Okay, this works and makes the rocket point toward the mouse.
mouseDirection = rotation2D(Math.atan2(gMouse.y-_y, gMouse.x-_x))*180/Math.PI;
//_rotation = mouseDirection; <This works Perfectly, but no easing...
//My failing attempts at easing...
Radians = _rotation*Math.PI/180;
_x += Math.sin(Radians)*5;
_y -= Math.cos(Radians)*5;
//ANother...
_x -= _root.rocketSpeed*Math.cos(mouseDirection*Math.PI/180);
_y -= _root.rocketSpeed*Math.sin(mouseDirection*Math.PI/180);