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NROL-44 failed to be in space for yet one more night last night, this time because of an automated hotfire abort triggered when one of the outboard (iirc) engines on the Delta IV Heavy failed to ignite. Turns out this isn't uncommon for the Delta IV (much to my surprise).

By what mechanism do RS-68s light and how do they fail to do so?

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Partial answer (covering By what mechanism do RS-68s light?):

The RS-68 uses electrically fired solid-propellant igniters in the gas generator and main combustion chamber.

It also uses a helium spin line (purple arrow) to start the turbopumps rotating.

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Sources:

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