I have a 3D environment with 2D gameplay. The player controls a rocket with rotate and thrust within the XY plane. The physics-based gameplay necessitates the rocket being a rigidbody. I want it to roll off collision models within the environment, which it does perfectly. But when I hit something at a high velocity, the rocket will be rotated on its local X or Y axis, setting it on an oblong plane rather than the XY plane of the environment.

I thought this code would keep the rotation locked to the local Z axis, but after some testing, I can still disengage the rocket from the XY plane.

rigidbody.angularVelocity = Vector3.Scale(rigidbody.angularVelocity, Vector3(0, 0, 1));

What is the answer?

Here is the current script for the rocket:

var thrust : float = 1;
var turnSpeed : float = 1;

private var rotation : float;

function Update ()
{
    //"rotation" is > or < 0 based on left and right arrows, multiplied by "turnSpeed"
    rotation = Input.GetAxis ("Horizontal") * turnSpeed * Time.deltaTime;
}

function FixedUpdate ()
{
    //"vert" is > 0 when the up arrow or W is pushed
    var vert : float = Input.GetAxis ("Vertical");

    //Adds a force in the direction of the rocket's local Y+ coord when "vert" > 0,    multiplied by "thrust"
    if (vert > 0)
    {
	    rigidbody.AddRelativeForce (Vector3(0, 1, 0) * thrust);
    }

    //Rotates rocket in local Z coord based on "rotation"
    transform.Rotate (0, 0, rotation);
}