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by inglor_cz 2 days ago
By the time that the booster returns to the landing zone, the Flight Termination System (FTS) is AFAIK no longer active and cannot be reactivated. Which makes sense because it is a bunch of high explosives, now nearing some valuable equipment.

The SpaceX solution is to steer carefully and have the default trajectory end in the sea. Only if everything looks good, is the booster redirected to the tower.
1 comments
wat10000 2 days ago
I wonder what happens when things go wrong after everything looks good. They punched a good number of holes in their landing barges when they were perfecting Falcon 9 landings. And I imagine this crazy catch tower is a lot more expensive than a barge with a flat surface.
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inglor_cz 2 days ago
Falcon 9 engines (Merlins) cannot throttle down, they either run on max power or they don't. Which means that any attempts to land are incredibly risky. You have to reignite them at the precise moment to slow down the rocket to zero at the height of the landing platform, or else. That is why the maneuver is called "suicide burn", and it is incredible that someone actually perfected something as crazy as this. The landing legs can absorb a tiny error, but anything over that = Rapid Unscheduled Disassembly (Monty Python theme starts to play in the background...)

Raptors are much more finely controllable and both stages of Starship can even hover when necessary, so the total control envelope, while not perfect, is much larger. That is why they can even try to land into a steely embrace, with no landing legs used.

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wat10000 2 days ago
Merlin and Raptor can both throttle to about 40%. The difference is that Super Heavy has way more engines, so 40% of one engine is a lot less thrust relative to the mass of the nearly empty booster.

Did the suicide burn actually play a role in the various barge-holing events? It’s been a while but I don’t remember that being the hard part.

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inglor_cz 1 day ago
I was mistaken, Merlins can indeed throttle down. Thanks for correction.

What they cannot is achieve thrust-to-weight ratio of <= 1 when the first stage is empty, so controlled descent is basically impossible and suicide burn is the only option to land.

When it comes to failed returns, I think that at least once they ran out of fuel several meters early(?)

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