Terminal Run

The starboard unit had received all its power from the launching ship until the switch-over. The gyro was warm, the self-checks were complete, and the unit was ready to go. At the start-engine order the unit pressurized its fuel tank and popped open a valve to admit the self-oxidizing fuel to the combustion chambers. The spark plugs flashed electrical energy into the chambers, and the fuel ignited, the temperature and pressure soaring inside the combustion chambers. With nowhere to go, the pressurized gases in the combustion chambers pushed hard against the “B-end” hydraulic-type turbine, a series of pistons set within cylinders, the pistons connected to a tilted swash plate. The pressure of the gas pushing against the pistons in a desperate attempt to expand made the pistons start to move in the direction to make the trapped volume larger, which caused the plate assembly to rotate. When a piston reached the bottom of its travel it left the area where it was exposed to the high-pressure gas and allowed the combustion gas trapped inside to vent to the exhaust manifold, which led to the seawater outside the torpedo. The first few loads of exhaust gases blew the water out of the manifold and cleared the way for the next pulses of exhaust gas. The rotating canted swash plate continued to bring low-volume cylinders in contact with the combustion gases, which expanded against the pistons and added rotational energy to the plate assembly as the pistons were forced outward to the exhaust port. The engine revolutions sped up, the first circle taking a full half second, the next an eighth of a second, then rapidly faster until one revolution took less than a sixtieth of a second, or thirty six hundred RPM. The turning plate of the turbine spun a propulsor turbine set into the shroud outside the torpedo at the aft end. As the propulsor came up to full speed, the torpedo surged against the outriggers from the thrust of the propulsor, until, at the required thrust, the outriggers let go and the torpedo was released to fly through the water.

At first it had only the velocity of the launching ship, but under the massive jet thrust of the propulsor, the starboard Mark 58 Alert/Acute torpedo accelerated away from the outriggers, sped up to attack velocity of sixty knots, and ascended toward the layer depth. Unit one did not ping on sonar but only listened to its nose-mounted broadband transducer. In its wake it streamed a length of guidance wire, a dental-floss thin electrical signal wire that reeled out of the torpedo body at the same speed the torpedo traveled. The wire input was quiet, with no new orders coming from the mother ship. At this speed, nothing could be discerned on the passive sonar except for the flow noise around the nose cone

The firecontrol solution — a set of theoretical “answers” about where the target was located, what direction it was traveling and at what speed — had been locked into the computer memory the second prior to the start-engine order. The target was out there, nineteen miles away, hovering at speed zero, in the shallows above the layer depth. The torpedo sped on, counting the distance that it had traveled and subtracting that from the firecontrol solution’s range to the target to determine the distance remaining to the target, and also to count out the yards to the enable point.

The run-to-enable was the torpedo flight from the launching submarine to a point on its trajectory where it would arm the warhead and begin to start pinging active sonar, and to begin its snake-pattern wiggle to search for the target. The run-to-enable was sixteen miles. All systems were nominal, and the torpedo emotionlessly clicked off the distance from the firing ship.

The guidance wire suddenly lit up in an electronic flurry of new instructions. The firing ship changed the attack plan. No longer was the Mark 58 ordered to proceed to the enable point and activate active sonar during the search phase and to search at high speed, but was now ordered to slow at the enable point to thirty knots and search in listen-only passive mode. Only when the target was detected and fully acquired could the unit speed back up to attack velocity. The unit did the electronic version of shrugging, accepting the orders and replying back to the launching ship that the orders were received.

Eventually the torpedo reached the enable point, three miles from the target solution. Unit one slowed the propulsor to thirty knots by partially shutting the throttle valve at the fuel feed, the combustion chamber gas flow lowering, the turbine coasting down, the propulsor slowing. The weapon armed the PlasticPak explosive train, rotating heavy metal plates to align a passageway between the volatile low explosive and the relatively inert high explosive. The warhead was a software signal away from detonation.

At this point the signal wire went dead, the circuit no longer established to the launching ship. The unit was operating fully independently.