Space Craft controller.

I have the following code for my space craft to allow it to fly in space but right now when I press the key to make it move it continues to move in that direction even after you relase the key. I want to change this code to make it so that once the button is released it causes the ship to stop moving in that direcion to give more of a exact contolle.

 var Speed = 1000;
 var TorqueSpeed = 20.0;
 function Update () {
 
 if(Input.GetKeyDown(KeyCode.W)){constantForce.relativeForce.z = Speed * Time.deltaTime;else{
 
 if(Input.GetKey(KeyCode.S)){constantForce.relativeForce.z = -Speed * Time.deltaTime;}else{
 constantForce.relativeForce.z = 0;}}
 //Turn right and left
 if(Input.GetKey(KeyCode.D)){constantForce.relativeTorque.y = TorqueSpeed * Time.deltaTime;}else{
 if(Input.GetKey(KeyCode.A)){constantForce.relativeTorque.y = -TorqueSpeed * Time.deltaTime;}else{
 constantForce.relativeTorque.y = 0;}}
 //Rotate Up and Down
 if(Input.GetKey(KeyCode.E)){constantForce.relativeTorque.x = TorqueSpeed * Time.deltaTime;}else{
 if(Input.GetKey(KeyCode.Q)){constantForce.relativeTorque.x = -TorqueSpeed * Time.deltaTime;}else{
 constantForce.relativeTorque.x = 0;}}
 //Rotate right and left
 if(Input.GetKey(KeyCode.Z)){constantForce.relativeTorque.z = TorqueSpeed * Time.deltaTime;}else{
 if(Input.GetKey(KeyCode.X)){constantForce.relativeTorque.z = -TorqueSpeed * Time.deltaTime;}else{
 constantForce.relativeTorque.z = 0;}}
 }