Defending the Superbomber: The B-29’s Central Fire Control System

Posted on Wed, August 12, 2020
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The Boeing B-29 Superfortress was conceived as a state-of-the-art, high altitude strategic bomber. In 1938, with World War II on the horizon, the United States Army Air Corps saw the need for a bomber that could carry heavy bomb loads over the vast distances of the Pacific. It was the threat of Nazi Germany, however, that spurred the development of the new “superbomber.” In 1940, Boeing submitted an advanced design for a four-engine bomber with a pressurized fuselage, which the Air Corps chose for production as the B-29. The B-29 program would become the most expensive of the war, outpacing the development of the atomic bomb by more than a billion dollars. Delays in developing the new bomber kept it from flying in the European Theater, but it would enter service in time to make an impact in the Pacific where it became an icon of the air war and would forever be associated with its part in dropping the world’s first — and only — atomic bombs used in combat.

Aircraft in flight

The Boeing B-29 was the most advanced heavy bomber of World War II. 

The advanced bomber concept included several cutting-edge features for the time. Besides a pressurized fuselage designed to ease crew fatigue on long-range missions, the B-29 had another element designed to enhance the capabilities of the human crew — a computerized central fire control system. With today’s modern, fly-by-wire warplanes, a centrally controlled, computer-corrected, remotely operated gun turret may not sound impressive, but in World War II it was revolutionary. 

drawing from manual "Combat Gunnery"

The Combat Crew Manual issued by XX Bomber Command, under the command of Maj. Gen. Curtis LeMay, included this cartoon representations of a B-29 gunner shooting down Japanese aircraft with the press of a button, underscoring the futuristic nature of the central fire control. 

In 1939, Air Corps commander, Lt. Gen. Henry H. “Hap” Arnold, fearing war in Europe, pushed for development of a Very-Long Range superbomber for which the Army issued a requirement in January 1940. Four companies submitted designs, but Lockheed and Douglas soon dropped out, and in August the Air Corps ordered two prototypes of  Boeing’s XB-29 Superfortress. Consolidated’s entry, the XB-32 Terminator (later named Dominator), was chosen as a backup in case the B-29 program ran into difficulty. As it turned out, the Dominator suffered more delays than the Superfortress and only entered combat in mid-1945.

Early models of previous heavy bombers, such as the B-17 and B-24, had gunner positions that were exposed to the open air where temperatures could reach -40 degrees Fahrenheit at altitude. Even enclosed turrets were not heated, exposing gunners to extremely cold and dangerous conditions. The specifications for the superbomber called for pressurized compartments that would provide a more comfortable environment for the crew. Rather than trying to also pressurize a revolving turret with a gunner, the turrets were made smaller and more streamlined, with the gunner aiming and firing from inside the pressurized compartment. Locating the gunner remotely from the turrets, however, compounded the problem of hitting a moving target. In keeping with the rest of the state-of-the-art bomber, it was clear that a computer guided system was needed.

Two men on top of aircraft

Armorers check the operation of the four .50 cal. guns in the forward, upper turret. Like many B-29s, this turret had two extra guns added to protect against head-on attacks

In the early 1940s, the widespread use of electronic digital computers was still in the future. Most computers at the time were analog devices that used changes in physical inputs to model the problems being solved (whereas digital computers use symbols to represent values). The military had used analog computers since about 1910 as fire control directors for warships, and by World War II, they were also used for fire control of ground based anti-aircraft guns and in the Sperry and Norden bombsights. But the B-29 was the first production aircraft to use them to remotely aim and fire aircraft gun turrets. The system in the B-29 employed analog electromechanical computers that used small, electrically-driven mechanical switches, called relays, instead of levers and gears. These devices were significantly faster than strictly mechanical computers. 

Computers in the B-29 central fire control system made corrections in three main areas to accurately aim the aircraft’s guns: ballistics, lead, and parallax. 

  • Ballistics is the behavior of a projectile once it leaves the muzzle of a gun. The two main considerations when calculating ballistics are the pull of gravity and the resistance from air pressure that could push the bullet back, which can greatly change in varying conditions
  • Lead refers to the compensation for how much a target moves in the time it takes a bullet to reach it. To hit a fast moving target, guns may have to be aimed as much as 200 yards ahead of the enemy.
  • Parallax error occurs because the angle to the target from the gunner’s sight is different from the angle to the target from a turret located several yards away. The computer calculates the parallax error and ensures that the guns fire at the target sighted by the gunner. The computers made these three corrections so that a gunner could simply point his sight at the target he wished to hit.

In March 1940, Bendix, General Electric, Sperry, and Westinghouse responded to the Air Corps’ request for bids on the B-29 central fire control system. In April, Sperry won a contract to develop the system. Sperry had extensive experience with analog fire control and bombing systems. Ever careful to hedge its bets, the Air Corps chose General Electric (GE) to develop its own fire control in case problems delayed the Sperry system. GE also had experience with analog computers, having built the first general purpose electromechanical computer based on Vannevar Bush’s design of a differential analyzer at MIT between 1928 and 1931. 

Sperry’s Central Station Computer System used periscopes that projected up and down from the gunners’ positions to sight enemy aircraft and operate turrets which were hydraulically operated and retractable. Sperry did not have the expertise to design the periscope optics and sub-contracted with Eastman Kodak for these components. Since the B-29 was just entering development, the Sperry system was tested in modified B-17 aircraft. Problems with the periscope sighting and the hydraulic turrets, however, plagued the system and in April 1942, the United States Army Air Forces, as it was now known, decided that the Sperry system would not be used on the B-29. With the failure of Sperry’s system, GE’s successful fire control became the standard installation in the B-29. GE would go on to manufacture nearly 4,000 fire control systems.

Man sitting at central gunnery control system

Industrial Aviation magazine, vol. 2, no. 3, March, 1945

GE’s central fire control was simpler than Sperry’s complicated periscope system. With the GE system, B-29 gunners observed and tracked attacking aircraft directly with their gunsights. There were five sighting stations througout the aircraft, each with its own computer. 

The front station was located in the nose, where the bombardier acted as the front gunner when not occupied on the bomb run. Just behind the wing were three stations, one on top of the aircraft and one on each side. Plexiglas “blisters” allowed these three gunners a larger field of view for spotting incoming fighters. The fifth station was  in the tail where the tail gunner watched for attacks from the rear. Each of the turrets contained two .50 caliber Browning machine guns. The tail originally held two .50s and a 20mm cannon, but the cannon was often removed and sometimes replaced with a third machine gun. Two more machine guns were frequently added to the forward upper turret to help ward off head-on attacks.

Airmen in gunner's seat in aircraft

The B-29 upper gunner sat in a raised position known as the “barber chair” From this position  he operated his sight with his viewing dome above and coordinated the fire of the other gunners. 

Each gunner accessed a gunsight and control system to enter information to calculate the range to a target. Two gyros on the sighting station provided the relative velocity of the target. Additional inputs came from the navigator’s handset. Using indicated air speed, barometric altitude, and temperature set by the navigator, the computer calculated true air speed and density altitude to make the corrections for ballistics, lead, and parrallax.

B-29 navigator handset

The B-29 navigator entered barometric altitude, indicated air speed, and outside air temperature into this handset which provided this information to a dedicated computer at each sighting station to properly aim the guns. 

One of the advantages of a remote firing system is that since the sights control the turrets electronically, a single sight can be used to train more than one turret on a target so each gunner  had the ability to control more than one turret through a series of switches mounted at each station. If an attack was coming from the front, the bombardier could operate the upper and lower forward turrets. It was also his responsibility to raise and lower the retractable lower forward turret. If an attack was coming from high on the side, the upper gunner could control both forward and rear upper turrets. The side gunners could operate both lower turrets and even the tail turret in the event of an attack from below. Each gunner had his primary turret and could take control of another (see diagram below). It was the responsibility of the upper gunner to act as the “quarterback”, coordinating the actions of all the gunners. Good communications were essential, as it was crtical each gunner knew what turret they were currently controlling. Two important safeguards were built into the system. The computers were programmed to prohibit a gunner from accidentally firing at parts of his own aircraft and each sight contained a “dead man’s switch.” If a gunner failed to grip the switch, control of his turrets was automatically turned over to other gunners, even if they had not been manually switched, so that the loss of a gunner did not mean a turret was put out of action.

diagram

The integrated fire control allowed the gunner with the best view of an attack to control the appropriate turrets. Also, if a gunner was incapacitated, another crewman could operate his turrets. This diagram shows which turrets each gunner could control. 

The GE fire control system provided valuable protection for the superbomber. With an effective range of 900 yards, it provided accuracy at 50 percent more range than non-computer sighted guns, and over twice the range of most enemy fighters’ guns. A post-war official report credited B-29s with 914 victories against 72 losses in over 31,000 sorties flown in the 13 months from August 1944 to August 1945.

Despite the effort that went into developing the B-29’s central fire control and its operational success, it was not always seen as an essential system. Ironically, the central fire control was removed from B-29s engaged in the aircraft’s most destructive missions. The Silverplate B-29s modified to carry the atomic bombs were stripped of all guns, except those in the tail, to accommodate the increased weight of the bombs. It was correctly reasoned that the Japanese would not waste resources attacking a group of three aircraft, so defensive systems were merely excess weight. Similarly, a reduced threat from a weak Japanese night fighter force resulted in the removal of guns and their associated fire control systems during the nighttime firebombing raids on Japanese cities in March 1945. Removing the guns allowed the aircraft to carry more fuel and bombs.

B-29 <em>Enola Gay</em> at the Udvar-Hazy Center

A view of the sighting control in the rear tail gunner’s position on the B-29 Enola Gay, which dropped the first atomic bomb. All other guns were removed on the aircraft modified to carry atomic bombs.

The B-29 soldiered on past the end of World War II and into the Cold War and Korean War with the new U.S. Air Force. Through all of these conflicts, central fire control remained a key feature of the B-29’s success. The appearance of MiG fighter jets in the Korean skies, however, signaled the end of the aging system. Though Superfortress gunners are credited with shooting down 27 aircraft during the Korean War, including a MiG 15, the B-29 had trouble keeping up with the speed of new aircraft. Although soon obsolete, the once state-of the-art B-29 fire control had been one of the revolutionary components of the first superbomber and would serve as the model for future bomber defense weapons systems.

Christopher T. Moore is a museum specialist in the museum’s Aeronautics Department and is responsible for aircraft armament, among other collections.

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