The driving force for the motion of any vehicle is friction, but in the case of rocket propulsion in space, the friction force is not present. The motion of a rocket changes its velocity (and hence its momentum) by ejecting burned fuel gases, thus causing it to accelerate in the direction opposite to the velocity of the ejected fuel. In this situation, the mass and velocity of the rocket constantly change along with the total mass of ejected gases. Due to conservation of momentum, the rocket's momentum changes by the same amount (with the opposite sign) as the ejected gases. However, as time goes by, the rocket's mass (which includes the mass of the remaining fuel) continuously decreases, and its velocity increases. Therefore, the principle of conservation of momentum is instrumental in explaining the dynamics of a rocket's motion.

Using the conservation of momentum principle, the velocity of the rocket at any given instant can be calculated using the ideal rocket equation. Similarly, the thrust acting on the rocket and its instantaneous acceleration can be estimated.

This text is adapted from Openstax, University Physics Volume 1, Section 9.7: Rocket Propulsion.