The most terrifying sound a submariner can hear is not the active “ping” of a sonar. It is the high-pitched “whine” of a torpedo going active.
Modern heavyweight torpedoes, like the US Mk 48 ADCAP or the Russian Futlyar, are smart, fast (50+ knots), and relentless. They have onboard computers that map the target’s hull, ignore decoys, and circle back for a second strike if they miss.
So, how does a submarine survive being hunted?
For decades, the strategy was “Don’t get found.” But in an era of ubiquitous sensors, detection is becoming inevitable. This has forced navies to develop a new “Shield” to counter the “Spear.” This article explores the cutting-edge world of Torpedo Defense Systems, from noisemakers that seduce missiles to “Anti-Torpedo Torpedoes” that shoot bullets with bullets.
The Old School: “Soft Kill” (Decoys)
The standard defense for the last 40 years has been “Soft Kill”—tricking the torpedo into hitting something else.
1. The Nixie (AN/SLQ-25)
If you look at the back of any US Navy ship or sub, you will see a small hole. Inside is the Nixie.
How it works: It is a towed decoy. The sub reels out a cable (thousands of feet long) with a “fish” (noise maker) at the end.
The Seduction: The Nixie emits loud acoustic signals that mimic the sound of the submarine’s propeller and engine.
The Goal: The torpedo hears the louder, juicier target (the Nixie), locks onto it, and detonates unharmed behind the sub.
2. Acoustic Jammers (Canto / ADC)
Modern torpedoes are smart enough to recognize a “looping tape” of noise. They check for Doppler shifts and physical size.
To counter this, navies use Acoustic Device Countermeasures (ADC) like the French naval group’s Canto.
The Confusion: Instead of just being loud, the Canto fills the ocean with hundreds of “ghost targets.” It uses “Dilution/Confusion” tactics. It creates an acoustic hall of mirrors. The torpedo’s computer gets overwhelmed trying to process 50 potential targets, causing it to run out of fuel before it finds the real one.
The New Frontier: “Hard Kill” (Interceptors)
What if the torpedo is too smart to be tricked?
You have to physically destroy it. This is Hard Kill.
The US “Tripwire” and CAT Program
The US Navy has spent years developing the Countermeasure Anti-Torpedo (CAT).
The Concept: It is a mini-torpedo (6 inches wide).
The Action: When a hostile torpedo is detected, the CAT is launched. It races toward the incoming weapon and detonates a small warhead directly on its nose.
The Challenge: Hitting a bullet with a bullet underwater is incredibly hard. Acoustic variance, water pressure, and the blinding speed of the engagement make this the “Holy Grail” of defense.
Status: The US Navy fielded a prototype on the carrier USS George H.W. Bush but withdrew it for further refinement due to reliability issues. However, the requirement remains critical.
The Russian Paket-E/NK
Russia is arguably ahead in this specific niche. The Paket-E/NK system is deployed on their newer steering corvettes and submarines.
Dual Role: It serves as both an offensive ASW torpedo and a defensive “Anti-Torpedo.” Russia claims it has a high success rate in intercepting incoming Mk 48s, though this has never been proven in combat.
Passive Defense: Armor and Chemistry
If the decoy fails and the interceptor misses, the last line of defense is the hull itself.
1. Anechoic Coating (The Tiles)
Most modern subs are covered in rubber tiles (up to 4 inches thick).
Absorption: They absorb the active sonar “pings” of the torpedo, reducing the range at which the torpedo can “lock on.”
Damping: They dampen the noise of the impact. While a direct hit usually dooms a sub, a “proximity detonation” might be survivable if the shockwave is sufficiently dampened.
2. Double vs. Single Hull
- Western Philosophy (Single Hull): US/UK subs have a high-yield steel pressure hull with internal ribs. This optimizes space but offers less protection against impact.
- Russian Philosophy (Double Hull): Russian subs (like the Kursk or Oscar class) have two hulls. A light outer hull and a strong inner hull, separated by water.
- Survival: A double-hull sub can technically survive a torpedo hit to the outer hull (as the gap acts as “spaced armor”), although the shock would likely shatter internal instruments.
Evasion Tactics: “Knuckles” and “Crazy Ivans”
Technology is useless without a captain’s nerve.
When a torpedo is in the water, the Captain orders “Flank Speed” and performs violent maneuvers.
- The Knuckle: The sub turns tight at high speed. This creates a vortex of swirling water (a knuckle) in its wake. This disturbed water reflects sonar, creating a temporary “false target” that can confuse the torpedo for a few critical seconds.
- Depth Excursion: Torpedoes have depth limits. A daring captain might dive below the torpedo’s crush depth (risking their own hull) or rush to the surface to hide in the noise of the waves.
The Future: Laser and Supercavitation
Navies are looking at Sci-Fi solutions.
Supercavitating Interceptors: Using the physics of the Russian Shkval torpedo (which travels at 200 knots inside a gas bubble), future interceptors could reach the incoming threat in seconds.
Blue-Green Lasers: While lasers lose power quickly in water, research is ongoing into high-energy blue-green lasers that could blind the optical or sonar seekers of torpedoes at short range.
Conclusion
The active defense of submarines is the final piece of the puzzle. For 100 years, the submarine relied solely on stealth. Today, with the ocean becoming “transparent” due to drone swarms and satellites, the ability to Fight the Torpedo is becoming just as important as the ability to hide from it.
As we move toward 2030, we will likely see submarines equipped with “Hard Kill” systems as standard, turning the underwater dogfight into a high-speed exchange of fire and counter-fire.
Disclaimer: Specific effectiveness of countermeasures like the CAT program is classified. Article uses publicly available defense contractor brochures and naval tactical journals.