[exDM69]

No, most rocket engines have quite limited amount of throttle capability and run near maximum thrust until cutoff.

The rocket yaws and pitches in the first seconds of flight while the vehicle is still subsonic, then flies a gravity turn trajectory at zero angle of attack (facing the direction of travel) at near maximum thrust. Any errors accumulated during early part of the flight will be corrected by adjusting the timing of the second stage cutoff based on radar tracking.

The initial pitch over is just a few degrees off vertical, but must be precise to a fraction of a degree (KSP tolerances are higher due to small planet).

You can get a pretty good circular orbit in Kerbal Space Program with one burn if you do a few attempts and trial and error binary search for the optimal initial pitchover angle, but it's very difficult to do without throttling the 2nd stage burn. If I recall correctly, the MechJeb mod can do a precise single burn to orbit.

[dotancohen]

Thank you. Yes, I figured that real life engines could not throttle enough for the maneuver as stated. I am familiar with the gravity turn, but I just don't see how energy can be added continuously, uniformly right up to a circular orbit. But I've not really put much effort into trying to understand that, I'll start looking more at real life pitch angles at various altitudes. Maybe I just need to start that gravity turn sooner - you mention that it already starts in the first few seconds. Thank you.

[exDM69]

In vanilla KSP1 with a reasonable orbital launcher, fly up to 1000m altitude and tap D on your keyboard 1 to 10 times. Then hands off until 2nd stage and then throttle down to avoid overshooting the apoapsis.

Finding the correct number of key taps to get the right pitch angle is the key. The throttle can only help so much.

[dotancohen]

Will do, thanks. 1000 m is in fact much lower than I usually pitch. Off the pad I've been giving just a touch of pitch to ensure that the rocket is on the right trajectory, but didn't start the gravity turn until maybe 5000 m or so to get out of the thick atmosphere sooner. Yes, I would have quite the AoA for a little while.

Thank you. Though I don't expect to open KSP any time in the near future, I love to know how the real rockets do it.

[exDM69]

1000m is much higher than real rockets pitch over, but good for KSP with forgiving aerodynamic stress. It's just a convenient round number and gives a few seconds of breathing room to make sure the fiery end of the rocket points to the ground before start of maneuvering.

[dotancohen]

I do remember the Shuttle pitch and roll real early, though I thought that was to point the antennas (K-band was it?) at the receivers.

[exDM69]

The roll is to change the orbital inclination, the pitch is to set ascent trajectory.

[ericbarrett]

MechJeb + Realism Overhaul offers a "Primer Vector Guidance" ascent controller that (I think) is based on the space shuttle's Powered Explicit Guidance. It's definitely designed to work with more realistic spacecraft; it can ullage with RCS, doesn't need to throttle, etc.