What exactly is a full-flow closed cycle staged rocket engine like the SpaceX Raptor?

Well, this gets a bit complex, since to understand what the sentence means you have to understand how a rocket engine works. I’m going to try to go from the basics up…

I mean, it’s not like it’s rocket science, right? //

In case you’re wondering, it’s only the third one ever built — really, only the second full engine since the “full-flow power-head demonstrator” never actually had a combustion chamber attached. The other is the Russian RD-270 engine.

It’s also the first full-flow, closed-cycle, dual-shaft, staged-combustion rocket to ever fly. So far, it’s only 20 meters or so, but it did fly. (Update: 12.5 kilometres now…)

So there’s your answer.