Rockets move by expelling burning gas through engines at their rear. The ignited fuel turns to a high pressure gas which is expelled with extremely high velocity from the rocket engines (fig. 3.15).

The rocket gains momentum equal to the momentum of the gas expelled from the engine but in opposite direction. The rocket engines continue to  expel gases after the rocket has begun moving and hence rocket continues to gain more and more momentum. So instead so travelling at steady speed the rocket gets faster and faster so long the engines are operating.

A rocket carries its own fuel in the form of a liquid or solid hydrogen and oxygen. It can, therefore work at great heights where very little or no air is present. In order to provide enough upward thrust to overcome gravity, a typical rocket consumes about 10000 kgs-1. In fact, more than 80% of the launch mass of a rocket consists of fuel only. One way to overcome the problem of mass of fuel is to make the rocket from several rockets linked together.

When one rocket has done its job, it is discarded leaving others to carry the space craft further up at ever greater speed.

If m is the mass of the gases ejected per second with velocity v relative to the rocket, the change in momentum per second of the ejecting gases is mv. This equals the thrust produced by the engine on the body of the rocket. So, the acceleration ‘a’ of the rocket is

Fig. 3.15: Fuel and oxygen mix in the combustion chamber. Hot gases exhaust the chamber at a very high velocity. The gain in momentum of the gases equals the gain in momentum of the rocket. The gas and rocket push against each other and move in opposite directions.

Where m is the mass of the rocket. When the fuel in the rocket is burned and ejected, the mass m of rocket decreases and hence the acceleration increases.