Principle

The principle of rocket propulsion depends on the following two laws: -

We have already read about these laws, and now we will see how they can be applied for propelling the rocket.

The motion of a rocket is an interesting application of Newton 's third law of motion & momentum principle. The rocket expels a jet of hot gases from its tail. This is say, an action force. The jet of hot gases exerts a force on the rocket, propelling it forward; this is the reaction force.

 Propulsion Principle

From the momentum point of view, the hot gases acquire momentum in the backward direction & the rocket acquires an equal amount of momentum in the forward direction.

The simplest example to understand the propulsion of rockets is that of a balloon.

A balloon shooting forward (when the mouth of the balloon filled with air is released) and a rocket hurtling into space are propelled by similar forces. The air in a closed balloon exerts a uniform outward force. But when air rushes out of its neck (similar to exhaust gases leaving rockets) disturbs this equilibrium. Thus an equal and opposite force is exerted on the surface opposite to the neck. This drives the balloon forward.

As we have seen in the previous section propellants are used to provide thrust to the rockets. These propellants on burning produces large amount of gas, which are allowed to pass through nozzle. On passing through the nozzle, high pressure is generated i.e. gas comes out with high pressure.

Now to increase the thrust, one basic property is used while designing the nozzle. The neck of the nozzle is kept very small as compared to the body of the rocket. So the pressure of the gas increases and so does the velocity. Thus high thrust is achieved.

Let us now see the different types of propulsion techniques classified according to present-day and future techniques.