What is a rocket combustion chamber?
Also called: thrust chamber
The combustion chamber is the part of the engine where fuel and oxygen actually meet and burn. On a Merlin the pressure inside reaches 9.7 MPa — about a hundred times the air pressure around you.
It is a surprisingly small space. Everything a rocket does — all 845 kN of push from one Merlin — comes out of a chamber you could put your arms around, and it comes out because the burning gas has nowhere to go except backwards through the nozzle.
Two things make it hard to build. The pressure, which is what the turbopump exists to overcome, and the heat, which is far above the melting point of the metal the chamber is made of.
The heat is dealt with by running the cold kerosene through channels in the chamber wall before burning it, so the engine is cooled by its own fuel. Higher pressure means a more efficient engine, so every design pushes the chamber as hard as the walls will take — which is why chamber pressure is one of the first numbers engineers quote about a new engine.
See it happen
Meet a RocketEvery part, namedSource: Merlin rocket engine family