Upper Stages
The upper stages of space launch vehicles are designed to operate at high altitude, and thus under little or no atmospheric pressure. This allows them to use lower pressure combustion chambers and still obtain near-optimum nozzle expansion ratios with nozzles of reasonable size. In many low pressure liquid rocket upper stage engines, such as the Aerojet AJ-10, propellants are pressure fed without need for complex turbomachinery. Low chamber pressures also generate lower heat transfer rates, which allow ablative or radiative cooling of the combustion chambers rather than more elaborate regenerative cooling.
Read more about this topic: Multistage Rocket
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