## 1 Introduction  
Fire accidents are unpredictable and can spread quickly. They often cause significant damage to people and property. Therefore, the development of improved and effective fire suppression systems always attracts much interest. Automatic fire-extinguishing and sprinkler systems have been widely used and have proven their efficiency in indoor fire accidents. Nevertheless, in many cases, firetrucks and firefighters need to directly battle flames to preserve property and the lives of humans or animals. The firefighters always have to contend with multiple difficulties--for example, carrying heavy hoses and tools in and around heavy pieces of unsafe structures. The brutal conditions also put the firefighters at risk of loss of life or injury. Moreover, in some tricky conditions, such as fire accidents happening on a narrow bridge, a crowded harbor, or on the sea, it is extremely difficult for the fire trucks and firefighters to approach the location to do their mission. Over the past decade, unmanned vehicles or remotely controlled robots have been deployed in fire accidents, not only to assist but sometimes to replace the manpower in firefighting missions. Several intelligent firefighting systems with different shapes, features, and extinguishing agents have been developed for different situations. Generally, they can be classified into two main groups according to the method of approaching the flames: terrestrial systems and aerial systems. In chronological order, the terrestrial systems were the ones that were first studied and used in fire accidents in residential areas, industrial fields, and wildfires [1, 2, 3]. They can be ground vehicles or wheeled, legged, and even humanoid robots. Commercial firefighting robots [4; 5; 6; 7] are equipped with modern technologies that greatly increase their durability, capability, and feasibility. For instance, the first commercialized firefighting robots, Thermite RS1 and RS3 (Textron, Providence, RI, USA) [4], the AirCore TAF35 (MAGIRUS, Ulm, Germany) [5], and the Shark-robotic Colossus (Shark Robotics, La Rochelle, France) [6], are wheeled robots that carry water hoses and spaying nozzles to attack the fire. They also provide reconnaissance and situational awareness in high-risk areas through camera systems, so they can operate remotely. The power engines and continuous track configurations allow them to carry a huge amount of water, overcome obstacles, and traverse difficult terrain. This enables them to battle the fire from inside or at a closer distance, which is too dangerous for a firefighter. In addition, the Firefighting Robot System (Mitsubishi Heavy Industries, Ltd., Chiyoda City, Tokyo, Japan) [7] includes a firefighting robot equipped with onboard sensors, such as RTK-GPS (SoftBank Group, Minato City, Tokyo, Japan), LiDAR (Velodyne Lidar, San Jose, CA, USA), IMU (Imugene Limited, New South Wales, Australia), and odometry, and a hose extension accessory. The completed system has the ability to self-drive directly to the scene of a fire and perform the firefighting mission effectively. However, the large dimensions and heavy weight of the wheeled robots make them hard to operate in narrow environments or high locations. Animal-inspired robots--for instance, snake firefighting robots [8] and bug firefighting robots [9]--have been considered in these situations. The snake robot designed by [PERSON] et al. [8] works as a self-propeller fire hose that can crawl into a burning narrowed space and extinguish a fire. [PERSON] [9] proposed the idea of bug-inspired legged robots going around in nature to detect the location of fire ignition and suppressing it to prevent large-scale wildfires. In addition, [PERSON] et al. [10] proposed deploying the humanoid robot SAFiR (Robotiq, Levis, QC, Canada) for firefighting in naval vessels. The animal-inspired and humanoid robots are flexible, so they suit the missions in narrow environments or complex structures.