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Towards the end of the JSSAT-14 webcast (around T+32:58), we see a thruster firing. Over the space of around 20 seconds, you can see it "icing up":

enter image description here

What is this that's forming? Is this a normal process for thrusters on spacecraft, or is it related to this being an upper stage that doesn't have much of a use beyond getting the payload to orbit - I would expect thrusters used in the longer term to not do this (especially since presumably anything that forms is not contributing to the actual thrust, reducing overally efficiency). What makes this different?

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  • $\begingroup$ The video link has a problem. The question has v=L0bMeDj76ig#t=62m50 but the video is only about 55 minutes long. I've commented on this kind of problem here and here. Is this becoming a pattern with SpaceX videos? I think any references should start containing the launch time code as well. $\endgroup$ – uhoh Jun 18 '16 at 1:33
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    $\begingroup$ Actually I noticed the image here is cropped so that the launch time code is now missing. I think this is a bad idea to crop it out in every image. You also loose the rest of the context, even which launch it is, and if you are not an expert, no clue it is from SpaceX. Images are frequently copied and/or relinked, so it's a good idea to keep the useful information that's in an image, and not crop it out before posting to the internet. $\endgroup$ – uhoh Jun 18 '16 at 2:20
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    $\begingroup$ There are more images, which are not cropped, here. $\endgroup$ – WBT Jun 19 '16 at 1:01
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    $\begingroup$ @uhoh Yes - certainly seems they're editing them. I've updated the link and edited in the actual mission time too now. I think I chose to crop the images this much originally since they were in the side-by-side view, and at the time I thought the view of the presenters etc wasn't useful to the question, but will certainly include mission time in any future questions now I'm aware they can and do edit these clips. $\endgroup$ – James Thorpe Jun 20 '16 at 7:46
  • $\begingroup$ Oh no! I had a few questions in the oven based on some just plain wrong things some of the newer "yo, dude!" commentators said while live. Maybe that's what they've gone back in to tidy up. :) $\endgroup$ – uhoh Jun 20 '16 at 8:33
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Is it normal for thrusters to “ice up”?

The thing circled in your photo is a LOX vent, not a thruster. The puffy white object is solid oxygen. Your question is directly answered at T+1.05:25 in the Iridium-6/GRACE-FO webcast:

Currently you can see the camera on Stage 2, looking at the upper stage engine, that white object you see is solid oxygen, very fluffy, lighter than a snow ball. That is normal at this stage of the flight.

Screenshot Iridium-6/GRACE-FO webcast

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    $\begingroup$ I hope you don't mind that I added the actual quote, and a screen shot so that readers don't have to use the link and watch the video to get the info. While I'm sure SpaceX and YouTube will be around for a while, it's best practice to include the key information from links directly into the answer. If it's not okay, I can move it to a separate answer. $\endgroup$ – uhoh May 24 '18 at 7:18
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When this was captured, it was at the end of the rocket flight. Looking carefully at this, I don't see any other similar nozzles. Furthermore, I didn't see any evidence of this being used in flight. I'm going to assume from all of this that it must be the vent value for the LOX, which was mentioned in the video recording immediately before.

What happens with such a release is that the temperature tends to drop when a compressed gas is released. Being in a vacuum, the material cannot be atmospheric, such as ice, but must be a frozen variety of whatever is being leaked. We know that the LOX tank is the lower of the two tanks. A phase diagram indicates that LOX would be solid if the temperature is around 80K, which is entirely possible.

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  • $\begingroup$ The linked duplicate shows apparently three similar nozzles on the same spacecraft, and those photos were taken before the completion of the primary satellite deployment mission. One can also see the gas escaping at multiple different times, though it's not clear if the pauses were controlled (at least some appeared that way) or caused by the icing. I don't think this answer answers the dupe, so I'm not sure it answers this question either. $\endgroup$ – WBT Jun 16 '16 at 18:25
  • $\begingroup$ Double checking - the white stuff is Oxygen ice? And it is forming around a vent used to release excess LOX to space? If the ice forms and stays a while, it also means that whatever it is stuck to (metal?) must also remain below about 80K otherwise the ice would fall of. Since conduction is a good heat transfer mechanism, it seems these should be short lived. $\endgroup$ – uhoh Jun 18 '16 at 1:38
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That LOX was vented before the final burn to chill in the engine. The final burn happens after payload separation to park Stage 2 away from the satellite.

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    $\begingroup$ Any source for that? $\endgroup$ – SE - stop firing the good guys May 7 '16 at 10:45
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    $\begingroup$ Saw it on reddit. Which, I know, is almost worse than no source. :) $\endgroup$ – Jeff Allen May 7 '16 at 11:22

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