Thrust

Thrust is the force applied to a body in the opposite direction of the exhaust flow, arising from the reaction described by Newton's third law. In propulsion, thrust propels a vehicle by expelling mass or deflecting it, thereby pushing the vehicle forward.

In physics terms, thrust results from a change in momentum of the propellant. For a device enclosing a control volume around the propulsion system, thrust equals the net rate of momentum leaving the volume plus the net pressure forces acting on its boundaries.

Common sources include rocket engines, jet engines (turbojets and turbofans), propellers, and ion thrusters. Thrust can be static (produced at zero forward speed) or dynamic (changing with speed and ambient conditions).

The general thrust equation is F = m_dot (v_out − v_in) + (P_out − P_amb) A_out. In many aircraft engines, inlet speed is small relative to exhaust velocity, making F ≈ m_dot v_e + (P_e − P_amb) A_e. In rocketry, where the exhaust velocity is very high, F ≈ m_dot v_e.

Thrust is distinct from power; the efficiency of propulsion is described by thrust-related metrics such as thrust-to-weight ratio and specific impulse. Design choices balance thrust, efficiency, mass, and operating conditions.