Rocket Engine Nozzle - Atmospheric Use

Atmospheric Use

The optimal size of a rocket engine nozzle to be used within the atmosphere is when the exit pressure equals ambient (atmospheric) pressure, which decreases with altitude. For rockets travelling from the Earth to orbit, a simple nozzle design is only optimal at one altitude, losing efficiency and wasting fuel at other altitudes.

Just past the throat, the pressure of the gas is higher than ambient pressure, and needs to be lowered between the throat and the nozzle exit via expansion. If the pressure of the jet leaving the nozzle exit is still above ambient pressure then a nozzle is said to be 'underexpanded'; if the jet is below ambient pressure then it is 'overexpanded'.

Slight overexpansion causes a slight reduction in efficiency, but otherwise does little harm. However, if the exit pressure is less than approximately 40% that of ambient then 'flow separation' occurs. This can cause jet instabilities that can cause damage to the nozzle or simply cause control difficulties of the vehicle or the engine.

In some cases it is desirable for reliability and safety reasons to ignite a rocket engine on the ground that will be used all the way to orbit. For optimal liftoff performance, the nozzle should be ambient at sea-level; however, if a rocket engine is primarily designed for use at high altitudes and is only providing additional thrust to another "first stage" engine during liftoff in a multi-stage design, then designers will usually opt for an overexpanded (at sea-level) design. This was the technique employed on the Space shuttle's main engines, which spent most of their powered trajectory in near-vacuum while the shuttle's two Solid Rocket Boosters provided the majority of the liftoff thrust.