1. Trailspotter

    Trailspotter Senior Member

    While watching the BBC live broadcast of the first UK astronaut heading for the International Space Station this morning (11 AM GMT),

    I noted a contrail that started forming behind the Soyuz rocket at about one minute after the lift-off and stopped about ten second later:
    Screen Shot 2015-12-15 at 12.06.41.
    Screen Shot 2015-12-15 at 12.07.54.

    This suggests that rocket engines also require favourable temperature and relative humidity conditions for contrail formation. The sky above the Baikonur Cosmodrome was clear at the time, but a 100% RH was forecasted at 250hPa:
    Screen Shot 2015-12-15 at 12.39.29.

    Presumably, the contrail was formed at the time when the rocket travelled through this humid (and cold) layer. The rocket's altitude at these times must be known exactly, but I haven't found these data yet to verify this suggestion.

    A simulated Soyuz TMA Launch Multi-view with MFD & PAO commentary (Orbiter 2010) video suggests the altitudes of 11.36 km and 16.29 km for the flight times 60 sec and 70 sec, respectively. These roughly correspond to the lower and upper boundaries of tropopause, in which the ambient air is the coolest and its temperature remains fairly constant in this altitude range.

    Curiously, the above simulation shows a contrail forming at 55 second mark, suggesting that this may be a recurrent feature. Indeed, watching the multitude of youtube videos of previous launches of the same (Soyuz-FG) rocket confirmed that an intermittent contrail commonly occurs between the 60 sec and 70 sec marks, but the times of its appearance and duration vary. The contrail in the beginning of this post appears to be the earliest and the longest amongst those I've checked so far.

    This observation suggests that the ambient air lowest temperature in tropopause alone may be sufficient for the formation of this contrail, and that higher ambient humidity may increase the altitude range for its formation.

    However, there is one additional factor that may be even more important for the contrail formation in that particular time interval. At 70 sec mark, the Soyuz rocket passes through the maximum dynamic pressure (the point at which the airframe undergoes maximum mechanical stress). It is the most likely that the rocket engines are programmed to reduce the thrust shortly before reaching this point. This would result in slightly cooler exhaust and therefore more favourable conditions for its water vapour condensation upon mixing with ambient air.

    Last edited: Dec 16, 2015
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  2. Trailblazer

    Trailblazer Moderator Staff Member

    Funny you posted this - I noticed exactly the same thing! I was also wondering what altitude this corresponded to.
  3. Arbitrary

    Arbitrary New Member

    Looking at this image:

    From here.

    60 seconds corresponds to about 10km or so. Hard to read but that looks about right.
    • Informative Informative x 3
  4. Trailspotter

    Trailspotter Senior Member

    Thank you for the info. Yes, this was my estimate from visuals and simple math. The ascent to 10 km in 60 seconds would produce the g-force of ~1.5 g that looks about right. But I thought about availability of the actual telemetry data.
    Last edited: Dec 15, 2015
  5. Mick West

    Mick West Administrator Staff Member

    Soyuz rockets are liquid fueled, and burn liquid oxygen and the hydrocarbon mixture RP-1 (similar to Kerosene, or jet fuel). So the rocket exhaust is very similar to jet engine exhaust, except there's a lot more of it. The contrail however is sometimes described as a "smoke" trail, when it's actually a contrail.

    Here's an older (Nov 7, 2013) two minute exposure photo of a Soyuz launch. The bright line is just from the long exposure, but then you can see the contrail starting higher up. It's blown away from the actual path of the rocket.
    Source: http://www.nasa.gov/content/arcing-towards-orbit
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  6. Trailspotter

    Trailspotter Senior Member

    This is a great photo, but I do not think that the trail starting higher up is a condensation trail. At the two minute mark (near the buster separation time) the rocket's altitude is 28 miles. It could be some stratospheric phenomenon. In the OP video, a similar (?) trail is formed near the end of the first stage (from 2:10). it corresponds to the two minutes of flight. A video of this flight Soyuz FG - TMA-11M launch - November 7, 2013 shows a similar contrail that started and stopped at about the same times as the OP one. It occurred soon after the rocket passed the highest point of its apparent trajectory, as in the photo. Perhaps, the two-minute exposure of this photo started about 40 seconds before the liftoff.
    Last edited: Dec 16, 2015
  7. Arbitrary

    Arbitrary New Member

    You're thinking of the trail seen around booster separation - looks faintly grey below?


    I'm not sure what it is, but I might have an idea based on other videos I've watched of launches. For example, from SpaceX's CRS-6 video:


    Since this offers two views alongside each other, I'd guess it is the blacker material visible on the onboard video, in which case it might be unburnt fuel used as coolant around the nozzle. Admittedly I'm not an expert but it seems plausible. Possibly like the dark gas below the nozzle before the bright part, taken from the on the pad video of Apollo 11:


    In which case it's turbine exhaust in the case of Apollo and possibly the Falcon 9, though I think the Soyuz doesn't use a fuel rich mix of LOX/RP-1 in it's pumps.
  8. skephu

    skephu Senior Member

    It may simply be the temperature. Contrail formation does not require ambient humidity.
    • Agree Agree x 1
  9. Arbitrary

    Arbitrary New Member

    The fact that a rocket burns through a lot more fuel - and therefore produces more water vapour - than an aeroplane probably helps too.
  10. Trailspotter

    Trailspotter Senior Member

    Yes, the temperature is probably a more important factor here than humidity. However, there is a probable third factor of even greater importance, the temporary reduction of the engine thrust. See the OP update for the explanation.
    PS I've also corrected my reply to @Mick West (#6)
    Last edited: Dec 16, 2015
  11. Ross Marsden

    Ross Marsden Senior Member

    No, it does not "require" ambient humidity, but if there is some present the ambient temperature does not need to be as cold as when there isn't any.
    • Agree Agree x 2
  12. Tumeni

    Tumeni New Member

    Six videos at least, on YouTube, showing a partial contrail from a recent SpaceX rocket.

    "SpaceX Falcon 9 CRS-12 Launch from 30000 feet on Delta flight 1420!!!"

    Source: https://www.youtube.com/watch?v=b-OfW3KED-k

    Seems obvious to me the rocket is passing through the atmospheric layer usually occupied by cruising airliners, and the exhaust is producing the same type of vapour trail (contrail) that an aircraft might do, if it were passing through the same area

    So, matching the height of the SpaceX rocket to the typical airliner cruising height ....
    Last edited by a moderator: Dec 4, 2017