This morning's launch of an ISS resupply mission by SpaceX happened in a beautifully clear sky just before dawn. As hoped, the not-yet-risen sun illuminated the entire flight path from just after launch to the eastern horizon.

The first stage was almost as usual, other than the last portion before separation being well illuminated. Additionally, the first stage was not returned, therefore no boost-back burn and it appeared to "tag along" with the second stage for most of the flight. That was great to see.

According to The Curious Droid's video, there's a two minute dissertation that states atmospheric pressure is a factor in engine nozzle bell shapes. As the rocket climbs, atmospheric pressure drops and the thrust becomes less efficient. More of the engine thrust is not parallel to the flight path and is "wasted."

spacex launch image 1

In this photo, you can see the contrail from the first stage, well illuminated by the sun, but also the beginning of what appears to be the expansion of the exhaust gases due to higher altitude. I'm not sure where in the photo-timeline the first stage separated, but I believe that this is after the first stage separated.

spacex launch image 2

In this image, the exhaust has expanded substantially. The camera did not capture all of the illuminated area, which would appear to the naked eye to be at least another fifty percent larger.

Certainly the approaching sunrise contributed to this effect, but is the flaring of the exhaust this immense?

If so, one would guess that it's simply not visible in lower light conditions or in full sunlight.

  • $\begingroup$ Chemical engines typically produce more thrust with higher efficiency in the upper atmosphere - see here $\endgroup$
    – Jack
    Jun 29, 2018 at 14:34
  • $\begingroup$ @fred_dot_u - I didn't understand what exactly is "unusual"? Is there any difference with Iridium 4 launch? (Link: youtube.com/watch?v=tE5C3O71Xqo). Is your question "why exhaust bubble becomes so big at the end?" $\endgroup$
    – Heopps
    Jun 29, 2018 at 14:43
  • $\begingroup$ That video is very nearly identical to what I viewed today. I suspect there is also loss of fidelity due to light capture limitations of the recording device. The main question is this effect the spreading of the exhaust gases as described in Curious Droid's video and summarized in the question? I'd seen this only once before and had not viewed CD's video at that time. $\endgroup$
    – fred_dot_u
    Jun 29, 2018 at 15:04
  • $\begingroup$ Rocket exhaust trails often look weird due to winds aloft, lighting, etc. There's nothing unusual about this one. Here's a really crazy shuttle one c1.staticflickr.com/3/2801/4494546794_6e697f266d_b.jpg BTW that bit about most of the engine thrust being wasted is totally wrong. And "thrust becomes less efficient" means nothing. $\endgroup$ Jun 29, 2018 at 15:44
  • $\begingroup$ Turbulences in the atmosphere may explain the shape of the exhaust. $\endgroup$
    – Uwe
    Jun 29, 2018 at 15:54

1 Answer 1


During the earlier parts of the flight, two things are different: the atmospheric pressure is higher, limiting the expansion of the exhaust, and the rocket is moving slower and ejecting its exhaust at a high velocity relative to the atmosphere, so there's a narrow, turbulent exhaust trail which "wanders" due to winds and the gimbaling of the rocket's engines as it adjusts its trajectory.

The exhaust velocity is more or less constant relative to the rocket, about 2.8-3 km/s. As the rocket accelerates to that speed (which is reached shortly after staging), the speed of the exhaust relative to the surrounding air drops, so the turbulence smooths out. At the same time, the rocket's rising out of the atmosphere, so the exhaust expands into a large plume.

The main difference from other launches is that the rocket's rising into sunlight, which illuminates the exhaust. They're always doing this, you just can't see it during daytime launches. It's not unique, SpaceX has done a number of launches that encountered the right lighting conditions. It's also more visible with the Falcon 9 than with other rockets because of its size and fuel. Liquid hydrogen (used by the Delta IV) burns cleaner, the lack of boosters that burn out at low altitude means the first stage is rather large (about double the thrust of the Atlas V), and the upper stage has an unusually large engine due to using a vacuum version of the first stage's Merlin engine.

As an aside, the linked video is incorrect. Rocket engines function more efficiently at low pressures. Sea level nozzles are less than optimum at high altitudes because larger nozzles could get even better performance without suffering from overexpansion, but they're still doing better than they do at sea level.

  • $\begingroup$ Curious Droid is wrong..? I'm quite shocked to hear that, normally he's very thorough & I'm surprised he'd get something so basic so backwards. Could you link the timestamp where he says that? $\endgroup$ Oct 6, 2020 at 19:27
  • $\begingroup$ at 4 minutes, @AntonHengst he says the engine loses thrust with altitude. This is an ambiguity issue, as it is both true the amount of thrust is increasing and the amount of thrust lost is increasing -- because the realised thrust isn't increasing as much as the potential thrust. $\endgroup$
    – user20636
    Oct 7, 2020 at 12:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.