Here’s why SpaceX’s SN8 Produced that Green Flame & Crashed
I was honestly just trying to figure out why SN8 produced that bright green flame.
Turns out it’s the same reason it crashed.
In a tweet, Elon Musk explained:
“…Fuel header tank pressure was low during landing burn, causing touchdown velocity to be high & RUD, but we got all the data we needed!! Congrats SpaceX team hell yeah!!”
“RUD” is an acronym for Rapid Unscheduled Disassembly.
The Raptor engine didn’t exactly fail, but due to insufficient fuel pressure, the engines became fuel starved and consequently produced inadequate thrust to maintain the necessary rate of deceleration for a safe landing.
At first, it appeared that the engine may have shut down and was being restarted with TEA-TEB (Triethylaluminum + Triethylboron), a mixture used to ignite liquid oxygen engines and this might explain the bright green flames.
However, that seems to not be the case since SN8 used spark/torch igniters to ignite it’s fuel.
So, what caused the green flame?
It seems there was “engine-rich combustion” which is my new favorite term if I can remember it when I need it!
Effectively, the low fuel header tank pressure caused the fuel/O2 mixture to become lean, generating much more heat and consequently, certain alloys in the engine internals began to break down and burn. Presumably some exotic, high-temp alloys. Hence: “Engine-rich”.
When this happens, the copper which lines the nozzles and/or “engine bell” begins to burn, as well, producing a green flame!