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I just found this incredible video KSP Doesn't Teach: Rocket Engine Plumbing and ran through it a few times. It's very helpful in the way it outlines the various issues first, and then designs the designs that address them.

For a Full FLow Staged Combustion Cycle engine there are two combustion chambers driving two turbines; one to drive the fuel pump, and the other to drive the oxidizer pump. This "two shaft" solution solves problems that can be caused by leaky seals as explained in the video.

If I understand correctly, these combustion chambers are run either fuel-rich or oxidizer-rich so that they can deliver enough power without running so hot that the turbine could not handle it. By running on either side of the optimal mixture, the unused gas still expands and produces power by absorbing some of the heat generated by the reacted gases. And since the system is closed, it gets used later anyway.

But I am still having trouble understanding this diagram. If oxidizer and fuel are both cryogenic, why is it only the fuel that gets pre-heated? And are there additional propellant feeds from the pumps to the main combustion chamber, or does ALL of the propellant enter via the turbines in the form of incompletely combusted exhaust from the two smaller combustion chambers?

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  • $\begingroup$ Does they leaky seals problem really justify an additional pre burner and all the trouble of piping hot, corrosive oxygen rich gas? Surely it is simpler to design a super good seal, no? $\endgroup$
    – user3596218
    Commented Oct 14, 2016 at 2:42
  • $\begingroup$ @user3596218 that is an interesting question - why not post it as a separate question? That will make it easier for someone to find it, and then write an answer. A comment here probably won't be widely noticed. $\endgroup$
    – uhoh
    Commented Oct 14, 2016 at 6:29

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1) If oxidizer and fuel are both cryogenic, why is it only the fuel that gets pre-heated?

I assume you are talking about the H2 that flows through the nozzle? You have it a bit backwards. This is to cool the nozzle, not heat the hydrogen (although both happen of course) and H2 is a great heat transfer agent.

2) And are there additional propellant feeds from the pumps to the main combustion chamber, or does ALL of the propellant enter via the turbines in the form of incompletely combusted exhaust from the two smaller combustion chambers?

The whole point of the staged combustion cycle is that all propellant leaves via the nozzle so that you get efficient thrusting from all of it. In the schematic you show, a "Full Flow" staged combustion engine, all the propellants flow through the turbines.

The Space Shuttle Main Engine, although staged combustion, was not full flow, because much of the O2 entered the main combustion chamber without passing through the turbines. They had enough problems without dealing with the O2 rich turbine gas.

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    $\begingroup$ I see! So that's what "Full Flow" indicates here - all of the engine's propellant flows through one or the other set of turbine + combustion chamber before entering the main combustion chamber. Actually now that I look really really closely, I can see that on each of those control valves there is a thick line and a thin line. But the thick line is so short it's hard to see if you don't know it's there. Thanks! $\endgroup$
    – uhoh
    Commented Oct 7, 2016 at 4:00
  • $\begingroup$ Yes, that's correct. The Space Shuttle Main Engine also had 2 preburners, but they were both H2 rich. The vast majority of the H2 went through them, but the vast majority of the O2 was dumped right into the main combustion chamber. So not "full flow". $\endgroup$
    – Organic Marble
    Commented Oct 7, 2016 at 4:04
  • $\begingroup$ Got it! (the dotted lines in this cropped part point to the flow of the majority of the propellants: i.sstatic.net/0beFf.png (fixed) $\endgroup$
    – uhoh
    Commented Oct 7, 2016 at 4:07
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    $\begingroup$ One more big error: "in the form of incompletely combusted exhaust" - absolutely not. The preburners mix the substances in ratios that are very far from stochiometric, meaning only a small part of it burns there. But the heat generated is sufficient to evaporate the rest, so cryo liquid enters, hot gas (with some water steam left over from partial combustion) leaves. $\endgroup$
    – SF.
    Commented Oct 7, 2016 at 7:42
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    $\begingroup$ @SF. I think I see what you mean. When I said "incompletely combusted exhaust" it sounds like it could have combusted some more. In fact it has mostly combusted as much as it can, but there's an overwhelming amount of either one, or the other propellant as "left-overs" for the main engine combustion chamber to have for dinner. Thanks! $\endgroup$
    – uhoh
    Commented Oct 7, 2016 at 13:18

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