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Referring to the question in this thread.

What is the advantage of full-flow staged combustion (FFSC) vs conventional staged combustion rocket engine?

In FFSC we feed the full volume of fuel and oxidizer through the plumbing of the preburner and pump turbines causing fluid friction losses. Also plumbing simply has to be larger and heavier thus incurring performance loss. Why is this a good design choice?

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  • $\begingroup$ I reckon what you're asking is, how is a FFSC like the Raptor better than “ordinary” staged combustion engines like the RS-25. Am I right that this is inspired by Scott Manley's video on the full-size Raptor unveiling? I actually had the same question there. $\endgroup$ – leftaroundabout Feb 5 '19 at 0:07
  • $\begingroup$ @Organic Marble. Yes, Let's compare FFSC to FSC (as in SSME) where only parts of oxidizer are fed through the prebruner turbine. )I can't make a case about fuel as all of it is passing through the turbine.) $\endgroup$ – Gert Gottschalk Feb 5 '19 at 0:18
  • $\begingroup$ @leftroundabout. Yes. $\endgroup$ – Gert Gottschalk Feb 5 '19 at 0:20
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This Aerospace Corporation article explains it beautifully.

First, all of the propellants are burned in the preburners, thus providing more mass flow for turbine drive power than the conventional staged combustion cycle. This additional power can be used to increase the chamber pressure and produce a smaller engine; alternatively, the preburner temperature can be reduced to provide the same power at lower temperatures. The lower turbine temperatures translate into longer turbine blade life—often the limiting factor on reusable engine life.

The second advantage is that the use of oxidizer-rich gas in the oxidizer turbine and fuel-rich gas in the fuel turbine eliminates the need for a complex propellant seal for the pumps. There is little risk with leaking liquid fuel into a fuel-rich gas or liquid oxygen into an oxidizer-rich gas. In contrast, the fuel-rich staged combustion cycle must use sophisticated purges and multiple seals in the oxidizer pump to prevent any liquid oxygen from leaking into the hot fuel-rich gas. A similar situation must be avoided in the oxidizer-rich cycle on the fuel pump side. The elimination of this failure mode increases system reliability

Your concerns with "feed[ing] the full volume of fuel and oxidizer through the plumbing of the preburner and pump turbines" are not well founded. The "full volume" of propellants is going to be fed through the engine plumbing somewhere. You also state that "Also plumbing simply has to be larger and heavier"; this is not necessarily so; the general decrease in engine size and elimination of seal systems and helium tankage to pressurize them may reduce the overall weight. Also, this paper compares a Raptor-like engine with a single-preburner fuel-rich staged combustion engine; the pressures in the two engines do not differ greatly; it is not clear that one or the other requires heavier plumbing.

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  • $\begingroup$ I believe this is because LH2 is especially pump-head consuming when it comes to all possible bi-propellant choices. As such, in SSME the FPB already consumes the overwhelming majority of the available mass flow and turbine HP, therefore in an equivalent, FFSC design adding further whatever little there is left of the warm H2 flow (i.e. the flow of warm H2 that passes through the OPB in SSME) to FPB yields little improvement. I expect a more drastic improvement when it comes to chamber pressure in a bi-propellant pair that is a little 50/50 when it comes to pump head, say metholox. $\endgroup$ – Meatball Princess Sep 6 '19 at 6:58
  • $\begingroup$ Also, this paper differs from SSME in a significant way: in the real SSME the LH2 flow that cools the MCC, 29lb/s out of a total of 155lb/s is not passed through any PB and is simply not available to turbine, whereas in the paper all LH2 is available to the single PB. This overestimates the chamber pressure because just like in SSME this flow is needed to drive the FBP, because again LH2 requires so much pump head, that even the booster pump needs to tap into warm gas, as opposed to just a stream of HP LH2 bleed from HPFTP discharge. $\endgroup$ – Meatball Princess Sep 6 '19 at 6:59
  • $\begingroup$ Also, the authors of this paper simply fixed the MR of FPB in the FFSC design and the single PB in the FRSC design to 0.7 (this MR is taken straight from SSME FPB MR). I don't think this is appropriate. If I were to do this paper, I would add another pass of optimization of completely free MR for both the FPB and OPB in the FFSC design with the constraint that the mass flow rate of both F and O conserves and the back end pressures of both FPBT and OPBT equal, the optimization goal being minimizing turbine temperatures. $\endgroup$ – Meatball Princess Sep 6 '19 at 7:11

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