Terminal Run

Once the weapon reached the passive-approach speed of thirty knots, it began its snake-pattern search, a three-degree wiggle to port and a slow wiggle back to the target bearing and continuing on to a three-degree angle to starboard. While it was wiggling horizontally, the unit was wiggling vertically, the pattern looking like a gentle corkscrew in three dimensions. Since the nose cone transducer was highly directional and looked only forward, the corkscrewing motion generated a conical examination of the ocean, the vertex of the “search cone” a narrow six degrees. If the target was outside that flashlight beam of the search cone, the torpedo would miss, which was one of its few flaws, and which required the launching ship’s firecontrol solution to be highly refined. On a bad day, with a poor solution, the launching ship could select a widening of the search cone, but that would make the snake pattern more of a defined sine wave, and the torpedo would slalom in the direction of the target much slower.

As the unit spiraled to the target, the target noise moved in the acoustic cone of the passive broadband sonar transducer. The search phase had begun, the weapon sensitized to the target, looking for changes in the received sonar noises as it wiggled back and forth. There was a slight signal-to-noise phenomenon beginning at a point just to the right of the solution to the target — the solution had been slightly in error. The weapon turned slightly right and began a new snake-pattern search centered on the slight signal noise. The weapon wiggled right and the noise faded left. The weapon wiggled left and the noise faded right. In the vertical dimension the same thing happened. After three confirmed “left-to-right-tag reversals the unit had confirmed that it had acquired a valid target. The unit had reached the end of the search phase at the acquisition point. The final phase of torpedo flight was known in the Navy as “homing,” but had a more technical term in the design files of the DynaCorp defense contractor that had built the unit. The phase was known as the “terminal run.” During unit one’s terminal run it opened the throttle valve fully on the fuel feed to the propulsor, spinning the shaft to the maximum achievable until it sped up to sixty-two knots. It energized the active sonar system and ordered the sonar transducer to form a sound wave pattern, aping, except this was not the unsophisticated single tone of decades past, but a shark tooth pattern wave that started as a low growl and ascended over the next fractions of a second to higher octaves, to a bell tone and beyond to a whistle and ending in a screeching shriek. It de-energized and turned its function to listening. The sound wave came back, quite distorted, but intact. The computer reset the exact range to the target from the bearing of the passive noise and the new bearing and range from the active ping, forgetting all the erroneous data from the launching ship. Had the wire been connected, the torpedo would have informed the launching ship at this point that it was homing, but there was no continuity.

The weapon drove in, hearing another wave form transmitted in the ocean. This had to be coming from another torpedo, the sound wave subtly altered, with a slight notch in the wave pattern so that unit one could tell which wave was its own and which was the other unit’s. Since there was a unit two out there, the programming for target impact shifted. If unit one had been alone, it would have aimed for the geometric center of the target. But since there was a unit two out there, it was necessary to detect the shape of the target and hit it one-third of the way from the extreme end. Unit two would aim for the opposite end, avoiding the explosion fireball generated by unit one. In this way, two torpedoes would do real work rather than impacting at the same point, the explosion from the first simply fizzling the high explosive of the second unit harmlessly into the sea.

Unit one was closer now. It pinged out again with the shark tooth wave pattern and went silent, hearing the return waveform and perceiving the target in three dimensions. It aimed for the left third of the target, leaving the right two-thirds for unit two. Less than a second of transit time was left. It was almost time to fulfill its mission.