Space Rokets:

In all rockets, the exhaust is formed from propellants carried within the rocket prior to use.[49] Rocket thrust is due to the rocket engine, which propels the rocket forwards by exhausting the propellant rearwards at extreme high speed.


Rocket thrust is caused by pressures acting on the combustion chamber and nozzle
In a closed chamber, the pressures are equal in each direction and no acceleration occurs. If an opening is provided at the bottom of the chamber then the pressure is no longer acting on that side. The remaining pressures give a resultant thrust on the side opposite the opening; as well as permitting exhaust to escape. Using a nozzle increases the forces further, in fact multiplies the thrust as a function of the area ratio of the nozzle, since the pressures also act on the nozzle. As a side effect the pressures act on the exhaust in the opposite direction and accelerate this to very high speeds (in accordance with Newton's Third Law).[49]
If propellant gas is continuously added to the chamber then this disequilibrium of pressures can be maintained for as long as propellant remains.[49]
It turns out (from conservation of momentum) that the speed of the exhaust of a rocket determines how much momentum increase is created for a given amount of propellant, and this is termed a rocket's specific impulse.[49] Because a rocket, propellant and exhaust in flight, away from any external perturbations, may be usefully considered as a closed system, then the total momentum is constant at all times. Therefore the faster the net speed of the exhaust is in one direction the greater the speed of the rocket itself in the opposite direction can become, and even more so because the rocket mass is typically far lower than the final total exhaust mass.
As the remaining propellant decreases, rocket vehicles become lighter and their acceleration per unit of propellant tends to increase until eventually they run out. This means that much of the speed change occurs towards the end of the burn when the vehicle is much lighter.[49]