Not long ago, the British Navy's unmanned submarine, the Excalibur, made a public appearance and attracted attention from the outside world. It is said that the unmanned submarine adopts a modular design and can sail underwater for more than 30 days, performing various tasks such as intelligence reconnaissance, anti mine operations, underwater warfare, and underwater electronic warfare. To a certain extent, the Shenjian represents the current level of research and development of unmanned submarines by some advanced countries. Currently, in addition to the UK, countries such as the United States, Russia, France, Germany, and Australia are also advancing related projects. Unmanned submarines developed by some countries have been delivered to the military. As is well known, submarines have always been a top priority for underwater power construction in various maritime countries. Why have many countries launched research and development on unmanned submarines in recent years? What is the current development status of unmanned submarines in various countries? Where will the future develop towards? Please refer to the interpretation in this issue. The expansion of new technologies into underwater battlefields is an important component of naval equipment in various countries. Submarines play an irreplaceable role in anti submarine anti-ship and deterrence against the enemy due to their stealth in underwater navigation, and have always been known as the "ghost of the deep sea". However, with the development of modern technology, there are more and more anti submarine means, advanced anti submarine equipment, and increasingly powerful anti submarine forces. The risks faced by submarines in various countries during operations are also increasing. The rapid development and use of unmanned equipment have triggered a series of changes: drones frequently appear in the sky; On the ground, unmanned vehicles enter the battlefield; On the surface of the water, unmanned boats begin to shine; Underwater, unmanned vehicles continue to develop. Especially with the development of communication technology, autonomous control technology, underwater deployment and recovery technology, energy and power technology, and underwater detection technology, unmanned underwater vehicles of all sizes have begun to emerge and demonstrate their abilities in many aspects. In this context, researchers from various countries are starting to ponder whether it is possible to build unmanned submarines on the basis of unmanned underwater vehicles? This can not only leverage the existing advantages of submarines, but also alleviate the "pressure" on manned submarines in some aspects. This idea is obviously very attractive. Compared with manned submarines, unmanned submarines do not need to consider life support systems, which can save a lot of manpower and material resources; Unmanned submarines can perform missions for long periods of time in harsh environments without considering the pressure on crew members in extreme environments; Its volume is sufficient, it doesn't need to be too large, so its movements are more concealed; It is an unmanned system that can withstand damage even during wartime. Due to these advantages, unmanned submarines have quickly become a focus of research and development in some countries. Nowadays, the research and development of various countries not only makes this idea a reality, but also makes the definition of unmanned submarines clearer in practice. Essentially, unmanned submarines are a product of the expansion of new technologies into underwater battlefields. They are intelligent combat platforms that rely on unmanned driving, remote control, or automatic control to navigate and perform tasks underwater. As an important subcategory of unmanned underwater vehicles, unmanned submarines specifically refer to high-end and large unmanned underwater vehicles used for military purposes. It does not need to be connected to the mother ship through cables, but relies on pre-set programs and artificial intelligence to autonomously complete complex route planning, target recognition, and other tasks, with a high degree of autonomy and even a certain degree of decision-making power. Some can be equipped with weapons such as torpedoes and missiles, like manned submarines, to anti submarine, anti-ship, or strike other targets. The British Navy's Excalibur unmanned submarine is like this. It is 12 meters long, 2 meters wide, and has a displacement of 19 tons. It is equipped with an artificial intelligence control system and can rely on its own power to hide and lurk in key waterways or sensitive waters for weeks or even months. It continuously collects hydrological data and constructs underwater maps using sensors such as optoelectronics and acoustics, and has a certain degree of autonomous combat capability. Multiple countries are competing to carry out research and development. Currently, many countries are accelerating the development and deployment of unmanned submarines. For example, the Australian led development of the "Ghost Shark" unmanned submarine has entered the production stage, and the Royal Australian Navy may become the first customer of this unmanned submarine; The US Navy received its first "Orca" unmanned submarine by the end of 2023, and according to the plan, the number of this type of unmanned submarine delivered to the US Navy will reach more than 6 by 2027; Russia's unmanned submarines currently have the "harpsichord" series; The French ECA group is building the "Alister" series of unmanned submarines in an attempt to assist manned submarines in carrying out combat missions. During this process, the development of unmanned submarines has shown some distinct characteristics. Firstly, the "physique" of unmanned submarines is generally similar. Their length is generally between 10 and 20 meters, and their diameter ranges from 1 to 2 meters. This is because such a "physique" can better balance concealment, economy, and carrying capacity, especially with a large space to carry power combinations, sensors, communication devices, and mission payloads. Of course, there are also some countries developing larger unmanned submarines, some with a length of over 40 meters and some with a displacement of up to 300 tons, hoping to give unmanned submarines more powerful capabilities. Secondly, there are differences in terms of functionality. Due to differences in research and development capabilities and specific needs among countries, the functional positioning of unmanned submarines developed also varies. For example, the "Blue Whale" unmanned submarine developed by Israeli authorities is primarily designed to perform intelligence, surveillance, and reconnaissance tasks. Therefore, the submarine's underwater perception system has good performance and is adept at collecting intelligence on target waters, detecting enemy submarines, and mapping the seabed. For ease of deployment, its length is designed to be 10.9 meters, which can be loaded into containers and deployed using various mobile methods such as land, sea, or air. The US "Orca" unmanned submarine is positioned to independently perform tasks such as anti submarine, mine laying, intelligence surveillance and reconnaissance, anti-ship, anti mine, and electronic warfare. It has a length of 26 meters and a displacement of about 63 tons. It adopts a diesel electric hybrid power system and has a modular mission module that is 10 meters long and can carry a payload of 8 tons. It can flexibly replace reconnaissance equipment, mines, heavy torpedoes, cruise missiles, unmanned underwater vehicles, sonar hydrophone arrays, communication network nodes, etc. The most prominent capability of Russia's "replacer-V" unmanned submarine is to act as an "underwater decoy", which can mimic the acoustic and electromagnetic signal characteristics of various manned submarines of the Russian military and NATO, lure enemy anti submarine forces into ambush zones in actual combat, or cover up our own strategic nuclear submarines for operations. Of course, many unmanned submarines also adopt modular design to combine other functions in order to carry out diverse missions. Once again, unmanned submarines aim for the future battlefield from the beginning. It is not difficult to observe that although the development of unmanned submarines is relatively rapid, they are still in the "initial" stage. Taking the example of the British Navy's unmanned submarine, the Shenjian, although it was put into use in the "Protective Sabre" multinational joint exercise in July and successfully completed underwater intelligence gathering tasks, it has just completed port and sea tests. The US "killer whale" unmanned submarine has been delivered for several years, but it has not achieved significant results in the media, and it is likely that some functions are still being improved. The development of unmanned submarines in other countries is also basically in the testing stage. However, it is worth mentioning that almost all unmanned submarines have a high starting point, aiming for the future battlefield since their development. The Australian led development of the "Ghost Shark" unmanned submarine participated in the "Autonomous Warrior" 2024 exercise last year, partially demonstrating its ability to collaborate with other manned and unmanned platforms. The remote operation of the Shenjian unmanned submarine by British Navy personnel in Australia also reflects to some extent the potential of the vessel to support distributed warfare. The "replacer-V" unmanned submarine developed by Russia reflects the Russian military's concept of accelerating the coordinated operation of unmanned intelligent equipment and manned platforms. Some countries' researchers also consider the prospect of cluster use when developing unmanned submarines. If these ideas are transformed into practical capabilities, unmanned submarines may change the way underwater warfare is conducted. From the perspective of development trends, the functions of unmanned submarines are undergoing a transformation from "catching up" to "catching up" and then to "surpassing" manned submarines in certain aspects. The purpose of countries vigorously developing unmanned submarines is generally twofold: first, to make unmanned submarines serve as the "eyes and ears" of manned ships, becoming their "assistants"; The second is to enable unmanned submarines to have the ability to autonomously engage in forward combat and replace manned submarines in some aspects. To achieve this goal, research and development personnel from various countries still have a long way to go. Overall, the development of unmanned submarines in the future may strive to achieve breakthroughs in the following areas. One is to make up for some of the shortcomings exposed by current unmanned submarines. As a new underwater combat force, unmanned submarines still face many problems, such as using lithium battery packs as power, slow speed, and limited range; Some unmanned submarines have not yet reached the requirements of deep diving and ultra quiet operation, and need to develop and use new materials to make up for their shortcomings; The underwater positioning, navigation, and communication capabilities are limited, and more efficient methods need to be developed to improve them. Only by further addressing these issues can the performance of unmanned submarines be further improved and they can safely swim in the deep sea. The second is to make unmanned submarines more "intelligent". The integration of automation technology and artificial intelligence provides conditions for unmanned submarines to achieve autonomous operations. However, from the expectations of various countries for future unmanned submarines, it can only be seen that future unmanned submarines will be more "intelligent" in order to take on the responsibility of "underwater combat force multiplier". Therefore, future research and development personnel will continue to empower unmanned submarines, especially by integrating artificial intelligence and machine learning technologies, enabling them to achieve autonomous navigation, target recognition, and mission planning in complex marine environments, and even respond to emergencies and adjust tactics according to the battlefield situation. The third is to vigorously enhance the collaborative combat capability of unmanned submarines. Currently, in the development of some unmanned submarines, designers have considered their collaboration with manned platforms, and in some exercises, it has also reflected the attempts of relevant personnel to promote the "collaboration" between unmanned submarines and some unmanned platforms. However, this collaboration is currently at a lower level and can only be improved by multiple parties to meet future task requirements. In addition, achieving cross domain collaboration and cluster operations is the direction for future development. In the future, under increasingly complex combat backgrounds, unmanned submarines can only efficiently complete tasks by further integrating similar architectures and systems to achieve higher-level collaboration. The fourth is to continue improving battlefield survivability. Most current unmanned submarines adopt stealth design, especially some unmanned submarines that use optimized hull configuration and sound-absorbing and demagnetizing coatings. In future naval battles, the means to counter unmanned submarines will gradually increase, so it is necessary to further enhance the battlefield survivability of unmanned submarines. In this regard, the "Manta Ray" unmanned submarine led by the US Defense Advanced Research Projects Agency has a certain representativeness. By adopting a biomimetic shape, the mechanism is expected to achieve more effective stealth penetration. In short, with the development of artificial intelligence, new materials, and energy technologies, unmanned submarines are likely to transform from auxiliary equipment to an important combat force in the future ocean. The outcome of future naval battles may not only depend on traditional naval fleets, but also on these new "deep-sea ghosts" lurking underwater. (New Society)