Torpedo Data Computer - AbsoluteAstronomy.com

The Torpedo Data Computer (TDC) was an early electromechanical analog computer

Analog computer

An analog computer is a form of computer that uses the continuously-changeable aspects of physical phenomena such as electrical, mechanical, or hydraulic quantities to model the problem being solved...

used for torpedo

Torpedo

The modern torpedo is a self-propelled missile weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with it or in proximity to it.The term torpedo was originally employed for...

fire-control

Fire-control system

A fire-control system is a number of components working together, usually a gun data computer, a director, and radar, which is designed to assist a weapon system in hitting its target. It performs the same task as a human gunner firing a weapon, but attempts to do so faster and more...

on American submarine

Submarine

A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

s during World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

(see Figure 1). Britain, Germany

Kriegsmarine

The Kriegsmarine was the name of the German Navy during the Nazi regime . It superseded the Kaiserliche Marine of World War I and the post-war Reichsmarine. The Kriegsmarine was one of three official branches of the Wehrmacht, the unified armed forces of Nazi Germany.The Kriegsmarine grew rapidly...

, and Japan

Imperial Japanese Navy

The Imperial Japanese Navy was the navy of the Empire of Japan from 1869 until 1947, when it was dissolved following Japan's constitutional renunciation of the use of force as a means of settling international disputes...

also developed automated torpedo fire control equipment, but none were as advanced as US Navy's TDC. These nations all developed torpedo fire control computers for calculating torpedo courses to intercept targets, but the TDC added the ability to automatically track the target. The target tracking capabilities of the TDC were unique for submarines during World War II and set the standard for submarine torpedo fire control

Fire-control system

at that time.

The TDC was designed to provide fire-control solutions for submarine torpedo

Torpedo

launches against ship

Ship

Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

s running on the surface (surface warships used a different computer for their torpedo launches). The TDC had a wide array of handcranks, dials and switches for data input and display. To generate a fire control solution, it required inputs on

submarine course and speed, which were read automatically from the submarine's gyrocompass
Gyrocompass
A gyrocompass is a type of non-magnetic compass which bases on a fast-spinning disc and rotation of our planet to automatically find geographical direction...

and pitometer log
Pitometer log
Pitometer logs are devices used to measure a ship's speed relative to the water. They are used on both surface ships and submarines...
estimated target course, speed, and range information (obtained using data from the submarine's periscope
Periscope
A periscope is an instrument for observation from a concealed position. In its simplest form it consists of a tube with mirrors at each end set parallel to each other at a 45-degree angle....

, Target Bearing Transmitter, radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

, and sonar
Sonar
Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...

)
torpedo type and speed (type was needed to deal with the different torpedo ballistics)

The TDC performed the trigonometric

Trigonometry

Trigonometry is a branch of mathematics that studies triangles and the relationships between their sides and the angles between these sides. Trigonometry defines the trigonometric functions, which describe those relationships and have applicability to cyclical phenomena, such as waves...

calculations required to compute a target intercept course for the torpedo. It also had an electromechanical interface to the torpedoes that allowed it to automatically set the torpedo courses while they were in their tubes, ready to be launched.

The TDC's target tracking capability was used by the fire control party to continuously update the fire control solution to the torpedoes even while the submarine was maneuvering. The TDC's target tracking ability also allowed the submarine to accurately launch torpedoes even when the target was temporarily obscured by smoke or fog.

The TDC was a rather bulky addition to the sub's conning tower

Conning tower

A conning tower is a raised platform on a ship or submarine, often armored, from which an officer can con the vessel; i.e., give directions to the helmsman. It is usually located as high on the ship as practical, to give the conning team good visibility....

and required two extra crewmen: one as an expert in its maintenance, and the other as its actual combat operator. Despite these drawbacks, the use of the TDC was an important factor in the successful commerce raiding

Commerce raiding

Commerce raiding or guerre de course is a form of naval warfare used to destroy or disrupt the logistics of an enemy on the open sea by attacking its merchant shipping, rather than engaging the combatants themselves or enforcing a blockade against them.Commerce raiding was heavily criticised by...

program conducted by American submarines during the Pacific

Pacific War

The Pacific War, also sometimes called the Asia-Pacific War refers broadly to the parts of World War II that took place in the Pacific Ocean, its islands, and in East Asia, then called the Far East...

campaign of World War II. First-person accounts published on the American submarine campaign in the Pacific often cite the use of TDC.

Two upgraded

Greater Underwater Propulsion Power Program

The Greater Underwater Propulsion Power Program was initiated by the United States Navy after World War II to improve the submerged speed, maneuverability, and endurance of its submarines....

US Navy World War II-era fleet submarines ( and ) with their TDCs continue in service with Taiwan's navy and US Nautical Museum

San Francisco Maritime National Historical Park

The San Francisco Maritime National Historical Park is located in San Francisco, California, USA. The park includes a fleet of historic vessels, a visitor center, a maritime museum, and a library/research facility...

staff are assisting them with maintaining their equipment. The museum also has a fully restored and functioning TDC for the , docked in San Francisco.

History

The problem of aiming a torpedo

Torpedo

has occupied military engineers since Robert Whitehead

Robert Whitehead

Robert Whitehead was an English engineer. He developed the first effective self-propelled naval torpedo. His company, located in the Austrian naval centre in Fiume, was the world leader in torpedo development and production up to the First World War.- Early life:He was born the son of a...

developed the modern torpedo in the 1860s. These early torpedoes ran at a preset depth on a straight course (consequently they are frequently referred to as "straight runners"). This was the state of the art in torpedo guidance until the development of the homing torpedo during the latter part of World War II

World War II

. The vast majority of submarine torpedoes during World War II were straight running and these continued in use for many years after World War II. In fact, two World War II-era straight running torpedoes — fired by the British nuclear-powered submarine — sank the ARA General Belgrano

ARA General Belgrano

The ARA General Belgrano was an Argentine Navy light cruiser in service from 1951 until 1982. Formerly the , she saw action in the Pacific theater of World War II before being sold to Argentina. After almost 31 years of service, she was sunk during the Falklands War by the Royal Navy submarine ...

in 1982.

During World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

, computing a target intercept course for a torpedo was a manual process where the fire control party was aided by various slide rule

Slide rule

The slide rule, also known colloquially as a slipstick, is a mechanical analog computer. The slide rule is used primarily for multiplication and division, and also for functions such as roots, logarithms and trigonometry, but is not normally used for addition or subtraction.Slide rules come in a...

s (the U.S. examples were colloquially called "banjo", for its shape, and "Is/Was", for predicting where a target will be based on where it is and was) or mechanical calculator/sights. These were often "woefully inaccurate", which helps explain why torpedo spreads were advised.

During World War II, Germany, Japan, and the United States each developed analog computer

Analog computer

s to automate the process of computing the required torpedo course.

In 1932, the Bureau of Ordnance

Bureau of Ordnance

The Bureau of Ordnance was the U.S. Navy's organization responsible for the procurement, storage, and deployment of all naval ordnance, between the years 1862 and 1959.-History:...

(BuOrd) initiated development of the TDC with Arma Corporation and Ford Instruments. This culminated in the "very complicated" Mark 1 in 1938. This was retrofitted into older boats, beginning with Dolphin

USS Dolphin (SS-169)

USS Dolphin , a submarine and one of the "V-boats", was the sixth ship of the United States Navy to be named for that aquatic mammal. She also bore the name V-7 and the classifications SF-10 and SSC-3 prior to her commissioning. She was launched on 6 March 1932 by the Portsmouth Navy Yard,...

and up through the newest Salmon

Salmon class submarine

The United States Navy Salmon-class submarines were an important developmental step in the design of the "Fleet Submarine" concept during the 1930's...

s.

The first submarine designed to use the TDC was , launched in 1940 with the Mark III, located in the conning tower

Conning tower

. (This differed from earlier outfits.) It proved to be the best torpedo fire control system of World War II

World War II

.

In 1943, the Torpedo Data Computer Mark IV was developed to support the Mark 18

Mark 18 torpedo

The Mark 18 torpedo was an electric torpedo used by the US Navy during World War II.The Mark 18 was built in competition to the Bureau of Ordnance electric torpedoes, which had been in development at the Newport Torpedo Station , Newport, Rhode Island, since the 1920s, in particular the Mark II,...

torpedo.

Both the Mk III and Mk IV TDC were developed by Arma Corporation (now American Bosch Arma).

The problem of aiming a straight-running torpedo

A straight-running torpedo has a gyroscope

Gyroscope

A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...

-based control system that ensures that the torpedo will run a straight course. The torpedo can run on a course different from that of the submarine by adjusting a parameter called the gyro angle, which sets the course of the torpedo relative to the course of the submarine (see Figure 2). The primary role of the TDC is to determine the gyro angle setting required to ensure that the torpedo will strike the target.

Determining the gyro angle required the real-time solution of a complex trigonometric

Trigonometry

equation (see Equation 1 for a simplified example). The TDC provided a continuous solution to this equation using data updates from the submarine's navigation sensors and the TDC's target tracker. The TDC was also able to automatically update all torpedo gyro angle settings simultaneously with a fire control solution, which improved the accuracy over systems that required manual updating of the torpedo's course.

The TDC enables the submarine to launch the torpedo on a course different from that of the submarine, which is important tactically. Otherwise the submarine would need to be pointed at the projected intercept point in order to launch a torpedo. Requiring the entire vessel to be pointed in order to launch a torpedo would be time consuming, require precise submarine course control, and would needlessly complicate the torpedo firing process. The TDC with target tracking gives the submarine the ability to maneuver independently of the required target intercept course for the torpedo.

As is shown in Figure 2, in general, the torpedo does not actually move in a straight path immediately after launch and it does not instantly accelerate to full speed, which are referred to as torpedo ballistic characteristics. The ballistic characteristics are described by three parameters: reach, turning radius, and corrected torpedo speed. Also, the target bearing angle is different from the point of view of the periscope versus the point of view of the torpedo, which is referred to as torpedo tube parallax. These factors are a significant complication in the calculation of the gyro angle and the TDC must compensate for their effects.

Straight running torpedoes were usually launched in salvo (i.e. multiple launches in a short period of time) or a spread (i.e. multiple launches with slight angle offsets) to increase the probability of striking the target given the inaccuracies present in the measurement of angles, target range, target speed, torpedo track angle, and torpedo speed.

Salvos and spreads were also launched to strike tough targets multiple times to ensure their destruction. The TDC supported the firing of torpedo salvos by allowing short time offsets between firings and torpedo spreads by adding small angle offsets to each torpedo's gyro angle. Before the sinking

ROKS Cheonan sinking

The ROKS Cheonan sinking occurred on 26 March 2010, when the Cheonan, a South Korean Navy ship carrying 104 personnel, sank off the country's west coast near Baengnyeong Island in the Yellow Sea, killing 46 seamen...

of South Korea

South Korea

The Republic of Korea , , is a sovereign state in East Asia, located on the southern portion of the Korean Peninsula. It is neighbored by the People's Republic of China to the west, Japan to the east, North Korea to the north, and the East China Sea and Republic of China to the south...

's ROKS Cheonan

ROKS Cheonan (PCC-772)

ROKS Cheonan was a South Korean Pohang-class corvette of the Republic of Korea Navy , commissioned in 1989. On 26 March 2010, it broke in two and sank near the sea border with North Korea...

by North Korea

North Korea

The Democratic People’s Republic of Korea , , is a country in East Asia, occupying the northern half of the Korean Peninsula. Its capital and largest city is Pyongyang. The Korean Demilitarized Zone serves as the buffer zone between North Korea and South Korea...

In 2010, the last warship sunk by a submarine torpedo attack, the ARA General Belgrano

ARA General Belgrano

, was struck by two torpedoes from a three torpedo spread.

To accurately compute the gyro angle for a torpedo in a general engagement scenario, the target course, range, and bearing must be accurately known. During World War II, target course, range, and bearing estimates often had to be generated using periscope observations, which were highly subjective and error prone. The TDC was used to refine the estimates of the target's course, range, and bearing through a process of

estimating the target's course, speed, and range based on observations.
using the TDC to predict the target's position at a future time based on the estimates of the target's course, speed, and range.
comparing the predicted position against the actual position and correcting the estimated parameters as required to achieve agreement between the predictions and observation. Agreement between prediction and observation means that the target course, speed, and range estimates are accurate.

Estimating the target's course was generally considered the most difficult of the observation tasks. The accuracy of the result was highly dependent on the experience of the skipper. During combat, the actual course of the target was not usually determined but instead the skippers determined a related quantity called "angle on the bow." Angle on the bow is the angle formed by the target course and the line of sight to the submarine. Some skippers, like the legendary Richard O'Kane, practiced determining the angle on the bow by looking at IJN

Imperial Japanese Navy

ship models mounted on a calibrated lazy Susan

Lazy Susan

A Lazy Susan is a rotating tray, usually circular, placed on top of a table to aid in moving food on a large table or countertop.- Origin :The term "Lazy Susan" made its first written appearance in a Good Housekeeping article in 1906, although their existence dates back to the 18th century...

through an inverted binocular barrel.

To generate target position data versus time, the TDC needed to solve the equations of motion for the target relative to the submarine. The equations of motion are differential equations and the TDC used mechanical integrators to generate its solution.

The TDC needed to be positioned near other fire control

Fire-control system

equipment to minimize the amount of electromechanical interconnect. Because submarine space within the pressure hull was limited, the TDC needed to be as small as possible. On World War II submarines, the TDC and other fire control equipment was mounted in the conning tower

Conning tower

, which was a very small space.
The packaging problem was severe and the performance of some early torpedo fire control equipment was hampered by the need to make it small.

TDC functional description

Since the TDC actually performed two separate functions, generating target position estimates and computing torpedo firing angles, the TDC actually consisted of two types of analog computers:

Angle solver: This computer calculates the required gyro angle. The TDC had separate angle solvers for the forward and aft torpedo tubes.
Position keeper: This computer generates a continuously updated estimate of the target position based on earlier target position measurements.

Angle solver

The equations implemented in the angle solver can be found in the Torpedo Data Computer manual. The Submarine Torpedo Fire Control Manual discusses the calculations in a general sense and a greatly abbreviated form of that discussion is presented here.

The general torpedo fire control problem is illustrated in Figure 2. The problem is made more tractable if we assume:

The periscope is on the line formed by the torpedo running along its course
The target moves on a fixed course and speed
The torpedo moves on a fixed course and speed

As can be seen in Figure 2, these assumptions are not true in general because of the torpedo ballistic characteristics and torpedo tube parallax. Providing the details as to how to correct the torpedo gyro angle calculation for ballistics and parallax is complicated and beyond the scope of this article. Most discussions of gyro angle determination take the simpler approach of using Figure 3, which is called the torpedo fire control triangle. Figure 3 provides an accurate model for computing the gyro angle when the gyro angle is small, usually less than < 30^o.

The effects of parallax and ballistics are minimal for small gyro angle launches because the course deviations they cause are usually small enough to be ignorable. U.S. submarines during World War II preferred to fire their torpedoes at small gyro angles because the TDC's fire control solutions were most accurate for small angles.

The problem of computing the gyro angle setting is a trigonometry problem that is simplified by first considering the calculation of the deflection angle, which ignores torpedo ballistics and parallax.
For small gyro angles, θ_Gyro ≈ θ_Bearing - θ_Deflection. A direct application of the law of sines

Law of sines

In trigonometry, the law of sines is an equation relating the lengths of the sides of an arbitrary triangle to the sines of its angles...

to Figure 3 produces Equation 1.
where

v_Target is the velocity of the target.

v_Torpedo is the velocity of the torpedo.

θ_Bow is the angle of the target ship bow relative to the periscope line of sight.

θ_Deflection is the angle of the torpedo course relative to the periscope line of sight.

Range plays no role in Equation 1, which is true as long as the three assumptions are met. In fact, Equation 1 is the same equation solved by the mechanical sights of steerable torpedo tubes used on surface ships during World War I and World War II. Torpedo launches from steerable torpedo tubes meet the three stated assumptions well. However, an accurate torpedo launch from a submarine requires parallax and torpedo ballistic corrections when gyro angles are large. These corrections require knowing range accurately. When the target range was not known, torpedo launches requiring large gyro angles were not recommended.

Equation 1 is frequently modified to substitute track angle for deflection angle (track angle is defined in Figure 2, θ_Track=θ_Bow+θ_Deflection). This modification is illustrated with Equation 2.
where

θ_Track is the angle between the target ship's course and the torpedo's course.

A number of publications state the optimum torpedo track angle as 110^o for a Mk 14 (46 knot weapon). Figure 4 shows a plot of the deflection angle versus track angle when the gyro angle is 0^o (i.e., θ_Deflection=θ_Bearing). Optimum track angle is defined as the point of minimum deflection angle sensitivity to track angle errors for a given target speed. This minimum occurs at the points of zero slope on the curves in Figure 4 (these points are marked by small triangles).

The curves show the solutions of Equation 2 for deflection angle as a function of target speed and track angle. Figure 4 confirms that 110^o is the optimum track angle for a 16 knots (31 km/h) target, which would be a common ship speed.

There is fairly complete documentation available for a Japanese torpedo fire control computer that goes through the details of correcting for the ballistic and parallax factors. While the TDC may not have used exactly the same approach, it was likely very similar.

Position keeper

As with the angle solver the equations implemented in the angle solver can found in the Torpedo Data Computer manual. Similar functions were implemented in the rangekeepers for surface ship-based fire control systems. For a general discussion of the principles behind the position keeper, see Rangekeeper

Rangekeeper

Rangekeepers were electromechanical fire control computers used primarily during the early part of the 20th century. They were sophisticated analog computers whose development reached its zenith following World War II, specifically the Computer Mk 47 in the Mk 68 Gun Fire Control system. During...

External links

USS Pampanito: Article on the Pampanito's TDC.
Torpedo Data Computer Mk IV
A. Ben Clymer: The mechanical analog Computers of Hannibal Ford and William Newell, IEEE Annals of the history of computing
US Torpedo History: Good description of operational use of the Mk 14, Mk 18, and Mk 23
Original Manual for the Torpedo Data Computer Mark 3, Historic naval ships association
Discussion of the torpedo ballistic and parallax corrections used by the Imperial Japanese Navy

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