Which are more efficient: oxidizer-rich or fuel rich rocket engines?

Question

Several internet articles claim that the high Isp of the Soviet RD-180 engine (as used in the Atlas III and V launch vehicles) was (at least partially) due to the fact the RD-180 ran oxidizer-rich, unlike its fuel-rich American contemporaries. These articles provided no references to support the assertion of higher efficiency for oxygen-rich combustion.

EDIT: According to Wikipedia, the combustion chambers of the RD-180 ran fuel-rich. It was their LOX pre-burner which ran oxygen-rich.

EDIT:https://ntrs.nasa.gov/search.jsp?R=19820002372 deals with relative advantages of oxidizer- and fuel-rich pre-burners but not combustion chamber mix ratios.

My understanding is that maximum temperature (and combustion efficiency) is attained with a stoichiometric mix of fuel and oxidizer. However, these high temperatures exceed the service temperature of available engine materials. This relationship is illustrated by data for methane in gas turbines:

https://www.researchgate.net/publication/267643923_DME_as_a_Potential_Alternative_Fuel_for_Gas_Turbines_A_Numerical_Approach_to_Combustion_and_Oxidation_Kinetics/figures?lo=1

As a result, chemical rocket combustion chambers are designed to run fuel- or oxidizer-rich for durability reasons. Either fuel- or oxidizer-rich conditions will lower combustion temperature, but fuel-rich has the added advantage of lower exhaust gas molecular weight and therefore higher exhaust velocity ( Isp).

As a result, I would expect rocket designers to favor fuel-rich conditions to maximize Isp efficiency.

Fuel-rich engines have coking problems. Oxidizer-rich engines are very hard on metal parts. Both these are disadvantages but do not affect theoretical efficiency. Which strategy is objectively more efficient (e.g.: Isp or TWR)? Are there any successful oxidizer-rich engines?

Answer 1

Regardless of whether oxidizer-rich engines would be more efficient, the RD-180's efficiency is not due to that, as the RD-180 runs fuel-rich. According to braeunig.us, for each carbon atom in RP-1 there are 1.953 hydrogen atoms, so a stoichiometric ratio would be something like 3.4:1. The RD-180 runs at a O:F ratio of 2.72:1, definitely on the fuel-rich side of stoichiometric.

What distinguishes the RD-180 and similar Russian engines is not that they run oxidizer rich (as they don't), but that they are staged combustion engines. This does several things, but the most significant is that the working fluid used to pump propellant is injected into the combustion chamber to be combusted further and used as reaction mass, so much higher pumping powers can be used, allowing higher chamber pressures than gas generator engines. Their preburners run oxidizer rich, which allows them to do staged combustion with carbon-rich fuels that would otherwise cause coking that would clog the turbine passages and engine injectors.

At the time these engines were developed, the only staged combustion engines developed in the US used fuel-rich preburners with hydrogen fuel which didn't produce coking, and the only kerosene-burning engines used gas-generator or other cycles that were better able to handle the coking issues. Being able to use staged combustion gave kerolox engines like the RD-180 a major performance edge over other kerolox engines, while retaining the thrust and ease of handling advantages of kerosene fuel.