We know that a rocket has excess of fuel in it and the fuel is burned and the gases are released from the bottom of the rocket where we see fire. As the action done by the rocket is to release the gases with high speed in downward direction and the reaction given by the gases is to move the rocket in upward direction which is the thrust force , according to Newton’s third law of motion action done by the rocket by releasing gases(downward) gives the rocket reaction(upwards) by the gases which is equal and opposite.

Therefore, the source of force that propels the rocket to move upwards is the reaction force given by the burned gases when they are released by the rocket, the force is called thrust force.

The rocket also moves with the principle of law of conservation of momentum because when it burns the fuel and exhaust the gases it actually releases the mass from the bottom tip of it and hence gets an upward thrust to conserve the momentum.