In October of this year, Indonesia's first domestically produced unmanned underwater vehicle launched a "piranha" light acoustic guided torpedo in deep sea areas. Also this year, according to the US Navy's 2026 fiscal year budget plan, it plans to invest $50 million to develop a containerized MK48mod7 acoustic self guided torpedo launch system codenamed "Liberator" and integrate it into unmanned surface vessels. The above two pieces of news, while confirming that acoustic guided torpedoes still have a "place" in modern naval warfare, once again draw people's attention to this ancient and evergreen weapon. It is said to be ancient because since the first acoustic guided torpedo was introduced, this type of weapon has a development history of more than 80 years; It is said to be evergreen because no matter how time changes, it remains active on the underwater battlefield. From the launch tubes of submarines, to the decks of surface vessels, to the wings of fighter jets, and even some unmanned platforms today, the presence of acoustic guided torpedoes can be seen. So, why have acoustic guided torpedoes always been favored by various countries? What is its current development status? In what directions will the future develop? Please refer to the interpretation in this issue. The reason why the Evergreen Self Directed Torpedo has become an evergreen tree in naval warfare weapons is due to its certain advantages in technology, tactics, and cost. From a definition perspective, as a type of torpedo, acoustic self guided torpedoes consist of a body, propulsion system, sonar system, guidance and control system, and warhead. In underwater combat, acoustic signals are received through the sonar system, analyzed by the guidance and control system to generate instructions, which drive the rudder to adjust the heading until it hits the target. They are known as "sound hunters". This guidance mechanism based on sound waves endows acoustic guided torpedoes with unique technological advantages. The propagation speed of sound waves underwater is about 1500 meters per second. Although it is lower compared to electromagnetic waves, its energy loss in water is smaller, and it can achieve long-distance signal transmission; Sound waves can effectively penetrate seawater, overcome the influence of underwater environment, and transmit information stably. Specifically, when it comes to acoustic guided torpedoes, since the guidance device is integrated inside the projectile, it generally does not require communication with the outside world, making it more covert and highly resistant to interference during operation. In combat operations, it can achieve "automatic enemy reconnaissance" by actively emitting sound waves for search or passively receiving target noise, and maintain high hit rates under complex hydrological conditions. The tactical value of acoustic guided torpedoes is mainly reflected in their ability to deal with various types of targets. From light acoustic guided torpedoes with a diameter of 324 millimeters to heavy acoustic guided torpedoes with a diameter of 533 millimeters or even 650 millimeters, most of them have dual functions of anti submarine and anti-ship. Especially with its strong destructive ability, when the acoustic guided torpedo explodes under the keel of a ship, it can not only generate high-temperature and high-pressure shock waves, but also form huge bubbles. The formation and rupture of these bubbles can cause the ship to repeatedly lift and fall, and even break the keel of the ship in an instant, causing fatal damage to the ship. For the development of weapons and equipment, the cost-effectiveness determines whether they can serve for a long time. Sound guided torpedoes have certain advantages in this regard. For example, the American MK48 series heavy torpedo, which mainly uses acoustic self guidance as its guidance method, has not undergone significant changes in its body design since its service in the 1970s. Most of its performance has been improved through software and hardware upgrades and iterations of sonar and guidance units. This upgrade mode of 'mostly stationary, locally active' makes the development cost of acoustic guided torpedoes much lower than that of developing a completely new weapon system. In recent years, countries have not slowed down the pace of research and development of acoustic guided torpedoes, but have instead utilized cutting-edge technology to achieve sustained development and present some new characteristics. Specifically, there are five aspects. Firstly, continuously strengthen detection and anti-interference capabilities. Modern acoustic guided torpedoes have generally enhanced their three-dimensional space detection capabilities, only to "see more clearly and hear more accurately" in complex underwater acoustic environments. For example, the German "Sea Cod" Mod4 torpedo uses conformal sonar array technology, effectively expanding the detection range of the sonar system and accurately constructing stereo acoustic images of the target. The upgraded "Sea Cod" mod 4 ER torpedo has stronger detection capability and longer range. Meanwhile, in response to acoustic decoys and other interference methods, broadband/multi band sonar systems have become standard equipment for acoustic guided torpedoes. This system is capable of transmitting and receiving a larger number of sound signals with a wider frequency range. By comparing the echo characteristics, it effectively identifies true and false targets. Secondly, the guidance and control system is highly digitized. Currently, many torpedo guidance and control systems have shifted to using high-speed digital signal processors and more advanced guidance algorithms. Taking the "Stingray" Mod2 acoustic self guided torpedo being developed in the UK as an example, one of its core upgrades is the installation of a brand new guidance and control system, which greatly improves the computing speed. This not only enables it to accurately recognize the target's voiceprint and effectively filter out background noise, but also allows it to autonomously conduct secondary searches and re attacks according to a preset program after the first attack fails. Thirdly, strive for high speed and quietness. Some advanced acoustic guided torpedoes optimize their fluid shape and use electric propulsion systems to significantly reduce navigation noise while ensuring higher speeds, making them less noticeable when approaching targets. The French F21 heavy acoustic self guided torpedo is like this, equipped with an aluminum silver oxide battery driven motor, which not only has a good level of noise reduction, but also can reach a maximum speed of 50 knots. In order to compress the opponent's reaction time and improve the terminal breakthrough capability, some acoustic guided torpedoes have begun to adopt a "dual speed system" design, which means silently sailing at an economical speed during the cruise phase, and when entering the attack terminal, activating backup energy or rocket boosters to launch a "charge" at a higher speed. Fourthly, a composite guidance mode is adopted. The traditional acoustic self guidance mode has a limited operating range, while the "wire guidance+acoustic self guidance" composite guidance method can partially solve this problem. In this regard, Sweden's SLWT light acoustic self guided torpedo is somewhat representative. It is connected to the launch platform through a fiber optic cable that is tens of kilometers long. In the early stage of the attack, operators transmit data through the fiber optic cable to real-time grasp the battlefield situation, correct the heading of torpedoes, and achieve precise control of the "human in the loop". When the torpedo approaches the target area, it cuts off the fiber optic cable and switches to acoustic self guidance mode for end attack. This composite guidance mode effectively extends the combat radius of torpedoes and improves strike accuracy. Fifthly, highlight the adaptability to complex marine environments. Traditional torpedoes are mainly designed for deep-sea and oceanic environments, and their performance is greatly reduced in shallow and nearshore areas with complex hydrological conditions. Therefore, countries have started to develop torpedoes with good coastal combat capabilities. The MU90 lightweight acoustic self guided torpedo jointly developed by Italy and France can adapt to waters up to 25 meters deep, effectively reducing the interference caused by strong ocean reverberation; The UK's "Sailfish" Mod1 heavy acoustic self guided torpedo has also improved its adaptability to complex marine environments, and can still stably track targets in coastal waters. Under the guidance of modern warfare characterized by intelligence, unmanned operation, and networking, the future prospects of acoustic guided torpedoes are accelerating their "evolution", and the following trends will emerge in future development. The level of intelligence continues to rise. The future acoustic guided torpedoes will not only be machines that execute programs, but also intelligent weapons with preliminary cognitive abilities. Through deep learning and artificial intelligence assistance, acoustic guided torpedoes will be able to autonomously learn and recognize the acoustic characteristics and maneuvering modes of various types of ships, and even predict the avoidance actions of targets based on the battlefield situation, dynamically optimizing attack paths. When encountering complex acoustic confrontations, intelligent acoustic guided torpedoes may be able to make autonomous decisions and respond to emergencies. Anti unmanned platforms have become a new choice. As unmanned surface vessels and unmanned underwater vehicles sail into the sea battlefield, they will inevitably become targets of acoustic guided torpedo strikes. These unmanned platforms are small in size, highly maneuverable, and have unclear acoustic characteristics, making it difficult for traditional heavy torpedoes to deal with them. In the future, there may be lightweight, highly maneuverable acoustic self guided torpedoes specifically designed to counter unmanned platforms. In June 2023, Russia exhibited UMT small torpedoes for the first time at the International Maritime Defense Exhibition. It is reported that the weight of this type of torpedo is about 100 kilograms, equipped with more accurate acoustic guidance equipment, and has a faster response speed. Modular design has been widely applied. The previously mentioned US Navy "Liberator" project is a manifestation of modular design, with its containerized launch system allowing any ship with a certain deck space to quickly acquire the ability to strike underwater targets. In the future, self guided torpedoes can quickly replace corresponding functional components like building blocks. For example, the United States has added an anchoring module to the MK54 light acoustic guided torpedo, allowing it to both anchor in water to detect information and "transform" back into a torpedo to strike. Or it may be able to achieve cluster based collaborative attacks. The success rate of single mine attacks will decrease when facing advanced soft and hard kill defense systems, and future acoustic guided torpedoes may adopt the "wolf pack" tactic. Relying on underwater communication networks, multiple acoustic self guided torpedoes can be launched to form a network and share target information. Some self guided torpedoes can be used as decoys to feign attacks, forcing enemy ships to maneuver or release interference, while other self guided torpedoes will silently wait on the flank or deep waters, capture vulnerabilities, and then conduct saturation attacks to enhance strike effectiveness. It can be foreseen that future acoustic guided torpedoes will develop with the development of the times, deeply integrated into underwater combat systems, and become important nodes in the underwater kill chain in conjunction with unmanned platforms and command system networks. (New Society)