Engineering the B-29's Armament

by H T Hokanson
Aviation Division,
and T S Lisberger
Aeronautics & Marine Engineering Div,
General Electric Company

Defensive fire power of B-29 makes it deadlier than any other aircraft

The armament of the B-29 presented many unusual and difficult problems. The fact that it is called "the best gunned" airplane even though it is not the heaviest armed airplane is the proof of engineered armament.

For its special mission the B-29 demands three unusual things of its armament. First of all, it must defend itself. There are no escorts and it was found that large formations previously used for mutual protection were impractical. Experience has shown that "throttle jockeying" to maintain these large formations used enough gasoline to cut the range in half. Adequate firepower and fire coverage is needed to permit small formations or lone flights.

But, weight and drag are the worst enemies of high-altitude, long-range, and large bomb load characteristics; and weight and drag increase for every gun added.

Too many guns or too heavy armament is bad. The same amount of gasoline must be carried because the distance to the target is the same, so the bomb load must be reduced to pay for the weight of armament. Thus more airplanes will be needed to drop the necessary tons of bombs, and the number of operational losses will increase. Also, the losses to enemy flak will be greater because the increased drag reduces the operational altitude. Even if this large number of guns gave perfect protection so that no airplanes were lost to enemy fighters, the other losses would prevent calling this a good design. The optimum design is somewhere in between somewhere between no guns and too many guns. To find it was another one of the problems,

Then, too, the gunner had to be in a pressurized comfortable compartment to withstand the grueling 14 hr of flight. But guns cannot shoot through a pressure-tight wall.

The guns must be kept on the outside but must still be aimed and fired from inside the pressurized cabin.

The large wind forces exerted against the barrels of machine guns protruding from high-speed airplanes like the B-29 makes power drive for these guns imperative. Without this assistance, the gunner could not instantly swing his guns onto a precise aim at a distant enemy fighter plane. The flow of power to drive the guns must be instantaneous in its response to the gunner's control.

Altitude Problems

To fully realize the problems that the high altitude and high speed of the B-29 created, it must be remembered that the armament system had to be able to instantly defend the airplane against a variety of attack that are as greatly different from each other as these are:

  1. Nose attacks in which only three seconds elapse from when the attacking fighter is six blocks away until he is gone behind.
  2. Side attacks in which the relative angular rate is so great that the guns must lead the fighter by as much as 200 yards.
  3. Slowly closing tail attacks that give the fighter plane a long time to fire, thereby demanding accurate return fire from the B-29 to win the duel.

Remote control armament offered a major part of the solution but remote control is not easily solved in itself. First, let us examine what it is:

  1. A turret in one place on the airplane and the sighting stations that control it in different and more convenient places in the plane with nothing but electrical connections between the turret and sight.
  2. A sight pointing at an enemy fighter and the guns pointing to hit that fighter.
  3. Remote control armament is also a comfortable gunner in a warm cabin at 8,000 feet with his guns in a subzero gale at 30,000 feet.
  4. A gunner's undisturbed finger-tip touch on a sight, pushing several heavy machine guns against a 300-mph wind.
  5. Guns strategically placed, even where a gunner can't be comfortably located.

It is not something added; it is engineered into the airplane; it is a part of engineered warfare. On the B-29, five turrets had to be incorporated into the design two upper, two lower, and one in the tail and as many as three of these turrets can be controlled and fired simultaneously by a single gunner from a single sight. Never before has a combat gunner been able to control more than one turret. More than 30 different combinations of turrets are available to the five gunners, and they can get any one of these combinations in the split-second required to throw the switches. Although apparently insignificant when compared to the greatness of the airplane itself, these turrets carry the 11 or 13 guns the power with which the B-29 demands to complete its mission.

The upper forward and lower forward turrets are placed ahead of the big propellers. This gives them unrestricted freedom in fighting off attacks in the forward hemisphere. And against those breath-taking head-on nose attacks they team together for concentrated fire power. The upper rear turret provides protection from overhead attacks, the lower rear from attacks underneath. These two work together in stopping attacks from either side. The tail turret, with its machine guns and cannon, stands guard against offenders from the rear. All of these turrets are directed by remote control from five sighting stations.

System Designed to Fulfill Exact Specification

A good turret is the first essential of a good fire control system. The turret carries and operates the bomber's guns. Without a good turret, the accessory equipment to control it would become useless and the safety of the airplane would be jeopardized.

For an optimum design of the B-29 armament each turret had to be provided with enough guns for good hitting power, but not too many guns, because the weight increases with each gun added. It was necessary to select a caliber of gun heavy enough to be lethal against the enemy fighter planes. Yet, it is also necessary to select the smallest caliber of gun that would be lethal to permit carrying the most ammunition for the allowable weight. More than two rounds of .50-cal ammunition can be carried for the weight of one round of 20-mm ammunition. After all factors had been considered, the .50-caliber machine gun was selected for the job with one exception. The Boeing tail turret was to be provided with .50-caliber machine guns and, in addition, a 20-mm cannon to give hard-hitting fire power against those slowly closing tail attacks. All other turrets would have two .50-caliber machine guns for the optimum design.

Careful attention was given to obtaining the maximum fire coverage from each gun in order that there might be a minimum number of guns and still "no-blind-spot" protection. Each turret must have great freedom of motion. Each gun must be able not only to fire on one side of the airplane, but on both sides, not only forward but also aft, straight up for the upper turret and straight down for the lower turret.

Each turret is mounted in the airplane in bearings in such a way that it can swing around about a vertical axis. The guns themselves are fastened to a saddle in the turret that permits them to swing up or down. With this freedom, the guns can be brought to bear in any direction in even more than a hemisphere.

Ammunition for the upper and lower turrets is carried in ammunition cans which are a part of the turret. Automatic ammunition boosters are installed for each gun to boost the ammunition along into the gun. Each gun is provided with an automatic gun charger which performs two functions:

  1. It fires the guns by remote control whenever the trigger on the gunner's sight is pressed, and
  2. it throws out a bad bullet and puts in the next one whenever the gun finds one that won't fire all automatically and all in a fraction of a second.

Six electric motors are used to operate each turret and point its guns as the gunner aims his sight.

A gunner concentrating on "tracking" a fast-moving enemy airplane might swing the line of fire from his guns through parts of his own airplane. Automatic fire interrupters stop the guns from firing at the gunner's own airplane and relieve him of this responsibility. At the same time, however, these interrupters must be accurate enough to permit him to fire within inches of parts of his own plane, so that he loses the least possible fire coverage.

Everything in the turret is built snug and compact. It is finally covered on the outside by a low sleek dome for aerodynamic cleanliness and low drag.

The gunner uses a sighting station to aim at and range the enemy fighter. This is truly a precision instrument. It must permit a gunner, who has a comfortable grasp on its handles, to "track" a fast moving enemy fighter smoothly and accurately without the slightest stickiness or roughness to disturb his steady touch and throw him off. One axiom well known to armament engineers is the fact that a bomber's gun fire is no more accurate than the gunner's aim at the enemy.

On the sight there is a trigger under the gunner's finger which will fire all guns on all turrets controlled by his sight.

Mounted on the sighting station is the actual sight through which the gunner looks when he aims at the target. In this sight he sees a tiny spot of light which is his aiming point. Whenever he puts this tiny spot of light on the enemy airplane, he knows that his aim is correct.

Around the sighting dot is a circle of other tiny dots of light. The size of this circle can be changed by twisting the range control on his handles. When the gunner has his aiming point on the center of the enemy airplane, he adjusts the size of the circle until it just spans the length of the target. Sun or sky filters and a brightness control on the sighting light give the gunner a wide range of adjustment. With the proper settings, he can sight at any target from a plane coming at him almost directly out of the sun, to a plane coming at him out of the blackness of night.

A set of position indicators called selsyns are a part of the sighting station; from these selsyns, electrical connections run to a similar set of selsyns on the turret. Whenever the gunner moves the sight, the selsyns on the sight and the selsyns on the turret compare automatically the line of sight with the line of fire. If a difference exists, an electrical signal representing this difference between directions is fed into an electronic servo amplifier.

The servo amplifier amplifies the signal it has received and decides which way the guns must be moved to bring them into alignment with the sight. It then applies electric current to the field of an amplidyne motor-generator. In doing this, it makes the amplidyne generate a voltage which causes the turret drive motor to run in the proper direction until the selsyns signal that the guns are lined, up with the sight.

Each gunner is provided with the necessary switches and controls to start up and operate all of his armament right from his station.

In these descriptions of a B-29 turret, sighting station, and associated equipment, it has been shown that wherever the sight points the guns point parallel to the line of sight. But this won't permit hitting the target when the gunner aims at it because

  1. The guns point parallel to the line of sight. The guns and sights are located a distance away from each other so the guns have to be moved to intersect the line of sight at the target in order to hit. This is known as correcting for parallax.
  2. The apparent wind as the 8-29 flies along can "blow" the bullets back a whole plane length behind the enemy fighter by the time they get out to him. In order to hit, the guns must be moved ahead so that the bullets blow back to the fighter.
  3. Gravity will make the bullets drop so that by the time they get out to the enemy fighter, they can be below him by a couple of times his thickness. The guns must be elevated to correct for this drop.
  4. The relative motion of an enemy fighter can require lead correction as large as 200 yards ahead of the fighter in the direction he seems to be moving. If a bullet takes a second to travel out to the fighter, he might have moved 200 yards in that time. This must also be corrected for when firing.

The B-29 computer was designed to solve automatically all of these problems and make the guns point to hit the enemy fighter when the gunner aims at him. The computer automatically calculates the parallax, windage, gravity drop, and lead corrections and then adds them together into a total correction. This total correction, which might be more than 10°, is superimposed on the sight direction signal that is sent to the turret and fools the turret into pointing its guns not at the target but so that bullets will hit the target. So, the gunner does not have to guess at the correct gun aiming point which can be more than 500 ft away from the target. He can aim right at the target and know his bullets will hit.

Because all sight signals are transmitted electrically, more than one turret can be connected to these signals; so each sight can control as many turrets as are connected to it. Turrets can be transferred from one sight to another by simply operating a switch. In this way, the firepower from all turrets can be distributed for overall protection or can be concentrated where it is needed the most. B-29 turrets need never be idle regardless of the direction from which the Japs might choose to attack. Fewer turrets can do the job. For these reasons the five turrets, the five sighting stations, and the accessories are interconnected electrically to form an integrated armament system.

The nose gunner controls both the upper forward and the lower forward turrets.

The upper gunner controls the upper rear turret and can simultaneously control upper forward when the nose gunner is not using it.

The side gunners control the lower rear turret and they can also control the lower forward when the nose gunner is not using it. The side gunners can also control the tail turret if the tail gunner is not using his guns.

The gunners set up these various control combinations with a few toggle switches and transfer of turret control between sighting stations is then automatic. The combination of control that is selected depends upon the current enemy tactics being encountered, whether the B-29 is in formation or alone, whether it is day or night, and other factors.

Changes in the armament system arrangement can be more easily made as enemy tactics change.

This article was originally published in the March, 1945, issue of Industrial Aviation magazine, vol 2, no 3, pp 24, 26-27, 96-97.
The original article includes one phantom diagram and 5 photos.
Illustrations are not credited.