Gas generator cycle is one of the most commonly used power cycle in a bipropellant liquid rocket engine. Let us understand the working of a typical Gas generator cycle based engine using the below diagram.
Let’s start from the top most part. Liquid fuel and Liquid Oxygen are stored in two separate tanks. These propellants are taken up by two pumps — Fuel pump and Oxidizer pump, which increases the pressure and temperature of the propellants by several times. After this both of the propellants are passed via a control valve which bifurcates the respective propellant line into two different channels.
One channel gets the majority component and goes downwards, while the other channel gets the minority component and goes towards “Pre-burner”. In the pre-burner, the fuel and oxidizer are ignited and the resulting hot exhaust is used to run the turbine which in turn runs the fuel and oxidizer pumps, thus completing the cycle. Finally the exhaust after passing through the turbine is completely discarded and thrown out. And since it is being thrown away, gas generator cycle is also called as “Open cycle”.
Great, now back to the topic.
Now, the fuel line which got the majority component is made to go over the nozzle to act as a coolant which reduces the temperature of the nozzle (and also warms up the fuel as a secondary effect). After passing over the nozzle, the warmed-up fuel is put in the combustion chamber.
On the other side, the majority channel of the oxidizer is directly fed into the combustion chamber where it meets the fuel and combustion takes place. The exhaust resulting from the combustion is made to pass through the typical de Laval nozzle and thrust is produced.
This is how a Gas generator cycle works.
The engines based on this cycle offers advantage of simplicity and lower weight compared to its counterpart — staged combustion cycle, in which the out put of the turbine is fed into the combustion chamber. That requires dealing with the high pressure inside the combustion chamber, leading to higher amount of plumbing and complexity.
The disadvantage of this cycle is lower fuel efficiency as the exhaust of turbine is being completely thrown away. This leads to lower specific impulse.
Do check the working of Raptor Engine, to understand another type of combustion cycle.