that the guns faced straight down. If the turret had lost power, it could be
turned with two hand cranks for manual operation. Operating it in this way was very slow and only
used to align the exit hatch so that the gunner could escape. After exiting, the gunner had
to reach back inside and adjust the turret so that the guns faced
backwards and were flush with the fuselage. This would be to prevent parachutes snagging on the turret guns if bailing out, or to stop the guns hitting the ground during a standard landing. If landing normally, the gunner would lock the turret in position, but it could be jettisoned if the aircraft was going to crash land, wheels up to minimize
structural damage to the aircraft. In order to do that, the 12 connecting nuts holding the four harness legs to the turret support assembly had to be removed. Then, the four safety retaining hooks coupling the
turret support assembly to the turret ring also had to be removed, allowing the turret to drop away. This could be done by using a wrench or simply smashing the hooks with a hammer. In all, this procedure took around 20 minutes. Ball turrets were technically complex and only three factories in the entire US had the advanced equipment needed to build them. They cost the equivalent in today's
money of $250,000 each, or $14,300 in 1944 money. 25% of this amount was just for the shoebox sized Sperry K-4 automatic computing gun sight that every ball turret gunner used. While extremely high tech for the day, the ball turret with gunner inside went out of fashion in post war years due to the emergence of remote control guns. In the end, the introduction of high speed jet fighters, electronic warfare and missile weapons largely put an end to the era of the bomber defensive turrets. And If you want to master complex technical topics or upgrade your skills, we recommend the Brilliant interactive learning platform. Brilliant's first-principles approach helps you build understanding from the ground up. Each lesson is filled with hands-on problem-solving—proven to be 6 times more effective than watching lecture videos. All the content on Brilliant is crafted by an award-winning team from MIT, Caltech, Duke, Microsoft, Google, and more. Brilliant helps you build critical thinking skills through problem-solving, not memorizing. You can explore mathematics, data analysis, computer science, science, and advanced math. Brilliant helps you build real knowledge in minutes a day — with fun lessons you can do anytime. Our team frequently uses Brilliant to understand complex topics, helping us create accurate and detailed videos. To try everything Brilliant has to offer for free for a full 30 days, visit the site by scanning the QR code or click on the link brilliant.org/BluePawPrint in the description. You’ll also get 20% off an annual premium subscription. If you spotted any errors, please let us know in the comments. We're looking to continuously improve. Also, if you like our video,
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