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Old December 7th 18, 01:15 PM posted to sci.space.policy
Jeff Findley[_6_]
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Default Falcon 9 Delivers Dragon Into Orbit, Flubs Landing

In article ,
lid says...

On 18-12-06 13:06 , Jeff Findley wrote:

Also, the stage managed to land very well on the ocean (as can be seen
in a video posted by an observer on social media). SpaceX also released
the on board camera footage from the stage which showed that once the
landing burn started, the engines were able to negate the roll caused by
the stuck grid fin.


I thought the landing uses only one engine, therefore probably the
center engine -- then how can the engine control roll? I don't understand.


Some landings start with one engine, then three engines, then go back to
one engine. If that was the case here, gimbaling the outer two engines
gives you roll control authority.

In the videos it seems that the roll rate does not decrease much during
the landing burn, until just before and mainly after the landing legs
are deployed. Deploying the legs moves mass out from the roll axis and
therefore decreases roll rate, independently of the engines.


Agreed. If the landing burn was indeed one engine, then that's where
the majority of the de-spin would had to have come from.

All in all it landed very well (just in the ocean).


It was significantly tilted at touch-down. Perhaps one or two legs would
have crushed their crushables if it had been a hard surface.


Agreed.

But I'm impressed that it managed to land at all with that roll rate.


Agreed. The guidance control system performed very well. In the video
taken from the stage, you could see at least one reaction control
thruster firing almost constantly in order to counter the aberrant grid
fin torque.

Before the entry burn, at about 29:45 in the SpaceX launch video, a
white, spiky ring of some sort came off the area around the left-hand
grid-fin in the on-board video. Commentators described it as some frozen
condensation, but it looked curiously regular for that... I haven't
seens anything like that in earlier launches.


Possibly ice from around the LOX fill connection? That would seem to
make sense given the size and shape of the ring of ice.

Jeff
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