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When a Falcon 9 first stage is returning to Earth to land, it performs a brief re-entry burn in order to slow down.

My question is, what criteria does the first stage control software use to decide when to stop the re-entry burn? I can think of several possibilities:

  1. It's set to simply stop the burn after a fixed amount of time, regardless.
  2. It's measuring its velocity and stops the burn when its velocity has decreased to the desired target value.
  3. It's measuring its altitude and stops the burn when it reaches the specified altitude.
  4. Some combination of the above, or (more likely) something more sophisticated that I haven't thought of
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    $\begingroup$ Note: it has a certain range of throttle to operate during the burn, so it doesn't need to start it super-precisely - if it starts it late, it will simply brake harder, opening the throttle more, if it starts earlier it runs throttled deeper. Of course it can't be too early or too late, but the margin is quite significant so the method doesn't need to be very sophisticated. $\endgroup$
    – SF.
    Aug 31, 2021 at 12:42
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    $\begingroup$ "it performs a brief re-entry burn in order to slow down" – That's actually not the main reason for the entry burn. The entry burn creates a protective "bubble" of exhaust gas around the bottom of the vehicle, protecting it from the hot plasma. Yes, you read that right: the Falcon 9 uses its flamey end to cool itself, quite literally fighting fire with fire. $\endgroup$ Aug 31, 2021 at 19:45
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    $\begingroup$ The deceleration is just as important as the shielding effect, it's why they don't need such protection at the end of the burn. In addition to these, I've also thought that the reentry burn might be used to burn off excess propellant, reducing terminal velocity and making landing burns as identical as possible. $\endgroup$ Sep 1, 2021 at 0:51
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    $\begingroup$ It's almost certainly "something more sophisticated that I haven't thought of". You omitted the use of Inertial Measurement Units, atmospheric sensors, and software such as Kalman filters. I'm making this a comment rather than an answer because what I wrote is an educated guess. SpaceX does not provide technical details. They provide what are at best glossy simplifications of their technical details. I can't blame them; they have competition, and they have to worry about releasing dual-use technology. This latter item can result in massive fines, lost contracts, and jail time. $\endgroup$ Sep 5, 2021 at 11:26
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    $\begingroup$ SpaceX attracts a lot of questions for technical details on how SpaceX does things, Many of these questions go unanswered because the only people who can answer the question definitively cannot answer. SpaceX almost certainly makes their employees sign nondisclosure agreements (NDAs). I don't know this definitively, but every high tech company in the US that I know of / have worked for makes their employees sign NDAs as a condition of employment (and sometimes even as a condition of interviewing). $\endgroup$ Sep 5, 2021 at 11:40

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