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If I open the valve on my nitrogen bottle there is a terrible noise but the gas jet is totally invisible.

On some Falcon 9 launches, the gas jets are very visible both on ground and in the low pressure environment immediately after second stage separation.

What is it we see? In the normal atmospere on ground I could guess it is air moisture condensation, but in vacuum?

marked-down
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Wirewrap
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  • It can't be moisture condensation, compressed nitrogen in bottles is very dry, the moisture was removed by the cryogenic separation process. The gas is expanded, not compressed, this should counteract condensation of moisture. But expansion lowers the temperature of compressed nitrogen, that is used to liquify air. Thus it might be condensation of nitrogen if the temperature of the nitrogen tank and the cold gas thruster was low. – Uwe Jul 08 '17 at 14:43
  • I assume there is a difference in pressure and gas speed. There is also quite a difference in altitude (so, atmospheric conditions). –  Jul 08 '17 at 14:58
  • Anyone know how much cooling you get from releasing the nitrogen? Could it be nitrogen droplets or even nitrogen ice? – Loren Pechtel Jul 08 '17 at 23:54
  • @Loren Pechtel Lets take the liquefaction of air as an example. Air is compressed to 200 bar and cooled to ambient temperature after that. If the compressed air is expanded properly, the temperature of liquefaction (about -170 °C) is reached. The nitrogen tank cools down when the cold gas thrusters are used frequently. At reduced air pressure, all the equipment for cold gas thrusters are better isolated thermally and stay cold between uses. – Uwe Jul 09 '17 at 08:52
  • Ground nitrogen tanks are just compressed nitrogen gas, the thrusters presumably use liquid nitrogen; I'd imagine what you are seeing is atomized liquid nitrogen. – 0xDBFB7 Dec 20 '17 at 16:42
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    Is there any reference confirming that the trusters use liquid nitrogen? I thought cold gas was in contrast to what comes out of the main engine, that is quite hot gas. – Wirewrap Dec 20 '17 at 17:19

1 Answers1

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It is nitrogen snow.

The nitrogen gas enters the rocket nozzle around room temperature. The expansion converts the gas' internal energy to kinetic energy of bulk motion. The large pressure ratio means almost all the internal energy is converted and the gas becomes extremely cold.

It cannot be droplets of liquid nitrogen, since that would not be stable at the low pressure existing at that altitude. It would spontaneously boil, become colder while doing so, making a combination of gaseous and solid nitrogen.

Liquid hydrogen vented from some of the Saturn V third stages, after they were jettisoned by the Apollo missions on the way to the moon, produced clouds of frozen hydrogen by this mechanism.

Paul D.
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    I suggest to write some reference. – peterh Dec 28 '17 at 02:54
  • Here are some links: 1, 2. – Uwe Dec 28 '17 at 10:14
  • This is on its way to being a good answer, but there are some loose ends. "...the gas jets are very visible both on (the) ground and..." still needs an answer. Also, since no specific altitude was named in the question, you'll need to name the altitude that "at that altitude" refers to. Can you also add (or at least check) a nitrogen phase diagram to explain why you think it is the pressure that is determining if the liquid boils or not? – uhoh Dec 28 '17 at 10:16
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    @uhoh let’s be kind to all the new people in this community – Jake Blocker Dec 29 '17 at 20:01
  • @JakeBlocker Of course my comment is quite kind, and helpful as well! I have not down voted, nor flagged this answer, and I've left a substantial comment with some depth, explaining several things that can be done to improve the answer. Here is an example of an approach which would not be classified as kind. Read that, then read my comment again. There is quite a difference! Helping people improve their answers is kind to both new users, the OP, and future readers. – uhoh Dec 29 '17 at 21:14
  • @JakeBlocker there is a difference between questions and answers, and between a stackexchange answer and answers on other fora (ok, forums). I think in most SE sites, questions are allowed a lot more latitude than answers. Answers are SE's deliverable, it's raison d'être, so they get a far higher level of scrutiny and polish than questions. I've helped edit both questions and answers to improve them many times. In this case, I've left detailed hits so that the user has an opportunity to learn a bit about phase changes. If they are going to be answering questions here it's good to learn! – uhoh Dec 29 '17 at 21:27
  • @JakeBlocker If nothing happens in a few days, I'd planned on making the edits myself. This way the user gets a second chance to collect reputation, and to see what a good SE answer looks like. Suggesting I'm not kind is way off the mark! (oh, another example). – uhoh Dec 29 '17 at 21:29
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    @uhoh I completely agree with you! I just interpreted your reply to the answer a little harsher than you actually meant it to be, sorry – Jake Blocker Dec 29 '17 at 21:29