How Does A Space Ship Travel In Space?

A spaceship uses a propulsion method that requires combustion of fuel at an enormous rate which propels it forth enabling it to cross hold the barriers imposed by earth's gravitational field. However, once a space shuttle reaches into space it is not required to burn fuel at the same rate since there is no opposing friction to it's motion in space or other forces to counter it's motion. The shape ship carries with itself a huge cache of liquid Oxygen to enable it to burn fuel since combustion requires the presence of Oxygen as there is no freely abundant oxygen in space. In case of unmanned crafts like the voyager I and II and the Cassini and Huygens probes however a slingshot method was employed which used planetary orbits of Venus and Mercury's gravitational fields in order to catapult them into the far reaches of space.