Shooting particle system?

Hi. My problem is when I try to shoot a particle system using this script the particle just flops to the floor. Any ideas would be highly appreciated thanks!

var projectile : Rigidbody;

function Update () { if (Input.GetButtonDown("Fire1")) {

     var clone : Rigidbody;
     clone = Instantiate(projectile, transform.position, transform.rotation);
 
     Destroy(clone, 5.0);
 }

}