For a liquid-fueled rocket engine to work, you need to pump the fuel and oxidizer into the combustion chamber at the right quantity, pressure, and ratio. Handling this task is a turbopump, which requires a lot of power. In a modern staged combustion engine, this power is generated by what’s called a pre-burner. Take the BE-4 engine, for example: It has a single pre-burner mounted on a shaft that drives both the oxidizer and the fuel pumps.
When the engine ignites, the oxidizer pump routes a full flow of oxidizer through the pre-burner. Simultaneously, a small amount of fuel is introduced to create combustion and fire up the pre-burner. The resulting hot gas runs a turbine that powers the turbopump and, via piping, travels to the main combustion chamber. The oxygen-rich gas is then mixed with more fuel, combust, and boom — thrust.
While the BE-4’s design is relatively simple, it has its downsides. For instance, extra sealing is required to prevent the oxygen-rich gas from “leaking” back through the fuel line and prematurely igniting. This sealing makes the engine more maintenance-intensive. This is where Raptor’s full-flow staged combustion (FFSC) design comes into play. Instead of just one pre-burner, the Raptor has two: an oxygen-rich pre-burner for the oxidizer pump and a fuel-rich pre-burner for the fuel pump.
Since these two systems are separate, there’s no need for sealing. This makes it more durable by design, which is a crucial requirement for a one-way trip to Mars.
