In this movie variables dx and dy have initial values of 0. In the frame event the rockets position is incremented by dx and dy. At start, because dx and dy are both 0, the rocket doesn't move.

When we press the button we assign values to dx and dy. In subsequent frame events, the rocket will move using the new values of dx and dy.

The code for this movie is shown below:

var dx:int=0;
var dy:int=0;
this.addEventListener(Event.ENTER_FRAME,frames);
function frames(e:Event):void {
//the rocket will not move as long as dx and dy are 0
rocket.x=rocket.x+dx;
rocket.y=rocket.y-dy;
} //frames
btnBlastOff.addEventListener(MouseEvent.CLICK, blastOff);
function blastOff(e:MouseEvent):void {
//after dx and dy are not 0, the rocket will move in the frame event
dx=2;
dy=4;
} //blastOff

Once the rocket blasts off, the show is over. Download this movie.
In the next part, we will add code to relaunch the rocket.