There are 6 PID controllers currently:
- Altitude by Throttle
- Orientation/Position by gimballing engines with controllers GimbalX/GimbalY and GimbalXSP/GimbalYSP
- Roll by adding a rotation to the gimbal action
More explanation from in-game:
When "Automatic Control" is toggled on, and nothing else, it just tries to keep the orientation upwards by gimballing the engines. The controllers "GimbalX" and "GimbalY" are used for this.
Altitude control throttles the engines to reach the given set point. It doesn't switch off single engines. You have to do this manually, if you have too much thrust.
Roll is for orienting the mounting points towards the tower. This is tricky. Use it only when the rocket is relatively steady and not gimballing much.
For position, target coordinates in North and East directions can be given. The launch mount is the origin.
The position controller provides a set point to the gimbal controllers to tilt the rocket into the direction of the target. Therefore they are called "GimbalXSP" and "GimbalYSP".
For the landing phase, the control depends on the object:
For the booster it is as follows: Before starting the raptors, a landing trajectory is calculated using the landing target position and current booster position and velocity. After the raptors are started the trajectory is fixed and used as reference for automatic control. The controller takes the closest point on the trajectory curve as target for the position control and the tangent at this point as target vector for the orientation.
Starship: As the flip is usually initiated relatively close to the target, the controller always uses the set target coordinates and altitude setpoint as reference. So the orientation target is always the world up axis. That makes it easier for the controller as the ship typically comes in with horizontal orientation or might even be pitching downwards (even though optimal pitch would be slightly upwards to maximize drag).
When the ship is in landing phase and the raptors are started, the front flaps are automatically opened, while the AFT flaps are folded. For the prototypes SpaceX also folded the front flaps after the flip. I don't think it does much, so it's up to you. If you want you can do it manually.
There is some sort of mass distribution with the current ship implementation. But it is not very realistic. If you increase the propellant mass, the center of mass is shifted to the AFT somewhat. For small changes of mass the controllers should be able to still work properly. But for larger changes it might become difficult, for example at some point the flaps just don't generate enough torque anymore to keep the pitch stable.