Rotate a Rigidbody to align with an orientation - unity3d

Question

I'm currently setting the rotation of a player object so that it "stands up" relative to the spheroid the user stands on.
However, that will break Rigidbody physics.
I already have the direction of gravity as a Quaternion, how can add torque to the Rigidbody so that it aligns with that direction?
Here's my code on Github, which does allow the player to move around the surface of the planet.
I have tried various approaches using AddTorque and Vector3.Cross, but they have all only ended in the Rigidbody spinning wildly.

Answer 1

Instead of using transform whenever a Rigidbody is involved rather first calculate the needed final Quaternion ad then apply it using theRigidbody.MoveRotation which does not break the physics.
The same also accounts to the position where you should rather use theRigidbody.MovePosition.
Both should be used in FixedUpdate so
either move the entire code from Update to FixedUpdate
or split the input and the physics and do your API stuff in Update, store the relevant values into fields but apply anything related to the Rigidbod transformations reactive in FixedUpdate.
Your code is quite complex so I will only give an example as a pseudo code and hope you can take it from there
So instead of doing e.g.
private void Update ()
{
player.transform.position = XYZ;
// Applies a world space rotation
player.transform.rotation = XYZW;
// Adds a relative rotation in local space
player.transform.Rotate(x,y,z);
}
You would rather do something like
[SerializeField] Rigidbody playerRb;
private Vector3 targetPosition;
private Quaternion targetRotation;
private void Awake ()
{
if(!playerRb) playerRb = player.GetComponent<Rigidbody>();
}
private void Update ()
{
targetPosition = XYZ;
targetRotation = XYZW * Quaternion.Euler(x,y,z);
}
private void FixedUpdate ()
{
playerRb.MovePosition(targetPosition);
playerRb.MoveRotation(targetRotation);
}