Since hydrogen-fueled rockets usually require Radial Outward Firing Igniters (ROFI), what is the temperature of the sparks from this system? When seen at RS-25/RS-68 ignition, they don't melt but seemingly flow down with the rocket exhaust into the flame trench.
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Radially Outward Firing Igniters (ROFI) are a subtype of Hydrogen Burn Off Igniter (HBOI). PACSCI EMC, a manufacturer of HBOI, specifies their HBOI output sparks “at least” 1,500*F. https://psemc.com/products/hydrogen-burn-off-igniters/#:~:text=Another%20common%20version%20of%20this,accumulation%20of%20hydrogen%20during%20launch
The ignition temperature of hydrogen in air is about 1100*F https://seshydrogen.com/en/safety-of-hydrogen-systems/#:~:text=The%20standard%20auto%2Dignition%20temperature,compared%20to%20long%20molecule%20hydrocarbons
However. The fact that sparks are visible as yellow light implies a black body temperature of over 3000*F
My understanding is that ROFI, an early design of igniter, used a small solid fuel rocket motor with an annular discharge ring replacing the nozzle. Hence the “radially outward firing”. The design quoted above has a linear discharge with a specified “throw” distance. Although it is not “radially outward firing”, they are still informally referred to as "ROFI"s since "HBOI" is harder to say.
I don’t have a solid reference for this, just Tweets by Tony Bruno (CEO United Launch Allaince) and Scott Manley https://twitter.com/torybruno/status/1311552569530081280?lang=en
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$\begingroup$ I'm not sure "H-boy" is actually harder to say than "rofi", but still, habits are hard to break. $\endgroup$ Commented May 4, 2023 at 14:30