Rotating a tower nozzle mesh in unity3D

Question

I am making my first game in Unity 3D something like tower defense game. I have imported a Fbx model of a tower and attached it to a prefab. Now I want the nozzle of the tower to rotate and follow the enemy as the enemy passes by . In the fbx model i imported i got I have two poly meshes one for the base of the tower that is fixed, and one for the top of the tower that will rotate. Now I tried to create two different gameObject with these two meshes but if I put them on same point they overlap. So I have to do manual alignment such that the nozzle sit correctly over the base. I was wondering if there is any other way such that entire towers remain is one gameObject and I can rotate the upper part.

Answer 1

I did manage to solve my problem. Not sure if was the best way , but it works.

To upgrade the towers and transform only the nozzle part I essentially did this.

public class tryFbx : MonoBehaviour {
    public GameObject[] ModelPrefab;
    GameObject modelInstance;
    Renderer rn = new Renderer();

    // Attaching the model to prefab at runtime by creating a array of prefabs
    public void AttachModelToPrefab(GameObject modelPrefab) {
       modelInstance = GameObject.Instantiate(modelPrefab) as GameObject;
       modelInstance.transform.position = transform.position;
       modelInstance.transform.rotation = transform.rotation;

       // Attach the model instance to the prefab shell
       modelInstance.transform.parent = gameObject.transform;
   }

    void Start () {

    }

    // Update is called once per frame
    void Update () {
            if (GameManager.upgrade){
            AttachModelToPrefab(ModelPrefab[GameManager.towerUpgradeLevel]);
            foreach ( Renderer r in modelInstance.GetComponentsInChildren<Renderer>()){
                    // "polySurface98" is the name of the mesh I want to rotate. The tower and its upgrade have the same name.
                    if (r.name == "polySurface98") 
                    rn = r; 
             }
        // apply any transformation to the partial fbx
        rn.transform.Translate(1,1,1);
       }
    }
}