propulsion

Propulsion refers to the means by which a vehicle is pushed or pulled to higher velocity. In physical terms, propulsion systems generate thrust, a force that counteracts resistance and produces forward motion. Thrust arises from momentum exchange: a propulsion device accelerates mass in one direction, producing an equal and opposite force on the vehicle. The efficiency of a propulsion system is often described by the specific impulse, which relates thrust to propellant consumption.

Propulsion methods can be grouped by how they generate thrust. Chemical propulsion uses chemical energy stored in fuel and oxidizer to rapidly produce high-temperature, high-pressure exhaust. Rockets fall into this category; air-breathing jet engines also use chemical energy but draw oxygen from the atmosphere. Electric propulsion uses electrical energy to accelerate propellant. Ion and Hall-effect thrusters, and other electrostatic or electromagnetic devices, provide high specific impulse but typically generate lower thrust, suiting spacecraft. External- or propellantless methods rely on momentum transfer with light or fields, such as solar sails (radiation pressure) and, in some concepts, magnetic or electric sails. For many vehicles, propulsion systems convert shaft power into thrust through propellers or water jets, or through wheels driving against a surface.

Applications range from aircraft, ships, and automobiles to spacecraft. In aviation, turbofan and turbojet engines are common; in naval use, diesel engines or gas turbines drive propellers or water jets. In rocketry and spaceflight, chemical rockets, electric propulsion, and experimental concepts enable maneuvering beyond Earth’s atmosphere. Propulsion remains a core area of engineering, balancing energy sources, thrust, efficiency, and mission requirements.