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Whenever Falcon 9's engines start, there's a green ignition flash. During the latest landing there were similar green flashes right after touchdown:

1st stage landing (3 camera angles):

Why were there ignition flashes AFTER the Falcon actually landed?

Note: It seems not all landings have this flash. See footage from the OrbComm mission landing:

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    $\begingroup$ The answer must involve the TEA-TEB ignition, though it does seem strange that any uncombusted igniter would be being released at that point. $\endgroup$
    – Russell Borogove
    May 25, 2016 at 4:20
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    $\begingroup$ Safing procedures so there is none volatile when the crew boards? $\endgroup$
    – jkavalik
    May 25, 2016 at 4:22
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    $\begingroup$ I have one "wild" theory - The valve is probably not directly adjacent to the chamber, so there is a nontrivial length of plumbing containing unburnt TEA-TEB which, after the valve closes, has less pressure than the fuel+LOX being pumped and burned in there. What if this residual amount starts dripping after the engine is stopped and the pressure drops enough, reacting with residual LOX and oxygen in the air. (Orbcomm used longer single-engine landing burn, but no idea if that might explain the difference.) $\endgroup$
    – jkavalik
    May 25, 2016 at 8:58
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    $\begingroup$ Possible duplicate of What causes the green flash during Falcon 9's ignition? $\endgroup$
    – user
    May 25, 2016 at 11:22
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    $\begingroup$ This question is not a duplicate. It's obvious why TEA-TEB is burning at ignition, but not obvious why at landing. $\endgroup$
    – Russell Borogove
    May 25, 2016 at 14:14

1 Answer 1

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The green colour is characteristic of TEB (Triethylborane) combustion, which is used in combination with TEA (Triethylaluminum) to ignite the Merlin 1D engines (those two substances are pyrophoric, and are used to kickstart the RP1/Oxygen combustion).

At least three of the nine engines are equipped with an onboard supply of TEA-TEB for in-flight relights. During the processing of the CRS-8 core in Port Canaveral, the remains of those reserves were dumped on the ground to safe the stage for transport. It's possible the first stages are now commanded to dump as much as possible as soon as the landing is confirmed, to reduce the risks & time needed once the stage is brought back to land (some was still left in JCSAT's first stage though, we could see the green flashes during Port Canaveral processing)

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  • $\begingroup$ @fasterthanlight: those substances are indeed pyrophoric, spontaneously igniting on contact with air, but they are used to ignite the Merlin because they are hypergolic with liquid oxygen. Notably, the upper stage engine ignites in near vacuum and may do additional ignitions in orbit, with no air. $\endgroup$
    – Christopher James Huff
    Jun 11, 2022 at 19:33
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    $\begingroup$ @ChristopherJamesHuff I am aware of this, but hypergolic leads to the incorrect assumption of TEA-TEB being hypergolic with each other. Pyrophoric does mean air, but more specifically oxygen as it is the main oxidization agent in air. $\endgroup$
    – fasterthanlight
    Jun 11, 2022 at 23:21

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