Helium is a rare and expensive noble gas. When it is loaded into a rocket to be used after launch, it is not possible to recycle it. But the helium used for sub-cooling the LOX before launch (see question Why would sub-cooled LOX tanks need to "topped-off" until the last minute or so?) could be recycled. The mixture of helium and oxygen should be exhausted from the tank and separated by liquefaction. Is recycling of helium done when using sub-cooled LOX or is it vented into the atmosphere?

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    $\begingroup$ As noted on that thread: do we have any indication SpaceX uses this helium cooling method? $\endgroup$ – Hobbes May 3 '17 at 11:22

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