Thrust is the reaction force produced when a propulsion system accelerates a working fluid (air, combustion gases, ions) in one direction, propelling the vehicle in the opposite direction. Newton’s third law is the governing principle: the momentum imparted to the exhaust equals the momentum gained by the vehicle.
For a rocket engine:
F = ṁ × vₑ + (pₑ - p₀) × Aₑ
where ṁ is mass flow rate (kg/s), vₑ is exhaust velocity, pₑ is nozzle exit pressure, p₀ is ambient pressure, and Aₑ is nozzle exit area. The first term (momentum thrust) dominates; the second term (pressure thrust) is a correction for imperfect nozzle expansion.
For a propeller or turbofan:
F = ṁ × (v_exit - v_inlet)
The thrust comes from accelerating a large mass of air by a small velocity increment (propeller) or a smaller mass by a larger increment (turbojet).
Thrust versus drag
In steady level flight, thrust equals drag. To accelerate, thrust must exceed drag. To climb, thrust must exceed drag plus the component of weight along the flight path. The thrust-to-weight ratio determines whether a vehicle can hover (T/W > 1 for vertical flight), climb steeply, or merely sustain level flight.
Thrust in vacuum versus atmosphere
A rocket engine produces more thrust in vacuum than at sea level because the pressure thrust term (pₑ - p₀) × Aₑ increases as ambient pressure p₀ drops to zero. A Merlin 1D engine produces 845 kN at sea level and 981 kN in vacuum — a 16% increase. This is why rockets with multiple engines often use different nozzle expansion ratios for sea-level and vacuum stages.
Related terms
- Thrust-to-Weight Ratio — thrust relative to vehicle weight
- Specific Impulse — thrust per unit propellant flow rate, measuring propulsive efficiency
- Dynamic Pressure — the aerodynamic pressure that generates the drag thrust must overcome