How to disable constant force over time? Rocket game

Here is all of my code in player.js...... // Player.js // // Takes care of everything to do with controlling the player (including animations) and checks for death // // // -------------------------------------------------------------------------------

 // if you want to change how high (or low) the player jumps, change this value
 public var jumpPower : float = 35.0;
 
 // this chunk o variables are just boring variables that the functions use to function
 private var theZ : float;
 private var jumpEnabled : boolean;
 private var canMove : boolean;
 private var hit : RaycastHit;
 private var mousePos : float;
 private var xVel : float=0.0;
 private var xPos : float=0.0;
 private var yVel : float=0.0;
 private var gameEnding : boolean;
 private var didGroundHitSound : boolean;
 
 // rocket fuel, burn rate, and velocity
 private var fuel = 200.0;
 private var burnRate = 50.0; // Units per second
 private var velocity = 0.5;
 
 private var jumpSound : GameObject;
 private var fallingSound : GameObject;
 private var groundHitSound : GameObject;
 
 // we store a reference to the gameController so that we can send it important messages like when the
 // player falls too fast (game over) or when the player hits the ground (game over)
 private var gameControl : GameController;
 
 // theTransformHit stores a reference to the last transform that the player jumped on so that we can
 // destroy it if all the conditions are met to start a jump
 private var theTransformHit : Transform;
 
 // we need to make sure we're grounded before starting a new jump
 public var grounded : boolean;
 
 // to use an animated character, we parent it to the player object. so that we can still talk to it and
 // play animations, the player object needs to know where the animated character is, so we store a reference
 // to its gameobject here (accessible through the editor)
 //public var animatedCharacter : GameObject;
 
 // change this to change the players movement speed on the x
 public var maxMoveSpeed : float = 0.4;
 
 // the width of the play area (used to keep the player within the play area - if you make the game wider, change the value in gameController.js since that's where we get its value from)
 private var gameWidth : float;
 
 function Start() {
     // set gravity here
     Physics.gravity = new Vector3(0, -45, 0);
     
     // grab default position data for the z position (to fix it wherever the player starts) and to
     // get a starting point for our xPos variable, which holds the players x position
     xPos=transform.position.x;
     theZ=transform.position.z;
 
     // we use this to freeze the player at the beginning or end of the game
     jumpEnabled=false;
     
     // we use this to freeze player movement at the end of the game
     canMove=false;
     
     // grab a reference to our game controller script
     gameControl=GameObject.FindObjectOfType(GameController);
     
     // grab the game width from the gameController.js instance
     gameWidth=gameControl.gameWidth;
     
     // grab a reference to a gameObject that has audio attached to it. we use this to make a sound when
     // we bounce off a platform
     jumpSound=GameObject.Find("SOUNDFX_platform_hit");
     fallingSound=GameObject.Find("SOUNDFX_falling_down");
     groundHitSound=GameObject.Find("SOUNDFX_ground_hit");
         
     // set our game ending flag to false
     gameEnding=false;
     didGroundHitSound=false;
     
     // fix our player on the spot when the game first starts (until gameController calls our gameStart function)
     rigidbody.isKinematic=true;
     
     
     
 }
     
 
 function FixedUpdate () {
     
     
     // always assume we're NOT grounded every step (and expect to be proved wrong by raycasting etc.)
     grounded=false;
     
             
     // check to see if we're on top of a platform - here we cast two rays, one on each side of the player
     // LEFT:
     if (Physics.Raycast (transform.position- Vector3.up * 0.5 + Vector3.right * 0.5, -Vector3.up, hit, 1, 1<<9)) {
         // we found ground, so set our grounded flag to true, so that the player will jump
         grounded=true;
         if(rigidbody.velocity.y<0){
             theTransformHit=hit.transform;
         }
 
     }
     // RIGHT:
     if (Physics.Raycast (transform.position- Vector3.up * 0.5 - Vector3.right * 0.5, -Vector3.up, hit, 1, 1<<9)) {
         // we found ground, so set our grounded flag to true, so that the player will jump
         grounded=true;
         if(rigidbody.velocity.y<0){
             theTransformHit=hit.transform;
         }
     }
         
     // do jumping, if we're on top of a platform
     if(jumpEnabled){
         if(grounded && rigidbody.velocity.y<0){
             doJump();
             // send a message to the platform to tell it to destroy itself
             theTransformHit.gameObject.SendMessage("hitPlatform");
             
         }
     }
     
     // grab the mouse position and take off the width of the screen /2 so that moving the
     // mouse to the left of the window will produce a negative number and moving it to the right of
     // the window will produce a positive number
     
     mousePos = (Screen.width/2)-Input.mousePosition.x;
     
     xVel=0;
     
     // now we check the mouse position and move our player accordingly
     if(mousePos>1 || mousePos<-1){
         xVel+=(mousePos*0.02);        
     }
         
     if(canMove){
         
         // add x velocity to our position
         xPos=transform.position.x;
         
             
         if(xVel>maxMoveSpeed)
             xVel=maxMoveSpeed;
     
         if(xVel<-maxMoveSpeed)
             xVel=-maxMoveSpeed;
             
         if(xPos>gameWidth){
             xPos=gameWidth;
             rigidbody.MovePosition(Vector3(gameWidth,transform.position.y,transform.position.z));
             if(xVel>0){
                 xVel=0;
                 rigidbody.velocity.x=0;
             }
         }
         
         if(xPos<-gameWidth){
             xPos=-gameWidth;
             rigidbody.MovePosition(Vector3(-gameWidth,transform.position.y,transform.position.z));
             if(xVel<0){
                 xVel=0;
                 rigidbody.velocity.x=0;
             }
         }
         
         // set our movement velocity
         rigidbody.velocity.x=xVel*5;    
 
         // lean our player some
         transform.eulerAngles.z=-(mousePos*0.2);
     
     }
         
     fuel -= burnRate * Time.fixedDeltaTime;
     
     if (fuel < 0.0)
        fuel = 0.0;
  
     if (fuel > 0.0001)
            rigidbody.velocity = Vector3(0.0, 10, 0.0);
     else
            rigidbody.velocity = Vector3.zero;
            
         
     // if we're falling too fast, we're falling from high up and it must be game over, so we tell
     // gameController to endGame()
     if(rigidbody.velocity.y<-20 && !gameEnding){
         
         // comment this line out if you don't want the player to go upside down when he falls
         transform.eulerAngles.z=180;
         // play falling sound
         fallingSound.audio.Play();
         // tell gameController.js to end the game
         gameControl.endGame();
         gameEnding=true;
     }
 
     if(gameEnding){
         // as we fall from the sky, we move the player back toward the center of the play area. we do this
         // simply so that we know where he is going to land and theres no chance of him landing on an
         // awkward bit of scenery!
         if(transform.position.x>0)
             rigidbody.velocity.x=-5;
         if(transform.position.x<0)
             rigidbody.velocity.x=5;
     }
     
     // if we fall below the ground at the bottom (which may happen when the physics body is falling super
     // fast), we correct it and hold the player in place.
     if(transform.position.y<-4.5){
         transform.position.y=-4.5;
         rigidbody.velocity.y=0;
         rigidbody.velocity.x=0;
         
         if(!didGroundHitSound){
             groundHitSound.audio.Play();
             didGroundHitSound=true;
         }
             
         // comment this out if you don't want the player to end up upside down when it gets stuck in
         // the ground at the end of the game
         transform.eulerAngles.z=180;
         
     }
     
     transform.position.z=theZ;
     
 
 }
 
 
 function doJump(){
     // force velocity change to our 'jumpPower' value
     rigidbody.velocity.y=jumpPower;
     // play character animations
 //    animatedCharacter.animation.Rewind("jump");
 //    animatedCharacter.animation.Play("jump");
     // disable jumping temporarily (to allow the character to move away from the platform)
     jumpEnabled=false;
     // schedule jumping to be re-enabled in 0.2 seconds, once we're clear of the platform
     Invoke("enableJump",0.2);
     // play a sound
     jumpSound.audio.Play();
 }
 
 function enableJump(){
     jumpEnabled=true;
 }
 
 public function gameStart(){
     // until gameStart is called, the player is frozen in place. once this function is called, we release
     // any holds we have over it
     jumpEnabled=true;
     canMove=true;
     rigidbody.isKinematic=false;
     // start the game ... with a jump!
     doJump();
 }
 
 function OnTriggerEnter(fule: Collider){
 
     if(fule.gameObject.tag == "fule"){
     Debug.Log("We hit a fule box!");
     fuel += 150.0;
     }
 }
 
 public function gameEnd(){
     // it's game over, so we stop any jumping from happening and lock out the movement controls with canMove=false
     jumpEnabled=false;
     canMove=false;
 }