Torpedo defense

Torpedo defense concerns the entire field of strategies, tactics, and practices which are intended to thwart the combat effectiveness of torpedoes. Torpedoes are subsurface naval weapons whose movement is limited to the underwater environment. As such, torpedoes are generally intended for attacking naval assets - broadly separated into surface ships, submarines, and potentially other waterborne assets (coastal installations such as floating docks, floating oil platforms, bridge foundations). The strategies for torpedo defense can dramatically differ for these categories, and the specific constituents thereof.

Such strategies include stealth, evasive maneuvers, passive defense like torpedo belts, torpedo nets, torpedo bulges, and sonar torpedo sensors, "soft-kill" active countermeasures like sonar decoys and sonar jammers, and "hard-kill" active defenses, like anti-torpedo torpedoes similar in idea to missile defense systems.^[1] Surface Ship Torpedo Defense and Countermeasure Anti-Torpedo systems are highly experimental and the US Navy ended trials on them in 2018.^[2]

As stated, defenses can be broadly classed as passive, active, and stealth-based. Further, the interdiction of a torpedo can be either classed as a soft kill or a hard kill. Soft kill refers to a partial or total reduction of the weapon's combat effectiveness, such as by distracting it with countermeasures or causing it to waste time until its propulsion system runs out of mobile endurance. Hard kill refers to a physical incapacitation of the weapon with damage.

As with all other defensive measures, torpedo defense falls under the categorization of the "survivability onion". The first lines of defense are always stealth and distance, ideally beyond any hope of engagement by the enemy. Secondary to these are speed and agility, the capacity to evade. These also include the emerging field of hard-kill countermeasures, which are kinetic interceptors designed to destroy or incapacitate the incoming torpedoes. The final lines of defense are various designs of armor, sheer size and bulk which can shrug off localized damage, and resiliency modifications such as distributed flotation compartments.

Stealth

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Stealth is one of the most effective ways to thwart a torpedo attack. It is the primary defensive asset of submarines, who rely upon it in order to survive. To that end, submarines can employ radical stealth-augmenting measures such as covering the entire hull in anechoic tiles and reducing self-noise to a minimum. By contrast, surface vessels are easily detected and tracked with electromagnetic signals, visually, and acoustically. The advantage of depth is greatest in regions of the ocean which are beyond the continental shelf, where sonar return from the ocean floor is faint and distant, and submarines are capable of diving to their operating depth. Out in the open of the high seas, where terrain cannot be used, stealth can be provided by other vessels; the noise and acoustic shadow of a vessel's hull can conceal a warship or submarine behind or beneath it. Acoustic homing torpedoes can be misled by other acoustic signatures which are audible to them. To this end, moving target simulators and noisemakers are employed. Maneuvers which generate acoustic anomalies (knuckles and vortices) can be an effective last-ditch defense against primitive acoustic homing torpedoes, but are largely useless against more advanced seeker heads which possess anti-countermeasure systems.

Changing the vehicle's depth grants the ability to pass through thermal layers, take advantage of acoustic shadows (regions of which sound waves tend to curve around), and conceal the vehicle's acoustic signature against nearby surfaces which mostly or partially reflect sound (the sea floor, the sea surface, thermal layers). The vehicle's acoustic signature can also be concealed by noise. Natural ambient noise during rough seas, other nearby vessels, active noisemakers, and noise sources which directly overlap the vehicle's signature (such as a running torpedo or mobile noise simulator directly inline with the hostile acoustic receiver) all contribute to such concealment. Modern computerized sonar, capable of isolating a frequency spectrum from a multiplexed acoustic signal, is far more difficult (or downright impossible) to trick with layered noise. If an attacker is actively guiding the weapon with a wire, especially when the wire also communicates sounds locally heard by the seeker head to the attacker, it is far more difficult to use stealth to defend against the incoming torpedo.

Soft-Kill Countermeasures

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Soft-kill torpedo countermeasures include a variety of decoys and jammers designed to deceive or disrupt the tracking systems of acoustic torpedoes. These countermeasures can be towed, stationary, or self-propelled, and typically rely on acoustic signals, magnetic signatures, or bubble screens to mislead incoming weapons. E.g.

• Bubble decoys: Create reflective noise via chemical or gas discharge (e.g., Bold, Sieglinde).
• Sonar jammers: Saturate acoustic sensors with disruptive noise (e.g., Siegmund, ADC).
• Signature decoys: Simulate ship acoustics or magnetic fields to lure torpedoes away (e.g., Foxer, Nixie, Maareech, MG-series).
• Mobile/swimming decoys: Self-propelled, often reusable systems simulating ship movement (e.g., Rafael's SCUTTER Mk 3, Mobile Submarine Simulator).
• Integrated systems: Combine sensors, jammers, and decoys in one suite (e.g., Leonardo C30/S, Ultra SSTD, Rafael's Torbuster).

Hard-kill Countermeasures

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Hard Kill Countermeasure anti-Torpedo (CAT) systems are generally divided into two categories: Antitorpedo Mines (ATM) and Anti-Torpedo-Torpedoes (ATT). Both types rely on Torpedo Detection, Classification, and Localization (TDCL) systems to identify incoming threats. E.g.

Torpedo Detection, Classification and Localisation (TDCL):

• Ultra Electronics DCL Technology Demonstrator programme
• Surface Ship Torpedo Defence
• Rafael's BlackFish/MonkFish HMS (DSIT)^[3]

Anti-Torpedo-Torpedo (ATT) systems:

• Rafael's TORBUSTER a fourth-generation torpedo defense countermeasure developed by Rafael Advanced Defense Systems. Designed to neutralize incoming torpedoes in both shallow and deep water, it combines soft- and hard-kill capabilities.^[4]
• The U.S. Navy’s Countermeasure Anti-Torpedo Torpedo (CAT), developed by Penn State’s Applied Research Laboratory. The CAT system was designed to counter wake-homing torpedoes, which detect a ship’s wake rather than its acoustic signature. The CAT torpedo operates autonomously after launch with “fire-and-forget” capability, using onboard sensors to search, target, and maneuver against threats. Developed under a high-priority program, CAT was produced within 24 months through extensive simulation-based design and modular construction, reducing costs significantly compared to legacy torpedoes. It was first deployed aboard the USS George H.W. Bush in 2013 and subsequently installed on additional vessels.^[5][6]
• The SeaSpider ATT system by ATLAS ELEKTRONIK has undergone successful live-fire trials. These tests involved detection and localization of incoming Mk 37-derived autonomous underwater vehicles (AUVs) and DM2A3 torpedoes using passive and active TDCL sonar systems. Upon detection, the SeaSpider interceptor was launched from an above-water launcher to intercept threats at their closest point of approach, with successful intercepts confirmed through acoustic and optical means.^[7]

Antitorpedo Mines (ATM)

• ATM systems are considered a low-cost, broad-spectrum defense solution. These systems deploy neutrally buoyant mines into a ship’s stern wake, where they maintain depth and detect incoming torpedoes using acoustic and magnetic sensors. The mines then detonate to disrupt torpedoes following predictable paths.^[8]
• In addition, the RBU-1000 anti-submarine rocket launcher is alleged to have anti-torpedo capability.

See also

• AN/SLQ-25 Nixie – sonar decoy that fakes passive, active and wake-homing returns