How Rocket Engine works?

A scaled pressure vessel, such as a balloon with the neck securely tied, can be used to illustrate how a rocket engine produces its thrust. The pressure in the balloon acts uniformly in all directions, and consequently there is no force trying to make the balloon move one way or another. If the neck of the balloon is suddenly opened, the air rushes out giving an unbalanced force. On the side of the balloon opposite the open aperture; this out of balance produces a force which propels the balloon.

There is another way of looking at the generation of this force by using Newton’s Laws of Motion. As the air accelerates through the aperture a force has to be applied to it to give it its increasing velocity. To every action there has to be an equal and opposite reaction that propels the balloon. The balloon is indeed a very simple form of rocket engine combustion chamber, but it is, of course, much too crude for any practical use. To begin with, the energy it contains is too small to produce any really worthwhile thrust. Secondly, the conversion of pressure energy of the air in the balloon to kinetic energy of the air leaving the balloon is very inefficient. Finally, there is no way of continuously recharging the balloon so that it goes on propelling itself for any length of time.

These limitations can, however, be overcome in the combustion chamber of a rocket engine, which can be linked to the balloon. To overcome the very short and rather explosive nature of the balloon’s behavior, chemicals can be continuously fed into the combustion chamber where they burn without interruption, generating a steady supply of gas. If the chemicals are liquids they can conveniently be injected into the chamber under pressure and, as a great deal of heat is generated in the combustion process, a large amount of high-temperature, high-pressure gas is therefore formed.

The chemicals used are known as propellants, and those are generally divided into two sorts. There are oxidants which contain oxygen or an oxidizing component, and fuels which burn with the oxidants, releasing energy in the process.

There are a number of added advantages in using liquid propellants in a rocket engine: To begin with, it makes it possible to vary the thrust by varying the flow of propellants into the chamber. Furthermore, by suitable design, it is possible to cut the flow completely, so stopping the rocket engine, and then to re-start it again later, perhaps a large number of times. In addition, one of the propellants can be used as a coolant to keep the combust on chamber at a reasonable temperature.

Whereas the performance of modern liquid propellant engines is not very different from those using solid propellants, those giving the highest performance, which will be needed for the more ambitious journeys into space, will almost certainly use liquid propellants, because they are the only known substances which contain enough available chemical energy.

The maximum amount of heat and pressure energy in the combustion chamber is converted into kinetic energy by a suitably designed nozzle or aperture which has a particular convergent/divergent shape with a throat of a comparatively small diameter, this shape being necessary when a supersonic jet has to be formed. The velocity of the gas stream at the throat is equal to the local speed of sound, and in the subsequent divergent part of the nozzle the gases go on accelerating to supersonic speeds. In actual fact, the size of the throat determines the pressure/flow relationship in the combustion chamber. As soon as the jet becomes supersonic no pressure waves, which themselves travel at the speed of sound, can pass up the gas stream , and this means that external pressure and temperature cannot in any way influence conditions in the combustion chamber. This is another reason why a rocket engine is independent of its surroundings.