Rocket Rambling
When starting a liquid rocket engine, the pragmatics of valve timing, dribble volume, turbopump spin up (if applicable) and other transient fluid dynamics mean that you're going to naturally have one propellant "lead" the other into the chamber.
It's common design practice to bias this intentionally toward one propellant or the other to give a consistent startup behavior. So, which one? Do you bias toward a fuel lead or an oxidizer lead? Well, that depends on the propellant.
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The disadvantage of an oxidizer lead is that you will have a transient operating condition where you have a high flame temperature AND an oxidizing environment. This is an environment in which metals burn. Steady state operating is usually fuel rich.
In hydrogen engines, these concerns have led designers to generally use a fuel lead startup. But hydrogen has a unique property that makes this the obvious choice, it has nearly symmetrical flammability limits.
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Try an ox lead with methane, and the result is similar. Ignites once you reach 5%vol, and reacts with 10%vol oxygen. 85%vol doesn't participate. Similar to a fuel lead with hydrogen.
Now, try a fuel lead with methane. Introduce oxygen into a chamber filled with gaseous methane. Ignition doesn't occur until you hit 40%vol oxygen, which reacts with 20%vol methane. 60% of the gas volume in your combustion chamber ignites.
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Now, this is all a very simplified model for very complex combustion physics, and the specific conditions of a given engine may vary, but it is still instructive. The takeaway is this. Most, if not all, hydrogen engines start with a fuel lead. Hydrocarbon engines often don't.
For hydrogen, it makes sense to avoid the hot, oxidizing transient point during startup, and precluding a hard start is relatively easy either way.
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