In Ignition, Clark describes an interesting approach to thermal control in rocket engines using oxygen as an oxidizer and hydrocarbon fuel: A small amount (few percent) of silicone oil or another silicon-bearing flammable chemical was mixed with the fuel, which would lay down a thermally-insulating layer of quartz on the inside of the engine. This was continuously ablated, but it also was continuously applied as the silicon carrier combusted.

Did anything happen to this idea? Did it ever get out of the laboratory, or become anything more than a curiosity? Is it pointless with modern developments in regenerative cooling?

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    $\begingroup$ I've seen it used with the Copenhagen suborbitals engine (the smaller one) but I'm not sure if the kept that in the final flight operated iteration. $\endgroup$ – Reuben Farley-Hall Dec 10 '20 at 13:01

In Sutton's History of Liquid Propellant Rocket Engines (2006) he states the following about the silicon oil method:

  • "This method was not fully reliable and was abandoned at GE, RMI, and Rocketdyne. However it apparently was successful at ARC, where the Agena engine (16,000 lbf) ran successfully with a 1% silicon oil addition to the UDMH fuel in 1971 and 1972."* (p.49)
  • This Agena engine is the only flying engine known to the author to successfully use this internal coating method of heat-transfer reduction. (p.523)

So it seems to be abandoned as a technology. However, I am not sure if anybody tried to use this method of thermal control after 2006.

A longer part from the book on the topic, going in to detail why most companies abandoned it:

  • The concept of a self-renewing internal silica insulation coating was developed originally at General Electric Company in the early 1960s, but with LOX/kerosene or LOX/jet fuel as propellants. It was also tried by Reaction Motors, Inc., and Rocketdyne, but with mixed results. Although ARC liked the silicon coating and seemed to have good results, the other companies did not pursue it because it caused occasional local burn-throughs. There is a plausible mechanism for these failures. The silicon-oxide insulation layer at GE and Rocketdyne was occasionally relatively hard and crusty and not always as fluffy and low in density as the coatings at ARC. When a piece of the brittle insulation broke off (and was discharged through the nozzle), a sharp edge formed at the remaining hard insulation layer next to the chamber wall. At this edge the hot gas can reach temperatures close to the stagnation temperature in the combustion chamber. The local heat transfer would increase greatly,and the local wall temperature could at times quickly exceed the melting point of the wall material, resulting in a local wall failure. Also the coating caused the chamber and throat diameters to decrease, causing an unpredictable modest rise in chamber pressure and a change in engine performance. The author does not know what happened at RMI, but this idea of a self-forming thermal insulation silicate layer was abandoned at General Electric and Rocketdyne. It might have been the propellant combination of IRFNA/UDMH that gave a fluffy coating without sharp edges.

Link to pdf of book: https://www.pdfdrive.com/history-of-liquid-propellant-rocket-engines-d158227714.html

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    $\begingroup$ Thanks for the link! $\endgroup$ – Organic Marble Dec 19 '20 at 17:43
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    $\begingroup$ Oh my, thank you. This is extremely interesting! Did flight versions of the agena actually use this? $\endgroup$ – ikrase Dec 19 '20 at 23:09
  • $\begingroup$ The first quote does say so. It should be noted that Sutton gives "Personal communication, Fred Baroody, ARC, 2003" as a reference for that statement, so not the most reliable, but I don't really see why he wouldn't speak the truth. $\endgroup$ – Ruben Dec 20 '20 at 15:23

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