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Do they control for angle of attack on the return flight of stage 1 of Falcon 9?

Once it reenters thick atmosphere, aerodynamic stresses would be a concern as during launch, maybe more so because you no longer have the stiffening effect of pressurized fuel inside---you're pretty much flying an empty soda can at this point.

How do they minimize the angle of attack then? Is it aerodynamics alone that keep it near zero? The center of mass is well in front of the center of pressure if you fly stage 1 with engines in front. This would tend to keep the rocket in the right orientation for the return flight.

Or is it feedback control actively monitoring the angle of attack and correcting it back to zero through cold gas thrusters and grid fins? The controllers must be very busy trying to keep the rocket on track toward its landing pad, so it would be challenging to control simultaneously for angle of attack, if you could do it at all.

My guess is aerodynamics take care of angle of attack while the controllers take care of staying on track, but does anyone know for sure?

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  • $\begingroup$ The angle of attack is actually pretty shallow, and F9 is using lift generated by its body to glide a fair amount cross-range to bleed off speed. At least after the entry burn. $\endgroup$ – Jörg W Mittag Jan 14 at 20:10
  • $\begingroup$ @JörgWMittag: Drag would bleed off speed, but how would lift, which acts perpendicular to velocity, help you slow down? Did you mean drag instead of lift? $\endgroup$ – user36480 Jan 14 at 20:12
  • $\begingroup$ Also, the angle of attack starts out very large right after the flip maneuver after MECO. The atmosphere is negligible then, so the large angle of attack is OK, but it isn't until later in the return flight that the angle of attack becomes small (as your velocity vector realigns with the rocket's length axis). $\endgroup$ – user36480 Jan 14 at 20:16
  • $\begingroup$ Alex note the reference to cross-range in @JörgWMittag's comment. $\endgroup$ – Organic Marble Jan 14 at 20:53
  • $\begingroup$ @Alex: If you fall straight down, you travel 100km through the air slowing you down. If you glide 200km cross-range, you travel 223km through the air slowing you down. Also, lift always induces drag. $\endgroup$ – Jörg W Mittag Jan 14 at 21:17

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