Type 91 torpedo#Kyoban stabilizer plates

The torpedo measured {{convert|18|ft|m|abbr=on|order=flip}} in length, with a diameter of {{convert|450|mm|in|abbr=on}}, and weighed {{convert|1841|lb|kg|abbr=on|order=flip}}, with an explosive charge of {{convert|452|lb|kg|abbr=on|order=flip}}. It had a range of {{convert|2200|yd|m|abbr=on|order=flip}} and a speed of {{convert|42|kn|kph|abbr=on|order=flip}}. A slightly-modified variant was used to sink HMS Prince of Wales and HMS Repulse, launched from Mitsubishi G4M "Betty" bombers in an action in the South China Sea three days after Pearl Harbor on 10 December 1941.{{cite web| url=http://www.navweaps.com/Weapons/WTJAP_WWII.htm| title=Japan Torpedoes of World War II| publisher=NavWeaps.com| access-date=2009-08-05| archive-url=https://web.archive.org/web/20090804165454/http://navweaps.com/Weapons/WTJAP_WWII.htm| archive-date=2009-08-04| url-status=live}}

File:Mitsubishi G4M1 bombers fly low through anti-aircraft fire between Guadalcanal and Tulagi on 8 August 1942 (NH 97766).jpg1s making a torpedo attack at Guadalcanal on August 8, 1942.]]

File:Nakajima B5N2s fly past USS South Dakota (BB-57) during the Battle of Santa Cruz Islands on 26 October 1942.jpg

File:752nd Kokutai Nakajima B6N2 Tenzan in formation.jpg2s in formation flight with torpedoes with box type tail stabilizers]]

File:B7A-Ryusei torpedo.jpg Ryusei carrying torpedo with cross type tail stabilizer plates, 1945]]

: Chronological Table

: 1931{{snd}}Type 91 aerial torpedo is put into service, production begins.

: 1936{{snd}}Revision 1. Self-detachable wooden plates are introduced.

: 1937{{snd}}Launch-tests at {{convert|500|and|1000|m|yd|abbr=on|sigfig=2}} with wooden damper.

: 1939{{snd}}Revision 2 starts production. Not running true after water entry is identified as a major problem.

: 1941{{snd}}Revision 2 clears the shallow water launching test due to the introduction of an anti-rolling controller. Battle of Pearl Harbor, sinking of HMS Prince of Wales and HMS Repulse.

: 1941{{snd}}Revision 3 starts production.

: 1942 {{snd}}Indian Ocean raid, Battle of the Coral Sea, Battle of Midway, Battle of the Santa Cruz Islands. 2 August: Type 91 torpedo technology reaches Nazi Germany via IJN sub I-30{{Cite web |url=http://www.combinedfleet.com/I-30.htm |title=Submarine I-30: Tabular Record of Movement |work=combinedfleet.com |access-date=15 September 2010 |archive-url=https://web.archive.org/web/20101117192703/http://www.combinedfleet.com/I-30.htm |archive-date=2010-11-17 |url-status=live }}

: 1943{{snd}}Revision 5 starts production.

: 1944{{snd}}Battle of the Philippine Sea, Aerial Battle of Taiwan-Okinawa.

Rear Admiral Seiji Naruse led the team in charge of the initial development of the Type{{nbsp}}91 aerial torpedo at the Yokosuka Naval Arsenal. The team was known as the Ninety One Association and included Lt Cmdr Haruo Hirota, Lt Cmdr Makoto Kodaira (Matsunawa), Naval Assistant Manager Iyeta, Naval Engineer Noma, Naval Engineer Moritoshi Maeda, Lieutenant Hidehiko Ichikawa, and Teruyuki Kawada, a university student who was a naval apprentice.{{Citation needed|date=November 2017}}

Captain Fumio Aiko was in charge of further development of the torpedo from 1931. Captain Aiko managed the team as it developed an effective aerial torpedo and anti-rolling controller. He considered the Type{{nbsp}}91 aerial torpedo to be his great achievement.{{Citation needed|date=November 2017}}

At the beginning of 1934, Kan-Pon or the Imperial Japanese Navy Technical Department, an operating division of the Ministry of the Navy of the Imperial Japanese government, which had the primary responsibility for naval weapon systems, had their own plan for a Japanese aerial torpedo. In their concept, a big flying boat was to carry a variant of the heavy Type{{nbsp}}93 oxygen torpedoes to launch at long range, and then turn back towards safety. This eventually proved to be an unrealistic desk plan. Kan-Pon confidentially developed their own Type{{nbsp}}94 torpedo and even ordered a halt to production of the Type 91. This significantly delayed the development schedule of the Type{{nbsp}}91 and frustrated the project members.{{Citation needed|date=November 2017}}

The project team developed Kyoban wooden aerodynamic stabilizer plates for the Type{{nbsp}}91's tail fins as revision{{nbsp}}1 in 1936. These stabilized the torpedo in flight to ensure the proper angle for water entry and were designed to shear off on entry to the water, preventing the torpedo from diving too deep. The team demonstrated their effectiveness in tests at altitudes of both {{convert|500|and|1000|m|yd|abbr=on|sigfig=2}} the following year.

The original Type{{nbsp}}91 was considered to have a frail body{{Citation needed|date=November 2017}}, and so this was strengthened in a new model in 1938 known as revision{{nbsp}}2.

Type 91 aerial torpedoes won admiration for their effective anti-rolling controller and acceleration control system.{{Citation needed|date=November 2017}} Before the anti-rolling controller was introduced, the early versions of the Type{{nbsp}}91 had serious problems, as did all other aerial torpedoes of the time. When released at high speed, it had a tendency to make a double-roll in the air. When released into heavy seas, a spin could be imparted by the hard impact on water entry. Other issues included: the running direction veering on water impact; not running horizontally after water entry, but continuing vertically to either stick in the bottom of shallow water or be crushed by the water pressure (at a depth of 100{{nbsp}}m or so); jumping back out of the water; skipping along the water surface; or even running backwards. Only very experienced aviators could be sure of a clean torpedo bombing run, and then only when operating over a calm sea.{{Citation needed|date=November 2017}} A tumbling torpedo will run out of control once it hits the water. The gyrocompass and the depth meter may work well, but the torpedo cannot control the running direction by tail rudders unless they are initially in the neutral position. Once the torpedo rolls, the horizontal and vertical rudders lose their positions, resulting in a runaway.{{citation needed|date=November 2022}}

The specification for the launch speed of aircraft was increased from {{convert|130|to|180|kn|kph|abbr=on|order=flip}} with the expectation that it would be increased again. The engineers and scientists of the Type 91 project concluded that any aerial torpedo needed an anti-rolling system with not only a damping stabilizer function but also an acceleration controlling function. Without these features any torpedo would be highly likely to fall into an unstable state. The idea of acceleration-control, or counter-steering, was at the time widely considered to be impossible.{{Citation needed|date=November 2017}}

A breakthrough on aerial torpedo design was made with the anti-rolling controller invented first by Iyeda, assistant manager of the arsenal workmen, in spring 1941. Ten days later, while the Iyeda system was being tested, Naval Engineer Noma invented another system. It functioned in a similar way, but with a different mechanism. During the prototype tests, Noma's system was found to be the better, having less time lag in its responses. So the Noma system was adopted for the next production version of Type{{nbsp}}91 and it went into final testing in August 1941, making practical the use of aerial torpedoes both in rough seas and in shallow waters.{{Citation needed|date=November 2017}} It enabled the Type{{nbsp}}91{{nbsp}}rev.2 to run under water no deeper than 20 meters, with experienced pilots learning to launch their torpedo so as to sink to a depth of no more than 10 meters.{{Citation needed|date=November 2017}}

The anti-rolling controller also made it possible for the Type{{nbsp}}91 to carry a heavier warhead. The Type{{nbsp}}91{{nbsp}}rev.1 warhead weighed {{convert|213.5|kg|lb|abbr=on}} with a high explosive charge of {{convert|149.5|kg|lb|abbr=on}}, but the rev.2 warhead weighed {{convert|276|kg|lb|abbr=on}} with {{convert|204|kg|lb|abbr=on}} of high explosive. Warhead rev.7, which was carried by twin-engine bombers, weighed {{convert|526|kg|lb|abbr=on}} and boasted a high explosive charge of {{convert|420|kg|lb|abbr=on}}; this was designed to pierce the reinforced armour plates of the latest US Navy ships.{{Citation needed|date=November 2017}}

The Type 91 was researched and developed at Yokosuka Naval Arsenal in Kanagawa Prefecture. It was first produced at the Mitsubishi-Urakami Ordnance Works division of Mitsubishi Heavy Industries. Later, the Imperial Japanese Navy established two manufacturing sites: Suzuka Naval Arsenal in Mie Prefecture; and Kawatana Naval Arsenal, a branch of Sasebo Naval Arsenal, in Nagasaki Prefecture. The Mitsubishi-Urakami Ordnance Works plant at Kawatana specialized in torpedo production and was destroyed by the atomic bomb dropped on Nagasaki.{{cite report |date=1 July 1946 |title=United States Strategic Bombing Survey Summary Report (Pacific War) |chapter=The Effect of the Atomic Bombs |location=Washington, DC |publisher=United States Government Printing Office |chapter-url=http://www.anesi.com/ussbs01.htm#teotab |page=24 |access-date=2018-09-15 |archive-url=https://web.archive.org/web/20080516014539/http://www.anesi.com/ussbs01.htm#teotab |archive-date=2008-05-16 |url-status=live }}

Germany approached Japan requesting the transfer of Japanese aerial torpedo technology. In a yanagi mission the Imperial Japanese Navy sent the plans and a number of Type{{nbsp}}91 aerial torpedoes on Japanese submarine I-30 (a large cruiser type submarine) which arrived in Lorient on August{{nbsp}}2, 1942. It was designated the Lufttorpedo LT 850 in German service.{{Citation needed|date=November 2017}} The weight of the LT 850 German version was somewhat lighter at {{convert|810|kg|lb|abbr=on}}, with a {{convert|5.43|m|ft|abbr=on}} length.{{citation needed|date=September 2022|reason=ref to unreliable source removed}}

Germany wished to acquire the knowledge behind the Imperial Japanese Navy Air Service's aerial torpedo technology in order to more effectively attack the Allied transport ships steaming in the Mediterranean Sea.{{harvc |p=13 |author=Fumio Aikō |c=__ |in1=Ichikawa |in2=Kodaira |in3=Kawada |year=1985}} It had previously imported Italian-made aerial torpedoes, which became unavailable following the Italian Armistice of Cassibile with the Allies in September 1943. The indigenous German aerial torpedo designs were badly restricted in launch speed and launch altitude.{{Citation needed|date=November 2017}}

The Type 91 (modification 2), was a shallow-water aerial torpedo that was designed for and used in the attack on Pearl Harbor in 1941. Wooden fins and a softwood breakaway nose cone were added to allow for launching into shallow water at low altitudes.{{cite book | last=Gannon| first=Robert| title=Hellions of the Deep| publisher=Pennsylvania State University Press| date=30 April 1996| pages=49–50| isbn=978-0-271-01508-8| url=https://books.google.com/books?id=5pJxc8Je2vsC&q=thunder+fish+torpedo&pg=RA1-PA49| access-date=2009-08-05}}

There were two versions in the Type 91 warhead rev.3, differing in designed maximum launch speeds.

Later, heavier models had a decreased range.

In spring 1944, the Yokosuka air arsenal began development of the Shisei Gyorai M (trial model torpedo M), or simply the "Two tonne torpedo". This was an enlarged version of the Type 91 aerial torpedo and was {{convert|533|mm|in|abbr=on}} in diameter, {{convert|7.10|m|ft|abbr=on}} long, weighing {{convert|2070|kg|lb|abbr=on}}, and carrying a {{convert|750|kg|lb|abbr=on}} warhead.{{sfnp|Minoru Akimoto|1995|p=383}} It would have been the largest aerial torpedo in the Imperial Japanese Navy Air Force, but the operating concept became outdated and the project was never completed.{{Citation needed|date=November 2017}} However, the Type{{nbsp}}91 aerial torpedo project members did not regard it as a part of the Type{{nbsp}}91 series.{{Citation needed|date=November 2017}}

Some 30 years after the war, surviving members of the development team raised money to privately publish a small book, Koku Gyorai Note or Aerial Torpedo Notebook.{{harvc |p=278 |last=Ichikawa |first=Hidehiko |c=Postface |in1=Ichikawa |in2=Kodaira |in3=Kawada |year=1985}}

Type 91 torpedoes are currently displayed at the Etajima school of Japan Maritime Self-Defense (the Maritime Self Defense Force 1st{{nbsp}}Technical School) and Shimofusa Base. They are missing the roll rudders. An excavated Type 91{{nbsp}}aerial torpedo is preserved at the Resource Museum in JGSDF Camp Naha, 1st Combined Brigade of The Western Army, JGSDF, located in Naha city, Okinawa. It retains the original features. It was picked up as unexploded ordnance by a bomb-disposal unit of the JGSDF. A captured Type{{nbsp}}91 aerial torpedo is displayed at the US Naval Academy, Annapolis, Maryland. It rests on two supports flanking a pathway in a small park in front of the Academy's Dahlgren Hall. Displayed on the other side of the pathway is a Type{{nbsp}}93 Japanese Long Lance ship-launched torpedo.{{Citation needed|date=November 2017}}

• British 18-inch torpedo#Mark XII

{{reflist}}

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{{citation style|date=August 2020}}

• {{cite book |last1=Ichikawa |first1=Hidehiko |last2=Kodaira |first2=Makoto |last3=Kawada |first3=Teruyuki |date=25 July 1985 |title=Kyu Ichi Kai – Koku Gyorai Note |trans-title=91 Association – Aerial Torpedo Notebook |publisher= Iyeno Hikari Private Publishing Service |location=Tokyo, Japan |language=ja}} Privately printed book.
• {{cite book |date=July 2002 |chapter=Warship Carrier Zuikaku Action Report No. 7, Battle of the Coral Sea |title=Kaigun Koku Bokan Sento Kiroku |trans-title=Naval Aircraft Carrier Action Reports |publisher=Athen-shobo |location= Tokyo, Japan |language=ja}} Photographic print copies of Imperial Japanese Navy Action Reports.
• {{cite book |year=1994 |last=Ozawa |first=Kyuno Joe |chapter=Mitsubishi Type 4 Army Bomber Aircraft |pages=196–222 |title=Document of Historical Aircraft with Japan Making |issue= Special Thanks 600 Issue of Aireview |publisher=Kanto-sha |location=Tokyo, Japan |language=ja |issn=0450-6669}} Ozawa is the designer of Ki-69.
• {{cite book |last=Seko |first=Tsutomu |date=December 1986 |title=Raigeki no Tsubasa |trans-title=Wings of Torpedo Bombers |publisher=Kojin-sha |location=Tokyo, Japan |language=ja}} Seko was one of the last torpedo bombardiers of B6Ns.
• {{cite book |author=Minoru Akimoto |date=June 1995 |title=Nihon Gunyoki Kokusen Zenshi |trans-title=Japanese military aircraft air combat complete history |volume=4 |publisher=Green Arrow sha |location=Tokyo, Japan |isbn=4-7663-3174-5 |language=ja}}
• (August 1945), Resources from Torpedo bombing section, Kawatana branch, Naval aerial technology arsenal, Imperial Japanese Navy.
• (August 1945), Resources from the 1st torpedo section, Kawatana naval arsenal production firm, Imperial Japanese Navy.

• [http://www.navweaps.com/Weapons/WTJAP_WWII.htm A page with many statistics on Japanese WWII torpedoes.]

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Category:Torpedoes of Japan

Category:World War II weapons of Japan

Category:World War II naval weapons

Category:Aerial torpedoes