I've been reading about some concerns about the densified (sub-cooled) propellants for Falcon 9 crewed missions in Spaceflight Now's article Lawmakers question safety, schedule for Boeing/SpaceX commercial crew ships.

Could this concern be sidestepped by launching with ambient temperature RP-1 and boiling point LOX? Taking a payload hit of course. If so, would doing so prevent reuse?

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    $\begingroup$ RP-1 needs to be pressurized, to provide uninterrupted supply to the turbopumps, otherwise you'd have a problem of cavitation, essentially bubbles of vacuum forming as supply from the tank can't catch up with suction of the pump. Obviously normal LO2 instead of supercooled could be used. I think safety-wise all it would change is appeasing the concerned individuals. $\endgroup$
    – SF.
    Commented Jan 18, 2018 at 7:10
  • $\begingroup$ Sf: fixed typo, meant to write ambient temperature not ambient pressure. $\endgroup$
    – lijat
    Commented Jan 18, 2018 at 7:14
  • $\begingroup$ Can you provide a link to something about these concerns? $\endgroup$ Commented Jan 18, 2018 at 16:30
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    $\begingroup$ It is mentioned in this article spaceflightnow.com/2018/01/17/… $\endgroup$
    – lijat
    Commented Jan 18, 2018 at 19:15
  • $\begingroup$ @lijat I've moved your link to the question itself. Comments are considered temporary, so it's always better to make the changes directly to your original post. $\endgroup$
    – uhoh
    Commented Jan 20, 2018 at 14:56

1 Answer 1


enter link description hereThe LOX is densified by 9.9% and the RP-1 by 2.6%. This means that if you bring both back to regular temperatures, for the same flow rates, the engines will see a different fuel ratio (producing a different amount of thrust and possibly being less stable, or a much different temperature). You will also consume different ratios of propellant than your tankage - this means you carry extra dead weight, either in unused, empty tankage or unused, surplus propellant.

Warming the propellants and retaining the same fuel ratio could require changes to propellant feed valves in the engines, the turbopump itself, and the common bulkhead location between the LOX & RP-1 tanks.

These aren't small changes, the rocket would require redesign to a hybrid of the current model and the v1.1 model. You could bring the LOX densification up to match the RP-1 densification, but you wouldn't avoid having a small window after fueling to launch - you would just change the size of that window.

These changes would cost them payload mass on regular flights and reduce the number of missions that could recover, and flying these changes on only Dragon 2 flights would mean it wasn't of a common design and SpaceX would have to expend more flights proving the design's safety and reliability, rather than simply using paid for commercial flights to prove the design.

  • $\begingroup$ Sub cooling propellants is done on the launch pad, but not during the flight. So the propellants will warm up during flight and the engines should work the full period from ignition to shut off. Heating of LOX will be limited by boiling temperature. $\endgroup$
    – Uwe
    Commented May 7, 2018 at 19:29
  • $\begingroup$ Stage one only operates for about 2 minutes - 10 if you count landing, the flight time isn't much compared to the waiting period between propellant load finishing and launch (seems like 20-30 minutes depending on mission). Stage two probably does need to handle some propellant warming between the initial burn and any later burns - but perhaps not, loitering in the vacuum of space. $\endgroup$
    – Saiboogu
    Commented May 7, 2018 at 19:53
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    $\begingroup$ @Christoph It came from a Reddit comment, linked. I believe it's just a matter of collecting public statements about temperatures, and referencing the ratio the propellants shrink at that temperature. I just relied on someone else's lookup. $\endgroup$
    – Saiboogu
    Commented May 8, 2018 at 13:26

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