Rocket Landing

Rocket landing controller, rocket is coming towards earth due to gravity its moving down, so i want to give thrust to rocket in such a way its slow down the falling the velocity(-ve velocity in y direction ) of rocket not make it to move rocket upward, but if i pressed the thrust button for longer time its should slowly move upwards....here is my code that i am using.

 private Rigidbody rg;
 public float thrustvalue;
 public float speed;
 public float constantforce;
 public bool iscollided=true;
 // Use this for initialization
 void Start () {
     rg=GetComponent<Rigidbody>();
     Physics.gravity=new Vector3(0,-15,0);


 }

 // Update is called once per frame
 void FixedUpdate() {

     if(iscollided){

         rg.AddRelativeForce(0,constantforce,0);

     }



     if(Input.GetKeyDown(KeyCode.Space)){



         rg.AddForce(transform.up*thrustvalue);
         //rg.AddForceAtPosition(transform.up*thrustvalue);

     }

     if(Input.GetKey(KeyCode.LeftArrow)){



         transform.Rotate(0,0, -Input.GetAxis("Horizontal")*speed*Time.deltaTime);


     }
     if(Input.GetKeyDown(KeyCode.RightArrow)){

         transform.Rotate(0,0, Input.GetAxis("Horizontal")*speed*Time.deltaTime);

     }

 }

 void  OnCollisionEnter(Collision collisionInfo)
 {
     iscollided=false;
 }