Add a small amount of initial movement to the rocket in its constructor. To do this, create a new vector with some arbitrary direction and a small length (I used 0.3 for my own version) and then use the SmoothMover's addForce method with this vector as a parameter to add this force to the rocket. (Make sure to use an int as your first parameter in the Vector's constructor, in order to use the correct constructor.)
Test. If all went well, the rocket should drift all by itself when the scenario starts. Don't make this initial drift too fast. Experiment until you have a nice, slow initial movement.