Missiles using rotary detonation engines. Recently, Pratt&Whitney announced the completion of testing for its rotary detonation engine (RDE), marking the transition of this technology from the experimental phase to the engineering application phase. As the core technology of the US Department of Defense's "Gambit" project, the rotary detonation engine is considered a solution for future hypersonic missile power systems and will have a significant impact on the development of long-range precision strike weapons. Unlike traditional turbojet or turbofan engines that rely on "detonation" (subsonic combustion), the core of a rotary detonation engine lies in "detonation" - the combustion speed is above supersonic, resulting in a sustained high-pressure shock wave. Its working principle is as follows: in the annular combustion chamber, fuel and air are mixed and ignited, generating high-speed rotating detonation waves that circulate in the combustion chamber, thereby converting chemical energy into thrust. Due to the absence of complex mechanical structures such as compressor blades, the components of the rotary detonation engine are greatly reduced, resulting in a more compact structure. The technological breakthrough of rotary detonation engines is mainly reflected in three aspects: firstly, the improvement of thermal efficiency. The pressure peak generated by detonation combustion far exceeds that of traditional engines, and its combustion efficiency is higher, providing the possibility to extend the range of missiles. The second is volume and weight optimization. The simplified structure reduces the size and weight of the engine, allowing the missile to carry more fuel and payload. The third is the potential for hypersonic speeds. The characteristics of detonation engines make it easier for weapons to achieve flight speeds above 5 Mach, meeting the demand for "instant strike" on future battlefields. There are two main technical obstacles that need to be overcome before the launch of the rotary detonation engine product. One is precise control over the contact between fuel and air. The stable propagation of detonation waves requires fuel injection with microsecond level accuracy, and any mistake may lead to combustion interruption. The second is materials and manufacturing processes. The ultra-high temperature and high pressure environment inside the combustion chamber places extremely high demands on heat-resistant materials, and advanced technologies such as 3D printing are expected to become the key breakthrough. The rotary detonation engine is not the only hypersonic propulsion solution, but its advantages such as simple structure and controllable cost make it more suitable for large-scale equipment needs, and its future development is worth paying attention to. (New Society)