After about 36:17 in part 1 of this recording of SpaceX's Hans Koenigsmann's talk at IAC 2018, he narrates the propulsive landing of a Falcon 9 first stage, and mentions that the engine shuts off at roughly two feet (~60 cm) above the ground.

I'm wondering how fast a liquid propellant Merlin-like engine shuts down when timing is critical. Is torque to the turbo pumps somehow stopped and everything just winds down, or are there valves in the line that stop the flow of the propellants more quickly?

I understand that Koenigsmann's estimate of two feet is an "effective" shutdown point and there is going to be some transition or ramp-down in thrust and that's okay. Here I'm just asking how it's done in a controlled, timed way, two feet from the ground, such that there is a "bump" as shown in the video and articulated by his "dzhh" sound.

Also, is there at least an estimate of how quickly quickly it shuts down, say value for

  1. latency: time between the command and the time where the thrust is reduced by half (of whatever it was at the moment)
  2. abruptness: time for thrust to drop from 80% to 20% (of whatever it was at the moment)

  • $\begingroup$ same descent, different moment: What flame is SpaceX's Hans Koenigsmann describing in his presentation? $\endgroup$
    – uhoh
    Oct 6, 2018 at 6:00
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    $\begingroup$ Stopping a rapid fuel flow through the pipes as fast as possible may destroy them. Redirecting the fuel flow back to the tank was used sometimes. $\endgroup$
    – Uwe
    Oct 6, 2018 at 9:39
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    $\begingroup$ I'd add that it is not too important to shut off quickly during landing. More important is to have a well defined ramp-down curve and get to a TWR < 1 at the right moment. $\endgroup$
    – asdfex
    Oct 6, 2018 at 11:23
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    $\begingroup$ @uhoh Rolling doesn't change the timing, rising/falling happens on time scales of several seconds and is not actually fast (say 1 m/s as long as weather conditions permit a landing) - compared to a rocket accelerating at 5 - 10 m/s² (or even more with a 3-engine landing burn) that's negligible. $\endgroup$
    – asdfex
    Oct 6, 2018 at 14:20
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    $\begingroup$ @asdfex ah, I see what you mean. It seems that at some point, say 3 seconds before touchdown (just as an example), the ability to make further adjustments or decisions becomes minimal and you're committed to stopping at a given vertical point, but in those 3 seconds the ships can't really deviate unpredictably from whatever it is doing by more than a small fraction of a meter. Hear that Mr. Anderson? That's the sound of inevitability. $\endgroup$
    – uhoh
    Oct 6, 2018 at 17:23

1 Answer 1


The Merlin is throttled by modulating the propellant flow to the gas generator and thus the turbopump speed. Engine shutdowns are carefully orchestrated and timed affairs but I'm reasonably sure it would begin by shutting off the gas generator.

Some deductions (since there is not much released information) about Merlin throttling, and good schematics, are found in the answer to this question: Merlin 1D Engine Throttling

Again, a guess, but after the turbopump spins down, the flow to the chamber would be terminated by closing the Main Oxidizer and Main Fuel valves. Tom Mueller, designer of the engine, is quoted in a video linked to the other answer as calling the MOV the "valve we use to turn the engine on and off" at about 01:40.

  • 1
    $\begingroup$ It took me a few times to catch the mention, so I've added the time. I enjoyed the simplicity and conciseness of the video, wish I could talk like that. $\endgroup$
    – uhoh
    Oct 8, 2018 at 6:37

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