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?

  • $\begingroup$ 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. $\endgroup$
    – Uwe
    Commented Jul 8, 2017 at 14:43
  • $\begingroup$ I assume there is a difference in pressure and gas speed. There is also quite a difference in altitude (so, atmospheric conditions). $\endgroup$
    – user10509
    Commented Jul 8, 2017 at 14:58
  • $\begingroup$ Anyone know how much cooling you get from releasing the nitrogen? Could it be nitrogen droplets or even nitrogen ice? $\endgroup$ Commented Jul 8, 2017 at 23:54
  • $\begingroup$ @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. $\endgroup$
    – Uwe
    Commented Jul 9, 2017 at 8:52
  • 1
    $\begingroup$ 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. $\endgroup$
    – Wirewrap
    Commented Dec 20, 2017 at 17:19

1 Answer 1


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.

  • 1
    $\begingroup$ I suggest to write some reference. $\endgroup$
    – peterh
    Commented Dec 28, 2017 at 2:54
  • $\begingroup$ Here are some links: 1, 2. $\endgroup$
    – Uwe
    Commented Dec 28, 2017 at 10:14
  • $\begingroup$ 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? $\endgroup$
    – uhoh
    Commented Dec 28, 2017 at 10:16
  • 1
    $\begingroup$ @uhoh let’s be kind to all the new people in this community $\endgroup$ Commented Dec 29, 2017 at 20:01
  • 1
    $\begingroup$ @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 $\endgroup$ Commented Dec 29, 2017 at 21:29

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