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On videos of Soyuz docking to ISS we can see exhaust from the control engines (if background is dark enough). For example here

Soyuz TMA-19M docked to ISS at December 15 2015. Exaust plume is circled at screenshot:

enter image description here

The textbook chemical formula of the NDMH and nitrogen tetroxide reaction is:

H2NN(CH3)2 + 2*N2O4 -> 2*CO2 + 3*N2 + 4*H2O

My question is:

How are visible ice crystals are forming in the exhaust? The molecules of exhaust products should be too hot and moving too fast... Or any other products occur in the reaction?

Also I wonder about the contamination of spacecraft by hypergolic exhaust products. The Hubble telescope has no reaction engines because of this concern. The ISS has limitations for use of reaction engines in close proximity. But if exhaust products are nitrogen, carbon dioxide and water - can they cause a long-term contamination, not just icing? Or they can react with metals because of their high speed?


EDIT

I have read "Rocket propulsion elements" by Sutton and Biblarz (google "rocket propulsion elements pdf"). A lot of interesting info, including chapter 18 about exhaust plumes. But not so much for my question.. About contamination of spacecraft the book only mentions "hydrazinum nitrate and other materials were found".

So, to reformulate the question:

  • What are the visible products of reaction engines' exhaust in the video? I suppose they should be in solid or liquid phase, because a gas could not scatter enogh light.

EDIT 2

Summarised some points in self answer

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    $\begingroup$ On videos of Soyuz docking to ISS we can see .... Can you add a link to one of these videos? $\endgroup$ – Digital Trauma Jun 4 '18 at 21:54
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    $\begingroup$ Yes it will be helpful to add a link to an example video. Also there's no reason to expect that chemical reactions are 100% complete, there could be incomplete reaction products as well, even traces of unreacted material. Also, things don't stay "hot" as they expand and radiate in the vacuum of space for \very long. $\endgroup$ – uhoh Jun 5 '18 at 1:40
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    $\begingroup$ 1) Who said those are ice crystals? 2) Thruster firings, especially short burps like that, often leave a fair amount of unburned propellant that can contaminate ISS surfaces, especially when it gets baked on by UV light from the sun. The restriction on engine firings near ISS are also due to plume loads and mechanical damage from propellant droplets. $\endgroup$ – Tristan Jun 5 '18 at 16:42
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    $\begingroup$ Something to keep in mind: Think the rocket burn is perfect? Perfect mixing so every bit of both chemicals is reacted? $\endgroup$ – Loren Pechtel Jun 6 '18 at 2:47
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    $\begingroup$ See this NASA paper about bipropellant thrusters with high speed droplets of unburned and partially burned propellant. $\endgroup$ – Uwe Jun 7 '18 at 17:14
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Well, after useful comments and some research I can summarise:

The chemical reaction of bipropellant hypergolic fuel is more complex:

1) There are more products of the reaction

2) Some part of the fuel remains unburned

In this link by @Uwe is stated that exhaust plume contains microdroplets of unburned propellant, with size up to several micrometers. Quote:

Contaminant mass is predicted to be less than 2% of total propellant

In this paper a formula of hypergolic bipropellant burning products is given as:

4a*CH3N2H3 + 5*N2O4 -> np1*CO2 + np2*H2O + np3*O2 + np4*N2 + np5*NO + np6*CO + np7*OH + np8*H2 + np9*O + np10*H + np11*N + np12*NO2 + np13*H2O2 + np14*HO2 + np15*HNO

where a, np1-np15 are coefficients to be found. But unburned propellant is not taken into account there.

In this pdf the basics of plume impingement are given. It has a picture of unburnt propellant droplets inside engine's nossle:enter image description here Also interesting detail - exhaust plume in vacuum has "back flow", so small part of it spreads in opposite direction and it should be taken to account as contaminant:enter image description here

In recent (Jun 8 2018) video of Soyuz MS-09 docking to ISS not only plumes could be seen, but also low-speed ice particles (start at 6.15)

I suppose that particles are frosen droplets fron engine's nozzle.

I would say the question is partly answered. But there is one thing that I have completely forget:

The first stage of Falcon-9 rocket has nitrogen thrusters. So, the plumes of them should contain only nitrogen. But on videos we see the plumes of the nitrogen thrusters! So, my initial suggestion that gases like nitrogen form ice crystals in exhaust plume is correct. But I wonder - how it happens?

This question deals with it. Links there are interesting, but not completely relevant, I think. The nitrogen is in gaseous phase when it exits a nozzle (of nitrogen thruster or hypergolic thruster). So it's rather "conterintuitive" that the gas spreading in vacuum forms ice crystals. The gas "should" only spread ever more in vacuum, not condense..

At this moment I could not find any reference on physics of this process. If anybody could explain it, the question would be completely answered.

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    $\begingroup$ On Earth compressed nitrogen may be liquified by expansion. This should be possible in space too. If the temperature is very low, you may get solid nitrogen. The LOX for rocket engines is made by compressing air, liquifying it by expansion and separating it by destillation. Without that process, no cryogenic propellants would exist. $\endgroup$ – Uwe Jun 9 '18 at 16:19
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In-short, the answer is that at the beginning and especially end of an RCS thruster firing, there are clouds of unburned reactants. Source (for the shuttle, which has a very similar chemistry):

In a discussion with a NASA aerospace engineer familiar with the space shuttle reaction control system, I learned that the thrusters never generate any light while operating, but they always emit a small cloud of unburned propellant just before the thruster fires and a much larger cloud immediately after the thruster shuts down.

[...]

When the valves are closed to shut the thruster down, small amounts of propellant are trapped in the tubes between the valves and the combustion chamber. [... In] the vacuum of space, the "microscopic snow" also forms after shutoff just as it does at startup. But[, in this case,] the dribble volume is large enough that the snow generated can be seen as a white plume in reflected sunlight. It is totally invisible without some external source of illumination.


Note: there are a few alternate theories I came across which, while apparently less-plausible, may still at least contribute to the effect. I have seen (less-credible) sources suggesting that either the fuel or the oxidizer is injected first (for safety purposes, presumably), leading to unconsumed propellant/oxidizer, and that in longer OMS burns the plume is invisible. Also, CO₂ ice would be present in addition to water ice.

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    $\begingroup$ Um, your source is a website called "the living moon" which claims to have pictures of buildings on the moon, etc. It is a fringe website and an unreliable source. $\endgroup$ – Organic Marble Jun 9 '18 at 2:28
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    $\begingroup$ @OrganicMarble Okay fair enough, that was the only article I skimmed from the website, and the root domain indeed looks pretty bonkers (I saw this explanation a couple other places too, but this seemed to be perhaps the original source). I contend that the explanation still makes sense, though. $\endgroup$ – imallett Jun 9 '18 at 5:12

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