This link Optimum Mixture Ratio shows a range of mixture ratios for Lox-Kerosene of 2.1 to 2.45. Its not clear to me where in this range is the stochiometric case. Some Russian designs are higher (RD-180 being ~2.7) while the Merlin 1D is 2.34 and the F1 was 2.27 (all according to Comparison_of_orbital_rocket_engines).

Question. In the relatively fuel rich cases is there unburned kerosene and what happens to it after leaving the engine?

My main interest is the environmental impact. I'm also interested, as an aside, to understand whether the main contribution of unburned kerosene is from the main exhaust or is more a feature of gas generator engines that dump the turbine exhaust separately.

Graph from the first link: enter image description here

EDIT inspired by comment: This link Liquid fuel / Oxygen proportions hints that stochiometric could be in the range 2.58-2.77.


TLDR: The combination burning of hot kerosene and sun-driven decomposition of any remainder gets rid of it quite quickly.

From an environmental point of view, this is the bottom line:

It is predicted from indirect photolysis modeling of C9 and C16 paraffinic, naphthenic, olefinic, and aromatic hydrocarbon compounds that volatile components in kerosenes/jet fuels will undergo atmospheric oxidation and not persist in the environment.

(See the Kerosene/Jet Fuel RSI)

Kerosene at even somewhat elevated temperatures (well below 100C) will evaporate, and the vapor will then start to oxidize. Anything in droplet form at 60C or above will flash burn instantly when it reaches atmospheric oxygen. Heat it to higher temperatures, above 120C, and it'll start to disassociate into smaller molecules and radicals, which makes the subsequent reactions even faster.

It's hard to imagine kerosene getting through the combustion path without reaching a temperature high enough to ignite it once it reaches air. That’ll reduce it to CO, CO2 and water pretty quickly.

  • $\begingroup$ Thanks for the link, very interesting and looks like a good start. However its clear that 60deg is the flash point with an ignition source rather than the auto-ignition point (210 degC according to Wiki). That doesn't invalidate your hypothesis in isolation. On a separate topic though, does the nozzle and expansion cone not result in the exhaust being substantially cooled by the time that it meets the atmosphere? $\endgroup$
    – Puffin
    Aug 3 '18 at 13:02
  • $\begingroup$ @puffin In-atmosphere engines have hot exhaust. Have you noticed the flame-like appearance of the exhaust? (Vacuum engines are somewhat different) Hydrocarbon vapor is going to ignite when it reaches air for the same reason it happens in a candle flame: it can't cool before it reaches air. And the quote shows that even small parts that make it to air won't persist, because they'll oxidize spontaneously. $\endgroup$ Aug 3 '18 at 13:47
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    $\begingroup$ @Puffin Substantially cooled, but still quite hot -- I couldn't find a good exhaust exit temperature figure for kerolox, but it's likely over 1000ºC. $\endgroup$ Aug 3 '18 at 13:57
  • $\begingroup$ @Puffin it’s not a well-defined single T: different parts of the flow have different histories, there’s shock rehearing, etc. but it’s well above ignition temperature. $\endgroup$ Aug 3 '18 at 13:59
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    $\begingroup$ @uhoh photolysis goes to “what if it doesn’t burn?” Or perhaps “even if a tiny part doesn’t burn, what happens to that? Does it hang around and Do Bad Things?” (C.f. A long tedious Quora thread on fuel dumping by aircraft). Kerosene has a short atmospheric lifetime, even absent combustion, due to photolysis. $\endgroup$ Aug 4 '18 at 2:21

This issue deserves a lot more attention than it currently gets. According to research, microscopic soot particles develop that could persist in the atmosphere for a long time. This is essentially different from soot produced on the ground, because it is not well-known what happens to those particles above the troposphere, i.e. when there is no rain to "wash it out" and less/no/different convection. Especially the absence of vertical winds might contribute to the long lasting of the soot.

Possible impacts include a heating of the upper layers of the atmosphere which could lead to degradation of ozone, with all the known implications.

Bottom line is that neither the remains nor their environmental impact are well understood.

Side notes, not directly addressing the question: the linked article article also talks about other exhaust products, such as aluminium oxide from solid fuels. Personally, I wouldn't be surprised if it turned out that even the water vapor has a significant negative effect on the Earth's climate. After all, it is a strong greenhouse gas, which supposedly does not leave the upper atmosphere well because it is lighter than nitrogen and oxygen.


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