why cant we use air breathing engines.
In rockets after achieving high speeds i.e first stage using solid propulsion and after achieving high mach using ram or scram jet.
In principle, you could use atmospheric air as oxidizer for the portion of the flight in the lower atmosphere where there is any decent density of oxygen. But it would add a lot of complexity because most of the kinetic energy is added long after the rocket is out of the air. So, basically, you would have to design, build and carry the mass of two separate oxidizer handling systems. The small benefit in reduced propellant just is not worth the extra expense.