Top-down space flight physics - AddForce - Object moves off center the faster it goes

Hi all,

I just started a little project where I want the player to control a ship from a top-down perspective. I want the whole thing to use physics-based movement.

My problem is that when I add force to a rigidbody, the object it is attached to seems to move off its position the more the faster it goes. However, transform.positions seem to be alright.

I put together this little package. If you press W, the ship will move upwards and off the center the longer you press W. S moves in the other direction.

The code is rather simple:

 using UnityEngine;
 using System.Collections;
 
 public class TestController : MonoBehaviour {
   public float mainThrust = 100;
   private Vector3 cameraOffset = new Vector3(0f, 0f, 150f);
   private new Rigidbody rigidbody;
 
   void Start () {
     rigidbody = GetComponent<Rigidbody>();
   }
     
   void FixedUpdate() {
     float axisVertical = Input.GetAxis("Vertical");
     Vector2 force = new Vector2(0f, axisVertical) * mainThrust * Time.fixedDeltaTime;
     rigidbody.AddRelativeForce(force, ForceMode.Impulse);
     Camera.main.transform.position = rigidbody.transform.position - cameraOffset;
     Debug.Log("camPos: " + Camera.main.transform.position
       + " - rbPos: " + rigidbody.transform.position);
   }
 }

Any help appreciated.

Thanks Nico