Torpedo Data Computer: Difference between revisions

Revision as of 03:01, 11 May 2020

Revision as of 09:25, 19 November 2020

Line 40:

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 |origyear=1950-02 |date=2006-04-16 |url= http://www.maritime.org/doc/attack/index.htm |pages=1–12 |chapter=Definitions |accessdate=2006-08-22 |ref=CITEREFCOMSUBATL1950}}</ref> 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)<ref name="spread">{{harvnb|COMSUBATL|1950|loc=&sect;§ 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 | origyear = 1944-02 | date = 2006-02-17 | pages = paragraph 4614 | chapter = Attacks -- General (Chapter IV, Section 1) | chapterurl = http://www.history.navy.mil/library/online/ss-doc-4.htm | url = http://www.history.navy.mil/library/online/sub_doctrine.htm | accessdate = 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 [[ROKS Cheonan (PCC-772)|ROKS ''Cheonan'']] by [[North Korea]] in 2010, the last warship sunk by a submarine torpedo attack, the [[ARA General Belgrano|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| accessdate = 2006-08-20| author = Nathan Decker | date = July 2005|archiveurl=https://web.archive.org/web/20070317172208/http://www.geocities.com/nmdecke/Submarines.html|archivedate=2007-03-17}}</ref>

Line 68:

*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.<ref name = positionkeeper>{{harvnb|COMSUBATL|1950|loc=&sect;§ "The Torpedo Fire Control Party", p. 4-2}}</ref>

====Angle solver====

Line 80:

[[Image:Intercept.svg|thumb|Figure 3: The torpedo fire control triangle]]

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=&sect;§ "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 | origyear = 1944-02 | url = http://www.history.navy.mil/library/online/sub_doctrine.htm | accessdate = 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) | chapterurl = 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=&sect;§ "Definitions", p. 1-2}}</ref>

For small gyro angles, {{math|''θ''_Gyro ≈ ''θ''_Bearing &minus; ''θ''_Deflection}}. A direct application of the [[law of sines]] to Figure 3 produces Equation {{EquationNote|1}}.

{{NumBlk|:|<math>\frac{\left \Vert v_{\mathrm{Target}} \right \| }{ \sin(\theta_{\mathrm{Deflection}}) } = \frac{\left \Vert v_{\mathrm{Torpedo}} \right \| }{ \sin(\theta_{\mathrm{Bow}}) } </math>|{{EquationRef|1}}}}

Line 93:

:{{math|''θ''_Deflection}} 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 [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=&sect;§ "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''_Track=''θ''_Bow+''θ''_Deflection}}). This modification is illustrated with Equation {{EquationNote|2}}.

Line 102:

[[Image:DeflectionAngle.png|thumb|Figure 4: Deflection angle versus track angle and target speed ({{math|1=''θ''_Gyro = 0°}}).]]

A number of publications<ref name = OptimumTrackAngle>{{harvnb|COMSUBATL|1950|loc=&sect;§ "Theory of Approach and Attack", p. 8-9}}</ref><ref name="Clear2">{{harvnb|O'Kane|1977|p=303}}</ref> state the optimum torpedo track angle as 110° 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° (''i.e.''., {{math|1=''θ''_Deflection=''θ''_Bearing}}).<ref name="track">Most work on computing intercept angles is done using track angle as a variable. This is because track angle is a strictly a function of the target's course and speed along with the torpedo's course and speed. It removes the complexities associated with parallax and torpedo ballistics.</ref> 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 {{EquationNote|2}} for deflection angle as a function of target speed and track angle. Figure 4 confirms that 110° is the optimum track angle for a {{convert|16|kn|km/h|0|sing=on}} target, which would be a common ship speed.<ref name = TargetSpeed>

{{harvnb|COMSUBATL|1950|loc=&sect;§ "Duties of the Fire Control Party", p. 5-25}}</ref>

There is fairly complete documentation available for a Japanese torpedo fire control computer that goes through the [http://home.comcast.net/~mbiegert/Work/HistOfTech/TDC/Model.htm 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.