he Navy Carrier Torpedo Bomber “Tenzan” or Nakajima B6N, was codenamed Jill by the Allies. It was the replacement of the famous Nakajima B5N (Kate) torpedo bomber and although more B6N’s were built than B5N’s, the plane never achieved the spectacular success of its predecessor.
It entered service in the second half of 1943 but wouldn’t be used on a large scale until June, 1944 during the air and sea battles in the Philippine Sea for the Mariana Islands. Later it would be used in Taiwan, the Philippines, Iwo Jima and Okinawa and for Kamikaze attacks. The crushing air superiority of the Allies and the loss of nearly all the Japanese aircraft carriers and their trained pilots led to many Tenzans being lost, while never having had a chance to prove their worth. In spite of the fact that the Tenzan never achieved any real successes, it was undoubtedly one of the best carrier based torpedo bombers of WWII.
Design, Development, Production
In December, 1939, two years after the Nakajima B5N1 had been approved for production and a short while before the B5N2 prototype had its maiden flight, Kaigun Koku Hombu1 drew up the newest set of technical and tactical guidelines for a Navy Experimental 14-Shi Carrier Attack Aircraft, or 14-Shi Kanko2 . Their intent was to have a replacement for the B5N within two years, thus insuring a constant supply of modern torpedo bombers to the Imperial Japanese Naval Air Force (IJNAF).
The guidelines set out in the 14-Shi3 specification were for a three-man, low wing, cantilevered, all-metal plane. It size must conform to the carrier deck elevators. A top speed of 463 km/h, a cruising speed of 370 km/h and a range of 1,852 km with maximum bomb load or 3,333 km clean was required. Bomb load was to be 800 kg or one torpedo. For self defense a flexible 7.7 mm type 92 (Shiki) machine gun was to be mounted. The 14-Shi Kanko specifications were more demanding than those for the B5N2 and called for an increase in top speed of 85 km/h, in cruising speed of 111 km/h, in range with full armament of 572 km/h and in range without armament of 1,053 km/h.
There was no contract bidding for the 14-Shi design, and the project was handed over directly to Nakajima Hikoki Kabushiki Kaisha (Nakajima Airplane Factory). It was assumed that the design team that had worked on the B5N would be most qualified to design its replacement. Engineer Kenichi Matsumura was designated head of the team which would work on the 3K (N-10) project. The 3K (N-10) was based on the airframe of its predecessor, while a radial engine with almost twice the power of the Nakajima “Sakae” 11 used on the B5N2 would deliver the increased performance. Kaigun Koku Hombu recommended a 14-cylinder “Kasei” radial engine made by Mitsubishi, but the designers insisted on using the Nakajima “Mamori” 11 radial engine which would deliver 1,870 hp. The Nakajima “Mamori” Model 11 (also know as Mamori 11-Gata or NK7A) was a completely new design and the 3K bomber would be one of the first airplanes to use it.
Work on the design went on through 1940 and the first prototype, the B6N1, was completed in March, 1941. It was a low wing, cantilevered, all-metal plane except for control surfaces, which were covered with fabric. The flat, tapered-rounded wings were in three sections; one central and two hinged for storage on an aircraft carrier, and had a dihedral of 6.5°. The wing span and area were similar to the B5N and the increase in overall weight due to the new powerplant caused a greater load on the wings. In order to compensate for this, the designers used Fowler flaps on the B6N instead of the regular flaps found on the B5N. The Fowler flaps were extended beyond the wing trailing edge on tracks and lowered 20° for take-off and 38° for landing. Although the flaps made a difference, the plane had a much higher stall speed than the B5N.
The B6N had a hydraulically retracted undercarriage with a tail wheel. The main landing gear was stowed into the underside of the wing center section towards the fuselage and the tail wheel was pulled up into the tail section. A tail hook hung in front of the tail wheel and was also retractable. The crew comprised of a pilot, navigator/bombardier and radio operator/gunner. As on the B5N the crew was under a seven part canopy. The torpedo rack was also like that of the B5N; mounted on the bottom of the fuselage 30 cm starboard from the centerline. The oil cooler was mounted on the bottom of the fuselage to port of the centerline, unlike on the B5N, where it was mounted on the centerline. A four-blade constant-speed propeller measuring 3.5 m in diameter was used on the prototype.
The maiden flight of the B6N took place on March 14, 1941. Soon after, test flights would be carried out by pilots from the Navy Arsenal in Yokosuka4 Hiko Jikkenbu and Yokosuka Kokutai squadron which exposed a number of serious problems with the design. The first problem to be fixed was the tendency of the plane to roll during flight due to the powerful torque of the four-bladed propeller. In order to correct this the tail fin was thinned and keyed 2° 6’ to port. Test flights made at the end of 1942 on the carriers “Ryuho” and “Zuikaku” revealed a weakness in the tail hook assembly, which was quickly fixed. The toughest problems for the engineers were those with the “Mamori” engine, which turned out to be highly undependable. At certain speeds it caused major vibrations, overheated and never achieved its planned power ratings. The engine wasn’t accepted for use until the end of 1942 and after many modifications had been made. After almost two years since the first prototype had been built, the B6N1 was officially accepted by Kaigun Koku Hombu for series production as the Kanko Tenzan Ichi-Ichi-Gata, or Navy Carrier Attack Aircraft Tenzan Model 11. The word Tenzan literally means “Heavenly Mountain” and was the Japanese name for the Tien Shan mountain range in China.
The B6N1 Model 11 differed from the prototypes as a result of further modifications introduced during production. It was fitted with a second flexible machine gun (Type 92) mounted in a ventral tunnel at the rear of the cockpit. A 7.7 mm Type 97 (97-Shiki) with a 400-round magazine was mounted in the center wing section on the port side. The effectiveness of this armament was questionable (small caliber arms were useless against either surface targets or enemy fighters) and from the seventy-first plane to roll off the assembly line, this type of armament was removed. The main landing gear was strengthened, the torpedo rack, positioned on the starboard side of the belly, was angled down 2° and thinner, longer exhaust stacks were used. Tests were conducted using rocket assisted take-off on a few B6N1’s for use on small carriers, but the concept was considered too complicated and offered no solution to the problem of landing such a heavy plane on a short deck. The designers proposed replacing the unprotected fuel tanks with self-sealing ones, but this resulted in a 30% reduction in the B6N1’s fuel capacity, and the Navy insisted on using the original fuel tanks to maintain range. A total of 133 B6N1 Model 11’s were built in the Nakajima plant in Koizumi between February and July, 1943
The planned date for replacing the B5N with the B6N was sometime in 1941, but the delays during development forced the Navy to renew production of the B5N2 in 1942. Meanwhile, problems with the new bomber continued. Soon after production of the B6N1 Model 11 had begun, Kaigun Koku Hombu ordered a halt to the production of the “Mamori” engine. It had been decided for logistical reasons to unify the engines being used by the Navy as much as possible. The Mitsubishi “Kasei” was dependable and widely used, and the new Nakajima “Homare” engine looked very promising and used the same pistons as the mass produced “Sakae” engine. Kaigun Koku Hombu ordered the B6N1 to be adapted to use the “Homare.” This engine would be used on many of the planes designed at the time, including the Nakajima C6N “Saiun” observation plane, the Kawanishi N1K-J “Shiden” fighter plane and the Kugisho P1Y “Ginga” bomber.
With the “Homare” just entering production and the need for bombers at a critical level, the Nakajima engineers first adapted the B6N to use the Mitsubishi “Kasei” engine, which was readily available. The 1,850 hp fourteen cylinder Mitsubishi “Kasei” Model 25 (Kasei 25-Gata or MK4T) was picked for its size, which was close enough to the “Mamori” to make installation trouble-free. It’s lighter weight forced the need to extend the nose to maintain the correct center of gravity. Following this were changes made to the oil cooler intake, the engine cowling and air intake. A new four-blade propeller with a smaller (3.4 m) diameter, wider blades and shorter spinner was fitted. The adaptations for the “Kasei” engine made this version of the plane somewhat longer than the B6N1.
Additionally, the exhaust stacks were changed from the original two to individual ones for each cylinder. This was to eliminate flames which were blinding the pilots during night maneuvers, and also helped by adding some drag. The oil reserve was reduced from 90 liters to 70 liters and the fuel capacity was increased by 10 liters. The tail wheel was mounted in a permanent position, as its retraction was considered too much of a luxury. The overall weight of the plane dropped by 140 kg, and even with the weaker engine, the plane reached a top speed of 482 km/h, and it had a better climb rate. The armament was not modified.
Kaigun Koku Hombu named the new plane Kanko Tenzan Ichi-Ni-Gata, or Navy Carrier Attack Aircraft Tenzan Model 12 (B6N2 Model 12), and production was begun in June, 1943. In November, 1944, changes were made to the armament based on suggestions from front line combat groups. The dorsal Type 92 machine gun was replaced with a 2‑Shiki (Type-2) 13mm machine gun, and the ventral machine gun was replaced with a 1-Shiki (Type-1) 7.92 mm machine gun. The radio was upgraded to the newer model 96-Gata Ku 3-Go 4-Gata. This model was named the Kanko Tenzan Ichi-Ni-Ko-Gata, or Navy Carrier Attack Aircraft Tenzan Model 12A (B6N2a Model 12A) and was produced till the war ended in August, 1945. Nakajima manufactured a total of 1,133 B6N2’s and B6N2a’s at their plants in Okawa in the Gumma prefecture and at Handa in the Aichi prefecture. According to factory records a total of 1,268 B6N’s including prototypes, B6N1’s, B6N2’s and B6N2a’s were built: the first 298 at Koziumi and 970 at Handa. According to Navy records only 1,262 planes were built. Production was slow, with only 18 B6N2’s built by October, 1943 and a maximum rate of only 90 planes per month was ever achieved. From the fall of 1943 about 1/3 of the planes leaving the factory were equipped with 3-Shiki Type 3 ship-seeking radar with Yaga antennas mounted on the wing leading edges and on the sides of the rear fuselage. The planes with radar were not marked in any special way.