@Uwe's answer to Why would sub-cooled LOX tanks need to “topped-off” until the last minute or so? explains how subcooled LOX is kept cold and dense by "in situ refrigeration"; helium is bubbled through and the evaporation of oxygen into the bubbles removes heat as the helium escapes.

The LOX tank therefore needs to be regularly "topped off" with makeup LOX as it evaporates.

(A cryogenic engineer would like to build rockets like giant dewars, vacuum flasks or thermoses but it adds too much additional weight and complexity and diameter.)

But the RP-1 is sub-cooled as well and is also subject to warming.

Question: Is there any refrigeration for the subcooled RP-1? If not, why would one of F9's cryogenic subcooled propellants require refrigeration but the other not?

  • $\begingroup$ To bubble liquid nitrogen through the RP-1 tank seems to be too cold. Liquid carbon dioxide may be too cold if there is solid "dry ice" too. $\endgroup$
    – Uwe
    Commented Jun 2, 2020 at 10:31
  • 1
    $\begingroup$ @Uwe nonuniform cooling of RP-1 could produce blobs of wax. That doesn't sound very friendly to injectors. $\endgroup$
    – uhoh
    Commented Jun 2, 2020 at 11:35

1 Answer 1


RP-1 isn't cryogenic actually. The subcooling for RP-1 is only to cool it to slightly below the freezing point of water, 20 F. At that temperature, no extreme cooling is required. The temperature difference is quite small compared to the larger LOX temperature difference, and the fuel is only there for 35 minutes. There likely isn't a need to cool the RP-1 any more than it already is cooled.

Also, the difference for cryogenic LOX is huge. A mere 20 degrees K increases the density by about 10%. For the chilled RP-1, the same only changes a few percent. Keeping the LOX cool is required, or else it would expend too much, but the temperature of RP-1 is much closer. It could also be that they put the RP-1 slightly cooler in the tank than is desired for flight purposes, which would mean it would be right on the mark for flight. In any case, continuing to cool the RP-1 in the vehicle seems unneeded.

For the record, RP-1 cooled is around 2.5%-4% denser, per Wikipedia. The LOX is closer to 9.7%.

  • $\begingroup$ Can you cite your source that shows a much larger coefficient of thermal expansion for subcooled LOX than for subcooled RP-1? They look pretty similar to me. $\endgroup$
    – uhoh
    Commented Jun 2, 2020 at 3:44
  • $\begingroup$ From Wikipedia: " The latest version of Falcon 9, Falcon 9 Full Thrust, also has the capability of sub-cooling the RP-1 fuel to −7 °C, giving a 2.5–4% density increase." $\endgroup$
    – Uwe
    Commented Jun 2, 2020 at 10:24
  • $\begingroup$ @Uwe I can't find the program I plotted with four years ago but they both look like about 4% per 25 °C i.sstatic.net/ikcVS.png $\endgroup$
    – uhoh
    Commented Jun 2, 2020 at 11:33
  • $\begingroup$ Added sources and the numbers. $\endgroup$
    – PearsonArtPhoto
    Commented Jun 2, 2020 at 11:53
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    $\begingroup$ Of course they pour the RP-1 in a little cooler so that it has the perfect temperature when consumed! It's what a good espresso machine does with the water, just in reverse. And that is no coincidence: As we know, the Falcon 9 is only the latest by-product of the quest for the perfect brew! $\endgroup$ Commented Jun 2, 2020 at 16:45

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