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Consider a Falcon 9 two-burn reentry maneuver.

My understanding is that the second, final, burn is always single-engine.

And I think the first burn can be either single-engine or three-engine. But I'm not sure. Do they ever fire just one engine for this first burn? Or is it always three engines?

I'm simulating reentry and finding that a single engine does little to slow the rocket stage down. Drag is basically doing most of the decelerating... and because the stage hits the dense atmosphere at high speed, my dynamic pressure is on the high side (100kPa).

A three-engine burn would help kill off the excess speed before the stage hits the denser atmosphere, and maybe then Q would peak closer to the 30-40 kPa seen during launch.

But if the first burn is sometimes single-engine, then it seems they must start the burn higher up... or maybe they're cool with much higher max-Q on descent, in the range of 100kPa, maybe?

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According to simulations by Declan Murphy, the creator and developer of Flight Club, the profile is as follows:

  1. Light center engine at 80% thrust.
  2. ~3.3s later, light additional two engines at 80% thrust.
  3. ~15.2s later, shut down additional two engines.
  4. ~1.5s later, shut down center engine.

[Sample values from Starlink 1.0 L22 mission.]

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  • $\begingroup$ Thanks! But where do they get these engine burn profiles? This info isn't exactly public. Are they just guessing it? Because I can guess it too, and I don't trust my guesses. $\endgroup$
    – user39728
    Mar 26, 2021 at 15:21
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    $\begingroup$ Declan has written a physical modeling engine that models things like thrust, throttle, drag, gravity, bla, bla, bla, … and he simply tweaks the parameters until his model gives exactly the same flight profile as you see on YouTube. Especially now that SpaceX puts both stage 1 and 2 telemetry on the screen. Some things, you can see on YouTube, e.g. they almost always show the entry burn, so you can just time it. Also, many news outlets and amateurs film the launches and entries and put the videos up on YouTube, in case you suspect that SpaceX are manipulating their feeds. $\endgroup$
    – Jörg W Mittag
    Mar 26, 2021 at 15:37
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    $\begingroup$ For example, NASASpaceFlight, Cosmic Perspective, USLaunchReport, and many others. Note that the launch cadence of F9 means that we now have a large amount of data points to validate the models. $\endgroup$
    – Jörg W Mittag
    Mar 26, 2021 at 15:39

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