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I was looking at the Thaicom 8 falcon 9 launch and landing, trying to get a bit more info about the trajectory the first stage takes. The camera is however damaged and we miss most of the atmospheric footage.

It seems however that the camera was damaged because a grid fin on the rocket was lit on fire by the monopropellant. You can see the event in this video (It'll start 07:18 which is T+00:06:53).

I don't know which RCS fuel they use but I'd imagine it coming out pretty hot and being able to light some materials on fire. It seems like the rocket is a bit sideways to the air flow, which brings the smut from the fire straight to the camera lens. Does anybody know what the grid fins are made of? The paint on them might only be the flammable part too.

And, as a side note the landing seems really badly filmed, I'd imagine the would have had more cameras on the rocket and the barge, not only one which doesn't catch the landing and then freezes a few seconds after completely (they just show the last image I think there. Of course the rocket might have damaged their antennas but seems quite easy to shield them from a nominal landing)

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As far as I know, the RCS uses nitrogen cold gas thrusters. The grid fins are covered in an ablative material, they are heated by atmospheric friction.

The landing is badly filmed because this is a recording of the live webcast. The transmissions from the barge get disrupted by the rocket landing.

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  • $\begingroup$ From what I've read, the ablative material is likely a paint, and it is also likely flammable. $\endgroup$
    – called2voyage ♦
    Commented Dec 15, 2016 at 15:04
  • $\begingroup$ I just looped through it ten times at 1080P (thanks to the YouTube back-arrow 5 second rewind) and at about T+ 06:56 (07:22 in video) I think I see a little blip of yellow in about the same position on the grid fin on the right as well. So it's probably just a coincidence the thruster fired at that particular time. $\endgroup$
    – uhoh
    Commented Dec 15, 2016 at 15:14
  • $\begingroup$ Okay it does seem they use CGT:s. Wouldn't have guessed, they have so much lower Isp than hypergolic mixes or monopropellants :o $\endgroup$
    – Perttu Yli-Opas
    Commented Dec 17, 2016 at 11:58
  • $\begingroup$ @RussellBorogove has linked here to a clearer video of F9 first stage decent showing small flames on the grid fins. $\endgroup$
    – uhoh
    Commented Feb 21, 2017 at 0:54
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You can see in the various photos of the grid fins after landing (a unique feature of a SpaceX first stage, you get to see it after flight) that they have ablated quite a bit.

The true proof will be when the first reused first stage flies, and the state of refurbishment on the grid fins. This should be the SES-10 mission in early 2017.

The fins have an ablative covering.

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