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[[Image:TDCfullview.jpg|thumb|right|U.S. Navy Mk III Torpedo Data Computer, the standard US Navy torpedo fire control computer during World War II. Later in World War II (1943), the TDC Mk III was replaced by the TDC Mk IV, which was an improved and larger version of the Mk III.]]
The '''Torpedo Data Computer''' ('''TDC''') was an early [[electromechanical]] [[analog computer]] used for [[torpedo]] [[fire-control system|fire-control]] on [[US Navy|American]] [[submarine]]s during [[World War II]]. [[Royal Navy#1914–1945|Britain]], [[Kriegsmarine|Germany]], and [[Imperial Japanese Navy|Japan]] also developed automated torpedo fire control equipment, but none were as advanced as the [[US Navy]]'s TDC,<ref name=JapanGermany>{{cite book | last = Friedman | first = Norman | title = US Submarines Through 1945: An Illustrated Design History | year = 1995 | publisher = Naval Institute Press | isbn=1-55750-263-3 | pages = 195
</ref><ref name=OtherTrackers>While the TDC's target tracking abilities were unique for submarine torpedo fire control during WWII, target tracking was used on surface ship torpedo fire control systems by a number of nations (see references in this article to [http://www.maritime.org/doc/destroyer/ddfc/index.htm US destroyer] and [http://www.fischer-tropsch.org/primary_documents/gvt_reports/USNAVY/USNTMJ%20Reports/USNTMJ-200F-0086-0124%20Report%20O-32.pdf Japanese torpedo fire control] {{Webarchive|url=https://web.archive.org/web/20070720142332/http://www.fischer-tropsch.org/primary_documents/gvt_reports/USNAVY/USNTMJ%20Reports/USNTMJ-200F-0086-0124%20Report%20O-32.pdf |date=2007-07-20 }}). The TDC was the first analog computer to miniaturize the capability enough for deployment on a submarine.</ref>
Replacing the previously standard hand-held [[slide rule]]-type devices (known as the "banjo" and "is/was"),<ref>Beach, ''Run Silent, Run Deep''</ref> the TDC was designed to provide fire-control solutions for submarine torpedo firing against [[ship]]s running on the surface (surface warships used a different computer).<ref>http://www.maritime.org/doc/destroyer/ddfc/index.htm</ref>
The TDC was a rather bulky addition to the sub's [[conning tower]] and required two extra crewmen: one as an expert in its maintenance, the other as its actual operator. Despite these drawbacks, the use of the TDC was an important factor in the successful [[commerce raiding]] program conducted by American submarines during the [[Pacific war|Pacific]] campaign of World War II. Accounts of the American submarine campaign in the Pacific often cite the use of TDC.<ref name=clear>{{cite book | last = O'Kane | first = Richard | title = Clear The Bridge:The War Patrols of the U.S.S. Tang | publisher = Bantam Books | year = 1977 | ___location = New York | isbn= 0-553-14516-9
Two [[Greater Underwater Propulsion Power Program|upgraded]] World War II-era U.S. Navy fleet submarines ({{USS|Tusk|SS-426|6}} and {{USS|Cutlass|SS-478|2}}) with their TDCs continue to serve with [[Taiwan Navy|Taiwan's navy]] and [[San Francisco Maritime National Historical Park|U.S. Nautical Museum]] staff are assisting them with maintaining their equipment.<ref>{{cite web|url=http://www.maritime.org/taiwan/index.htm|title=Museum documents an operating US, WW II built submarine in Taiwan.|
==Background==
===History===
The problem of aiming a [[torpedo]] has occupied military engineers since [[Robert Whitehead]] 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]].<ref name=othertorps>There were other forms of torpedo guidance attempted throughout WWII. Notable are the Japanese human-guided ''[[Kaiten]]'' and German [[G7e#G7e/T3|pattern running]] and [[acoustic homing]] types for attacking convoys. Today, most submarine-launched torpedoes are wire-guided with terminal homing.</ref> 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.<ref name=USMk14his>{{cite web|url = http://www.geocities.com/Pentagon/1592/ustorp5.htm|title= Part Five: Post WW-II Submarine Launched/ Heavyweight Torpedoes|
During [[World War I]], computing a target intercept course for a torpedo was a manual process where the fire control party was aided by various [[slide rule]]s<ref name=fleetsub>{{cite web | title = Torpedo Data Computer | work = FleetSubmarine.com | year = 2002 | url = http://www.maritime.org/tdc.htm |
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During World War II, Germany,<ref>{{cite web |url=http://www.msichicago.org/exhibit/U505/virtualtour/photo_tour/contower.html |title=Archived copy |
In 1932, the [[Bureau of Ordnance]] (BuOrd) initiated development of the TDC with [[Arma Corporation]] and [[Ford Instruments]].<ref name="Holwitt, p.147">Holwitt, p.147.</ref> This culminated in the "very complicated" Mark 1 in 1938.<ref name="Holwitt, p.147"/> This was retrofitted into older boats, beginning with [[USS Dolphin (SS-169)|''Dolphin'']] and up through the newest [[Salmon class submarine|''Salmon'']]s.<ref name="Holwitt, p.147"/>
The first submarine designed to use the TDC was {{USS|Tambor|SS-198|2}},<ref name=Tambor>{{cite web | last = Mohl | first = Michael | title = Tambor (SS-198) | work = NavSource Online: Submarine Photo Archive | year = 2006 | url = http://www.navsource.org/archives/08/08198.htm |
In 1943, the Torpedo Data Computer Mark IV was developed to support the [[Mark 18 torpedo|Mark 18]] torpedo.<ref name=Mk18>The Mark 18 was electric and therefore wakeless and difficult for surface forces to trace. On the downside, it was slower than the Mark 14. This made it more difficult to aim accurately because larger gyro angles were involved. Even so, thousands of them were fired during WWII.</ref><ref name=clearMk18>{{harvnb|O'Kane|1977|p=221}}</ref>
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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.<ref name="GyroPointing">Torpedoes were developed by the United States with this capability during WWI. However, without automated fire control it was difficult to realize the full advantages of this approach.</ref> 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.<ref name = parallax>{{cite book |editor=Commander Submarine Force, United States Atlantic Fleet |title=Submarine Torpedo Fire Control Manual |
Straight running torpedoes were usually launched in salvo (i.e. multiple launches in a short period of time)<ref name="spread">{{harvnb|COMSUBATL|1950|loc=§ Definitions pp 1–9}}</ref> or a spread (i.e. multiple launches with slight angle offsets)<ref name="spread"/> 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.<ref name = doctrine>{{cite book | title = Current Submarine Doctrine | editor = Commander Submarine Force, Pacific Fleet |
[[Image:Torpedo Data Computer, interior.jpg|thumb|right|A look inside the TDC showing the motors driving the Position Keeper. <!-- Ref: http://www.maritime.org/doc/tdc/pg105.htm -->]]
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*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 [[Dick O'Kane|Richard O'Kane]], practiced determining the angle on the bow by looking at [[Imperial Japanese Navy|IJN]] ship models mounted on a calibrated [[lazy Susan]] through an inverted binocular barrel.<ref name="Okane">{{cite book | last = O'Kane | first = Richard H. | title = Wahoo: The Patrols of America's Most Famous World War II Submarine |
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.<ref name=CBC>{{cite web | last = Bromley | first = Allan | title = Analog Computing Devices | work = Computing Before Computers | year = 1990 | url = http://ed-thelen.org/comp-hist/CBC.html |
The TDC needed to be positioned near other [[fire-control system|fire control]] 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]], which was a very small space.<ref name=silent>{{cite video | people = Wise, Robert (Director-One scene shows how cramped a conning tower could be.) |date = 1958 | title = Run Silent, Run Deep | medium = Film | ___location = Pacific Ocean}}</ref>
The packaging problem was severe and the performance of some early torpedo fire control equipment was hampered by the need to make it small.<ref name=USSubHis>{{harvnb|Friedman|1995|p=350}}</ref> It had an array of handcranks, dials, and switches for data input and display.<ref>http://www.fleetsubmarine.com/tdc.html</ref> To generate a fire control solution, it required inputs on
*submarine course and speed, which were read automatically from the submarine's [[gyrocompass]] and [[pitometer log]]
*estimated target course, speed, and range information (obtained using data from the submarine's [[periscope]], [[Target Bearing Transmitter]] (TBT),<ref>{{cite web |url=http://www.bowfin.org/website/bowfin/bowfin_systems/TBT/tbt.htm |title=Archived copy |
*torpedo type and speed (type was needed to deal with the different torpedo ballistics)<!--This was accounted for by changing cams in the machine, but can't recall if the source is Blair, Grider, O'Kane, or Beach...or somewhere else...-->
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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.<ref name="clear"/><ref name = "wahoo"/> Figure 3 provides an accurate model for computing the gyro angle when the gyro angle is small, usually less than 30°.<ref name = SmallGyro>{{harvnb|COMSUBATL|1950|loc=§ "Theory of Approach and Attack", pp. 8-8, 8-9}}</ref>
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.<ref name = Doctrine>{{cite book | editor = Commander Submarine Force, Pacific Fleet | title = Current Submarine Doctrine |
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.<ref name = Deflection>{{harvnb|COMSUBATL|1950|loc=§ "Definitions", p. 1-2}}</ref>
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