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I'm trying to do some background research on the coking process that occurs in rocket engines, but haven't found any resources that fit what I'm looking for.

To be more specific, I'm just looking for any resources that give a background introduction to coking, the different ways that coking can occur, maybe the chemical formula that drives its formation, and the parameters that affect coking, etc.

I found a PDF by ExxonMobile that gives some basic information on coking, but I'm unable to find any further resources. If anybody has an in-depth understanding of coking and can point me to any resources, that'd be greatly appreciated!

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  • $\begingroup$ Well, there's this semanticscholar.org/paper/… but it's paywalled. Sutton references this paper but his discussion of coking is very brief. Another brief discussion here ntrs.nasa.gov/citations/19820002372 focuses on why it's less of a problem in ox-rich combustors. $\endgroup$ May 4 at 19:20
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    $\begingroup$ @OrganicMarble I found an online PDF for the paywalled article, thanks for the references! Will begin reading :) $\endgroup$
    – Frank W
    May 4 at 22:03
  • $\begingroup$ I found another paper which was openly available, so I posted an answer. $\endgroup$ May 4 at 22:39

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In addition to the paywalled paper Investigation of Heat Transfer and Coking Characteristics of Hydrocarbon Fuels (which is cited by Sutton among others), I found this openly available one.

Heat Transfer and Deposition Behavior of Hydrocarbon Rocket Fuels

A major issue in fuel cooling by hydrocarbons is deposit formation ("fouling", "coking").

Addressing "parameters that affect coking", here's dependence on temperature and flow velocity.

enter image description here

Some of the references in this paper may also be useful.

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    $\begingroup$ Do you know of any resources on the process that leads to coke forming? This PDF I found (apps.dtic.mil/dtic/tr/fulltext/u2/a533333.pdf) says that coking forms due to oxidative, catalytic, and pyrolytic mechanisms and I haven't been able to find anything else about either of those mechanisms $\endgroup$
    – Frank W
    May 5 at 0:07
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    $\begingroup$ @FrankW I don't but I will keep looking. $\endgroup$ May 5 at 0:12
  • $\begingroup$ It looks to me like the Rosenberg paper above is concerned with coking in the use of hydrocarbon fuels as a rocket cooling media; not in the combustion chamber itself. $\endgroup$
    – tckosvic
    May 5 at 14:37
  • $\begingroup$ @tckosvic that's exactly what coking is defined as."Kerosene is a hydrocarbon fuel that becomes gummy and deposits a hard film on engine components as its temperature increases – a process known as coking. " sciencedaily.com/releases/2002/08/020806080142.htm $\endgroup$ May 5 at 15:09
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RP-1 can be simplistically thought of as a mixture of hydrocarbon chains with an average carbon chain length of 12 and an average of 26 hydrogen.

enter image description here

In idealized stoichiometric combustion with oxygen, the products are only CO2 and H2O. In real world combustion, particularly with inadequate oxygen, the carbon chains can combine into large molecules composed mainly of carbon.

enter image description here

https://www.ariyancorp.com/petroleum-coke-petcoke

As airborne particles, this is known as soot. As a solid, it is coke.

A stoichiometric mixture of RP-1 and oxygen can burn at 3801C which is significantly above the melting point of stainless steel (1530C). This temperature would consume the preburner turbines for rocket propellant pumps. As a result, preburners are run deliberately oxygen-rich or fuel-rich. Fuel-rich preburners produce very sooty exhaust, as seen in this Merlin test fire: enter image description here

Fuel-rich conditions inside an engine can deposit coke as seen in this jet engine fuel nozzle:

enter image description here

Credit: NASA https://www.omnia.ie/index.php?navigation_function=1&navigation_collection=National+Archives+at+College+Park+-+Still+Pictures&repid=2

The conversion of kerosene to coke can occur outside combustion chambers any time it is heated above the coking limit of about 300C. This can happen when fuel is in contact with hot engine parts, particularly during shut-down when cooling flow ceases and latent heat remains.

Coking is particularly a problem in passages used for fuel cooling of combustion chambers. Coke deposits can also change clearances between mechanical parts.

Fuel coking was a significant risk for the SR-71 since the fuel was used as airframe coolant and engine lubricant. It was heated to 270C before being burned as fuel. The SR-70 burned JP-7 to counter the potential coking problem. https://en.wikipedia.org/wiki/JP-7

Exxon has a .pdf primer on coking at https://www.exxonmobil.com/en/aviation/knowledge-library/resources/engine-oil-coking

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    $\begingroup$ This doesn't answer the question, which asks for references, and also is describing the wrong phenomenon. Coking takes place in the fuel, not in the combustion products. "Kerosene is a hydrocarbon fuel that becomes gummy and deposits a hard film on engine components as its temperature increases – a process known as coking. " sciencedaily.com/releases/2002/08/020806080142.htm $\endgroup$ May 4 at 21:36

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