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Watching the first (only?) test flight of SN8 we saw them start flying with three Raptor engines firing and then one looked like it flamed out, with lots of fire inside the engine bay. Then later a second engine shut down.

Since they restarted two engines for the flip turn and landing, it is clear they did not really lose an engine, but it would be nice to know for certain.

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Elon Musk posted a tweet on this topic:

The Raptors were well below max thrust or the ship would have blown through the altitude limit. As we hit min throttle point, an engine would shut off.

The issue was that three Raptors at full thrust would have gone too high, too fast. They needed to control speed and altitude to stay inside their allowed flight environment.

But you can only throttle a Raptor down so much (to 40% I believe). Eventually you shut an engine down and ramp the others up to compensate and then throttle down again until you need to shut another engine.

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    $\begingroup$ Maybe worth saying that the reason you need less and less thrust is that the ship is getting lighter as it burns fuel. $\endgroup$ – Steve Linton Dec 16 '20 at 21:37
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    $\begingroup$ Your 40% figure seems accurate - here are references if you want to incorporate them in your answer. In 2019, Elon tweeted that "50% is hard, but manageable" and in Aug 2020 Elon tweeted that Raptor could do max thrust of 225 tons and min thrust of 90 tons which works out to 40%. I found that tweet from Wikipedia. $\endgroup$ – mgarey Dec 16 '20 at 22:28
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    $\begingroup$ Technically speaking, the bright green exhaust during the landing burn seems to indicated that the inside walls of the combustion chamber were being ejected as part of the exhaust, which to me sounds like losing an engine. $\endgroup$ – Jörg W Mittag Dec 17 '20 at 4:43
  • $\begingroup$ @JörgWMittag the engine with the bright green exhaust was still running at impact. The other engine may have been lost...it's not clear if it was intended to shut down when it did, it may have been intended to be a single-engine landing. In any case, the cause was fuel tank pressure, not the engines. $\endgroup$ – Christopher James Huff Dec 17 '20 at 17:04
  • $\begingroup$ @ChristopherJamesHuff: I wonder if it tries to fix a pressure problem by adding teb into the fuel. Wouldn't work very long as they don't have much but when you have a problem on a landing burn it's better than not trying. $\endgroup$ – Joshua Dec 17 '20 at 20:29
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A couple days ago I watched this video which does a pretty good job of explaining the whole test.
Basically:

  • Take-off needed all three engines to reach enough altitude, so all three engines were initially fired up.
  • As the ship was climbing up, higher altitude and lower weight due to fuel consumption meant less thrust was needed to reach the apogee and turn back down. One engine was enough to do so, so two of the engines were sequentially shut down on purpose.
  • Once the apogee was reached, the remaining engine was shut down as well.
  • Of the three engines, two were fired up again for the landing maneuver. However, due to insufficient fuel pressure, they could not get enough fuel to function properly, resulting in one of them burning itself from the inside out in the process (hence the green flame). The other one shut itself down moments before touching ground, but it is not clear whether this was intended or not. This other video analyzes the landing sequence really well too (@ 8'00").
  • As a result of this, SN8 reached the ground too fast and crashed instead of landing.

So no engines failed during the flight, but at least one (maybe two) failed on landing because of too low fuel pressure.

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  • $\begingroup$ That is interesting. My understanding was that they only needed 2 on landing. It was a pressure issue that reduced thrust on those that caused the too fast landing and thus crash. $\endgroup$ – geoffc Dec 17 '20 at 11:40
  • $\begingroup$ Both landing engines clearly fired up for the landing maneuvers. Their ignition, independent gimbaling to control roll, and shutdown of one of them after the landing flip while the exhaust of the other went green are all unmistakably visible in the various videos of the event. $\endgroup$ – Christopher James Huff Dec 17 '20 at 12:44
  • $\begingroup$ @ChristopherJamesHuff You are right, I rewatched the video and @ 6:30 it says "two of them fire up as planned". Seems like I did not remember it correctly, I'll edit the answer. $\endgroup$ – walen Dec 17 '20 at 12:58

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