Nakajima B6N Tenzan (Heavenly Mountain) 'Jill'

Introduction and Development
Variants
Combat Record
Specifications

Introduction and Development

The Nakajima B6N Tenzan (Heavenly Mountain) Jill was designed to replace the Nakajima B5N 'Kate', but delays meant that it didn't enter service until 1943, two years after originally planned, and too late for it to have any real impact on the fighting in the Pacific.

The B6N was designed in response to a 14-Shi (14th year of the Emperor Hirohito's reign) specification for a new carrier attack aircraft, which was to replace the Nakajima B5N. The new specification called for a three-seat low-wing all-metal monoplane, small enough to fit o the deck elevators of current Japanese carriers. The new aircraft was to have a top speed of 288mph, a cruising speed of 230mph, a range of 1,151 miles with a 1,764lb bomb load or one torpedo and a range of 2,072 miles with no bomb load. It was to be lightly armed, with a single rear firing flexibly mounted 7.7mm machine gun. It was hoped that the new aircraft would be ready in two years.

The B6N was designed by the same design team as the B5N, and had a very similar air frame to the earlier aircraft. It was to be powered by a radial engine, had a circular fuselage, and a long glazed cockpit with a greenhouse canopy. The main external differences were on the wing - the wing on the B5N had a flat central section with dihedral beyond the folding point, while the B6N wing had dihedral along its entire length; and on the vertical tail surface, which appeared to leaning slightly forward (although its leading edge did actually lean back slightly). 

The increase in performance came entirely from an increase in available engine power. The B5N2 used a 1,000hp Sakae 11 radial engine, but the B6N was to get an engine producing at least 1,800hp. Here Nakajima disagreed with the Japanese Navy, which had suggested that the new aircraft use the Mitsubishi 'Kasei' 14-cylinder radial engine. Understandably Nakajima wanted to use one of their own engines, the new Nakajima Mamori Model 11 radial engine. Nakajima cited the new engine's lower fuel consumption and better growth potential, but the financial implications of using one of their competitors engines must also have played a part. This would prove to be a poor decision - problems with the new engine fatally delayed the B6N, which was unavailable for the battles of 1942, and Nakajima were eventually forced to use the Mitsubishi engine anyway.

The first prototype, a B6N1, was completed in March 1941, and was soon followed by the second prototype. Both were powered by the Nakajima engine. The first aircraft made its maiden flight on 14 March 1941, and was then used for flight tests with the Navy. These revealed a number of problems with the new aircraft. The most significant problem with the B6N itself was that the torque from the four-blade propeller made it roll in flight. This flaw was solved by moving the vertical tail 2 degrees 10 minutes to the left. The Mamori engine caused longer delayed, and proved to be very unreliable. It wasn't accepted for use until the end of 1942, delaying the B6N's carrier trials. The aircraft's higher performance also came at a cost - its higher landing speed meant that it could only be used from the larger carriers.

The first carrier trials took place on Ryuho and Zuikaku at the end of 1942. This time the arrester hook mounting caused problems, and had to be strengthened. Finally, early in 1943 the B6N was finally accepted for production, as the Navy Carrier Attack Aircraft Tenzan (Heavenly Mountain) Model 11 (B6N1).

Variants

B6N1 Model 11
 
The B6N1 Model 11 production aircraft differed from the prototype in a number of ways. The single large exhaust of the prototype was replaced with multiple smaller exhausts, reducing the glare at night. The torpedo rack was angled down by 2 degrees. A second 7.7mm machine gun was added, in a rear firing ventral position, while earlier aircraft had a single fixed forward firing 7.7mm gun in the port wing. The main landing gear and tail plane were both strengthened.

One chance that was considered but not made was to replace the unprotected semi-integral fuel tanks with self sealing tanks, but this would have reduced the range by around 30%, and the change was rejected by the navy.

A total of 133 production B6N1 Model 11s were built by Nakajima before they were ordered to cancel production of the Mamori engine. Their own 'Sakea' engine was still in wide-spread use, and their new 'Homare' engine was considered to be more useful and easier to produce than the 'Mamori'. The Navy ordered Nakajima to modify the B6N to use the Homare, but supplies of that engine were limited.

B6N2 Model 12

Instead Nakajima were forced to use the 1,850 Mitsubishi 'Kasei' Model 25. This was similar in size to the Mamori, and the switch was fairly easy. The nose had to be lengthened to compensate for the lighter weight of the new engine, and a new propeller used. The retractable rear wheel was also removed, and was replaced with a fixed wing. The B6N2 was lighter and faster than the earlier B6N1. Production began in June 1943 and continued through 1944.

B6N2a Model 12A

The B6N2a saw a slight increase in the defensive armament. The two 7.7mm machine guns were replaced, the dorsal gun with a 13mm machine gun and the ventral gun with a 7.92mm machine gun. This aircraft was produced from late in 1944 until the end of the war. A total of 1,133 B6N2s and B6N2as were built, at Okawa and at Handa.

B6N3 Model 13

The B6N3 was an unofficial designation given to two prototypes for a land based version of the B6N, designed in response to the loss of most Japanese aircraft carriers. They were produced by modifying the 751st and 752nd production aircraft. They were given a 1,850hp Mitsubishi MK4T-C Kasei 25c engine, had the arrestor hook removed and a retractable tail wheel installed. The engine cowling and cockpit canopy were also modified. The B6N3 never entered production.

Combat Record

The B6N1 made its combat debut during the fighting at Bougainville on 5 November 1943. The aircraft involved had originally been intended to reinforce the air fleet at Rabaul, but the American invasion of Bougainville forced the Japanese to change this plan. On 5 November fourteen B6N1s with an escort of only four Zeros attacked the American fleet south of Bougainville. As was so common in the later part of the war, Japanese claims were wildly exaggerated. On this occasion they claimed to have sunk one large aircraft carrier, two small aircraft carriers, two large cruisers and two smaller cruisers or large destroyers. Four B6N1s were lost.

This would have been a major victory for the Japanese if it has actually happened, but in fact the Japanese had attacked a force of two large landing ships (presumably the 'carriers') escorted by one gun ship, and all three American ships survived the attack. By the time the Japanese carrier force returned to Rabaul all but six of its B6Ns had been lost. Again as was often the case later in the war, the Japanese high command believed the exaggerated claims, and considered the B6Ns combat debut to have been very successful.

The B6N's first major test came during the fighting around the Marianas Islands, in June 1944. On 15 June the Americans landed on Saipan, and over the next few days were involved in battles with land-based and carrier based aircraft. The first B6N attack came on 15 June, when eleven aircraft from Truk claimed to have hit one carrier and sunk one transport ship, but at a cost of six aircraft lost. The remaining five reached relative safely on Truk.

The first truly large scale use of the B6N was during the Battle of the Philippine Sea. On 19 June 44 B6Ns took part in the large scale carrier attack on the US fleet (37 in the first wave of attacks and 7 in the second). Both attacks were disasterous failures - 27 of the 37 B6Ns in the first wave were lost and 4 of the 7 in the second wave. Only 14 aircraft survived the attack, and none of their torpedoes struck home. By the end of the second day of the battle the Japanese carrier force had been destroyed - three carriers were lost, and others damaged. Only 35 carrier borne aircraft survived, including two B6Ns. The heavy losses of the more experienced Japanese aircrews were even more important, and they proved to be almost irreplaceable. 

Significant numbers of aircraft still existed elsewhere. In October aircraft in Taiwan came under attack during preparations for the invasion of the Philippines. In mid-October large numbers of aircraft were sent against the American fleets, but with little or no success. One formation of seventeen B6Ns that did make contact with an American force on 14 October was nearly wiped out, with only one survivor.

The last major naval battle in the Pacific came in late October, after the American invasion of Leyte, in the Philippines. The Battle of Leyte Gulf saw the once-mighty Japanese carrier force reduced to a decoy, carrying just over one hundred aircraft. Even with the help of B6Ns based on the Philippines, the Japanese torpedo bombers were unable to achieve anything in the face of overwhelming American fighter defences.

The B6N had one minor success early in 1945. On 21 February 1945 three B6Ns managed to score a hit on the USS Saratoga. The carrier was never fully operational again, but by this stage the US had so many aircraft carriers in the Pacific that this didn't matter.

The B6N was also used during the battle of Okinawa, where is served as both a conventional bomber and a kamikaze aircraft. The suicide attacks were the most successful, sinking three destroyers and damaging the carrier USS Intrepid, but once again they were unable to affect the course of the battle.  

Specifications

B6N1
Engine: Nakajima NK7A Mamoru 11 fourteen-cylinder air-cooled radial
Power: 1,800hp at take off, 1,750hp at 4,595ft, 1,600hp at 16,075ft
Crew: 3 - pilot, observer/ navigator/ bomb-aimer, radio-operator/ gunner
Wing span: 48ft 10 3/8in
Length: 34ft 0 1/16in
Height: 12ft 1 21/32in
Empty Weight: 7,105lb
Loaded Weight: 11,464lb
Max Speed: 289mph at 15,750ft
Cruising Speed: 207mph at 13,125ft
Service Ceiling: 28,380ft
Climb to 16,405ft: 11min 1sec
Range: 909 miles normal, 2,142 miles maximum
Armament: Two flexibly mounted rear-firing 7.7mm machine guns, one in dorsal and one in ventral position
Bomb-load: 1,764lb of bombs or one torpedo

B6N2
Engine: Mitsubishi MK4T Kasei 25 fourteen-cylinder air-cooled radial
Power: 1,850hp at take-off, 1,680hp at 6,880ft, 1,540hp at 18,040ft
Crew: 3 - pilot, observer/ navigator/ bomb-aimer, radio-operator/ gunner
Wing span: 48ft 10 3/8in
Length: 35ft 7 3/4in
Height: 12ft 5 19.32in
Empty Weight: 6,636lb
Loaded Weight: 11,464lb
Max Speed: 299mph at 16,075ft
Cruising Speed: 207mph at 13,125ft
Service Ceiling: 29,660ft
Climb to 16,405ft: 10 min 24 sec
Range: 1,085 miles normal, 1,892 miles maximum
Armament: Two flexibly mounted rear-firing machine guns, one 7.7mm ventral gun and one 13mm dorsal gun
Bomb-load: 1,764lb of bombs or one torpedo

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How to cite this article: Rickard, J (5 July 2011), Nakajima B6N Tenzan (Heavenly Mountain) 'Jill', http://www.historyofwar.org/articles/weapons_nakajima_B6N.html

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