Gas-generator cycle

The gas-generator cycle, also referred to as the GG cycle, is one of the most commonly used engine cycles in bipropellant liquid rocket engines.

Propellant is burned in a gas generator (analogous to, but distinct from, a preburner in a staged combustion cycle) and the resulting hot gas is used to power the propellant pumps before being exhausted overboard and lost. Because of this loss, this type of engine is considered an open cycle (note other open cycles exist, e.g. the tap-off cycle or the expander bleed cycle).

The gas generator cycle exhaust products pass over the turbine's wheel(s) first. Then they are expelled overboard. They can be expelled directly from the turbine, or are sometimes expelled into the nozzle (downstream from the throat) for both a small gain in efficiency, and can serve as film cooling. An advantage of this cycle is the high pressure drop available to the turbine (GG chamber pressure down to ambient) for extracting work from the drive gas; at the cost of needing to be sparing with the total mass flow. For this reason, turbines in GG cycles are commonly of the impulse type, rather than the reaction turbines common in staged combustion cycles.

The main combustion chamber does not use these products. This explains the name of the open cycle. The major disadvantage is that this propellant contributes little to no thrust because they are not injected into the combustion chamber. The major advantage of the cycle is reduced engineering complexity compared to the staged combustion (closed) cycle.