View source for Type 91 torpedo

{{Short description|Aerial torpedo of the Imperial Japanese Navy}}
{{use dmy dates|cs1-dates=ly|date=January 2021}}
{{Multiple issues|
{{More footnotes needed|date=November 2017}}
{{Cleanup|reason=unpublished papers referenced; Japanese readers needed for citations verification|date=November 2017}}
}}
{{Infobox weapon
|name = Type 91 torpedo
| image = Type 91 torpedo.JPG
| image_size = 250
|caption = Type 91 torpedoes aboard an aircraft carrier.
|origin = Empire of Japan
|type = [[Aerial torpedo]]
|is_missile = yes
|is_explosive = yes
|service = 1931–1945
|used_by = Japanese Navy
|wars = [[World War II]]
|designer = Rear Admiral [[Seiji Naruse]] and his team
|design_date = 1930–1945
|manufacturer =
|unit_cost = 20,000 yen (in 1941)
|spec_label =
|weight = {{convert|848|kg|lb|abbr=on}}
|length = {{convert|5.270|m|abbr=on}}
|diameter ={{convert|45|cm|in|abbr=on}}
|velocity =
|range =
|max_range = {{convert|2000|m|yd|abbr=on}}
|filling_weight = {{convert|323.6|kg|lb|abbr=on}} high explosive, {{convert|235|kg|lb|abbr=on}} for warhead rev.3
|detonation =
|yield =
|engine = Wet-heater type, 8-cylinder radial engine
|engine_power = {{convert|200|hp|kW|abbr=on|order=flip}}
|payload_capacity =
|wingspan = {{convert|69|cm|in|abbr=on}} in the air, {{convert|66|cm|in|abbr=on}} in the water
|propellant =
|fuel_capacity = {{convert|4|L|usgal|abbr=on}}
|speed = {{convert|42|kn|kph|abbr=on|order=flip}}
|guidance =
|steering = Gyrocompass-guided vertical-rudder control system, gyroscope-guided anti-rolling controller system
|accuracy =
|launch_platform = Single-engine carrier-based attack aircraft, twin-engine land-based attack aircraft
|transport =
}}
The '''Type 91''' was an [[aerial torpedo]] of the [[Imperial Japanese Navy]]. It was in service from 1931 to 1945.  It was used in [[naval battles]] in [[World War II]] 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 [[PID controller|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 {{convert|180|kn|kph|abbr=on|order=flip}} at an altitude of {{convert|20|m|ft|abbr=on}}, but also in a power-glide torpedo-bombing run at the maximum speed of the [[Nakajima B5N]] 'Kate', {{convert|204|kn|kph|abbr=on|order=flip}}

The Type 91 torpedo was an {{convert|450|mm|in|abbr=on|disp=flip}} diameter torpedo, similar in size to other nations. There were five models put into service, with high-explosive warheads weighing from {{convert|213.5|to|526.0|kg|lb|abbr=on|sigfig=3}} with effective ranges from {{convert|1500|to|2000|m|yd|abbr=on}} at {{convert|42|kn|kph|abbr=on}}.

Since the Type 91 torpedo was the only practical aerial torpedo of the [[Imperial Japanese Navy]], it was simply known as the ''Koku Gyorai'' or "aerial torpedo". Surface warships and submarines used other types of torpedoes, namely the [[Type 93 torpedo|Type&nbsp;93]] and [[Type 95&nbsp;torpedo|Type 95]] respectively, while the [[Type&nbsp;97 torpedo]] was designed for use by [[midget submarine]]s.

==Specifications==
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 (53)|HMS ''Prince of Wales'']] and [[HMS Repulse (1916)|HMS ''Repulse'']], launched from [[Mitsubishi G4M]] "Betty" bombers in [[sinking of Prince of Wales and Repulse|an action]] in the [[South China Sea]] three days after Pearl Harbor on 10 December 1941.<ref>{{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}}</ref>

== Type 91 history ==
[[File:Mitsubishi G4M1 bombers fly low through anti-aircraft fire between Guadalcanal and Tulagi on 8 August 1942 (NH 97766).jpg|thumb|[[Mitsubishi G4M]]1s 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|thumb|right|Nakajima B5N2s making a torpedo attack at Santa Cruz on 26 October 1942]]
[[File:752nd Kokutai Nakajima B6N2 Tenzan in formation.jpg|thumb|[[Nakajima B6N]]2s in formation flight with torpedoes with box type tail stabilizers]]
[[File:B7A-Ryusei torpedo.jpg|thumb|[[Aichi B7A]] ''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. [[Attack on Pearl Harbor]], [[Sinking of Prince of Wales and Repulse|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 [[Japanese submarine I-30|IJN sub I-30]]<ref name="combinedfleet.com">{{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 }}</ref>
: 1943{{snd}}Revision 5 starts production.
: 1944{{snd}}[[Battle of the Philippine Sea]], [[Aerial Battle of Taiwan-Okinawa]].

=== Initial development ===
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}}

===Delayed development===
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}}

=== Wooden tail stabilizers added ===
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''.

=== Anti-rolling controller developed ===
Type 91 aerial torpedoes won admiration for their effective anti-[[Yaw, pitch, and roll|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}}

=== Increase in explosive weight ===
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 [[United States Navy|US Navy]] ships.{{Citation needed|date=November 2017}}
<!--
== Main components (Type 91.rev.2) ==
[[File:Type91 AerialTorpedo Rev3 StructuralDrawing.svg|360px|right|Type91_aerial torpedo rev3, structural drawing]]

=== Warhead ===
Length = {{convert|1460|mm|in|abbr=on}}

When a torpedo hits a ship, the inertia forces the initiator to thrust forward and ignite its high explosive. The high explosive in the warhead will not detonate unless initiated as designed. An aerial torpedo, released at an altitude of 100{{nbsp}}m, is falling at nearly [[Mach number|Mach]]{{nbsp}}0.5 on water entry and receives over 100[[gravitational acceleration|G]] at the hard impact on the water surface.{{Citation needed|date=November 2017}} The Type{{nbsp}}91 warhead had five reinforced bands on the front-bottom of the inner shell, lap welded in the shape of cut lower half star, or the superposition of the letter{{nbsp}}''T'' and the letter{{nbsp}}''Λ''.{{Citation needed|date=November 2017}}

=== Air chamber ===
L = {{convert|1068|mm|in|abbr=on}}

The air chamber is a thin-shelled cylinder made of nickel-chromium-molybdenum steel. This tough steel alloy was originally developed for the steel armor plate of battleships. The chamber is charged with highly compressed air at {{convert|175|–|215|atm|psi|abbr=on}}, which burns with fuel oil to produce the driving power. Its pressure drops to around {{convert|50|atm|psi|abbr=on}} while running {{convert|2000|m|yd|abbr=on}}.{{Citation needed|date=November 2017}}

=== Front float ===
L = {{convert|733|mm|in|abbr=on}}

The front float section has a pure water tank, a fuel oil tank and a depth meter. The depth meter is placed at the bottom of the section to detect the water depth. It detects the displacement level and controls the horizontal tail rudders' (or [[Elevator (aircraft)|elevators]])' accordingly, so that the torpedo maintains level running under water.{{Citation needed|date=November 2017}} The ''horizontal tail rudder controller'' is operated by the rod connection mechanism from the depth meter in the front float section.{{Citation needed|date=November 2017}} The horizontal tail rudders are locked at their uppermost position while the torpedo falls to the water surface.{{Citation needed|date=November 2017}}

=== Engine housing ===
L = {{convert|427|mm|in|abbr=on}}

This section is exposed to incoming water to help cool the engine. It has a starter, a ''Chowaki'' or [[pressure regulator]], a wet-heat chamber and a main engine. The ''starter'' starts controllers, one for vertical tail rudders, and another for roll rudders for anti-rolling in both side wing rudders.

The pressure regulator is called a ''Chowaki'' or ''harmonizing system''. It is a two-stage pressure regulator with twin pressure-tunable regulation valves. It steps down the pressure of compressed air at {{convert|215|to|50|atm|psi|abbr=on}} in the air chamber to a constant flow of high-pressure air at {{convert|10|atm|psi|abbr=on}}. While the air pressure is declining as the torpedo is running under the water, the pressure regulator feeds the constant high-pressure air to the engine intake aspirator and keeps the running speed constant at {{convert|43|kn|kph|abbr=on|order=flip}}.{{Citation needed|date=November 2017}}

The ''wet-heat chamber'' is made of heat-resistant steel. Type{{nbsp}}91 aerial torpedoes use a [[Torpedo#Wet-heater|wet-heater engine]] like almost all other torpedoes in World War II. The general wet heater burning method drastically improved the combustion efficiency of torpedo engines. It burns a mixed gas of fuel oil and the high-pressure air with a spray of pure water in the wet-heat block to produce burning steam gas which is fed to the engine. The high-pressure fuel oil gas is burnt at a temperature of {{convert|800|C|F|sigfig=1}}. The sprayed pure water mists into the combustion gas, which produces a vapour explosion, resulting in completely gasified fuel oil combustion.{{Citation needed|date=November 2017}}

The main engine is an 8-cylinder single-row radial [[reciprocating engine|piston engine]]. A single [[drive shaft]] runs to the tail and the screws. The main engine is started when the torpedo hits the water. A thick safety bolt is inserted into the starter when the weapon is loaded on an aircraft. The bolt is pulled out from the torpedo when it is released and remains underneath the fuselage of aircraft.{{Citation needed|date=November 2017}}

=== Rear float ===
L = {{convert|1002|mm|in|abbr=on}}

This rear float section has a machine oil tank, a rudder controller, an anti-rolling controller, and roll rudders on both sides. The ''machine oil tank'' is centre-mounted in the rear float section. The ''rudder controller'' is a general [[gyrocompass]] controlled system, which steers the vertical rudders to keep the longitudinal axis of the torpedo in the sensed direction straight. Both the vertical rudder controller and the anti-rolling controller had their own [[gyroscope]], which start rotating when the torpedo is released from an aircraft. Each gyro has dual ring support mechanisms to allow them to move freely.{{Citation needed|date=November 2017}}

=== Anti-rolling controller ===
{{multiple image |direction=vertical |width=220
|image1=Type91 AerialTorpedo MainRollController.svg |caption1=Type91 aerial torpedo, main roll controller
|image2=Type91 AerialTorpedo RollControllerMovement.GIF|caption2=Type91 aerial torpedo, roll controller movement
|image3=Type91 AerialTorpedo RollRudder.svg|caption3=Type91 aerial torpedo, roll rudder
}}

The anti-rolling controller is a gyro-controlled air valve system which steers the roll rudders (or ailerons) on both sides of a torpedo and is composed of a gyroscope, a main controller, and an output booster.

A spinning gyroscope senses the roll angle of the torpedo, and the controller then centers the roll by steering roll-rudders on both sides at an angle in the range of ±22.5°. The main controller controls two output air valves to steer and countersteer the roll rudders, according to the roll angle and its rate of change. It countersteers to correct the angle and its time derivative.{{Citation needed|date=November 2017}} The output booster or auxiliary valve has two inlets and two outlet ports. The output booster works as a pair of air shutoff valves. It is connected in cascade to the two output ports of the main controller. It switches on and off directly the two powerful high-pressure controlling air flows, one for clockwise twist and the other for counter-clockwise twist of the roll rudders. This is largely to ensure proper operation in heavy impact conditions.{{Citation needed|date=November 2017}}

=== Tail section ===
L = {{convert|530|mm|in|abbr=on}} (to the tip end of propelling screw hub)

[[Bevel gears]] drive coaxial contra-rotating double 4-bladed screws to propel the torpedo under the water and keep it running straight. The tail section has vertical and horizontal stabilizer fins in a cross. Each fin has a controlling rudder in aft. Horizontal fins and rudders have a wide span in a longitudinal direction and work proportionally, while vertical fins are small, and rudders have a very short span.{{Citation needed|date=November 2017}}

=== Screws ===
The [[Propeller|propeller screws]] were coaxial contra-rotating double screws, with 4 propeller blades each. Each screw was wrought from a cubical mass of SK chromium-molybdenum alloy steel into a bold cross shape and punched through the centre. Hammering punches of 1{{nbsp}}tons and 3{{nbsp}}tons shaped the 4 blades. The propeller sections were compactly designed to allow the front screw and the rear screw to be only 5&nbsp;mm apart.{{Citation needed|date=November 2017}}

=== ''Kyoban'' stabilizer plates ===
[[File:Type91 AerialTorpedo TailSection and AeroDynamicTailStabilizer.PNG|thumb|left|Type 91 aerial torpedo, tail section and aerodynamic tail stabilizer plates]]

The Type{{nbsp}}91's tail fins were fitted with ''Kyoban'' wooden aerodynamic stabilizer plates.  Introduced in 1936 these stabilized the torpedo in flight and helped to ensure the proper angle for water entry. The plates were designed to shear off on water entry, absorbing energy and preventing the torpedo from diving too deep. The aerodynamic wooden plates stabilized the torpedo in both the vertical and horizontal axes and provided drag to ensure the torpedo struck the water at or near the proper water entry angle despite the inevitable variations in drop altitudes and air speeds encountered in combat, and to reduce yaw during its fall.  However, it was the wooden fins added to the roll-control fins in front of the tail that allowed the torpedo to be used in the shallow water of Pearl Harbor. The roll-control fins would roll the torpedo upright after entry to the water, but were too small to control roll in the air. Adding breakaway wooden wings to these fins allowed the roll-control system to work in air, keeping the torpedo upright during its drop, which allowed the horizontal rudder to pull the torpedo out of its dive much faster. The structure was simple and worked well, as seen at Pearl Harbor, which was generally considered too shallow for aerial torpedoes prior to the attack.  The ''Kyoban'' was so effective the US Navy copied it for their Mark{{nbsp}}13 torpedo after observing it in action at the Battle of the Coral Sea. {{Citation needed|date=November 2017}}

Two versions of the ''Kyoban'' stabilizer were used: A box-shaped version for single-engine carrier-based torpedo-bombers [[Nakajima B5N]] and [[Nakajima B6N]] and a cross-shaped version For twin-engine land-based torpedo-bombers [[Mitsubishi G3M|G3M]], [[Mitsubishi G4M|G4M]], [[Yokosuka P1Y|P1Y]], and [[Mitsubishi Ki-67|Ki-67]].  The cross-shaped version used longer plates to lower drag resistance but needed more clearance under the fuselage. In the case of land-based torpedo-bomber aircraft, a plate was set inside the bomb bay to smooth the airflow, otherwise the vortex coming in the bomb bay would disturb the torpedo at release.{{Citation needed|date=November 2017}}

==Steering mechanism==
There are three separate steering systems:
* The full steering system: The vertical rudder system steers the torpedo left or right by switching the rudders to one of full-right, neutral or full-left in response to signals from the gyroscope. This system responds relatively slowly to deviations from the correct running direction.
* The proportional steering system: The horizontal rudder system changes the angle of the rudders to cause the torpedo to run at a deeper or shallower depth in response to signals from the depth meter. This system has a moderately rapid response to deviations from the appropriate running depth.
* The angular velocity steering system: The two roll-rudders switch to one of full-up, neutral or full-down in response to signals from the anti-rolling controller. When the controller detects the torpedo returning to the centre position, the system counter-steers the roll rudders in opposite directions. This system has a rapid response to deviations in running direction, and also serves to roll the torpedo upright after entering the water, so that the vertical and horizontal rudders provide the proper course corrections.
The three systems operate simultaneously to maintain the appropriate direction, depth and attitude of the torpedo while running.
-->

==Production==
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 [[Fat Man|the atomic bomb dropped on Nagasaki]].<ref>{{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 }}</ref>

==Technology transfer to Germany==
Germany approached Japan requesting the transfer of Japanese aerial torpedo technology. In a [[Yanagi missions|''yanagi'' mission]] the Imperial Japanese Navy sent the plans and a number of Type{{nbsp}}91 aerial torpedoes on [[Japanese submarine I-30|Japanese submarine ''I-30'']] (a large cruiser type submarine) which arrived in [[Lorient]] on August{{nbsp}}2, 1942.<ref name="combinedfleet.com"/> 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]].<ref>{{harvc |p=13 |author=Fumio Aikō |c=__ |in1=Ichikawa |in2=Kodaira |in3=Kawada |year=1985}}</ref> It had previously imported Italian-made aerial torpedoes, which became unavailable following the Italian [[Armistice of Cassibile]] with the [[Allies of World War II|Allies]] in September 1943. The indigenous German aerial torpedo designs were badly restricted in launch speed and launch altitude.{{Citation needed|date=November 2017}}

==Variants==

{| class="wikitable"
|+Type 91 Aerial Torpedo and Type 91 Warhead, operational models<ref>{{harvnb |Ichikawa| Kodaira |Kawada|1985|p=24}}</ref>
<!-- Header line -->
|-
! Main body
! Warhead<br>type
! Warhead<br>weight
! Speed
! Range
! Total Length
! Diameter
! Total Weight
! Head Length (m)
! Head Weight (kg)
! Comments
<!-- Row1 line -->
|-
| Type 91
| Type 91
| {{cvt|149.5|kg}}
| {{cvt|42|kn|km/h|order=flip}}
| rowspan="5" | {{cvt|2000|m|yd}}
| rowspan="7" | {{cvt|5.270|m}}
| rowspan="7" | {{cvt|450|mm}}
| {{cvt|784|kg}}
| 0.958
| 213.5
| –
<!-- Row2 line -->
|-
| Rev.1
| Rev.1
| {{cvt|149.5|kg}}
| {{cvt|42|kn|km/h|order=flip}}
| {{cvt|784|kg}}
| 0.958
| 213.5
| Supported shedding wooden tail-plates in 1936, first model considered for German {{ill|Deutsche Abwurfmunition des Zweiten Weltkrieges#Lufttorpedo (LT)|lt=LT 850|de}} version
<!-- Row3 line -->
|-
| Rev.2
| Rev.2
| {{cvt|204.0|kg}}
| {{cvt|42|kn|km/h|order=flip}}
| {{cvt|838|kg}}
| 1.158
| 276.5
| Body reinforced in 1938, anti-rolling controller added in 1941, 2nd version considered for German LT 850 version
<!-- Row4 line -->
|-
| Rev.3
| Rev.3
| {{cvt|235.0|kg}}
| {{cvt|42|kn|km/h|order=flip}}
| {{cvt|848|kg}}
| 1.460
| 323.6
| –
<!-- Row5 line -->
|-
| Rev.3
| Rev.3_rev.
| {{cvt|235.0|kg}}
| {{cvt|42|kn|km/h|order=flip}}
| {{cvt|848|kg}}
| 1.460
| 323.6
| Reinforced warhead
<!-- Row6 line -->
|-
| Rev.5
| Rev.3_rev.
| {{cvt|235.0|kg}}
| {{cvt|41|kn|km/h|order=flip}}
| rowspan="2" | {{Convert|1,500|m|yd|abbr=on}}
| {{cvt|848|kg}}
| 1.460
| 323.6
| Precision forging and stainless steel cast body
<!-- Row7 line -->
|-
| Rev.5
| Rev.7
| {{cvt|420.0|kg}}
| {{cvt|41|kn|km/h|order=flip}}
| {{cvt|1080|kg}}
| 1.900
| 526.0
| Warhead designed to breach the armor of US battleships
|}
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.<ref name="hellions">{{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}}</ref>

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]] 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}}

==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.<ref>{{harvc |p=278 |last=Ichikawa |first=Hidehiko |c=Postface |in1=Ichikawa |in2=Kodaira |in3=Kawada |year=1985}}</ref>

Type 91 torpedoes are currently displayed at the [[Naval Academy Edashima|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 [[Naha Airport|JGSDF Camp Naha]], 1st Combined Brigade of The Western Army, [[Japan Ground Self-Defense Force|JGSDF]], located in [[Naha, Okinawa|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}}

==See also==
* [[British 18-inch torpedo#Mark XII]]

==References==
{{reflist}}

==Notes==
{{notelist}}
{{citation style|date=August 2020}}

===Bibliography===
* {{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.

== External links ==
* [http://www.navweaps.com/Weapons/WTJAP_WWII.htm A page with many statistics on Japanese WWII torpedoes.]
{{WWIIJapaneseNavalWeapons}}
{{IJN}}

{{DEFAULTSORT:Torpedo, Type 91}}
[[Category:Torpedoes of Japan]]
[[Category:World War II weapons of Japan]]
[[Category:World War II naval weapons]]
[[Category:Aerial torpedoes]]