selfpropulsion

Self-propulsion refers to the ability of a system to generate motion using its own energy and internal actuation, without relying on external forces to move. In practice, this means converting stored energy into thrust or forward momentum by interacting with the surrounding environment, such as by expelling mass, pushing against a medium, or deforming in a way that yields net movement.

From a physics perspective, movement of a self-propelled object involves transferring momentum to the surroundings. Propulsion can occur by ejecting propellant (as in rockets and jet engines), by accelerating or deflecting surrounding fluid (as in watercraft or some underwater devices), or by pushing against a surface (as in wheels turning against the ground). At very small scales and low Reynolds numbers, propulsion often requires non-reciprocal actuation or chemical gradients, a constraint highlighted by the scallop theorem in fluid dynamics.

Examples of self-propulsion span both natural and engineered systems. Rockets and missiles propel themselves by expelling exhaust; aircraft use engines to accelerate air backward; ships and submarines rely on propellers or water jets; land vehicles move by pushing against the ground through wheels or tracks. Biological examples include bacteria and other microorganisms driven by rotating flagella or undulating bodies and sperm cells utilizing flagellar propulsion. Synthetic and robotic systems explore self-propulsion through chemical reactions, light- or magnetic-driven actuation, and deformable micro- or nanoswimmers.

Applications of self-propulsion encompass space launch and aviation, automotive and marine propulsion, and the development of autonomous robots and microscale devices designed to move without external tugs or tethers.