The force that propels a rocket forward is called thrust. A rocket produces thrust by accelerating exhaust gases in the opposite direction, and Newton’s third law gives the rocket an equal and opposite reaction force forward.
Rocket engines burn propellant in a combustion chamber, creating high-pressure gas. The nozzle directs and accelerates that gas backward. The resulting momentum change produces thrust on the engine and the rocket attached to it. Unlike an airplane propeller or jet intake, a rocket carries both its fuel and oxidizer, so it can operate in the vacuum of space.
Thrust is not the same as lift, drag, or weight. Lift is an aerodynamic force, drag resists motion through an atmosphere, and weight is the gravitational force on the rocket. During launch, thrust must exceed the rocket’s weight for the vehicle to accelerate upward, while drag usually adds another opposing force.
Rocket thrust is measured in newtons. It can vary during flight as propellant is consumed and engine conditions change. The word also applies to electric propulsion systems, which create much smaller thrust by ejecting charged particles at very high speeds.