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Max Q for a Falcon 9 is on the order of 30-40 kPa during launch.

What would the number be on descent?

Do they allow for greater dynamic pressures in that portion of the flight? Or is the rocket still controlled for a max Q on the order of 30-40 kPa?

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  • $\begingroup$ naively I'd guess they want as little as possible going up and plenty going back down $\endgroup$
    – uhoh
    Mar 25, 2021 at 6:20
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    $\begingroup$ How much would plenty be? I ask because on reentry velocity is much higher than during launch and dynamic pressure can easily reach multiples of 30-40 kPa. I'm seeing 100 kPa in my simulations. On launch, this would be outrageous. But how unreasonable is it on landing? $\endgroup$
    – user39728
    Mar 25, 2021 at 6:31
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    $\begingroup$ Some of the slowing down that the F9 does comes from aerodynamic drag, flying downwards blunt-end first. They do use propulsion for much, perhaps most, but drag is free so why not use it? $\endgroup$
    – uhoh
    Mar 25, 2021 at 7:33
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    $\begingroup$ @uhoh: In fact, F9 actually glides a fair amount in the horizontal axis instead of falling straight down, using the rocket body as a lifting surface, in order to bleed off even more speed. $\endgroup$
    – Jörg W Mittag
    Mar 25, 2021 at 16:01
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    $\begingroup$ I believe the speed reduction is mostly from experiencing the drag for a longer time than if they were falling straight down. But any lift always induces drag as well. Not sure about the angle. I think I remember seeing a video on YouTube where an amateur had tracked the stage with a telescope camera during a drone ship landing, and you can really see it "gliding". I remember I was surprised about the angle as well, but I don't remember what it was. $\endgroup$
    – Jörg W Mittag
    Mar 25, 2021 at 16:12

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According to a simulation at FlightClub.io, aerodynamic pressure for the first stage reentry peaks at about 112 kPa.

plot of aerodynamic pressure versus time for a Falcon 9 flight; there's an initial peak of about 30kPa on ascent at about T+75 seconds, and a sharper peak of about 112kPa during reentry at about T+440 seconds.

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  • $\begingroup$ Wow that is close to what I'm seeing. Still could be coincidence or that they're wrong like me. Any idea how accurate their numbers are? Their drag coefficients seem unrealistic and could easily skew their Q numbers. I wonder how they estimated those coefficients? $\endgroup$
    – user39728
    Mar 25, 2021 at 17:08
  • $\begingroup$ Yeah, I don't know what's up with that drag coefficient plot. $\endgroup$
    – Russell Borogove
    Mar 25, 2021 at 18:00
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    $\begingroup$ The drag coefficient plot shows a Cd of ~0 during retropropulsive burns and 0.85-0.95 for a Falcon 9's butt, depending on the flow velocity (higher number when transonic). It also doesn't show any data above 85km $\endgroup$
    – Declan Murphy
    Mar 25, 2021 at 18:26
  • $\begingroup$ Above 85 km you have near vacuum so I wouldn't expect drag to matter there. But the cD plots I've seen suggest very basic cD estimates, and I have to worry how realistic those are. Q is proportional to cD, so if cD is off by a factor of two, then so is Q... $\endgroup$
    – user39728
    Mar 25, 2021 at 18:56
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This is not directly related, but here's a graph from measurements of Apollo. From NASA tech report 19690029435

The report includes this graph of dynamic pressure as the Apollo capsule returned to Earth. (One pascal = 0.0208854 lb/ft^2 ) These values roughly 29 kPa (peak).

enter image description here

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  • $\begingroup$ Thanks, Carl! This is helpful. Wondering what that reentry Q-Max peak would be like for an unmanned stage, though... Could it be higher than seen during launch? $\endgroup$
    – user39728
    Mar 25, 2021 at 16:07
  • $\begingroup$ @user39728 I see there is a comment pointing out the stage falls "sideways" for a while for increased drag. But in any case, the dynamic pressure during launch is a function of the top of the entire assembly, while the booster stage returns "headless" . $\endgroup$
    – Carl Witthoft
    Mar 25, 2021 at 17:45
  • $\begingroup$ Wouldn't drag act to rotate the rocket until it's near zero angle of attack, though? Would the grid fins be enough to keep it flying "sideways" at angles of attack much greater than zero? A few degrees, sure, but a few tens of degrees... at those speeds, as soon as you enter the thick atmosphere, rotation from drag would be difficult to overcome with grid fins alone... it seems? $\endgroup$
    – user39728
    Mar 25, 2021 at 18:03

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