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In this Twitter post of the plumbing line diagram(left half) by LauncherSpace, one can notice that Helium is used for pressurising LOx while Nitrogen is used for pressurising Kerosene.

Why the dichotomy? Why not use the same gas, preferably the cheaper Nitrogen gas to pressurise LOx?

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    $\begingroup$ I don't have a reference handy, but I'm pretty sure that the N2 diffuses into the LOX too much. We had a problem in the Space Shuttle integration office where the engine tag data from the test stand was always slightly worse than the flight performance. There was a strong feeling from some that the difference was because the flight vehicle used He/autogenous gas and the ground test system used N2 to pressurize the tanks. $\endgroup$
    – Organic Marble
    Sep 26 '18 at 16:48
  • $\begingroup$ @OrganicMarble do you mean to say N2 “dissolves” in LOx? And thanks for sharing your experience. Very interesting and informative! $\endgroup$
    – karthikeyan
    Sep 26 '18 at 17:05
  • $\begingroup$ Does the dissolved N2 bubble off at any site in the LOx plumbing line?or will it bubble off in engine and cool the gases resulting in reduced performance? $\endgroup$
    – karthikeyan
    Sep 26 '18 at 17:14
  • $\begingroup$ @OrganicMarble - is this a good reference - dtic.mil/dtic/tr/fulltext/u2/272377.pdf? perhaps this was the one you were looking for $\endgroup$
    – karthikeyan
    Sep 26 '18 at 17:23
  • $\begingroup$ You would think that the He would dissolve even easier - it's a much smaller molecule than N. Would it not easily get in? $\endgroup$
    – Martin James
    Sep 26 '18 at 19:31
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As suggested by OrganicMarble in a comment, nitrogen is miscible with oxygen (you can thus make liquid air). According to NASA Technical Paper 2464, this is a major concern because using "enriched air" instead of pure oxygen as the oxidizer degrades the performance of the engine:

The transfer of liquid oxygen (LOX) from a storage vessel to a rocket engine generally requires the use of a pressurizing gas at high pressures. The primary criteria for the choice of gas are low cost, safety, and immiscibility with liquid oxygen. Among the common gases, helium, nitrogen, and oxygen itself have been considered. Helium is expensive, and oxygen is hazardous at high pressures. The remaining gas, nitrogen, unfortunately is miscible with oxygen and causes dilution and loss of engine performance. (emphasis mine)

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  • $\begingroup$ It also increases the vapour pressure of the lox. This could result in cavitation and therefore might destroy the feed system. $\endgroup$
    – Christoph
    Sep 26 '18 at 19:21
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    $\begingroup$ @Christoph: True, although I'm not sure if the effect is large enough to matter. I don't have any numbers, but I'd assume the turbopumps work with a sizable safety margin, because the inlet pressure can fluctuate quite a lot (due to changes in total acceleration of the rocket) and any cavitation would be disastrous. $\endgroup$
    – TooTea
    Sep 26 '18 at 19:29
  • $\begingroup$ Thank you for the edit on the other question ;-) I ofton mipsell wrods. $\endgroup$
    – uhoh
    Nov 16 '18 at 13:39

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