In space rocket travels by the application of Newton's 3rd law of motion. Whenever the rocket has to change its speed or direction,it burns the fuel.The burning of the fuel produces gas exhaust which provides the necessary force. The rocket will be accelerated according to the 2nd law of motion. If it has to move in a uniform motion, no force is required. The inertia of motion of the rocket will keep the rocket moving, according to the 1st law of motion.