The Type 91 was an
aerial torpedo
An aerial torpedo (also known as an airborne torpedo or air-dropped torpedo) is a torpedo launched from a torpedo bomber aircraft into the water, after which the weapon propels itself to the target.
First used in World War I, air-dropped torped ...
of the
Imperial Japanese Navy
The Imperial Japanese Navy (IJN; Kyūjitai: Shinjitai: ' 'Navy of the Greater Japanese Empire', or ''Nippon Kaigun'', 'Japanese Navy') was the navy of the Empire of Japan from 1868 to 1945, Potsdam Declaration, when it was dissolved followin ...
. It was in service from 1931 to 1945. It was used in
naval battles in
World War II
World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
and was specially developed for attacks on ships in shallow harbours.
The Type 91 aerial torpedo had two unique characteristics. Firstly, it used wooden stabilizers attached to the tail fins which were shed upon water entry. Secondly, it engaged an
angular acceleration control system to control rolling movements, which was very advanced for its time. This system made it possible to release the Type 91 not only at a cruising speed of at an altitude of , but also in a power-glide torpedo-bombing run at the maximum speed of the
Nakajima B5N
The Nakajima B5N (, World War II Allied names for Japanese aircraft, Allied reporting name "Kate") was the standard Carrier-based aircraft, carrier-based torpedo bomber of the Imperial Japanese Navy (IJN) for much of World War II. It also served ...
'Kate',
The Type 91 torpedo was an diameter torpedo, similar in size to other nations. There were five models put into service, with high-explosive warheads weighing from with effective ranges from at .
Since the Type 91 torpedo was the only practical aerial torpedo of the
Imperial Japanese Navy
The Imperial Japanese Navy (IJN; Kyūjitai: Shinjitai: ' 'Navy of the Greater Japanese Empire', or ''Nippon Kaigun'', 'Japanese Navy') was the navy of the Empire of Japan from 1868 to 1945, Potsdam Declaration, when it was dissolved followin ...
, it was simply known as the ''Koku Gyorai'' or "aerial torpedo". Surface warships and submarines used other types of torpedoes, namely the
Type 93 and
Type 95 respectively, while the
Type 97 torpedo was designed for use by
midget submarine
A midget submarine is any submarine under 150 tons, typically operated by a crew of one or two but sometimes up to six or nine, with little or no on-board living accommodation. They normally work with mother ships, from which they are launched an ...
s.
Specifications
The torpedo measured in length, with a diameter of , and weighed , with an explosive charge of . It had a range of and a speed of . A slightly-modified variant was used to sink
HMS ''Prince of Wales'' and
HMS ''Repulse'', launched from
Mitsubishi G4M
The Mitsubishi G4M is a twin-engine, land-based medium bomber formerly manufactured by the Mitsubishi Aircraft Company, a part of Mitsubishi Heavy Industries, and operated by the Air Service (IJNAS) of the Imperial Japanese Navy from 1940 to ...
"Betty" bombers in
an action in the
South China Sea
The South China Sea is a marginal sea of the Western Pacific Ocean. It is bounded in the north by South China, in the west by the Indochinese Peninsula, in the east by the islands of Taiwan island, Taiwan and northwestern Philippines (mainly Luz ...
three days after Pearl Harbor on 10 December 1941.
Type 91 history

: Chronological Table
: 1931Type 91 aerial torpedo is put into service, production begins.
: 1936Revision 1. Self-detachable wooden plates are introduced.
: 1937Launch-tests at with wooden damper.
: 1939Revision 2 starts production. Not running true after water entry is identified as a major problem.
: 1941Revision 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''.
: 1941Revision 3 starts production.
: 1942
Indian Ocean raid,
Battle of the Coral Sea
The Battle of the Coral Sea, from 4 to 8 May 1942, was a major naval battle between the Imperial Japanese Navy (IJN) and naval and air forces of the United States and Australia. Taking place in the Pacific Theatre of World War II, the battle ...
,
Battle of Midway
The Battle of Midway was a major naval battle in the Pacific Ocean theater of World War II, Pacific Theater of World War II that took place on 4–7 June 1942, six months after Japan's attack on Pearl Harbor and one month after the Battle of t ...
,
Battle of the Santa Cruz Islands
The Battle of the Santa Cruz Islands, fought during 25–27 October 1942, sometimes referred to as the Battle of Santa Cruz or Third Battle of Solomon Sea, in Japan as the Battle of the South Pacific ( ''Minamitaiheiyō kaisen''), was the fourt ...
. 2 August: Type 91 torpedo technology reaches
Nazi Germany
Nazi Germany, officially known as the German Reich and later the Greater German Reich, was the German Reich, German state between 1933 and 1945, when Adolf Hitler and the Nazi Party controlled the country, transforming it into a Totalit ...
via
IJN sub I-30
: 1943Revision 5 starts production.
: 1944
Battle of the Philippine Sea
The Battle of the Philippine Sea was a major naval battle of World War II on 19–20 June 1944 that eliminated the Imperial Japanese Navy's ability to conduct large-scale carrier actions. It took place during the United States' amphibious r ...
,
Aerial Battle of Taiwan-Okinawa.
Initial development
Rear Admiral
Seiji Naruse led the team in charge of the initial development of the Type91 aerial torpedo at the
Yokosuka Naval Arsenal
was one of four principal naval shipyards owned and operated by the Imperial Japanese Navy, and was located at Yokosuka, Kanagawa, Yokosuka, Kanagawa Prefecture on Tokyo Bay, south of Yokohama.
History
In 1866, the Tokugawa shogunate govern ...
. 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.
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 Type91 aerial torpedo to be his great achievement.
Delayed development
At the beginning of 1934, ''Kan-Pon'' or the
Imperial Japanese Navy Technical Department
The was the externally operating division of the Ministry of the Navy of Japan responsible for the administration of naval vessel construction. From 1923 onward, it took on the role of a research institution for the research and development of n ...
, 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 Type93 ''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 Type94 torpedo and even ordered a halt to production of the Type 91. This significantly delayed the development schedule of the Type91 and frustrated the project members.
Wooden tail stabilizers added
The project team developed Kyoban wooden aerodynamic stabilizer plates for the Type91's tail fins as ''revision1'' 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 the following year.
The original Type91 was considered to have a frail body, and so this was strengthened in a new model in 1938 known as ''revision2''.
Anti-rolling controller developed
Type 91 aerial torpedoes won admiration for their effective anti-
rolling
Rolling is a Motion (physics)#Types of motion, type of motion that combines rotation (commonly, of an Axial symmetry, axially symmetric object) and Translation (geometry), translation of that object with respect to a surface (either one or the ot ...
controller and
acceleration
In mechanics, acceleration is the Rate (mathematics), rate of change of the velocity of an object with respect to time. Acceleration is one of several components of kinematics, the study of motion. Accelerations are Euclidean vector, vector ...
control system. Before the anti-rolling controller was introduced, the early versions of the Type91 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 100m 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. 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.
The specification for the launch speed of aircraft was increased from 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.
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 Type91 and it went into final testing in August 1941, making practical the use of aerial torpedoes both in rough seas and in shallow waters. It enabled the Type91rev.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.
Increase in explosive weight
The anti-rolling controller also made it possible for the Type91 to carry a heavier warhead. The Type91rev.1 warhead weighed with a high explosive charge of , but the rev.2 warhead weighed with of high explosive. Warhead rev.7, which was carried by twin-engine bombers, weighed and boasted a high explosive charge of ; this was designed to pierce the reinforced armour plates of the latest
US Navy
The United States Navy (USN) is the naval warfare, maritime military branch, service branch of the United States Department of Defense. It is the world's most powerful navy with the largest Displacement (ship), displacement, at 4.5 millio ...
ships.
Production
The Type 91 was researched and developed at
Yokosuka Naval Arsenal
was one of four principal naval shipyards owned and operated by the Imperial Japanese Navy, and was located at Yokosuka, Kanagawa, Yokosuka, Kanagawa Prefecture on Tokyo Bay, south of Yokohama.
History
In 1866, the Tokugawa shogunate govern ...
in
Kanagawa Prefecture
is a Prefectures of Japan, prefecture of Japan located in the Kantō region of Honshu. Kanagawa Prefecture is the List of Japanese prefectures by population, second-most populous prefecture of Japan at 9,221,129 (1 April 2022) and third-dens ...
. It was first produced at the Mitsubishi-Urakami Ordnance Works division of
Mitsubishi Heavy Industries
is a Japanese Multinational corporation, multinational engineering, electrical equipment and electronics corporation headquartered in Tokyo, Japan. MHI is one of the core companies of the Mitsubishi Group and its automobile division is the prede ...
. Later, the Imperial Japanese Navy established two manufacturing sites: Suzuka Naval Arsenal in
Mie Prefecture
is a Prefectures of Japan, prefecture of Japan located in the Kansai region of Honshu. Mie Prefecture has a population of 1,781,948 () and has a geographic area of . Mie Prefecture is bordered by Gifu Prefecture to the north, Shiga Prefecture an ...
; and Kawatana Naval Arsenal, a branch of
Sasebo Naval Arsenal
was one of four principal naval shipyards owned and operated by the Imperial Japanese Navy.
History
The Sasebo Naval District was established at Sasebo, Nagasaki in 1886, as the third of the naval districts responsible for the defense of the ...
, in
Nagasaki Prefecture
is a Prefectures of Japan, prefecture of Japan, mainly located on the island of Kyūshū, although it also includes a number of islands off Kyūshū's northwest coast - including Tsushima and Iki. Nagasaki Prefecture has a population of 1,246,4 ...
. The Mitsubishi-Urakami Ordnance Works plant at Kawatana specialized in torpedo production and was destroyed by
the atomic bomb dropped on Nagasaki.
Technology transfer to Germany
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 Type91 aerial torpedoes on
Japanese submarine ''I-30'' (a large cruiser type submarine) which arrived in
Lorient
Lorient (; ) is a town (''Communes of France, commune'') and Port, seaport in the Morbihan Departments of France, department of Brittany (administrative region), Brittany in western France.
History
Prehistory and classical antiquity
Beginn ...
on August2, 1942.
It was designated the ''Lufttorpedo LT 850'' in German service. The weight of the LT 850 German version was somewhat lighter at , with a length.
Germany wished to acquire the knowledge behind the
Imperial Japanese Navy Air Service
The (IJNAS) was the air arm of the Imperial Japanese Navy (IJN). The organization was responsible for the operation of naval aircraft and the conduct of aerial warfare in the Pacific War.
The Japanese military acquired its first aircraft in ...
's aerial torpedo technology in order to more effectively attack the Allied transport ships steaming in the
Mediterranean Sea
The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean basin and almost completely enclosed by land: on the east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern Eur ...
. It had previously imported Italian-made aerial torpedoes, which became unavailable following the Italian
Armistice of Cassibile
The Armistice of Cassibile ( Italian: ''Armistizio di Cassibile'') was an armistice that was signed on 3 September 1943 by Italy and the Allies, marking the end of hostilities between Italy and the Allies during World War II. It was made public ...
with the
Allies
An alliance is a relationship among people, groups, or states that have joined together for mutual benefit or to achieve some common purpose, whether or not an explicit agreement has been worked out among them. Members of an alliance are calle ...
in September 1943. The indigenous German aerial torpedo designs were badly restricted in launch speed and launch altitude.
Variants
The Type 91 (modification 2), was a shallow-water
aerial torpedo
An aerial torpedo (also known as an airborne torpedo or air-dropped torpedo) is a torpedo launched from a torpedo bomber aircraft into the water, after which the weapon propels itself to the target.
First used in World War I, air-dropped torped ...
that was designed for and used in the
attack on Pearl Harbor
The attack on Pearl HarborAlso known as the Battle of Pearl Harbor was a surprise military strike by the Empire of Japan on the United States Pacific Fleet at Naval Station Pearl Harbor, its naval base at Pearl Harbor on Oahu, Territory of ...
in 1941. Wooden fins and a softwood breakaway nose cone were added to allow for launching into shallow water at low altitudes.
There were two versions in the Type 91 warhead rev.3, differing in designed maximum launch speeds.
Later, heavier models had a decreased range.
Further development
In spring 1944, the
Yokosuka
is a city in Kanagawa Prefecture, Japan.
, the city has a population of 373,797, and a population density of . The total area is . Yokosuka is the 11th-most populous city in the Greater Tokyo Area, and the 12th in the Kantō region. The city i ...
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 in diameter, long, weighing , and carrying a warhead. 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. However, the Type91 aerial torpedo project members did not regard it as a part of the Type91 series.
Post-war commemoration
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.
Type 91 torpedoes are currently displayed at the
Etajima school of Japan Maritime Self-Defense (the Maritime Self Defense Force 1stTechnical School) and Shimofusa Base. They are missing the roll rudders. An excavated Type 91aerial torpedo is preserved at the Resource Museum in
JGSDF Camp Naha, 1st Combined Brigade of The Western Army,
JGSDF, located in
Naha
is the Cities of Japan, capital city of Okinawa Prefecture, the southernmost prefecture of Japan. As of 1 June 2019, the city has an estimated population of 317,405 and a population density of 7,939 people per km2 (20,562 persons per sq. mi.). ...
city, Okinawa. It retains the original features. It was picked up as
unexploded ordnance
Unexploded ordnance (UXO, sometimes abbreviated as UO) and unexploded bombs (UXBs) are explosive weapons (bombs, shell (projectile), shells, grenades, land mines, naval mines, cluster munition, and other Ammunition, munitions) that did not e ...
by a bomb-disposal unit of the JGSDF. A captured Type91 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 Type93 Japanese Long Lance ship-launched torpedo.
See also
*
British 18-inch torpedo#Mark XII
References
Notes
Bibliography
* Privately printed book.
* Photographic print copies of Imperial Japanese Navy Action Reports.
* Ozawa is the designer of Ki-69.
* Seko was one of the last torpedo bombardiers of B6Ns.
*
* (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.
External links
A page with many statistics on Japanese WWII torpedoes.
{{DEFAULTSORT:Torpedo, Type 91
Torpedoes of Japan
World War II weapons of Japan
World War II naval weapons
Aerial torpedoes