This spray seemed to be there throughout the entirety of the Stage 2 burn but was especially visible once Stage 2 cutoff had happened.

As that nozzle uses radiant cooling, it can't be cooling fluid, so what could it be?

enter image description here

Video here.

  • $\begingroup$ I think it is the gas generator exhaust and is being used to vent residual fuel. But I cannot find confirmation of that yet. (Also noticed it). This was a very light payload so probably lots of fuel left in second stage to vent before reentering. $\endgroup$
    – geoffc
    Commented Feb 23, 2018 at 13:29
  • 1
    $\begingroup$ I've asked a couple of related questions about this thruster/exhaust/whatever it is before - I'm unaware of an answer on this site yet that categorically states precisely what it is with sources. $\endgroup$ Commented Feb 23, 2018 at 13:54
  • $\begingroup$ I don't think it can be a thruster, as it was there all through the entire 2nd stage burn. $\endgroup$
    – Rory Alsop
    Commented Feb 23, 2018 at 13:54
  • $\begingroup$ @geoffc They shouldn't be venting much before all satellites get deployed imho. And they might need some fuel for the deorbit burn. So probably not venting all residual fuel. But def looks like some purge at least. $\endgroup$
    – jkavalik
    Commented Feb 23, 2018 at 16:33
  • 2
    $\begingroup$ I don't think that's the gas generator exhaust: the GG points down, not radially, and the exhaust sits higher. $\endgroup$
    – Hobbes
    Commented Mar 21, 2018 at 12:46

1 Answer 1


The thing circled in the photo is a LOX vent. The puffy white snow-like stuff is solid oxygen. It was described at T+1.05:25 in the Iridium-6/GRACE-FO webcast.

  • $\begingroup$ Puffy white object? Where is that? $\endgroup$
    – Rory Alsop
    Commented May 23, 2018 at 17:28
  • $\begingroup$ Sorry, poor phrasing. Clarified. $\endgroup$ Commented May 23, 2018 at 17:53
  • $\begingroup$ Ah - brilliant. I was hunting through the video for a solid object :-) $\endgroup$
    – Rory Alsop
    Commented May 23, 2018 at 18:10

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