Thrust-to-weight ratio

Thrust-to-weight ratio is a dimensionless ratio of thrust to weight of a reaction engine or a vehicle with such an engine. Reaction engines include, among others, jet engines, rocket engines, pump-jets, Hall-effect thrusters, and ion thrusters – all of which generate thrust by expelling mass (propellant) in the opposite direction of intended motion, in accordance with Newton's third law. A related but distinct metric is the power-to-weight ratio, which applies to engines or systems that deliver mechanical, electrical, or other forms of power rather than direct thrust.

In many applications, the thrust-to-weight ratio serves as an indicator of performance. The ratio in a vehicle’s initial state is often cited as a figure of merit, enabling quantitative comparison across different vehicles or engine designs. The instantaneous thrust-to-weight ratio of a vehicle can vary during operation due to factors such as fuel consumption (reducing mass) or changes in gravitational acceleration, for example in orbital or interplanetary contexts.