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I'm learning about rocket engines and full flow engines, and I'm a little confused about what goes on in the combustion chamber. It was my understanding that injectors were pretty much necessary to diffuse the oxidizer and fuel and maintain a stable reaction. How is the hot gas from the preburner or turbopump sent into the combustion chamber?

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Yes, they still use injectors. They are gas injectors instead of the liquid or mixed phase ones in other engines. The gas injectors serve the same purpose of metering and mixing the propellants that liquid or mixed phase ones do.

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Schematic source from SpaceX reddit (validity unknown, but it looks convincing).

Elon Musk also mentioned the Raptor injectors in a tweet.

The SSME wasn't a FFSC engine, but the H2 came into the main combustion chamber mixed with steam as a gas. It went through the main combustion chamber injector as a gas, and the O2 went through it as a liquid.

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Injectors are still used, or the propellants will at some point seep backward. Kaboom. A lot of engine options would be possible if we could get rid of injectors, and their head loss, thermal constraints, corrosion issues, manufacturability and testing, etc. but we can’t- kaboom.

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  • $\begingroup$ That issue could theoretically be fixed by making the turbopumps powerful enough and the pipes narrow enough that the flow velocity prevents any backwards propagation. However, injectors are also there to ensure rapid and thorough mixing of the two streams. Just using a massive wye to join the two flows would result in half of the combustion chamber running very fuel rich, the other half running very oxidizer rich (often also called engine-rich), and the stoichiometry pretty much uncontrolled overall. The PC term for that is "combustion instability", but "kaboom" will do as well. $\endgroup$
    – TooTea
    Mar 6, 2023 at 7:42
  • $\begingroup$ Let’s see, severe power gradient, across narrow pipes… that’s an injector, t. And since turbopumps cannot be made arbitrarily powerful (often the constraining/pacing component of its project), something that would be called ‘an injector’ by a reasonable person is called for. Except for some trivial cases, which would likely skip turbopumps anyway (hard to generalize for these outliers). $\endgroup$ Mar 8, 2023 at 21:15
  • $\begingroup$ Right, I wanted to add "(at which point you have reinvented an injector)" into my first comment but couldn't squeeze it into the character limit :) $\endgroup$
    – TooTea
    Mar 9, 2023 at 8:03

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