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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?

mhoran_psprep
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  • naively I'd guess they want as little as possible going up and plenty going back down – uhoh Mar 25 '21 at 06:20
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    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? –  Mar 25 '21 at 06:31
  • no idea, I just think it's interesting that resistance is actively sought on the way down while it's pretty much nothing but trouble on the way up. – uhoh Mar 25 '21 at 06:59
  • Is it sought on the way down? Why? I thought at best they’d have more margin for high q but that it still would be something to avoid! –  Mar 25 '21 at 07:05
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    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? – uhoh Mar 25 '21 at 07:33
  • Shuttle dynamic pressure was way lower on entry than ascent. https://space.stackexchange.com/q/31866/6944 – Organic Marble Mar 25 '21 at 14:00
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    @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. – Jörg W Mittag Mar 25 '21 at 16:01
  • @JörgWMittag: Would that speed bleed be from drag or from lift? Also curious what the angle of attack would be (roughly)---I'd always imagined they'd want to keep it near zero to minimize structural stresses, especially since the tanks are nearly empty and the rocket is therefore more susceptible to buckling? –  Mar 25 '21 at 16:04
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    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. – Jörg W Mittag Mar 25 '21 at 16:12
  • Thanks! I expected significant drag, but how much lift would a cylindrical stage get? I can't imagine it being much...? –  Mar 25 '21 at 16:25
  • @user39728 If you fall straight down the time you have to bleed off speed is short. In space-words the term "lift" applies to any force that is not parallel to velocity. I think the point is that some of the downward momentum is converted to sideways momentum, slowing the rate of descent, increasing the time it has to loose speed in the atmosphere. It doesn't have to be "airplane wing lift". As long as it falls at some angle pushing air to one side more than the other, it's called lift. If a launched rocket is pointed slightly, say 1° away from it's velocity vector, sideways force is "lift". – uhoh Mar 25 '21 at 22:29
  • @user39728 see Is aerodynamic lift ever useful in rocket flight? and also Rocket drag and lift based on flight direction - in which frame of reference? – uhoh Mar 25 '21 at 22:32

2 Answers2

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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.

Russell Borogove
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  • 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? –  Mar 25 '21 at 17:08
  • Yeah, I don't know what's up with that drag coefficient plot. – Russell Borogove Mar 25 '21 at 18:00
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    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 – Declan Murphy Mar 25 '21 at 18:26
  • 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... –  Mar 25 '21 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

Carl Witthoft
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  • 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? –  Mar 25 '21 at 16:07
  • @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" . – Carl Witthoft Mar 25 '21 at 17:45
  • 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? –  Mar 25 '21 at 18:03