In the context of staged combustion, I understand that burning ox-rich creates complications associated with having hot, high pressure, oxidizer. Some engines, notably the RD-170 and RD-180 family run ox-rich. The wikipedia page for the RD-180 explains this by saying:

The engine runs with an oxidizer-to-fuel ratio of 2.72 and employs an oxygen-rich preburner, unlike typical fuel-rich US designs. The thermodynamics of the cycle allow an oxygen-rich preburner to give a greater power-to-weight ratio, but with the drawback that high-pressure, high-temperature gaseous oxygen must be transported throughout the engine.

However this block isn't cited and I haven't been able to find much in the way of corroborating sources. I have however found some message boards that mention soot buildup offhand, but again, not a lot of further reading that I can find.

Can someone provide a conclusive answer on either the soot buildup, an explanation of where a power-to-weight ratio advantage comes from, or some combination of the two?

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    $\begingroup$ I note that LOX is ~70% denser than kerosene, so you'd get more mass flow through smaller/lighter pipes going ox-rich. The gaseous expansion characteristics would be different as well but I don't know how that plays out. $\endgroup$ – Russell Borogove Sep 8 '17 at 13:57
  • $\begingroup$ oh that's an interesting take. I'd been thinking about how the density comparison might play out but never thought about pipe weights. $\endgroup$ – randomUsername Sep 8 '17 at 21:01
  • $\begingroup$ Yeah, it's probably a small thing compared to the factors Organic Marble covers in his answer, though. $\endgroup$ – Russell Borogove Sep 8 '17 at 21:04
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    $\begingroup$ @RussellBorogove Higher density working fluids lead to less work potential in turbines. Higher density is a con here. The coking is the big issue with fuel-rich preburners. Oxygen rich isn't particularly Russian rockets. The Russians just do staged combustion with everything, the west just does it with O2/H2. $\endgroup$ – Rikki-Tikki-Tavi Sep 8 '17 at 22:05
  • $\begingroup$ hey @everyday-astronaut I recently watched your raptor video (for at least the 3rd time) wherein you mention coking as one of the primary factors for methalox selection. Out of curiosity was this post a source for that project or did you find other sources? $\endgroup$ – randomUsername Apr 4 '20 at 2:11

According to the super-detailed and very informative NASA-CR 165404 Fuel/Oxidizer-Rich High-Pressure Preburners, the primary advantages of hydrocarbon-fueled oxidizer-rich staged-combustion engines is that "carbon formation, coking, and the attainment of ignition are no longer issues." (page 133)

This useful rocket engine cycle overview presentation from Purdue University's AAE 539: Advanced Rocket Propulsion class also mentions only the coking issue as an advantage.

The book "Progress In Astronautics and Aeronautics: Liquid Rocket Thrust Chambers", excerpted at Google Books here, may provide a clue as to the claimed power-to-weight advantage. It shows an analysis that concludes for equal turbine temperatures and pump discharge pressures, an oxidizer-rich cycle has an 87% increase in chamber pressure over a fuel-rich cycle. This would seem to imply that a smaller oxidizer-rich engine could produce the same thrust as a larger fuel-rich engine. Unfortunately, the book is only excerpted there, and I don't own a copy.


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