Recoil of a gun:-
A gun and a bullet constitute one isolated system. On firing the gun, the bullet moves out with a very high velocity . The gun experiences a recoil. It moves in the opposite direction. Velocity of the recoil gun can be calculated by the application of law of conservation of momentum.
2. Rocket and jet plane:-
Fuel and oxygen is burnt in the ignition chamber. As hot gases escape from a rear opening, with some momentum, the rocket moves in the forward direction with the same momentum.