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A rocket launch normally begins with a roll to align with the orbital plane and a pitch to allow gravity to gradually torque the rocket down as it climbs. The combined roll and pitchover maneuver last a few seconds. The gravity turn maneuver that follows lasts a few minutes.

But what then? In a gravity turn, the rocket controls primarily for angle of attack, and this leaves it vulnerable to disturbances and irregularities that need correcting before entering orbit, beside the inevitable differences that will arise between flight simulations on the ground and the actual flight conditions during launch.

So it seems clear that the launch sequence must include at least one last maneuver. What would that maneuver be? What is it called and how does it manage to take the rocket to just the right speed, altitude, and angle of attack for orbit?

Any references for more info? Thanks!

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This has essentially been cleared up, but unfortunately nobody posted an answer. Since that's an unfortunate situation on SE, here's a quick summary.

There's indeed a need for such a manoeuvre, the term used is "Powered Explicit Guidance".

A very simple launch model is:

  1. Do a gravity turn.
  2. When the trajectory arc reaches the target orbital altitude, stop thrusting
  3. Coast until apogee
  4. Do a circularisation burn at apogee.

The clear problem with this strategy is both that the circularisation burn is not instantaneous, so the thrust must be angled during the burn to preserve apogee, and also that "coasting for a while" is not necessarily the most efficient utilisation of your rocket engines.

Then it turns into an optimisation problem. It helps starting the circlarisation "early" instead of coasting, and its sometimes even worth it to start doing so inside the atmosphere since the benefit may counteract the drag penalty.

This whole optimisation problem after leaving the gravity turn but before entering orbit is Powered Explicit Guidance.

For a more technical treatment, see for instance this NASA technical note: https://ntrs.nasa.gov/citations/19660006073

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  • $\begingroup$ Thanks! That's a nice clear answer. Question: to enter a desired orbit, you have to control for altitude as well as velocity and angle of attack. If you reach your target altitude before you reach your target velocity, then shouldn't you just keep firing the engines and just keep the rocket level with the horizon so that velocity goes up and altitude doesn't? Or is this all part of what you call a circularization maneuver? Thanks again! $\endgroup$
    – user36480
    Commented Sep 28, 2020 at 2:24
  • $\begingroup$ @Alex if you are below your target velocity, and fire the engines level with the horizon, you're going to start losing altitude. To keep altitude, you would need to tilt the rocket upwards. That is a circularisation manoeuvrer, yes. $\endgroup$
    – SE - stop firing the good guys
    Commented Sep 28, 2020 at 6:10
  • $\begingroup$ Right, so there's the angle of attack that you have to factor in also. OK, so you do your pitchover immediately after launch to enter a gravity turn, which you continue until you hit your target altitude, following which you do a circularization maneuver to reach your target velocity and angle of attack, at which point you've reached your target orbit. But when do you start the circularization maneuver? I'm guessing at apogee, but why not right away once you reach target altitude? Thanks! $\endgroup$
    – user36480
    Commented Sep 29, 2020 at 16:03

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