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Towards the end of the JCSAT-14 live webcast, after the satellite was deployed, we could see small thrusters firing on the second stage, starting to rotate it:

enter image description here

The webcast ended shortly afterwards, with (I believe) no explanation of what was happening1, so I'm wondering what the purpose of this was. Perhaps to get ready for an engine relight to re-test that capability / deorbit the second stage more quickly?


1Note that before the deployment, they did mention rotating the stage, but that was to stabilise it before deployment. I believe this later rotation is unrelated.

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At the end of the webcast, they mention the last step is to safe the spacecraft. I suspect all they were doing was getting rid of the remaining fuel on the spacecraft. Rotating it might lead to a slightly quicker reentry as it will be more likely to be broadside for longer to the atmosphere, increasing drag, but I think that is a secondary concern.

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  • $\begingroup$ So are you saying that it wasn't a thruster per se, rather they were actively venting the remaining fuel? Does that explain why it seemed to "build up" a bit on the thruster (if you watch it, you can see it "icing" up around the nozzle) - I wouldn't expect a normal thruster to ice up in this fashion (almost submitted another question about that) $\endgroup$ – James Thorpe May 6 '16 at 11:58
  • $\begingroup$ It's obviously some kind of a thruster. That would be a valid second question to ask. I suspect it's a cold gas thruster, which is basically compressed air, and will cool for the same reason a can of compressed air will cool when you spray it, but I'd have to do some more research. It could also be venting the super cool fuel, which would have a similar effect. $\endgroup$ – PearsonArtPhoto May 6 '16 at 12:01
  • $\begingroup$ Right, that makes sense. I'll go re-get my screengrabs of the buildup... $\endgroup$ – James Thorpe May 6 '16 at 12:03
  • $\begingroup$ Disagreed with 'that is obviously some kind of a thruster.' It's shown to leak continuously through almost all Falcon orbital operations, plenty of armchair guesses that it's a LOX vent from MVac chilldown, and I'm pretty sure it's even been referenced in webcasts as a 'vent' because it's a frequent point of interest as it grows and sheds solid oxygen ice. It likely even has a symmetrical twin on the far side to ensure it's a nonpropulsive vent - this can be guessed from best practice plus the symmetrical vent hoses in the interstage. $\endgroup$ – Saiboogu Feb 4 at 20:31
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The thing circled in the photo is LOX vent. The puffy white object is solid oxygen. It was described at T+1.05:25 in the Iridium-6/GRACE-FO webcast. As such, it's not intended to rotate the spacecraft.

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Likely it was to aim the stage for an orbital change either to reenter sooner, or a parking orbit much higher and out of the way. Do not want to leave clutter around GEO/GTO orbits.

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    $\begingroup$ Considering it was GTO, I'm fairly confident it wasn't a parking orbit. And it wouldn't make that much sense to perform a reentry maneuver so low in the orbit. Hmmm... $\endgroup$ – PearsonArtPhoto May 6 '16 at 11:48

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