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