The engine cycle of a rocket engine describes the primary method chosen to transport propellants into the combustion chamber. Real rocket engines can have multiple different engine cycles active during their operation in sequence, for optimal performance at different thrust levels or during ignition and cut-off.
The goals of an engine cycle are, to deliver the right amount of propellants to the combustion chamber and do that at the right pressure for the injector. If the maximum pressure of the propellants at the injector is lower than the chamber pressure, the propellant cannot be injected into the chamber and combustion stalls.
The simplest engine cycle is the pressure-feed. It relies on the ullage pressure of the tanks to push the propellants into the combustion chamber. It is very robust and such an engine can easily be started and cut-off, which makes this cycle preferred for RCS and restartable main engines.
See also: Gas-generator cycle
The gas-generator cycles uses a gas generator and a turbine to drive a pump, which transports the fuel into the combustion chamber. The exhaust of the gas generator is vented outboards, contributing only little thrust. The gas-generator cycle has an upper limit for chamber pressure, as a higher chamber pressure requires more power for the pump, which means more fuel has to be burned inside the gas generator without producing thrust.
Staged combustion cycle
The propellants get partially combusted in one or more preburners and the exhaust products are routed through one or more turbines and into the main combustion chamber, where they are finally combusted. This cycle allows high chamber pressures and this very high specific impulse.