The Tech Behind SpaceX’s New Engine
The article is a bit incomplete. The space shuttle main engine already had two main turbopumps. IIRC this is because the optimal pump speed is different for pumping hydrogen and oxygen. They have very different densities. You want to avoid gearing as much as possible.And the main problem problem in the staged combustion cycle - in feeding the gas generator exhaust to the main chamber is not that it's fuel rich or oxidizer rich - it's that it's usually much lower pressure than in the main combustion chamber, because it had to go through the turbine, which causes the pressure to lower.That's why engines like NK-33 have a separate boost pump for the gas generator.
RD-180 has more pump stages for the fuel entering the gas generator (all oxidizer passes through the gas generator).Full flow staged combustion is another way to solve this - put all the propellants through the gas generators and turbines.I realize it's hard to write popular technical articles about medium complexity subjects and sometimes you have to take some shortcuts.https://en.wikipedia.org/wiki/Space_Shuttle_main_engine#/med...