[exDM69]

No, that's inefficient and real spacecraft don't do that for typical low earth orbits. Works in Kerbal Space Program, though.

Normal orbital insertion is a single burn to orbit (with staging). With the correct initial roll and pitch, the spacecraft follows a perfect gravity turn and ends up in a near circular orbit at main engine cut off.

[numpad0]

IIUC real launchers do a single burn to orbit because S2 TWR is not very high and relight is finicky. Launchers that has relightable S2 and/or hypergolic S3 routinely do circularization burns.

There are reasons "apogee" is more recognized word than "apoapsis".

[lupusreal]

> There are reasons "apogee" is more recognized word than "apoapsis".

What are you getting at? The -gee suffix means Earth. Apogee is apoapsis of an orbit around Earth. People playing KSP speak of apoapsis because Kerbin isn't Earth.

[numpad0]

IRL they always did and therefore discussed "apogee kicks" after payload release into elliptical transfer orbit(literally GTO). That's where I'm getting at.

If you think about it, there can't be an ellipse that intersects a circle while also being fully encompassed by the latter. That's to say the periapsis can't be higher than the maneuver altitude that the off-apsis burn takes place as the other guy is suggesting.

On real rockets they add small shelf-stable stage such as the spinny boi Star-48 or the notoriously narcoleptic Fregat, inside fairings between S2 payload interface to actual payload, or let the payload pull itself into the final orbit at a great expense.

Yes, you can keep the S2 burning all the way to Ap and then burn at -45deg at Ap to deform the ellipse back into a circle, or fly a by the book gravity turn trajectory, but that's not the most energy efficient insertion, only what are situationally beneficial when there is land below and acceleration is limited.

[firebaze]

They do this while performing the burn of the (in this case) 2nd stage. "No, that's inefficient" is misleading, IMHO. They also waste some delta-V by starting the circularization burn a little bit early.

You surely know that, but mathematically it is unavoidable to have a 2nd burn at the apoapsis to have an orbit (with a periapsis > the initial launch altitude, minus gravity and atmospheric losses), albeit it may start earlier (while wasting a slight amount of delta-v).

[dotancohen]

Do they typically burn softly until near apogee, then put on the power until raising the orbit on the other side to their current altitude? In KSP, I can easily get an orbit on a single burn (with staging) but getting it circular obviously requires adding energy at apogee.

[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.

[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.