Reports suggest that the centre core of the FH launch crashed because it couldn't get the necessary three engines relit. If this isn't a misunderstanding (there was something about the shrapnel taking out two thrusters of the drone ship, which might easily have led to confusion) why did it need three engines. I know F9 boosters sometimes land on three engines as a fuel-saving device, but that seems like an off choice for this mission, where they had no specific target orbit. Is the core that much heavier? Or is there some other reason?
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asked Feb 7, 2018 at 9:53
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$\begingroup$ My guess is just more testing. $\endgroup$– jkavalikFeb 7, 2018 at 10:25
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$\begingroup$ Related: Why were three engines used for the F9 1st stage landing burn (Bulgaria-Sat 1)?, Can Falcon 9 first stage landing final burn using one, two, and three engines? Tradeoffs? $\endgroup$– EdlothiadFeb 7, 2018 at 12:11
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1 Answer
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The last mission before the Falcon Heavy tested a 3 engine landing burn. They did not have an ASDS underneath it, since it launched 6 days before the Falcon Heavy mission and to get the barge back to Florida, unload, and then back to station for a landing would have delayed the flight.
But they launched the SES16/GovSat1 mission with legs and tested a three engine landing burn regardless. They were testing to see if they could do it, since it saves fuel by avoiding gravity losses.
The less time you spend decelerating the less fuel you use. So a fast landing is more parsimonious on fuel.
The center core needs more fuel to slow itself down since it is going faster than the side cores, and thus this was an experiment to see if they could land it with less fuel.
Alas, they had a TEA-TEB ignition issue. This is interesting as they have landed 23 or so cores so far, successfully and all the engine restarts have been good.
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1$\begingroup$ I believe in the press conference Musk mentioned that they ran out of fuel for re-ignition, and only the center engine restarted. $\endgroup$ Feb 7, 2018 at 10:46
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$\begingroup$ nice answer, any chance of linking to some sources? $\endgroup$– user20636Feb 7, 2018 at 10:47
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$\begingroup$ @geofic "The center core needs more fuel to slow itself down since it is going faster than the side cores, and thus this was an experiment to see if they could land it with less fuel." This is the bit I find surprising. You would have thought they could afford to launch the car a bit less far in order to have a simpler landing for the centre core, and have the centre core left to inspect afterwards. $\endgroup$ Feb 7, 2018 at 11:22
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1$\begingroup$ @SteveLinton Big payloads are the money makers. So testing the top end of performance is key. $\endgroup$– geoffcFeb 7, 2018 at 11:27
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2$\begingroup$ @SteveLinton They absolutely could have landed the center core more conservatively, but they wouldn't have learned anything about their three-engine landing burn protocol by doing so. $\endgroup$ Feb 7, 2018 at 17:36