Definitions

Torpedo Data Computer

The Torpedo Data Computer (TDC) was an early electromechanical analog computer used for torpedo fire-control on American submarines during World War II (see Figure 1). Britain, Germany, and Japan 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 WWII and set the standard for submarine torpedo fire control at that time.

The TDC was designed to provide fire-control solutions for submarine torpedo launches against ships running on the surface (surface warships used a different computer for their torpedo launches). The TDC had a wide array of 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 and pitometer log
• estimated target course, speed, and range information (obtained using data from the submarine's periscope, target bearing transmitter, radar, and sonar observations)
• torpedo type and speed (type was needed to deal with the different torpedo ballistics)

The TDC performed the trigonometric 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 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 program conducted by American submarines during the Pacific campaign of WWII. First-person accounts published on the American submarine campaign in the Pacific often cite the use of TDC.

Two upgraded US Navy WWII-era fleet submarines (USS Tusk and USS Cutlass) with their TDCs continue in service with Taiwan's navy and US Nautical Museum staff are assisting them with maintaining their equipment. The museum also has a fully restored and functioning TDC for the USS Pampanito, docked in San Francisco.

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. The vast majority of the torpedoes launched during WWII were straight running torpedoes and these continued in use for many years after WWII. For example, the standard U.S. WWII torpedo remained in service until 1980 and still serves with foreign navies today. In fact, two WWII-era straight running torpedoes, fired by nuclear powered submarine HMS Conqueror sank the ARA General Belgrano in 1982, the last ship sunk by a submarine in combat to date.

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 rules (the U.S. examples were colloquially called "banjo", for its shape, and "Is/Was") and clever mechanical sights. During World War II, Germany, Britain, Japan, and the United States each developed analog computers to automate the process of computing the required torpedo course,). The first US submarine designed to use the TDC was the USS Tambor, which deployed in 1940 with the Torpedo Data Computer TDC Mk III(see Figure 1). In 1943, the Torpedo Data Computer Mk IV was developed to add support for the Torpedo Mk 18 and semi-automatic use of radar data. Both the TDC Mk III and Mk IV were developed by Arma Corporation (now American Bosch Arma).

The Problem of Aiming a Straight Running Torpedo

A straight running torpedo has a gyroscope-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 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 launching of torpedo salvos by allowing short time offsets between firings and torpedo spreads by adding small angle offsets to each torpedo's gyro angle. The last ship sank by a submarine torpedo attack, the ARA 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 WWII, 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

• estimate the target's course, speed, and range based on observations.
• use the TDC to predict the target's position at a future time based on the estimates of the target's course, speed, and range.
• compare the predicted position against the actual position and correct 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 ship models mounted on a calibrated lazy Susan 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 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 WWII submarines, the TDC and other fire control equipment was mounted in the 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 exact equations implemented in the angle solver have not been published in any generally available reference. However, the Submarine Torpedo Fire Control Manual does discuss 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 < 30o. 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. US submarines during WWII 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, θGyro ≈ θBearing - θDeflection. A direct application of the law of sines to Figure 3 produces Equation 1.

(Equation 1)
$frac\left\{left Vert v_\left\{Target\right\} right | \right\}\left\{ sin\left(theta_\left\{Deflection\right\}\right) \right\} = frac\left\{left Vert v_\left\{Torpedo\right\} right | \right\}\left\{ sin\left(theta_\left\{Bow\right\}\right) \right\}$
where
vTarget is the velocity of the target.
vTorpedo is the velocity of the torpedo.
θBow is the angle of the target ship bow relative to the periscope line of sight.
θDeflection is the angle of the torpedo course relative to the periscope line of sight.

Observe that 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 WWI and WWII. 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 accurately, 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, θTrackBowDeflection). This modification is illustrated with Equation 2.

(Equation 2)
$frac\left\{left Vert v_\left\{Target\right\} right | \right\}\left\{ sin\left(theta_\left\{Deflection\right\}\right) \right\} = frac\left\{left Vert v_\left\{Torpedo\right\} right | \right\}\left\{ sin\left(theta_\left\{Track\right\}-theta_\left\{Deflection\right\}\right)\right\}$

where

θTrack is the angle between the target ship's course and the submarine's course.

A number of publications state the optimum torpedo track angle as 110o for a Torpedo Mk 14 (46 knot weapon). Figure 4 shows a plot of the deflection angle versus track angle when the gyro angle is 0o (i.e. θDeflectionBearing). 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 110o is the optimum track angle for a 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 the exact same approach, it was likely very similar.

Position Keeper

As with the angle solver, the exact equations implemented in the position keeper have not been published in any generally available reference. However, 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.