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The SpaceX Falcon 9 FT is using subcooled liquid oxygen. I understand that LOX gets significantly denser at lower temperatures so more propellant can be filled in the rocket's tank.

Can NASA SLS RS-25s use subcooled liquid oxygen in the first stage to improve mass-to-orbit performance?

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  • $\begingroup$ If the supercooled liquid oxygen is significantly denser, it might be a problem for the LOX turbopump, the injectors and the combustion chamber. Without modification, the mass flow rate of oxygen might be too high, the mixture gets oxygen rich and might damage the combustion chamber and nozzle. If the thrust does not rise proportionaly to the increased mass flow rate, a part of the additional oxygen in the tank is lost. If the best mixture of hydrogen and oxygen is maintained with supercooled LOX, some extra hydrogen must be loaded to use the additional oxygen. $\endgroup$
    – Uwe
    Commented Feb 13, 2017 at 19:40

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You have an additional problem: SLS fuel is LH2. I am not sure if there is any experience at all using supercooled hydrogen (which is cold enough at boiling temperature).

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  • $\begingroup$ It would be possible (at least in theory) to use boiling LH2 and subcooled LOX. You just change the relative sizes of the tanks to get the right ratio. You could get more of the extra cold oxygen to a smaller tank and have a bit bigger tank for usual-temperature hydrogen. $\endgroup$
    – jkavalik
    Commented Feb 13, 2017 at 17:50
  • $\begingroup$ Changing only the relative sizes of the tanks to get the right ratio does not work. The flow rates of the turbo pumps must fit to get the right ratio. But if the density of LOX is different, the flow rate may be different too. $\endgroup$
    – Uwe
    Commented Feb 14, 2017 at 7:51
  • $\begingroup$ Yep. I was just trying to say you do not have to densify the propellant too and still could get some benefits from oxidiser subcooling. Not saying it is as simple with the rest of the hardware. $\endgroup$
    – jkavalik
    Commented Feb 14, 2017 at 8:21
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For the Space Shuttle, liquid oxygen was used at boiling temperature (95 K). Rocketdyne did some successful tests with LOX subcooled to 88.9 K. In a quick search I've found no indications this was used operationally in the Shuttle, or will be used for the SLS.

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  • $\begingroup$ You are correct that it was not used in the STS program. $\endgroup$
    – Organic Marble
    Commented Feb 13, 2017 at 13:28

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