An airbreathing engine takes in air from the surrounding atmosphere, compresses it, and mixes it with fuel for combustion. This differs from rocket engines, which must carry their own oxidizer. Jet engines, ramjets, and turboprops all fall into this category.
We can't use an airbreathing design for the upper stage since it will operate in a vacuum.
Airbreathing engines are highly efficient in the lower atmosphere but become useless once the air thins out at high altitudes.
Related terms
Ramjet
An airbreathing jet engine that uses the vehicle's forward motion to compress incoming air.
Turbojet
An airbreathing jet engine that generates all its forward thrust directly from high-velocity exhaust gas.
Scramjet
An airbreathing jet engine that operates with supersonic airflow through its combustion chamber.
Chemical rocket
A propulsion system that generates thrust through the exothermic reaction of chemical propellants.