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In electric-pump-fed engines fuel pumps are electrically powered and batteries are used as source of electricity. Yet there is another source of electrical energy which could be used in theory - fuel cells.

Fuel cells are very versatile in terms of fuels they can use (hydrogen, methane, hydrocarbons, hydrogen peroxide, hydrazine etc.). Also they are comparable (with potential to be even better) in terms of power density with batteries which are already employed electrical driven power cycles.

There is also a huge potential to increase power density of electrical motors (already satisfactory around 15-20KW/kg) from commercial ones which employ permanent magnets to cryogenic which employ high temperature superconductors. Liquid coolant is already available onboard.

Is this combination of fuel cells and electrical motors ever used in electrical pump-fed engines?

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    $\begingroup$ I see you edited, but it would be nice to clarify that you are talking about engines. $\endgroup$
    – Organic Marble
    Nov 7, 2020 at 13:41
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    $\begingroup$ Fuel cells are expensive so unless the booster is reusable, probably a poor choice. $\endgroup$
    – Organic Marble
    Nov 7, 2020 at 14:46
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    $\begingroup$ There are downs and ups with fuel cells. I doubt that they are more expensive than pre-burner + turbo-pump systems especially because of high temperature & pressure requirements and demand for super-alloys, precise machining and all the valves and plumbing. But yes they fall way behind in terms of power densities which means a lot of dead weight. Still potential for improvement is huge but there is lack of investments because even in car market, manufacturers find batteries to be more suited. $\endgroup$
    – WOW 6EQUJ5
    Nov 7, 2020 at 15:12
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    $\begingroup$ Yes that's certain, fuel cells are more expensive in this case. $\endgroup$
    – WOW 6EQUJ5
    Nov 7, 2020 at 15:18
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    $\begingroup$ @WOW6EQUJ5 High performance fuel cell catalysts are things like platinum and palladium, rather more expensive than superalloys. A complete Merlin 1D engine, turbopump and all, costs something less than $1M to make. It won't take much in terms of fuel cells to exceed that. $\endgroup$
    – Christopher James Huff
    Nov 7, 2020 at 16:01

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Partial answer.

At least not for a main engine for an orbital rocket, as the Rutherford engine is currently the only such engine using an electric pump. And it uses batteries.

However, conclusively ruling out all smaller secondary thrusters and propulsion systems is much harder. Electric feed pumps are beneficial for reducing complexity by not having to handle chemical fuels, and using a fuel cell negate this somewhat. It is however not unimaginable that some satellite using fuel cells for the onboard electrical system has small electrical pumps hooked up to thrusters. But at that scale simple pressure fed engines may be able to reduce complexity even further.

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  • $\begingroup$ One idea which comes to my mind to reduce complexity for handling fuel is maybe to create a middle layer of Solid Oxide Fuel Cell in the wall of combustion chamber where temperature would be high. Inner layer should be of porous material permeable to hydrogen, thus fuel rich cycle would be required. Outer layer of combustion chamber should be of material with high tensile strength. Oxygen should be used for regenerative cooling and for FC operation.This would be some kind of electric pump fed analogue to expandera cycle. But even if beneficial this would be hard to master which i am aware. $\endgroup$
    – WOW 6EQUJ5
    Nov 7, 2020 at 15:46
  • $\begingroup$ @WOW6EQUJ5 great amounts of effort go into getting heat from the combustion chamber wall into the cooling fluid as quickly as possible. A hot intermediate layer that itself produces heat is not conducive to that, and I have a hard time seeing how it reduces complexity. $\endgroup$
    – Christopher James Huff
    Nov 7, 2020 at 16:09

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