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From videos of both the SES-10 launch shown in @RussellBorogove's question, Cause of apparent plume deflection on SES-10?" and the Block 5 launch Bangabandhu Satellite-1 Mission at T+ 01:48 it appears the Falcon 9 uses non-zero angles of attack (or displacement angle) during the 1st stage burn.

The exhaust plume is aligned with the flight path (high dynamic pressure) and it's obvious the vehicle's roll axis isn't.

The accepted answer states:

I believe the visual effect is due to the rocket flying a small positive angle of attack at the end of the first stage burn, combined with a steeply foreshortened camera view.

According to the simulation data at FlightClub (https://www.flightclub.io/results/?code=SS10), the rocket flies a positive angle-of-attack during the end of the first stage burn, peaking at about 4.6 degrees AoA.

Is this advantageous?

enter image description here enter image description here

left: screen shot from Bangabandhu video, right: SES-10 from here. Click for larger size.

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  • $\begingroup$ Can you add a screen shot? See for example these questions 1, 2, 3, 4 $\endgroup$
    – uhoh
    May 13, 2018 at 4:48
  • $\begingroup$ Would do it if I knew how. As I mentioned a few days ago, I've been doing professional space exploration for a long time, but I'm new to online stuff. Still on the learning curve. $\endgroup$
    – Tom Spilker
    May 13, 2018 at 5:05
  • $\begingroup$ Can see the result of what you did (time tag icon, screen shot inserted) but not how you did it. $\endgroup$
    – Tom Spilker
    May 13, 2018 at 5:40
  • $\begingroup$ I did everything using the keyboard except for importing the screenshot. To import images, you use the "image" tool at the top of the edit window. See this guide: i.stack.imgur.com/BcK5Q.png To get a short link to a question or an answer, look for and click share below the post. It pops open a small window. You just use your keyboard shortcut for copy and then it's in your copy/paste buffer. See this guide: i.stack.imgur.com/GuUF0.png $\endgroup$
    – uhoh
    May 13, 2018 at 5:45
  • $\begingroup$ slightly related: Is aerodynamic lift ever useful in rocket flight? $\endgroup$
    – uhoh
    May 13, 2018 at 6:01

1 Answer 1

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It is theoretically possible to fly an ascent trajectory with zero angle of attack throughout -- this would be an ideal gravity turn. In practice, launches have a fairly specific main engine cut off (MECO) state vector that they are guided to however. This requires some angle of attack, usually commanded via gimbaling and limited by q-alpha, to alter the trajectory.

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  • $\begingroup$ I see. This tells me that the control methodology is that there is a defined ascent trajectory (and thus a defined state vector at MECO) that the vehicle is trying to match. If at any time the actual position of the vehicle deviates from that trajectory (say, from unanticipated upper-level winds), the vehicle begins canceling the deviation via a non-zero AoA. This is the methodology the Saturn V used. An alternative methodology, one that requires smaller AoA's, is to calculate, on-the-fly, "What is the best trajectory to my desired end state from where I am now?", and fly that. $\endgroup$
    – Tom Spilker
    May 16, 2018 at 18:54
  • $\begingroup$ Pretty much. There are all kinds of first stage guidance techniques -- open loop, closed loop, etc. I believe Falcon 9 has closed loop first stage guidance however, so your summary is accurate. Sometimes, first stage guidance will also do trajectory shaping for aborts and to avoid downrange regions -- all of which will require angle of attack changes. I am sure F9 does all of these things. Cheers. $\endgroup$
    – Erik
    May 16, 2018 at 20:03
  • $\begingroup$ Most/all of the answers to Is aerodynamic lift ever useful in rocket flight? state that deviations from non-zero AoA are quite undesirable and minimized, especially for this long, skinny flying noodle. But it sounds like as long as the altitude is sufficient, q-alpha can be kept in check. $\endgroup$
    – uhoh
    May 17, 2018 at 2:21

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