I hadn't noticed it before, but after seeing yesterday's recap video of the recent Launch and Landing of the Falcon 9, I'm wondering what the plume visible during ascent is.

You can see it clearly immediately after launch, when the video changes camera angles at around 1:04, but I've highlighted it below:

enter image description here

I assume it's the fuel gases continuing to be vented, and the plume appears to extend/originate from up the stack.

However, is the lower, circled part, just being illuminated by the light from the engines, or is it actually being ignited? Does the plume have any measurable affect on the attitude that the rocket has to correct for, either through the venting or ignition (if indeed it is alight)?

  • $\begingroup$ That just looks like condensation from a coolant port or something equally benign. I am fairly certain that is not being ignited, just illuminated by the rocket exhaust. $\endgroup$ Commented Jan 13, 2016 at 14:15
  • $\begingroup$ Yeah I didn't think it was likely to be being ignited, but wanted to mention it to complete the question fully - ie what exactly is it, and is there enough of it venting to need to be corrected for. Since asking I've also remembered that the fuel is RP-1, so is unlikely to be vapour from that, I'm guessing it's most likely LOX boiling off. $\endgroup$ Commented Jan 13, 2016 at 14:28
  • $\begingroup$ First, is that your name or are you a fan of the NFL player? Second, if that were LOX, wouldn't it affect the rocket exhaust? Hmm... Oh, probably not, never mind... $\endgroup$ Commented Jan 13, 2016 at 14:44
  • $\begingroup$ @honeste_vivere It really is my name - being in the UK, I wasn't aware of the NFL player until it was mentioned several times to me on the SE network :) $\endgroup$ Commented Jan 13, 2016 at 14:45
  • $\begingroup$ It actually continues much higher into flight and can be seen on the new video they released, the slick Orbcomm mission summary one. I initially thought gas generator exhaust except there are 9 of those not one. $\endgroup$
    – geoffc
    Commented Jan 13, 2016 at 15:30

1 Answer 1


I checked the CRS-6 launch (which launched at daytime, so you can see more on the video).

Around T+1m, the camera pans up and you can see vapor coming out of at least two ports at the top of the first stage. These ports are 90 degrees apart, so they aren't both umbilical connections. In the Jason-3 launch, you can see these plumes before liftoff (this video from 15:10 to 20:00). It looks like 2 plumes from the top of the second stage, and 2 from the top of the first stage. There is at least one LOX vent at the top of the first stage. Some redundancy would be logical, so 2 LOX vents is a plausible explanation as the source of the 2 plumes.

The vapor occurs during the entire sequence from T+0 to T+1m, I'd say that rules out ascent heating as the main driver for the vapor. It surprises me that LOX boiloff is rapid enough to require venting even while the tank is being drained at a gigantic rate.

There's also some ice coming off the LOX tanks, you can see chunks coming off from launch to T+1 minute. But that doesn't seem to be the source of the vapor. The gas generator exhausts are right next to the engine bells, so that's definitely not it.


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