How do rockets work?
As the rocket engines operate, some kind of fuel (liquid or solid) is burnt. The engines continuously eject burned fuel gases. As these exhaust gases have both mass and velocity, they have some momentum as well. By the principle of conservation of momentum, the rocket’s momentum changes by this same amount (with the opposite sign).
Hence, this can be said that the rocket gains momentum in the forward direction which is the opposite direction of the exhaust gases. The rate of change of momentum of the rocket in turn causes a force applied to the rocket in the forward direction. And this force makes the rocket accelerate in the forward direction. Thus the rocket propulsion works.
Rockets work on the principle of conservation of momentum. How?
If we define our system to be the rocket + fuel, then this is a closed system. As the rocket is in deep space, there are no external forces acting on this system.
As a result, momentum is conserved for this system. Thus, we can apply the principle conservation of momentum to explain how the rocket works.
If you are interested in how rockets work, then you must read our new post on Rocket Equation Derivation & the Rocket Acceleration formula.