Addforce to RigidBody problem

Hey, I just wanted this projectile to fly forward using "Addforce" but all it does is drop straight to the floor. Can someone help me? It would be really appreciated!

     //Variables Start___________________________________
  
  
     //The explosion effect is attached to this
     //in the inspector
  
     public GameObject mudParticle;
  
  
     //A quick reference.
  
     private Transform myTransform;
  
  
     //The projectiles flight speed.
  
     private float projectileSpeed = 500;
  
  
     //Prevent the projectile from causing
     //further harm once it has hit something.
  
     private bool expended = false;
  
  
     //A ray projected in front of the projectile
     //to see if it will hit a recognisable collider.
  
     private RaycastHit hit;
  
  
     //The range of that ray.
  
     private float range = 1.5f;
  
  
     //The life span of the projectile.
  
     private float expireTime = 10;
  
  
     //Variables End_____________________________________
  
     // Use this for initialization
     void Start()
     {
     myTransform = transform;
         
  
    //As soon as the projectile is created start a countdown
    //to destroy it.
  
     Destroy (gameObject, expireTime);
     
      //Translate the projectile in the forward direction (the pointed
     //end of the projectile).
     rigidbody.AddForce(myTransform.forward * projectileSpeed * Time.deltaTime);
     }
  
     // Update is called once per frame
     void Update () 
     {
        //If the ray hits something then execute this code.
  
       Debug.DrawRay( myTransform.position, Vector3.forward * range, Color.red );
       if(Physics.Raycast(myTransform.position, Vector3.forward, out hit, range) && expended == false)
       {
          //If the collider has the tag of Floor then..
  
          Debug.Log( "Ray Hit (floorMud) : " + hit.collider.transform.tag );
          {
           expended = true;
  
  
           //Instantiate an explosion effect.
  
           Instantiate(mudParticle, hit.point, Quaternion.identity);
  
  
           //Make the projectile become invisible.
  
           myTransform.renderer.enabled = false;
          }
        }
     }
  
  
     IEnumerator DestroyMyselfAfterSomeTime()
     {
        //Wait for the timer to count up to the expireTime
        //and then destroy the projectile.
  
        yield return new WaitForSeconds(expireTime);
  
        Destroy(myTransform.gameObject);
     }
 }