A rocket is a device that moves forward by expelling particles of a fuel mixture in a downward direction.
The principle behind the rocket propulsion is the conservation of momentum. When the rocket is fired, the exhaust gases rush downward with a tremendous speed, which gives the opposite momentum to the rocket, and the rocket is pushed upward by the reaction.
Thus the thrust on the rocket is provided by the reaction forces of high-speed gases ejected at the rear. So, the impulse on the rocket is due to the momentum change in a very small time.
Therefore, A rocket works on the principle of conservation of momentum.