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While watching today's live stream of the launch of the China National Space Administration (CNSA) Long March 3B rocket carrying Chang'e 3 lunar exploration probe and the Yutu rover to the Moon, @Everyone has noticed (chat room transcript with to launch relevant links and video screengrabs) that the rocket exhaust appeared unusually reddish during early launch phase while the rocket was clearing the launch tower.

Rewatching some launch videos (with English commentary, and without), I've established that this red gas only appears at the first few seconds of the burn of the first stage and booster engines, and the exhaust plume later appears normal. Some cameras show it slightly more yellowish than others, but that could be, I guess, put down to calibration and color profile of individual cameras used for the transmission. Knowing what I was looking for, I found this nice photograph of the early launch phase of Long March 3B on Chang'e 3 mission:

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   Long March 3B rocket on Chang'e 3 mission lifting off and clearing the launch tower (Image credit: CCTV / CNSA)

This red gas somewhat reminds me of the cold expansion gas used to launch ballistic and cruise missiles, and if that's it, can also be seen during some rocket launches of into orbital launch vehicle converted ICBM (Intercontinental Ballistic Cruise Missile) systems, for example the silo-launched ISC Kosmotras' Dnepr-1 rocket that pops out of an underground silo with the help of some red and presumably cold expansion gas and ignites its main engine mid-air (also seen in e.g. this YouTube video):

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                                    Silo-launched Dnepr-1 rocket on Terra SAR-X mission (Source & gallery)

So my question is:

What is the red gas at the exhaust of the Long March 3B rocket during liftoff, and does it have anything to do with that red expansion gas used for silo-launched and likely submarine-launched ICBMs and converted into orbital launch vehicle ICBMs? What is it, and why would the first stage or perhaps booster rockets of the Long March 3B use it?

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  • $\begingroup$ A few more photographs of the early launch of the Long March 3B rocket on the Chang'e 3 mission can be seen in this Spaceflight101 mission overview article. $\endgroup$
    – TildalWave
    Commented Dec 1, 2013 at 20:49
  • $\begingroup$ The German Spiegel magazine was running a photo of the daytime launch of a Beidou mission on a Long March 3B. It shows a striking red cloud, clearly not from illumination by a flame, which is why I came here. By the way, that N2O4 is pretty poisonous. $\endgroup$
    – Peter - Reinstate Monica
    Commented Jun 23, 2020 at 11:10

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That's unburned $\mathrm{N}_2\mathrm{O}_4$. The zeroth and first stages of the Long March use $\mathrm{N}_2\mathrm{O}_4$ and $\mathrm{UDMH}$. The Dnepr uses the same propellants. I doubt that the red gas you see is from the cold launch system. That system uses a black powder mortar.

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  • $\begingroup$ Cheers, didn't know dinitrogen tetroxide is red. So the color at the exhaust actually shows how well it is mixed with hydrazine at fuel / oxidizer injectors and the efficiency of the engine? I don't mean these first few seconds, but also later on. And another funny thing, dinitrogen tetroxide is apparently completely white at -196°C and below and gets darker red as temperature increases. I learn each day, thanks again! :) $\endgroup$
    – TildalWave
    Commented Dec 2, 2013 at 0:59
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    $\begingroup$ Yes, whenever rocket engines start up, it takes a little bit for them to get to equilibrium chamber pressure, temperature, and flow rates. Until they do, you're bound to get all kinds of stuff coming out, including unburned oxidizer or unburned fuel, partial combustion products, etc. If a storable-propellant biprop engine is deliberately running oxidizer rich, then yes, you will see some red all the time. However I am not aware of any like that. I'd expect that they are run even or a little fuel rich. $\endgroup$
    – Mark Adler
    Commented Dec 2, 2013 at 1:40
  • $\begingroup$ @tidalwave dinitrogen tetroxide is actually colourless, however it is in equilibrium with its dimer nitrogen dioxide which is brown. This equilibrium is affected by temperature, the higher the temperature the more brown nitrogen dioxide is present. $\endgroup$
    – Mike H
    Commented Sep 12, 2020 at 20:33
  • $\begingroup$ Sorry, a typo I was too late to edit. Dinitrogen tetroxide is the dimer, nitrogen dioxide is the monomer. $\endgroup$
    – Mike H
    Commented Sep 12, 2020 at 20:44
  • $\begingroup$ @MikeH You can always delete comments and rewrite them, to make it look you got it all correct the first time. :-) $\endgroup$
    – Mark Adler
    Commented Sep 12, 2020 at 21:47
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From to @TildalWave's comment

So the color at the exhaust actually shows how well it is mixed with hydrazine at fuel / oxidizer injectors and the efficiency of the engine? I don't mean these first few seconds, but also later on. And another funny thing, dinitrogen tetroxide is apparently completely white at -196°C and below and gets darker red as temperature increases.

enter image description here

Source

enter image description here

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