Nakajima B5N ‘Kate’

Introduction and Development
Service Record
Variants
Statistics

Introduction and Development

The Nakajima B5N 'Kate' was the most successful Japanese torpedo bomber of the Second World War, playing a major part in every victory in the year after Pearl Harbor. The B5N was the first truly successful Japanese torpedo bomber. The Mitsubishi B2M was unreliable, and the Navy Type 92 Attack Bomber B3Y1 was unimpressive. In 1932 a 7-Shi (7th year of the Emperor Hirohito's reign) was issued, but this failed to produce a satisfactory aircraft. Two years later a 9-Shi specification led to the Yokosuka B4Y, a biplane with a reasonable performance, but this was only ever seen as a stop-gap measure.

Nakajima B5N 'Kate' taking off from Shokaku to attack Pearl Harbor
Nakajima B5N 'Kate'
taking off from
Shokaku

to attack Pearl Harbor

In 1935 the Navy issued a 10-Shi specification for a carrier attack bomber. The new aircraft was to be a single engined monoplane, with a wing span of no more than 52ft 4 29/32in, and a folded wing span of less than 24ft 7 9/32in (to fit on the elevators on Japanese aircraft carriers). It was to have a top speed of 205mph, an endurance of 4 hours normally or 7 hours maximum, to carry a crew of three and to be powered by either the Nakajima Hikari or Mitsubishi Kinsei radial engine.

The Nakajima design team, led by Katsuji Nakamura, produced an aircraft with a number of advanced features, some of which didn't make it into the production version. The aircraft had a circular fuselage, with the three crew carried in a long cockpit covered by a greenhouse canopy. It was one of the first Japanese aircraft to use a hydraulically operated retractable undercarriage, and also had a hydraulic wing folding mechanism. The wings were hinged so that they folding at different angles, allowing their tips to overlap when fully folded (with one wing tip in front of the other). The prototype used Fowler flaps, which increased lift when extended, reducing the aircraft's landing and take-off speeds. The prototype was powered by a Nakajima Hiukari 2 engine 9-cylinder single row radial engine, which powered a variable pitch propeller.

The first prototype was completed in December 1936 and made its maiden flight in January 1937. The new aircraft reached a top speed of nearly 230mph, well above the Navy's requirements. The hydraulic systems caused some problems, but these were soon solved in the undercarriage.

The Japanese navy was worried about the number of innovations being introduced in the B5N, and decided to remove the hydraulic wing folding mechanism and replace the Fowler flaps with more standard flaps. The second prototype also had an engine change, to the Hikari 3, and used a constant-speed propeller. The range was increased by adding integral fuel tanks in the wings.

The B5N came up against the Mitsubishi B5M in competitive trials to decide which aircraft would be ordered into mass production. The B5M used a similar basic layout to the B5N, but with manual wing folding from the start and a fixed undercarriage. It's 1,000hp Mitsubishi Kinsei engine meant that it was a little faster than the B5N, but the Navy decided to order the Nakajima design instead. The B5N was seen as the more modern design, with more potential, and Mitsubishi were busy working on the A5M Navy Type 96 Carrier Fighter and the upcoming A6M Zero.

The B5N was ordered into production as the Navy Type 97 Carrier Attack Bomber Model 1 (later renamed the Model 11), while the B5M was ordered in much smaller numbers as the Navy Type 97 Carrier Attack Bomber Model 2. Both aircraft received the Allied code name 'Kate'. A total of 1,149 were produced, by Nakajima, Aichi and the Dai-Juichi Kaigun Kokusho. Nakajima built 669 aircraft of all types between 1936 and 1941. Aichi made 200 B5N2s in 1942-1943 and Dai-Juichi Kaigun Kokusho made 280 B5N2s in the same period.

Service Record

After an initial period of preparation in Japan the B5N was issued to the Navy's carriers and to land based units. Its combat debut came in China in late 1938, where it was used as a land based tactical bomber, escorted by A5M Navy Type 96 fighters. The aircraft performed well in China, and its lack of crew armour, self-sealing fuel tanks or a useful defensive armament didn’t cause any problems. A small number of aircraft were posted to French Indo China in the autumn of 1940, from where they took part in operations in China.

Nakajima B5N1 'Kate' taking off from Akagi
Nakajima B5N1 'Kate'
taking off from Akagi

The B5N came to the world's attention on 7 December 1941, during the attack on Pearl Harbor. The B5N was armed with improvised weapons for this attack - the torpedoes had to be given wooden fins to prevent them from hitting the bottom of the shallow harbour, while the 'bombs' were actually 16in naval shells with tail fins. There were 40 torpedo armed B5Ns and fifty bomb-armed B5Ns in the first wave and fifty-five bomb armed B5Ns in the second wave. At this point the Japanese aircraft were manned by highly skilled crews, and the torpedo bombers claimed a 90% hit rate.

Over the next twelve months the B5N played a part in most Japanese victories, both at sea, and on land where they were used to support amphibious assaults. At sea B5Ns played a role in the sinking of the carriers Lexington, Yorktown and Hornet. Lexington was lost during the battle of the Coral Sea in May 1942, the loss of the Yorktown was the only Japanese success during the otherwise disastrous Battle of Midway.

After a series of battles around the Solomon Islands, there was a gap in the war at sea. When carrier battles resumed in 1944 the Americans had introduced new, much more capable fighter aircraft, the Hellcat and the Corsair, outnumbered the Japanese in both carriers and in aircraft, and had the better trained crews. The B5N was meant to have been replaced by the B6N Tenzan, and although this had been delayed, it had entered service by 1944. A handful of B5Ns took part in the Battle of the Philippine Sea, and others were used in their intended role during the fighting in the Philippines, but by this point they were outdated, vulnerable and outnumbered, and casualties were high.

Like most Japanese aircraft the B5N was used for kamikaze attacks, but its main function in the last year of the war was as a maritime reconnaissance and anti-submarine aircraft, operating in areas away from Allied fighters.

Variants

B5N1 Navy Type 97 Carrier Attack Bomber Model 11

The B5N1 was the first production aircraft, and was based on the second prototype, with simple flaps, manual wing folding and the Hikari 3 engine.

B5N2 Navy Type 97 Carrier Attack Bomber Model 12

The B5N2 first appeared during 1939. It was very similar to the B5N1, but with a more powerful Nakajima Sakae 11 radial engine. This was a fourteen-cylinder double-row radial engine, and had a smaller radius than the single row Hikari 3 used in the B5N1. This allowed Nakajima to install a smaller cowling, which improved the pilot's forward view. At first this aircraft was designated as the Navy Type 97 Carrier Attack Bomber Model 3 (The B5M being the Model 2), but this was soon changed to Model 12. The new engine had surprisingly little impact on the aircraft's performance, but it was more reliable, a valuable attribute in an aircraft expected to fly long missions over water. The B5N2 rapidly replaced the B5N1 in front-line units, although some of the older aircraft took part in the attack on Pearl Harbor.

B5N1-K

The B5N1-K was a training aircraft developed by Nakajima. It was used by training and target-towing units, and was later supplemented by retired B5N2s.

Statistics

B5N1
Engine: Nakajima Hikari 3 nine-cylinder air-cooled radial
Power: 770hp at take-off, 840hp at 9,845ft
Crew: 3 - pilot, observer/ navigator/ bomb-aimer, radio operator/ gunner
Wing span: 50ft 10 15/16in
Length: 33ft 9 1/2in
Height: 12ft 1 21/32in
Empty Weight: 4,643lb
Loaded Weight: 8,157lb
Max Speed: 229mph at 6,560ft
Cruising Speed: 159mph at 6,560ft
Climb to 9,845ft: 7 minutes, 50 seconds
Service Ceiling: 24,280ft
Range: 679 miles normal, 1,404 miles maximum
Armament: One flexibly mounted rear-firing 7.7mm machine gun
Bomb-load: 1,764lb of bombs or one 1,764lb torpedo

B5N2
Engine: Nakajima NK1B Sakae 11 fourteen-cylinder air-cooled radial
Power: 1,000hp at take-off, 970hp at 9,845ft
Crew: 3 - pilot, observer/ navigator/ bomb-aimer, radio operator/ gunner
Wing span: 50ft 10 15/16in
Length: 33ft 9 1/2in
Height: 12ft 1 21/32in
Empty Weight: 5,024lb
Loaded Weight: 8,378lb
Max Speed: 235mph at 11,810ft
Cruising Speed: 161mph at 9,845ft
Climb to 9,845ft: 7 minutes 40 seconds
Service Ceiling: 27,100ft
Range: 608 miles normal, 1,237 miles maximum
Armament: One flexibly mounted rear-firing 7.7mm machine gun
Bomb-load: 1,764lb of bombs or one 1,764lb torpedo

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How to cite this article: Rickard, J (7 July 2011), Nakajima B5N ‘Kate’, http://www.historyofwar.org/articles/weapons_nakajima_B5N.html

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