Torpedo evasion tactics were designed for the submarine to depart the area before the torpedo got close enough to detect the submarine, often deploying decoys and jammers to distract or inhibit the torpedo’s sonar.
In this case, Plecas figured the air-dropped torpedo was doing a circle search, using both active and passive sonar to detect Kazan. If Plecas ordered ahead flank, Kazan’s main engines and screw would put an enormous amount of noise into the water, which was of little concern against submarine-launched torpedoes, which typically began their transit miles away. This wasn’t the case with the air-dropped torpedo. It was likely very close, and increasing Kazan’s speed to ahead flank would light up the torpedo’s passive sonar. Against a lightweight torpedo dropped nearby, playing possum worked better.
However, modern torpedoes also employed active sonar, and an object as large as Kazan would easily register as a valid sonar return, prompting the torpedo to travel toward it to investigate. Plecas needed a way to interfere with the torpedo’s active sonar without giving away the submarine was nearby to its passive sonar or the aircraft crew above, which meant ejecting a broadband sonar jammer wasn’t an option.
As Plecas searched for a solution, he recalled that during Kazan’s trip to the surface to cut away the tangled net, they had passed through a strong thermocline, with its lower boundary at sixty meters.
The thermocline was a thin layer of water where the temperature transitioned rapidly between the warm surface heated by the sun and the cold water beneath. Submarines used thermoclines to their advantage because the rapid temperature change bent sound waves as they traveled through the layer, reflecting the sound back toward its source like light reflecting off a window. Depending on the frequency and angle of the sound wave, some tonals wouldn’t make it through. If Plecas could place Kazan on the other side of the layer before the torpedo detected it, they’d have a chance.
“Hydroacoustic, Command Post. Report torpedo detection angle.”
“Command Post, Hydroacoustic. Spherical array detection angle is down ten degrees.”
The torpedo was below Kazan, which meant it was also below the layer.
“Steersman, all stop. Compensation Officer, hover to forty meters.”
Kazan’s main engines went silent as the Compensation Officer pumped water from the variable ballast tanks. As Kazan rose toward the surface, the questions in Plecas’s mind were — would they rise above the layer before the torpedo’s circle search lined up toward Kazan, and was the layer strong enough to deflect the torpedo’s sonar pings?
Plecas turned up the audio from the spherical array hydrophones. Unlike the powerful surface ship sonars that transmitted reverberating pings, the torpedo sonar had far less power and pinged at a higher frequency, which sounded more like a bird chirp. The strength of the chirps was steadily increasing — the torpedo was turning toward them.
“One hundred meters,” the Compensation Officer reported.
Kazan rose steadily, the torpedo chirps increasing in volume as well. When Kazan passed through the layer, the chirps faded. Plecas breathed a sigh of relief; if they couldn’t hear the torpedo, it couldn’t hear its return pings either.
The Compensation Officer flooded water back into the tanks, halting Kazan’s ascent.
“On ordered depth,” he reported. “Steady at forty meters.”
Now that Kazan was shallow and the torpedo on the opposite side of the layer, Plecas worried about detection from above. At forty meters, the top of Kazan’s sail was only twenty meters from the surface. However, the torpedo had likely been dropped by a P-8A maritime patrol aircraft, which lacked the magnetic anomaly detection equipment carried by its predecessor. With twenty meters of water between Kazan and the surface, it was most likely safe from visual detection as well.
It was quiet in the Command Post as Plecas and his crew waited — until the silence was broken by a report.
“Command Post, Hydroacoustic. Have regained the torpedo’s sonar.”
On the spherical array speaker, Plecas detected the faint chirp.
The torpedo was rising above the layer to take a look.
“Compensation Officer, flood all tanks at maximum rate. Hover to one hundred meters.”
Plecas ordered Kazan to just below the layer, the best place to hide from someone, or something, searching above it.
As Kazan dropped below the layer, the torpedo pings faded again. Thus far, they’d been lucky, vacating the water column the torpedo was searching in before they were detected.
Silence returned to the Command Post as Plecas and his crew waited tensely for indication the torpedo had locked on to their submarine. After a while, Plecas checked the Command Post clock. The torpedo had been dropped thirty minutes ago and had likely run to fuel exhaustion by now.
Erik Fedorov, Kazan’s First Officer, approached. “What now?” he asked.
“We wait. We convince the air crew that it was a false detection.”
65
USS NORTH CAROLINA
“Conn, Radio. Download in progress.”
Murray Wilson sat in the Captain’s chair in the Control Room, listening to the report. North Carolina had just arrived in its assigned operating area and had gone to periscope depth to download the latest round of naval messages, which Wilson hoped included information on Kazan’s location.
Submarines were the most effective platform against other submarines, but the detection range was still limited. Had Wilson been forced to build an effective ASW barrier across the Gulf with just submarines, it would have taken at least thirty. Thankfully, the P-8As had been quickly repositioned, establishing the desired barrier, and North Carolina had been assigned a station in the center of the second layer. However, the probability Kazan would travel through North Carolina’s operating area was low, and Wilson and his crew awaited queuing information.
That information finally arrived. A radioman handed Wilson the message board with the pertinent message on top. A P-8A had detected a contact classified as POS SUB Medium and dropped two torpedoes, neither of which homed to detonation. The P-8A in question was monitoring their sonobuoy field and hadn’t regained the contact, and had reclassified their initial detection as a false target.
This wasn’t a surprise to Wilson. Most people untrained in anti-submarine warfare had no appreciation for how difficult it was for surface and air crews to detect a submerged contact, much less verify it was a submarine. In the Falklands War between the United Kingdom and Argentina, for example, the British Navy expended more than one thousand torpedoes without sinking a single submarine. After the war, following an analysis of the Argentinian submarine movements, the British Navy reluctantly concluded that all one thousand attacks had been against false targets. A common joke after the war was that the British Navy had launched a torpedo against every whale fart in the ocean.
Submarines, however, had an advantage over their surface and air brethren. They operated in the same environment as their opponent, with better sensors and better trained crews when it came to acoustic processing. Below the surface, submarines operated blind and sonar was their lifeblood. While surface and air crews often couldn’t identify a submarine as friend or foe — they attacked anything submerged in their waterspace if they were weapons free — submarine crews could analyze down to the individual submarine class and sometimes even to the specific submarine if it had a unique tonal.