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One of the benefits of the SpaceX Raptor full-flow staged combustion engine is the extra power generated by having all of the fuel and all of the oxidizer flow through the turbines to produce power to run the fuel and oxidizer pumps.

At first, I thought having two pre-combustion chambers and two turbo pumps would double the power compared to an RD-180 style oxygen-rich-only staged combustion engine. But then I did this calculation (see below) of fuel mass and concluded that the fuel has only 25% of the mass of the oxygen, and so would only add 25% to the total pump power available.

${CH_4 +2O_2 = CO_2 + 2H_2O}$

${CH_4}$molecular weight = 16
$2O_2$ molecular weight = 64

Fuel to Oxidizer ratio is 1:4 (Assuming Stoichiometric for simplicity)

Conclusion: Using the fuel as well as the oxygen to drive the turbines adds 25% to the available pump power.

Is that the correct conclusion? If so, is it worth the extra effort to get this 25% extra power?

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    $\begingroup$ It is not possible to drive the turbine with only methane alone or only oxygen alone. You need to burn methane with oxygen to get a hot gas to drive the turbine. But it may be necessary to burn fuel rich to limit the temperature to a value acceptable for the turbine. Burning oxygen rich may be possible if the turbine will not be damaged by the excess hot oxygen. $\endgroup$
    – Uwe
    Dec 7, 2017 at 18:25
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    $\begingroup$ In an aerospace system, 25% is an enormous increase. $\endgroup$
    – Organic Marble
    Dec 7, 2017 at 19:42
  • $\begingroup$ @OrganicMarble Your comment above is helpful about the significance of adding 25% extra power, but does your comment imply that you are confirming my conclusion of the power increase actually being 25%? If so, perhaps put that in an answer vs a comment? $\endgroup$
    – nyDavid21
    Dec 7, 2017 at 20:50
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    $\begingroup$ My understanding is that the advantage of full flow has more to do with fuel/oxidizer isolation. If you have a fuel rich turbine pumping an oxidizer, if anything leaks through the bearing, BOOM. So you need really good gaskets, which will have high wear, which hurts reuse. With fuel rich turbines pumping fuel and oxidizer rich turbines pumping oxidizer, you don't need to worry about leakage across the bearing. I don't remember the source for this information though, which is why I made it a comment. $\endgroup$
    – Lex
    Dec 27, 2017 at 17:02
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    $\begingroup$ The mass flow rate through the turbines isn't the only, or even main, factor in the power they can generate. It’s a more complicated thermodynamic problem, but at the simplest level, you should be looking at the change in enthalpy between the turbine inlet and outlet. The specific (i.e. per unit mass) enthalpy change is multiplied by the mass flow rate (and some efficiency terms) to calculate turbine shaft power. So what the fluids are matters, not just the mass. Engine performance is also much more complicated than maximizing pump power or engine cycle. $\endgroup$
    – Adam Wuerl
    Mar 17, 2018 at 5:34

1 Answer 1

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Dual turbo pumps does not improve the power (they just make each of the dual pre-burner chambers smaller); being FFSC, Full Flow Staged Combustion, does.

There are to several reasons to separate the LOX path and the Methane path:

  1. Gaseous injection into the primary chamber improves combustion efficiency and mixing resulting in a smaller combustion chamber and a smoother less problematic burn .... to get both into a gaseous form means two separate pre-burners are required. (or a heavy heat exchanger)

  2. The dynamic seal on a 50,000 rpm or so turbo pump shaft will always leak especially when re-use is paramount. Separate turbo pumps means that you don't have to worry about accidentally mixing O2 and CH4, since static seals on the outside of the turbo-pump do the sealing work...thus improving reliability.

  3. The metals are different for O2 burner and O2 power wheel and likely will have different thermal expansion rates (hot, high pressure Oxygen is quite corrosive) .. SpaceX is formulating a new Supper Alloy for this.

  4. For start up, and at other times, they can change the mixing ratios dynamically via controlling the liquid valves without over pressuring the gas control valves....in the extreme, they may even remove the pre-burner to combustion-chamber control valve all together or make it a smaller-and-lighter shutoff valve instead .... (not too sure of this one)

Much of the improvement in power and efficiency comes from a) higher primary combustion chamber pressures; that is why they are shooting for 300+ bar, maybe, if they can get the metals to not corrode in the O2 pre-burner and b) in an FFSC design, you get to eject all of the fuel at Mach ?30? instead of dumping it out at sub Mach 1.

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  • $\begingroup$ Do you have any references to back this up? I'm particularly interested in the "gas control valves" and the "pre-burner to combustion chamber control valve". Are you suggesting there is a valve in the hot gas manifold between the preburner and the main combustion chamber? $\endgroup$
    – Organic Marble
    Jul 3, 2018 at 23:24

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