Science students are more or less familiar with Newton's third law. From this we get the law of conservation of momentum. The statement of the formula goes something like this: 'If no forces act between two objects other than action and reaction, their total momentum in a given direction remains unchanged.'

Multistage rocket

All rockets are launched based on this principle. A large amount of gas is produced by burning the fuel in the lower part of the rocket. Those gases are released through the underside of the rocket. Initially, both the fuel and the rocket are at rest. That is, the sum of their momentum at this time is zero. The emission of gas creates some momentum of the fuel towards the emission. According to the formula, to keep the total momentum of the whole system unchanged, an equal amount of momentum will be created in the front of the rocket by the fuel. As a result the rocket will move forward.

Modern rockets used to send artificial satellites into space do not hold all the fuel together. A few are kept in separate steps. When a stage runs out of fuel en route, that segment is disengaged. The debris is destroyed by collision with the atmosphere. When the fuel of one stage is exhausted, the fuel of the other stage is used. These rockets are called multistage rockets. Now the question is, why do we need a multistage rocket?

The first problem is with the weight. The tank inside which the rocket fuel is kept is very heavy. After the fuel is burned it is no longer needed. If all the fuel is kept together, then the tank only needs to be carried to the end of the journey. As a result, valuable fuel is wasted due to carrying extra weight. On the other hand, if the fuel is stored in several stages, then the empty tank can be isolated at the end of each stage. As a result, more distance can be covered using second stage fuel.

The second problem is with the rocket engine's nozzle (the pipe coming out of the engine). Let's explain a little. In the engine, the fuel is detonated and the resulting gases are ejected through a nozzle. Capitalizing on the resulting momentum, the rocket moves forward. When the rocket is inside the atmosphere, the nozzle size must be smaller. Because it is very dangerous to allow the gas flow from the nozzle to spread far into the atmosphere.

Gas flow is more linear in small nozzles. On the other hand, if a large nozzle is used, the flow of emitted gas can spread over a large area regardless of the air pressure. Turbulence can be created as a result. Due to its effect, the entire rocket is in danger of being destroyed due to continuous shaking. So there is no alternative to using smaller nozzles inside the atmosphere. However, it is not possible to generate the necessary momentum to go into space with only one engine with small nozzles. So there are some engines that have to be used.

Now let's find out what size nozzle engine needs to be used in space. Air pressure does not exist in space. So if desired, two types of nozzle engines - small or large - can be used. However, using engines with larger nozzles results in higher fuel output. Therefore, it is advisable to use large nozzle engines in space. So it appears that we need to use two types of engines in two situations. Only multistage rockets can meet this demand.