The early versions of the B-36 had been criticized for insufficient maximum speed and for a too-long takeoff run. On October 5, 1948, Convair proposed that these problems could be addressed by the fitting of two pairs of turbojets in pods underneath the outer wings. These turbojets would be used for takeoff and for short bursts of speed during the bombing run, and would have only a minimal effect on the range.
These changes resulted in the B-36D version. The B-36D featured two pairs of General Electric J47-GE-19 turbojets in pods underneath the outer wings to assist the six R-4360-41 engines. These pods were quite similar to those fitted underneath the inner wing of the Boeing B-47 Stratojet. The jet engines increased the maximum speed to 435 mph and the ceiling to more than 45,000 feet. In addition, they reduced the takeoff run by almost 2000 feet.
Conversion of a B-36B to D configuration was authorized on January 4, 1949. The prototype B-36D was obtained by converting B-36B serial number 44-92057. It had four Allison J35-A-19 engines in the pods in place of the later J47-GE-19s. It flew for the first time on March 26, 1949. Vibration problems required that an external brace be fitted for the nacelle. The last four B-36Bs on the original contract (44-92095/92098) were completed as B-36Ds. The modification was sufficiently successful that the USAF contracted for additional bombers as B-36Ds during FY 1949, and opted to modify existing B-36Bs to D configuration.
The first true production B-36D flew on July 11, 1949. The first B-36Ds were accepted by the USAF in August of 1950, and were initially sent to Eglin AFB for testing. By June of 1951 26 B-36Ds had been delivered. The last B-36D was accepted in August of 1951. A total of 81 B-36Ds were delivered to the USAF, 22 built as B-36Ds from the start, and 59 others were converted from B-36Bs.
The B-36D had a K-3A bombing and navigation system that replaced the B-36B's K-1 system. The K-3A permitted a single crew member to act as both radar operator and bombardier. The K-1 system had experienced its share of reliability problems, chiefly due to vacuum tube failures. A quarter of the B-36 mission aborts were caused by radar failures of one sort or the other. During later modernization programs, the K-1 system was replaced by the much more reliable K-3A system. This included the Farrand Y-3 periscope bombsight, an A-1A improved bombing/navigation computer, and an improved version of the Western Electric AN/APS-23 radar. The Sperry A-1A bombing computer could be used between altitudes of 4700 and 50,000 feet, at grounds speeds between zero and 760 knots. The Farrand Y-3 periscopic bombsight offered magnifications of up to 4 power with a 76 degree field of view. The sighting lens could be moved forward by 90 degrees, aft by 35 degrees, and laterally by 54 degrees in either direction. The Western Electric APS-23 radar was of an improved variety with a rapid scan antenna, high-definition radar scopes, data storage tubes that could hold an image on display for a considerable amount of time, K-band tunable radar heads, and flush-mounted antennae. It could scan either 360 degrees, or in 40 to 180 degree sector scans, with a range of five to 200 miles, using different pulse durations and pulse repetition frequencies. The 60 inch antenna could be rotated at up to 60 rpm. At 30,000 feet, large cities could be detected at a range of up to 200 miles and shipping could be detected at ranges of 50 to 100 miles. The APS-23 radar was equipped with anti-jamming features.
The B-36D featured AN/APG-32 radar to control the tail turret. The aircraft was fitted with snap-action split bomb-bay doors as opposed to the sliding type doors fitted to the preceding B-36As and Bs. These doors could open and close in only two seconds. Metal covered control surfaces were fitted, and bladder-type outer panel fuel cells were installed. Takeoff and landing weights were up to 370,000 and 357,000 pounds respectively. The maximum bombload was increased to 86,000 pounds, and the crew complement increased to 15.
The maximum bombload was 86,000 pounds, consisting of two 43,000-pound bombs. Smaller alternative loads consisted of three 22,000 pound bombs, four 12,000 pound bombs, 12 4000-lb bombs, 28 2000-lb bombs, or 132 500-lb bombs. Such loads were not equaled until the "Big Belly" B-52D modifications during the Vietnam War.
The B-36D had a crew of 15: commander, two pilots, two engineers, navigator, bombardier, two radio operators, and an observer forward (the first radio operator handled ECM while the second radio operator, the copilot, and the observer operated the three forward turrets. The rear compartment accommodated five gunners, including one for the AN/APG-3 (later AN/APG-32) radar controlling the tail turret.
On January 16, 1951, 6 B-36Ds were flown from Carswell AFB to the United Kingdom, landing at RAF Lakenheath after having staged through Limestone AFB in Maine. The flight returned to Carswell on January 20. This marked the first time that B-36s had flown outside US territory. A flight to French Morocco was made on December 3, when 6 B-36s of the 11th Bombardment Wing landed at Sidi Slimane, having flown nonstop from Carswell AFB.
Gradually, most of the problems with the B-36 were identified and corrected. An early major B-36 problem was leakage in the fuel system. In addition, the electrical system was unreliable and caused frequent fires. Improved containers and better sealants reduced fuel tank leakages. Changes in the electrical system reduced fire hazards during ground refuelling operations. Landing gear and bulkhead failures were almost totally eliminated.
However, even by October of 1951, the B-36D's defensive armament system was still performing poorly. In April of 1952, the Air Force ordered a series of gunnery missions to see if the cause of the failures could be determined. This test was completed in July. The K radar system was difficult to operate and maintain, and the training for the gunners was found to be inadequate.
In August and September of 1953, B-36s of the 92nd Heavy Bombardment Wing completed the first mass flight to the Far East, visiting bases in Japan, Okinawa, and Guam. This flight took place shortly after the hostilities ended in Korea, and was an effort to demonstrate US willingness to maintain operations in the Far East. On October 15 and 16, 1953, the 92nd Heavy Bomb Wing left Fairchild AFB in Washington for a 90 day deployment to Guam. This was the first time an entire B-36 wing had been deployed overseas.
The B-36 flew fairly well on just four or even three piston engines, so it was common practice to shut down some of the engines during cruise. The turbojets were normally used only for speed dashes over the target area or for takeoff.
Several B-36Ds were modified as lightweight, high-altitude aircraft by being stripped of all armament except the tail turret. All non-essential flying and crew comfort equipment was taken out. The crew was reduced to 13, 2 fewer than the standard B-36D. These planes were identified as Featherweight B-36D-IIIs. The Featherweight program was carried out in three phases: Model I, which included a general weight reduction effort, followed by Model II which further reduced weight but kept the defensive armament intact, and ended with Model III, which removed all the defensive armament, making it possible for the B-36 to reach altitudes in excess of 50,000 feet.
26 B-36Ds were built from scratch. In addition, some 64 B-36Bs were converted at Convair's San Diego facility to B-36D configuration. The last B-36Ds were taken out of service in 1957.
44-92095/92098 Consolidated B-36D-1-CF Peacemaker Originally ordered as B-36B. 49-2647/2654 Convair B-36D-5-CF Peacemaker 49-2655 Convair B-36D-35-CF Peacemaker 49-2656/2657 Convair B-36D-15-CF Peacemaker 49-2658/2663 Convair B-36D-25-CF Peacemaker 49-2664/2668 Convair B-36D-35-CF Peacemaker
Engines: Six 3500 Pratt & Whitney R-4360-41 Wasp Major air cooled radial engines, plus four 5200 lb.st. General Electric J47-GE-19 turbojets. Performance: Maximum speed 439 mph at 32,120 feet. Cruising speed 225 mph. Initial climb rate 2210 feet per minute. Service ceiling 45,200 feet. Takeoff run 4400 feet, 5685 feet over 50-foot obstacle. Combat radius 3525 miles. 7500 miles range. Weights: 161,371 pounds empty, 250,300 pounds combat, 370,000 pounds maximum. Dimensions: Wingspan 230 feet 0 inches, length 162 feet 1 inches, Height 46 feet 8 inches, wing area 4772 square feet. Armament: Two 20-mm M24A1 cannon each in six retractable, remotely-controlled fuselage turrets, tail turret and nose mounting, with 9200 rounds of ammunition. Normal bomb load up to 72,000 pounds. Maximum bomb load 86,000 pounds