Self-Contained Subassemblies
Feature B-29 Four-Gun Turret

by P E Humphrey,
Aeronautics & Marine Engineering Division, General Electric Company

Part I

Specially designed to meet demands for increased fire power, GE's four-gun turret for the Boeing Superfortress incorporates compact components which make for ease in both manufacturing and servicing in the field… Initial installment of a graphic two-part article.

Urgent requirements for more fire power have been created by the tremendous increase in speed of combat aircraft. This increased fire power can be obtained by three different means:

  1. Increased range of guns and ammunition,
  2. improved aiming to give a greater percentage of hits, and
  3. increased rate of fire, either more rounds per minute per gun, or more guns firing simultaneously.

To meet the specific call for greater fire power in the B-29, a special remote-control turret system was deigned — to make the most efficient use of the .50-cal machine gun. In this system, remotely located direct-aiming sighting stations, fitted with computers, make possible better aiming and enable a single gunner to control several turrets. To further increase the strength of the upper forward position, the four-gun turret was added, which has all four guns firing on the same target simultaneously. Remote control makes possible the most compact turret design. Fig 1 shows the B-29 four-gun turret which mounts in a 41½"-dia ring.

Comparison of Controls

Essential difference between the local control and remote control systems of gunnery is illustrated in Fig 2. In the local control turret, the gunner rides in the turret with the guns and operates the turret and mechanically connected sight by turning power on and off with control handles. In the remote system, the gunner aims the sight directly and the turret is driven electrically to follow the sight-position signal. The remote system requires that the turret be completely automatic, since the gunner is not present to perform any operations in the turret manually.

The functions that must be performed by the remote control turret mechanism are:

  1. Position guns accurately in azimuth and elevation to agree with the position of the sight,
  2. supply ammunition to the guns,
  3. interrupt fire when guns are pointed at parts of own plane,
  4. mechanically maneuver guns to clear parts of plane in which crew is carried, and
  5. automatically reload guns in case of ammunition jam or stoppage due to a dud. Essential parts of the four-gun turret that perform these functions are shown schematically in Fig 3.

For ease in manufacturing and servicing in the field, the four-gun turret was designed to be built up of six self-contained subassemblies:

  1. Elevation and azimuth drive units,
  2. interrupter and collector assembly,
  3. ammunition cases,
  4. frame and chute assembly,
  5. ring assembly and support castings, and
  6. dome and enclosure. These separate units are brought together at final assembly.

Details of the turret ring assembly are shown in Fig 4. The stationary outer ring is made as a stamping from ½" aluminum alloy plate. Outer flange of the Z section serves as a mounting flange for the turret, and the inner flange carries the stationary internal ring gear with which the azimuth final-drive pinion meshes. The inner ring is rolled up from an extruded aluminum alloy angle section. End are gas-welded to form a solid ring while the inside surface is machined for mounting support brackets, and the outside is machined for a tight fit with the stainless steel channel roller track.

Special needle bearing rollers mounted on removable bearing blocks are supported in the outer ring. These blocks are assembled from the outside, and washer-head screws are used in spline nuts for fastening. Aluminum alloy castings provide the main support for the guns and gear drive units. The saddle support castings are mounted with body bound bolts on aluminum forgings riveted to the inner ring.

Turret Saddle

The saddle (Fig 5) on which the guns are mounted is carried on two special ball trunnion bearings which are of especially large bore to provide for the ammunition feed. The elevating gear sector is bolted to the right side of the saddle. Guns are mounted on adapters with a rear slider unit providing adjustment or alignment. On this turret, the guns are so close together that they are aligned parallel. Adjustment is necessary to make up for bracket tolerances and variation between guns.

The saddle support brackets provide bolting surfaces for the elevation and azimuth drive units, and the mounting arrangement is such that the final drive pinion mesh is adjustable at assembly. Thus, very close backlash tolerances can be maintained. The elevation drive pinion meshes with the elevating gear sector and the azimuth drive pinion meshes with the stationary internal ring gear.

Drive Units

Details of the drive units are shown in Fig 6 and Fig 7. To obtain most efficient operation at all temperatures, simple spur and bevel gear reductions are used, and all shafts are ball bearing mounted. To provide a suitable compromise between strength, light weight, and easy machining, the gears are made of alloy steel, heat-treated to approximately 27 Rockwell C before machining.

For lubrication in the gear boxes, low-temperature grease is applied to the gear teeth at assembly. No further lubrication is provided, except at infrequent overhaul periods. The design is arranged so that there is only one gear mesh between the position transmitter Selsyn and the final drive.

The elevation drive is equipped with an overload relief clutch for protection when the guns are run full speed into the limit stops. Although limit switches in the sight cut off motor power, there would be sufficient inertia at high speeds to damage the gearing on overtravel. The clutch is a simple spring-loaded disk arrangement set to slip at approximately 1½ times the maximum wind load. No clutch is provided in azimuth, for the turret has unlimited azimuth rotation.

Frame and Chute Assembly

The frame and chute assembly (Fig 8) provides support for the ammunition cases and the collector and interrupter assembly, with the chute portion serving to collect and guide the empty cases and links from the turret. The frame is attached to the inner turret ring and to the saddle support castings.

Load carried on the frame is in excess of 1,300 lb when the ammunition cases are fully loaded. The access door in the chute allows the gunner to get at his guns in flight, if necessary. All parts of the frame are formed from aluminum alloy and stainless steel sheet, the latter being used in parts on which ejected cases and links fall directly.

Ammunition cases are supported on angle brackets, with upper and lower brackets on the ends of the cases engaging brackets on wings of the frame to prevent movement either up or down. Clamps hold the cases in place against rubber bumpers around the cylindrical frame. Ammunition case are of spot-welded stainless steel construction; supporting brackets are riveted to the ends of the cases. Top covers are welded in place, ammunition being loaded through the sliding side door. On each side, the ammunition belts feed up parallel from the fore and aft cases near the ammunition booster chutes. In Fig 9 is shown cutaway of ammunition feed to two guns of the B-29 four-gun turret.

Ammunition Booster

A double ammunition booster uni (Fig 10) is mounted on the outside of each trunnion bearing. On each side, the ammunition belt from the front case feeds over the lower booster sprocket into the outside gun, and the belt from the aft case feeds over the upper sprocket into the inside gun. Booster gear drives are shown in Fig 11. Both sprockets are driven from the same motor. Free-wheeling units make it possible to turn the sprockets toward the gun when loading ammunition in the turret, and an anti-reversing latch prevents the ammunition belt from tightening up on the gun when booster power is off. The booster motor is energized each time the firing trigger is pulled, and booster sprockets are driven at a speed somewhat faster than the rate of fire of the guns. Teeth of the sprocket are made of flexible rubber so that they can slip inside the ammunition belt when the belt becomes slack at the gun. Ammunition guides or strippers prevent overfeeding; thus, no switches or controls are required.


Concluding his revealing part-by-part study of GE's firepower-plus turret, Author Humphrey gives details of the gun charger, compressed air, collector, and fire interrupter units, then describes how the completed four-gun assembly is installed on the Boeing Superfort.

Next among the four-gun turret's compact components — each specially designed both for ease in manufacture and for convenience in field servicing — is the gun charger (Fig 12).

This automatic electro-pneumatic unit is fitted to each gun for initial loading, operation of the trigger, clearing jams, and removing duds. It comprises three operating units:

  1. Firing solenoid,
  2. solenoid-operated air valve and power cylinder assembly, and
  3. motor-driven timer or control unit.

When the firing trigger is closed, the firing solenoid is energized and the timing motor starts. If the gun fires, the timer is continually reset and the barging unit remains inactive. But if the gun fails to fire after about 1/3 sec, the timing mechanism closes a switch energizing the air valve solenoid, and high-pressure air admitted to the charging cylinder operates the gun through one cycle. At the end of the stroke, the bolt stud resets the timer, and another charging operation will not take place until the trigger switch is held closed for another full time delay.

If the gun does not fire after five consecutive charges, a counter in the timing mechanism cuts off the charging circuit and prevents further operation until the hand reset button is pushed. This arrangement is necessary to prevent wasting the high-pressure air.

Air at 1,000-psi pressure is supplied to the gun chargers by an automatic compressed air system (Fig 13). The compressor and pressure cylinder with the automatic pressure switch are mounted on the lower part of the turret frame between the ammunition cases. Here, the compressor is a two-stage high-speed unit designed to operate at high altitudes and extreme temperature limits, and the pressure cylinder has a volume of about 39 cu in. This provides sufficient air to operate all four gun chargers simultaneously through five charging cycles with a pressure drop from 1,000 psi to 40 psi. However, under normal operation, the compressor is started as soon as the pressure drops below 1,000 psi and it keeps the system fully charged

All the control and power circuits are brought into the turret through one connector at the bottom of the collector or slip-ring assembly (Fig 14). Wires from the connector go up to brushes which are mounted on a nonrotating bracket. This portion of the collector is held against rotation by an anti-rotation bar connected to the airplane.

Slip rings are mounted below the top plate of the collector under the terminal board and rotate with the turret. Wires run up through the center of the rings and are fastened to the terminal board. Connections from this board are made through conduit assemblies to the various units.

A fire interrupter unit (Fig 15) is built around the collector assembly. The enclosing drum around the brush assembly serves as a stationary interrupter cam drum. As for the switch carriage, it is mounted on vertical slide rods, and it travels around the drum with the turret and is moved up and down the drum by linkage connected to the elevation motion of the guns. Three switches are provided, one for each outside gun and one for the two middle guns. Thus, the most complete fire coverage is allowed each gun.

The interrupter switches are actuated by bevel-pointed plungers which ride up over the interrupter cams in areas where the gun is pointed at parts of the airplane. Since the cams are mounted on the stationary drum, their contour is simply a reproduction of the pattern traced by a bore-sight through the gun barrel following around the contour of the airplane. A margin of safety of 1½° is allowed to take care of errors in the system and to cover time for the firing solenoid to drop out when the turret is moving at high speed.

The interrupter cam drum is an aluminum alloy casting machined inside and outside to a 1/8" wall thickness. Stainless steel 1-1/16" thick is machined to agree with a master drum (by an accurate profile milling process), after the cam surfaces are ground concentric with the drum mounting surfaces. The complete interrupter and collector assembly is enclosed in a fabricated sheet aluminum case.

At installation in the airplane, the complete turret is lowered into the turret well, with only the dome needing to be removed to allow the hoisting sling to be attached. For additional reference, Fig 16 shows the upper half of the gun enclosure being removed from the turret, while Fig 17 reveals the turret (without guns) with the dome removed.

Thirty ¼" bolts fasten the turret flange to the airplane structure. To complete the installation, the anti-rotation bracket is coupled to the interrupter and the power cable connector is plugged in. Then following installation, the turret is synchronized with the sighting stations by adjusting the Selsyn position transmitters.

This two-part article was originally published in the September and October, 1945, issues of Aviation magazine, vol 44, nos 9 and 10, pp 133-137 (Part I), 152-153 (Part II.)
The original article includes 9 photos and 8 drawings (17 figures.)
Figures are not credited but are probably from GE.

Figure captions: