Is this a late landing burn problem? it seems that the engine touch the OCISLY ground as the landing legs flex.
Landing occurs at about T+0:08:20
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asked Jan 29, 2020 at 14:29
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5$\begingroup$ Might be nice to add a little more context, so that the question will still make sense in the future when they day is no longer today. $\endgroup$– Starfish PrimeJan 29, 2020 at 14:36
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1$\begingroup$ @StarfishPrime: We might need to wait some for that. The event is still being streamed, so one can't even link to a specific timestamp of the video. The landing did, indeed, seem harder than usual, but it seems more like the burn started slightly too early, and the engine had to cut off barely as the legs touched down, the whole mass of the rocket dumped suddenly onto the fully extended legs. (landing occurs at T+0:08:20, stream.) $\endgroup$– SF.Jan 29, 2020 at 14:57
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2$\begingroup$ I don't see contact between the engine nozzle & the deck. Looking at the flex of the landing legs, my impression is that there was a smooth, elastic recovery of their position, implying no hard contact elsewhere. I may be optimistic :-) $\endgroup$– Carl WitthoftJan 29, 2020 at 15:35
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1$\begingroup$ Yeah just saw that as well. Not sure if the engine bell touched the floor though. Could it be a smoother suspension system? I remember they had a crushable honeycomb structure to absorb the shock. Wondering if they changed that. $\endgroup$– pastulloJan 29, 2020 at 16:52
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$\begingroup$ Maybe because this time there is no interference in the video link? Maybe the legs always flex that way? $\endgroup$– Guilherme BechtingerJan 29, 2020 at 19:33
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2 Answers
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Scott Manley talks about this landing in his most recent video presenting a 'time-slice' image of the landing - and concludes the landing burn was too early. Merlin engines can't be throttled low enough to provide slow descent, and as the booster stopped decelerating still above the deck, the only thing that could be done to prevent loss of the booster at that point was to cut off the engine, which caused a rapid drop and grounding it rather hard.
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No, this is not a late landing burn problem, this is an early landing burn problem. By understanding the technology behind the landing of Falcon 9 rockets and the design of Merlin engines can help us to understand what happened there.
Falcon 9, uses one to three of its Merlin engines to slow down the vehicle as it is free-falling from space. To be able to have a controlled descent, SpaceX controls the throttle of the engines, gradually lowering it until the Falcon is landed. However, rocket engines, being very complex have a minimum amount of thrust that they can provide, after which they cut off. For Merlin 1D engines after refinements in 2013, this minimum has dropped to 40% instead of 70%. In the other landing videos we can see that the engines cut off only after the booster is landed. That's because the descent speed was correct and the minimum amount of thrust from the engine was just right to slow down and genuinely land the booster.
However, in this case the landing burn was done sooner, then supposed to. As a result, the speed of booster when approaching the droneship was lower than expected. As a result, the lowest thrust of 40% was too much, and the computer had no other decision then to cut them off. As a result, the booster had a bit more rough landing, but still survived.
