While in contact with the enemy, the gunner simply needed to properly size, range, and track the target with his gunsight and then fire at the appropriate effective range. The CFC system moved the guns while the computer continuously calculated all the corrections needed for the fired projectile to hit the target. With computer-calculated targeting, the effective range of the guns was 900 yards, 50 percent farther than manually sighted guns and over twice the effective range of most enemy fighters’ guns.
The computer introduced the correction as a deviation from the parallel mirroring movements between the gunsight and the guns as transmitted by the system of selsyns. The total calculated correction was the sum of individual corrections for ballistics, parallax, and lead. Ballistics corrections compensated for the deflection of the projectile caused by gravity and by wind as the projectile exited an aircraft traveling around 250 miles per hour. Since the guns themselves were remote from the gunsights, parallax correction allowed for the distance along the length of the aircraft between the gunsight and the gun barrels. Lead correction allowed for the travel distance of the target aircraft during the time the projectile was in the air.
To perform its correction calculations, the computer required a number of pieces of input information obtained from several different sources. These inputs included the current position of the guns in both elevation and azimuth; the aircraft’s true air speed, altitude, and outside temperature, which were input by the aircraft’s navigator; the range to the target obtained from the gunsight as the gunner tracked the target and adjusted the range wheel; and the relative velocity of the target received from two gyroscopes located on the gunsight as the gunner tracked the target.
Depending on the type of parallax, two different models of computers were used. A single-parallax computer, the General Electric model 2CH1C1, was used at sighting stations that had parallax between the gunsight and only one gun location, such as the tail gunner station, which controlled only one turret, or the nose gunner station, whose two turrets were located above and below each other almost equidistant longitudinally from the gunsight. A double-parallax computer, the model 2CH1D1, was used for the three top and side gunners’ sighting stations since all three gunners could simultaneously control two turrets of differing parallax from their gunsight. Although the side gunners were capable of controlling three different turrets via the control switching system, they could only operate two at any one time.
All inputs to the computer were electrical, but the computer itself performed its calculations mechanically since a purely electrical calculator lay outside the reach of existing technology in the era of vacuum tubes with their appetite for electrical current and enormous heat output. Aboard an aircraft, size and weight were further concerns. Conversion of the electrical inputs into the mechanical calculation system was achieved through an array of selsyns and potentiometers.
The computer itself was really several separate but interconnected calculating units contained within the same chassis, which was the size of a suitcase and weighed more than 50 pounds. The ballistic calculating unit was programmed with the known effects of gravity and the ballistic characteristics of a .50-caliber shell fired from the Browning M2, such as its muzzle velocity. To make the ballistics correction calculation, the ballistic unit needed to know the current gun position for the angle of initial velocity, the range to the target, the current true air speed, which affected windage, and the aircraft’s current altitude and outside temperature, which affected the air density and thus the drag on the bullet. Similarly, the parallax computing unit was programmed with the longitudinal distance(s) between its gunsight and the turret(s) that sight could control, but it needed to know the current gun position and the range to the target to compute the parallax correction trigonometrically. Finally, the lead calculating unit computed the lead correction from the range and the relative velocity of the target along with the ballistic characteristics, which affected the time of the projectile to the target.
The computer’s output, which consisted of the parallel signal received from the gunsight selsyn adjusted for the sum of the three calculated corrections, was then converted back into electrical impulses, which fed a servo amplifier, or feedback controller, that drove the two gunpositioning motors, one for elevation and one for azimuth. Thus the computer’s correction was introduced as an alteration to the position of the guns and of the turret from that which would have been exactly parallel to the gunsight’s position.
Because they were connected electrically, the computers did not have to be physically located with either the gunsights or the turrets. The nose gunner’s computer was located in the forward pressurized cabin aft of the pilot’s armor, while the other four computers were placed under the floor in the radar operator’s compartment near the back of the middle pressurized cabin surrounded by armor. In case of a computer failure or combat damage, an override switch allowed the gunner to bypass the computer completely and operate the guns without its correction using a retractable flip-down peep sight on the gunsight.
According to U.S. Army Air Forces records, a total of 3,760 production Superfortresses were delivered, around 70 percent of which were built by Boeing at its plants in Wichita, Kansas, and Renton, Washington. The rest were produced in Marietta, Georgia, by the Bell Aircraft Company and in Omaha, Nebraska, by the Glenn L. Martin Company, later part of Martin-Marietta.
After assembly was complete but before the aircraft was combat ready, the guns were harmonized and the targeting computers tested. First, both guns in a single turret were aligned to parallel target marks by use of a bore sighting tool with alignment made by changes to adjustment screws. Then each turret was aligned with each of the sighting stations that could control it by using a predefined harmonization target placed at least 100 feet away from the aircraft. Adjustments were made to either the selsyn at the sighting station or to the one at the turret, depending on which gunsight and turret combination was being adjusted. After the guns and sighting stations were harmonized, the targeting computers along with all the input systems and calculating components were tested using a comprehensive testing device containing on its face over 50 dials, meters, and switches.
Aircraft production quickly outpaced the manufacturers’ capacity to set up the revolutionary CFC systems, so the USAAF began training its own crews to ensure that the otherwise combat-ready bombers were not delayed in deployment. USAAF Corporal Robert W. “Bob” Truxell of Lansing, Michigan, was part of the first class to graduate from the B-29 CFC and computer training schools at Lowry Field in Denver, Colorado. Truxell had previously washed out of air cadet pilot training due to a three-month bout with rheumatic fever, which permanently disqualified him from serving in a flight crew. The illness proved to be fortuitous since most of his air cadet squadron was later lost in the ill-conceived raid on the Ploesti, Romania, oil refineries on August 1, 1943. After recovering from his illness, Truxell was reassigned from air cadets to aircraft armament, where he completed the remote control turret mechanic course.
After the turret course, Truxell was tapped for the 16-week B-29 CFC specialist course because he had begun studying engineering, including taking trigonometry, at the General Motors Institute in Flint, Michigan, before enlisting in February 1943. Corporal Truxell finished first in his CFC class, earning entrance into targeting computer school and a third stripe upon its completion. Shortly thereafter he was given command of the first graduating class and a staff sergeant’s rocker. The new crew’s first assignment was in Georgia.
Truxell writes, “We were all sent to the [Bell Aircraft] B-29 factory in Marietta, Georgia, where completed B-29s were lined up for a mile awaiting proper alignment of the gun turrets and central computer.” He vividly recalls using a fire extinguisher to flush civilians sleeping on the clock at government expense out of the bombers’ pressurized crew tunnels so his crew could access the aft areas of the aircraft. Once the backlog in Georgia was cleared, the crew leapfrogged to various stateside air bases where B-29 squadrons were ready for overseas deployment—except for the final adjustment of their gunnery systems. After a particular squadron deployed, the gunnery crew moved on to the next base. Truxell calls his service a “pretty safe and interesting job.”
The B-29’s revolutionary gunnery system made a Japanese fighter pilot’s job neither pretty safe nor interesting. According to the Army Air Forces Statistical Digest published in December 1945, in the 13-month period from August 1944 to the war’s end in August 1945, B-29s were responsible for the destruction of 914 enemy aircraft in the air with a loss of just 72 of their own to enemy aircraft during more than 31,000 combat sorties flown.
This article first appeared on the Warfare History Network.
Image: Wikimedia Commons