constant force

hi I've been toying around with unity struggling to make a jet pack system and have hade very little success so I decided to do every thing piece by piece I made a script (BELOW) and what it was supposed to do is make my third person controller character move up constantly but it doesn't work on my character and I tried it on a cube but it only gows up little by little when I press the button each time your help hear will be greatly appreciated

 var target : Transform;
 
 
  
 
 
 function Update () {
 
 
     
 
 
     if(Input.GetKeyDown("left shift")) {
 
 
     target.position += Vector3.up;
 
 
     }
 
 
 }