page_title: Torpedo Data Computer
section_title: The problem of aiming a straight-running torpedo
hierarchical_section_title: Torpedo Data Computer / Background / The problem of aiming a straight-running torpedo
caption_attribution_description: English: The inner gears of that do the calculations on a Torpedo Data Computer. This photo is of the fully restored and operational TDC Mark III on the USS Pampanito, docked in San Francisco, CA
context_page_description: The Torpedo Data Computer was an early electromechanical analog computer used for torpedo fire-control on American submarines during World War II. Britain, Germany, and Japan also developed automated torpedo fire control equipment, but none were as advanced as the US Navy's TDC, as it was able to automatically track the target rather than simply offering an instantaneous firing solution. This unique capability of the TDC set the standard for submarine torpedo fire control during World War II.
  Replacing the previously standard hand-held slide rule-type devices, the TDC was designed to provide fire-control solutions for submarine torpedo firing against ships running on the surface.
  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 campaign of World War II. Accounts of the American submarine campaign in the Pacific often cite the use of TDC.
context_section_description: 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 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 of South Korea's ROKS Cheonan by North Korea in 2010, the last warship sunk by a submarine torpedo attack, the ARA General Belgrano in 1982, 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, speed, 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 h
caption: A look inside the TDC showing the motors driving the Position Keeper.
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