propulsions

Propulsion is the act of creating motion by producing thrust that pushes a body in the opposite direction. It is fundamental to transportation and spaceflight, enabling aircraft, ships, cars, and spacecraft to overcome inertia, lift, drag, and gravity.

Historically, propulsion evolved from human and animal power to steam engines, and then internal combustion engines, which enabled powered flight and global shipping. In rocketry, propulsion relies on expelling reaction mass at high velocity to generate thrust, a principle described by Newton's third law and applicable from missiles to interplanetary probes.

Modern propulsion encompasses several families. Chemical propulsion releases energy through propellant combustion and provides high thrust for rockets and missiles. Mechanical propulsion converts energy into motion with engines, turbines, and propellers used in airplanes, ships, and some land vehicles. Electric propulsion uses motors and/or electric fields to accelerate propellant, and is dominant in most electric aircraft concepts and in-space thrusters such as ion and Hall-effect devices, which offer high specific impulse but require substantial power. Nuclear propulsion uses fission or fusion energy to heat propellants or drive electric systems, studied for ships and deep-space missions. Another approach uses photon pressure, as in solar sails, or other advanced concepts under research.

Key performance metrics include thrust, specific impulse, propellant mass flow, and overall efficiency. Thrust is the instantaneous force produced; specific impulse measures how effectively propellant is used over time; higher exhaust velocity generally improves performance but increases energy demands and system complexity.

Applications vary by domain. Air propulsion relies on turbofan and piston engines, with ongoing moves toward electrification and hybrid systems to reduce emissions. Marine propulsion uses diesel or gas turbines, with growing interest in electric and hybrid drives. Space propulsion combines chemical rockets for launch with electric or other advanced methods for in-space maneuvering and exploration.