In this answer, the attached quote, says that ground-supplied helium was made to bubble up through LOX lines which mixed it and kept it from boiling and geysering.

What exactly this means? By ground supply does it mean only in launch pad? Will not helium just rise up in the LOX ? And how can it keep things mixed and avoid boiling?

The special problem of the LOX tank involved the feed lines leading to the thirsty engines about 15 meters below the fuel tanks. To do the job, the S-IC used five LOX suction lines, which carried oxidizer to the engines at 7300 liters (2000 gallons) per second. To achieve such high rates of flow, the lines could not be bent around the outside of the fuel tank; therefore, designers ran them right through the heart of the fuel tank. This in turn caused considerable fabrication problems, because it meant five extra holes in both the top and bottom of the fuel tank and presented the difficulty of avoiding frozen fuel around the super-cold LOX lines. The engineering fix on this included a system of tunnels, each one enclosing a LOX line, especially designed to carry an effective blanket of insulating air. Even so, the warmer fuel surrounding lines created some thermal difficulties in keeping the LOX lines properly cool. So the S-IC used some of its ground-supplied helium to bubble up through the LOX lines, and kept the liquid mixed at a sufficiently low temperature to avoid destructive boiling and geysering, or the creation of equally destructive cavities in the LOX pumps

Note: emphasis mine

  • 1
    $\begingroup$ See my answer to the related question. $\endgroup$
    – Uwe
    Commented Oct 18, 2019 at 15:24
  • $\begingroup$ @Uwe From answer, it is still not clear to me what is topping off and is it done only on launch pad by helium supply from outside the vehicle? $\endgroup$
    – zephyr0110
    Commented Oct 18, 2019 at 16:05

1 Answer 1


To acheive full thrust of the first stage F-1 engines of the Saturn V only liquid oxygen should be pumped into the combustion chambers. A mix of gaseous and liquid oxygen would reduce the desired mass flow of oxygen to the engines and may damage or even destroy the oxygen pumps. A reduced mass flow of oxygen would reduce thrust and endanger a successful liftoff.

But the LOX lines through the fuel tank had a bad ratio of surface to volume, much worse than the large oxygen tank. The heat flow to the LOX lines would cause the LOX inside to boil heavily.

By bubbling of cold gaseous helium through the LOX lines and tank the LOX was cooled before launch below the boiling temperature. The gaseous helium and oxygen at the top of the tank are exhausted through umbilicals and the evaporated LOX is replaced by toping of more LOX from the large tanks at the launch pad.

Just before ignition the helium bubbling is finished and the very cold LOX within the lines and tank does not boil as long as its temperature is below the boiling point. So the oxygen pumps and combustion chambers could be fed with pure liquid oxygen free from bubbles.

For the cooling of LOX by helium bubbling see my answer to this question.


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