The nozzle exhaust air velocity for a turbojet engine is the speed at which the combustion products and air exit the engine’s nozzle. It is a crucial factor in determining the thrust produced by the engine and is influenced by the specific impulse of the engine.
The velocity of the exhaust gases at the nozzle exit for a turbojet engine, as per the rocket equation, can be expressed by adapting the basic rocket equation. The rocket equation is derived from the conservation of momentum and mass.
In the context of a turbojet engine, m0 includes not only the mass of the propellant (fuel) but also the mass of the air ingested by the engine.
The formula for calculating the nozzle exhaust air velocity using the rocket equation is:
Where:
- Â is the change in velocity of the rocket (in meters per second, m/s).
- Â is the specific impulse of the rocket engine (in seconds, s).
-  is the acceleration due to gravity at the Earth’s surface (approximately 9.81 m/s2).
- is the initial mass of the rocket (including propellant, air and structure, in kilograms, kg).
- Â is the final mass of the rocket (after propellant burnout, in kilograms, kg).