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. 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. Surface Ship Torpedo Defense and Countermeasure Anti-Torpedo systems are highly experimental and the US Navy ended trials on them in 2018.
As stated, defenses can be broadly classed as passive, active, and stealth-based. Strategies can be classed as reactive and proactive. 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

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 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, and conceal the vehicle's acoustic signature against nearby surfaces which mostly or partially reflect sound. 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 all contribute to such concealment. Modern computerized sonar, capable of isolating a frequency spectrum from a multiplexed acoustic signal, is far more difficult 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.

Speed

A vehicle's ability to move is among the simplest and most effective methods of defending against torpedoes, ever since the weapon's inception during the 19th century. For surface ships, reactive torpedo defense has consisted mainly of special high-speed maneuvers to avoid the inbound threat. If a torpedo is detected, it can be dodged, which means that the weapon also employs a measure of stealth in order to be functionally effective. This is especially true for torpedoes which are actually slower than the top speed of their targets. The original Whitehead torpedo of 1866 had a speed of a mere, slower than basically all steam-powered vessels of its era. The German G7e torpedo, employed in great numbers during the Second World War, had a top speed of ; homing versions of the same weapon lowered that to approximately since the weapon's seeker head was incapable of hearing the target at higher speeds. The G7e TIIId "Dackel" variant of the same was even slower, moving at in exchange for greatly increased endurance, since it was optimized for attacking convoys using a ladder search pattern. The Royal Navy Mark 20 Bidder torpedo, which remained in service until the late 1980s, had a speed of - this was slower than practically all surface warships, and even many civilian vessels. Against a weapon of this kind, escaping in a straight line would have been an effective strategy. Detection of an incoming weapon is paramount to evasion. The use of electric propulsion, or thermal propulsion which did not leave a surface bubble trail - such as the Japanese Type 93 or Type 95 - specifically served to thwart the target's capacity to detect the approach of the weapon.

Straight-Running Torpedoes

Torpedoes which run in a straight line are the oldest type, tracing their heritage back to the earliest specimens produced by Whitehead. These torpedoes run in a straight line, as best as their guidance systems allow. Straight-running torpedoes were the primary type used up until the end of the Second World War in 1945. Earlier propulsion technologies such as compressed air and compressed steam would leave a visible trail of bubbles on the surface; more advanced technologies, such as electric propulsion and high-test peroxide engines would not emit a surface trail. The emergence of advanced sonar technology has rendered straight-running torpedoes mostly obsolete, due to the great ease with which they can be dodged if their location and course can be determined. There are some modern examples of straight-running torpedoes which remained viable against surface targets during the Cold War, such as the RN Mark VIII, the USN Mark 14, the JMSDF Type 72, and the Russian VA-111 Shkval. The Shkval was originally an exclusively straight-running design, but is no longer such: it runs straight at high speed, then switches to a speed of approximately and activates an acoustic seeker head.
Straight-running torpedoes remain entirely viable against targets which are defenseless, motionless, or both. By their nature, straight-running torpedoes would be cheaper and easier to manufacture than ones with sophisticated guidance.
In order to defend against straight-running torpedoes, it is crucial to determine their location and direction of travel, at least approximately. If the torpedoes have been launched from a great distance, even a slight change in the speed or direction of travel of the target vessel can cause them to miss. Traveling perpendicular to the direction of the torpedoes' movement can be the quickest way to evade them; alternatively, traveling parallel to the torpedoes may be the viable course of action, if there are multiple incoming torpedoes in a spread or fan pattern. Such tactics were employed during the Second World War. If the torpedoes have been launched from a short distance, such as from a torpedo boat attacking a larger vessel, depending on the circumstances, little to nothing can be done. Maneuvers can be undertaken to affect the location of the impact, or potentially cause the impact to be at an acute angle, potentially causing the torpedo to malfunction and not detonate. Inertial fuzes cannot be thwarted with a glancing impact.
Straight-running torpedoes are almost totally useless against submarines, since such torpedoes are designed to operate at a single depth only, or at most two depths - the depth at launch, and a subsequent activation depth. They are almost totally dedicated for use against surface targets. Paradoxically, the only historically acknowledged sinking of one submarine by another while both were submerged was with straight-running torpedoes.

Pattern-Running Torpedoes

Historical examples of pattern-running torpedoes of the Second World War employed a ladder search pattern in order to facilitate lobbing a number of torpedoes at an enormous target, one much bigger than a solitary vessel, i.e. a convoy. The same strategies employed against straight-running torpedoes would have been effective, once the movement pattern was understood. In a modern sense, virtually all torpedoes possess acoustic homing, and their capacity to run a pattern is used in conjunction with homing to search for a target. In this sense, pattern-running torpedoes in the contemporary sense are a subtype of homing torpedoes, and the same strategies for defending against those would be applicable.

Soft-Kill Countermeasures">Sonar decoy">Soft-Kill Countermeasures

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. Such countermeasures include:
  • Bubble decoys: Create reflective noise via chemical or gas discharge.
  • Sonar jammers: Saturate acoustic sensors with disruptive noise.
  • Signature decoys: Simulate ship acoustics or magnetic fields to lure torpedoes away.
  • Mobile/swimming decoys: Self-propelled, often reusable systems simulating ship movement.
  • Integrated systems: Combine sensors, jammers, and decoys in one suite.