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Accompanying a newsreader's report of North Korea's launch of a new long-range (claimed up to 10,000km) missile is footage of a launch in which the rocket produces clouds of orange/brown smoke. According to this Wikipedia page, it would seem to be the Hwasong-14, said to be based on a Russian design burning dinitrogen tetroxide and UDMH.

Assuming the preceding is correct, what are the combustion products or by-products producing that nasty-looking and presumably toxic smoke? One might make the simplistic assumption that the fuel components would react to produce carbon dioxide, water, and nitrogen. Even if those are the primary reaction products, there is clearly other stuff being emitted. What is it, and in what sort of quantity? Would it be a chemical hazard for anyone downwind of the launch outside the range of explosion hazard?

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  • $\begingroup$ Closely related to space.stackexchange.com/questions/2982/… $\endgroup$
    – Russell Borogove
    Sep 4, 2017 at 3:27
  • $\begingroup$ Please add a reference to that newsreader's report, and a picture as well. $\endgroup$
    – user10509
    Dec 1, 2017 at 12:24
  • $\begingroup$ @JanDoggen It was a TV news broadcast. Now too long ago to remember any detail, but you can easily see what I was asking about in any of a number of Youtubes, such as this very recent one: youtu.be/dq4jfpZ56eA?t=326 $\endgroup$
    – Anthony X
    Dec 2, 2017 at 2:27

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The visible reddish-brown component is mostly nitrogen dioxide, produced by breakdown of the nitrogen tetroxide oxidizer used in the engine.

Ideal combustion of N2O4 with hydrazine-family fuels like UDMH indeed produces mostly water, nitrogen, and H2, plus ammonia if run fuel-rich, but it's not unusual for the engine to run with incomplete combustion at startup, as Mark Adler describes in a comment here:

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.

You can see it in launches of the Russian Proton launcher or US Titan II as well.

Both the fuel and the oxidizer in these hypergolic motors is toxic and nasty stuff. When burned completely the resultants aren't bad, but the plume at startup is definitely hazardous.

You can conveniently compare the size of the initial cloud to the size of the explosion hazard in this video of a failed Proton launch.

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This answer clarifies a detail over Russell Borogove's answer.

At typical surface conditions N2O4 is colourless but dis-associates readily into NO2. The latter is the gas that has the reddish brown colour; it is also a hazardous (toxic) gas.

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