## 1 Introduction  
The use of remote-controlled robots and unmanned vehicles in hazardous environments has been attracting much interest. In such environments, human exposure would either represent a threat to life or a long-term health concern. In particular, in a firefighting mission, firefighters have to carry heavy hoses and tools over a significant distance and under extreme conditions of heat, smoke, and structures that could collapse at any moment. Over the past decade, terrestrial robots and multi-propeller aerial vehicles have been deployed in these missions. Unfortunately, many of their characteristics make their use may ineffective. In terms of terrestrial firefighting robots, the most preferable configuration is in the form of an all-terrain tracked utility vehicle. They usually have a highly durable yet customizable chassis so that they can carry different tools for different tasks: firefighting, heavy load transportation, taking measurements, protective water shield creation, etc. For solely firefighting purposes, the robots equip with a motorized water cannon and carry a fire hose conveying water from a fire hydrant. These robots can be operated remotely, battling flames from close distances or in difficult locations, tunnels and explosives depots, for example. Commercial products, such as Colossus and Rhyno Protect robots from Shark Robotics (La Rochelle, France) [1], Thermite robots from Howe & Howe Technologies (Waterboro, ME, USA) [2], and TEC800 from Angatec (Perigny, France) [3], have been developed and deployed. However, moving on uneven terrain, for example, a pile of rubble, is not easy, not to mention the robot also has to pull a large fire hose with water. For fire events in high places, water areas, and intricate structures, a terrestrial robot cannot approach. Then, the only solution is to shoot a powerful jet of water from a far distance, which is not only ineffective, but can also cause property damage. Other types of terrestrial firefighting robots have also been studied, such as legged robots [4; 5], snake robots [6], and even humanoid robots [7; 8], but practicability is still low. An aerial firefighting system can overcome the above problems easily. Hence, multi-propeller aerial vehicles have been used to carry firefighting tools and perform firefighting missions. The flight maneuverability and flexibility of the vehicles allow them to reach almost every fire location. The aerial systems also have a high-ground advantage over the flame, thus improving the fire suppression efficiency. They also vary in design, fire extinguishing material, and firefighting tactics. Numerous related problems have been studied [9; 10; 11; 12; 13], and prototypes have been tested [14; 15; 16; 17]. However, the operation of these systems is still limited in range and duration, and the size of the fire they can fight. For larger-scale fires, multiple systems must cooperate to perform the task efficiently. Especially, the airflow generated by the propellers may also blow the flames into neighboring areas. Some systems, such as [17], avoid this problem by using a long extinguishing nozzle and battling the flame from a distance. Eventually, this reduced the maneuverability of the system. High-speed rotating blades are also not a good choice in terms of reducing human injury or property damage. Therefore, in this study, we propose the use of novel firefighting systems that take advantage of aerial ones and overcome the limitation of the multi-propeller configuration. Instead, we use waterpower, not only to put out the fire, but also to propel and maneuver the systems. The systems are aimed to battle fire events in the body or border of water areas, such as on a narrow bridge, in a crowded harbor, and over a boat on the sea. The enormous amount of water in these areas allows water-powered systems to operate successfully. Meanwhile, it is difficult, or in some cases impossible, for human firefighters and other firefighting robots to access and perform their duties.