(i) Before launch, the total momentum of a rocket and its fuel is zero. During launch, the downward momentum of the expanding exhaust gases just equals in magnitude the upward momentum of the rising rocket, so that the total momentum of the system remains constant—in this case, at zero value. In a collision of two particles, the sum of the two momentum before collision is equal to their sum after collision.

(ii) In airplane, the turbine works the compressor and drives the gas through tail piece of engine at larger speed. This results in forward momentum and jet moves forward. The forward momentum is equal to backward momentum of gases. The rates of burning and ejection of gases in the fuel burning chamber is adjusted to get desired increase momentum in the forward direction.