n a day of wire-guide directional control, inertial guidance and GPS positioning it is worthy of note that the basic gyroscope once was the sole guidance feature of a torpedo. During the Second World War the gyro played a pivotal role in submarine fire control accuracy. At the same time its malfunction cost the American submarine force the loss of several submarines while several more unexplained losses might well be traced back to gyro problems.

Gyros and depth control mechanisms came as a unit in Mark 14 and 15 torpedoes. Mounted on a single base platform, the pair of control devices rested in the torpedo's lower afterbody. Linkage arms extended aft to upper and lower rudders and stern planes.

Spinning at high speed by virtue of a small compressed-air turbine the gyro maintained a constant directional attitude in respect to its host torpedo. Resisting lateral moments, the torpedo was forced to turn to conform to the gyro's set course. The Torpedo Data Computer generated an electrical input into a servo-driven spindle which penetrated the torpedo tube and torpedo. When the torpedo left the tube its course was straight until the exploder arming impeller reached a distance to arm. At this point the gyro was uncaged and its pre-set course attitude was taken.

Since the gyro and depth mechanism was on the same mount, a short explanation of depth control is appropriate. A hydrostatic diaphragm was calibrated in feet below the surface. Depth determination was by spring tension attached to the diaphragm. Depth was set in the torpedo room by a spindle on the torpedo tube. To dampen pitch fluctuations a pendulum stabilized the linkage to the stern planes.

Information emanating from the gyro and depth mechanism were mechanical. Course and depth engines were small and alterations to rudder and planes were correspondingly limited.

Below is a Mk 12-3 gyro and depth mechanism in the Mk 15 torpedo. It is typical of the type used during the Second World War and after until more sophisticated guidance techniques were developed.