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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 | orig-year = 1944-02 | date = 2006-02-17 | pages = paragraph 4614 | chapter = Attacks -- General (Chapter IV, Section 1) | chapter-url = http://www.history.navy.mil/library/online/ss-doc-4.htm | url = http://www.history.navy.mil/library/online/sub_doctrine.htm | archive-url = https://archive.today/20121212215116/http://www.history.navy.mil/library/online/sub_doctrine.htm | url-status = dead | archive-date = December 12, 2012 | access-date = 2006-07-02 }}</ref> 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 [[ROKS Cheonan sinking|sinking]] of [[South Korea]]'s {{ship|ROKS|Cheonan|PCC-772|6}} by [[North Korea]] in 2010, the last warship sunk by a submarine torpedo attack, ARA ''General Belgrano'' in 1982, was struck by two torpedoes from a three torpedo spread.<ref name=belgrano_attack>{{citation| url=http://www.geocities.com/nmdecke/Submarines.html| title = Submarines 1950-2000, a study in unused potential| access-date = 2006-08-20| author = Nathan Decker | date = July 2005|archive-url=https://web.archive.org/web/20070317172208/http://www.geocities.com/nmdecke/Submarines.html|archive-date=2007-03-17}}</ref>
[[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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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 | orig-year = 1944-02 | url = http://www.history.navy.mil/library/online/sub_doctrine.htm | archive-url = https://archive.today/20121212215116/http://www.history.navy.mil/library/online/sub_doctrine.htm | url-status = dead | archive-date = December 12, 2012 | access-date = 2006-08-19 | publisher = Department of the Navy | date = 2006-02-17 | id = USF 25(A) | pages = paragraph 4509 | chapter = Attacks -- General (Chapter IV, Section 1) | chapter-url = http://www.history.navy.mil/library/online/ss-doc-4.htm. }}</ref>
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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:{{math|''θ''<sub>Deflection</sub>}} is the angle of the torpedo course relative to the periscope line of sight.
Range plays no role in Equation {{EquationNote|1}}, which is true as long as the three assumptions are met. In fact, Equation {{EquationNote|1}} is the same equation solved by the mechanical sights of [https://web.archive.org/web/20060902191228/http://www.history.navy.mil/photos/images/h41000/h41761.jpg 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.<ref name = AccurateRange>{{harvnb|COMSUBATL|1950|loc=§ "Theory of Approach and Attack" p. 8-10}}</ref>
Equation {{EquationNote|1}} is frequently modified to substitute track angle for deflection angle (track angle is defined in Figure 2, {{math|1=''θ''<sub>Track</sub>=''θ''<sub>Bow</sub>+''θ''<sub>Deflection</sub>}}). This modification is illustrated with Equation {{EquationNote|2}}.
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