How does a rocket work?

Law Of Conservation of Momentum

The basic principle that all rockets work by is law of conservation of momentum that is the total momentum of two objects before collision is equal to the total momentum of the two objects after collision that is the momentum lost by object 1 is equal to the momentum gained by object two.

In case of rocket science the rocket launches itself by ejecting some of its mass downwards. In fact while a rocket is being accelerated upwards its engine is continuously ejecting some mass and forcing it downwards. Because this mass is forced downwards, by conservation of momentum the rocket must then be forced upwards. In terms of forces the rocket has to exert some force on the ejected mass to push it downwards which then results in an equal and opposite reaction force pushing the rocket. Greater is the momentum, more is the thrust developed. That is how law of conservation of momentum or Newton’s third law of motion is responsible for launching of rocket.

Types of Propulsion System

There are only two types of propulsion system which can get a rocket escape the earth’s atmosphere. They are   i.) Liquid Propellant Rocket Engine  ii.)Solid Propellant Rocket Engine

In this article we will understand about how a liquid propellant rocket engine works.

Inside the rocket engine we have the combustion chamber, the throat and the nozzle. Inside the combustion chamber the fuel and the oxidizer mix. The liquid fuel before entering the combustion chamber travels through the nozzle body as it helps to reduce nozzle cover temperature and also covers some energy savings. To pump the fuel and the oxidizer in adequate rate, two pumps are used. These pumps are driven by a turbine which is connected to the same shaft as the pumps. The pump turbine unit is referred to as turbo pump. A gas generator produces hot gas to rotate the turbine. The gas generator uses bypassed fuel and bypassed oxidizer for the purpose of combustion. Exhaust from the turbine is mixed with main rocket exhaust.

Even as the liquid fuel converts into gas, the mass of the fuel remain the same. The massive acceleration of this transformation from liquid to gas causes the momentum. The mathematical representation of this energy transformation is  E=MV² (m=mass of gas, v=velocity of gas) which implies faster is the velocity of the gases more is the energy released. Lighter is the fuel better is it as lighter fuels can move faster. Hence the engine uses the mass and the velocity of the fuel as it burns to create thrust.

For example heavier but slower fuel mixture like kerosene and liquid oxygen won’t give as much energy as mixture of liquid hydrogen and liquid oxygen. The engine’s efficiency can increase by adjusting the hydrogen and oxygen ratio by making it more hydrogen rich. So we conclude that if we can take a really light fuel moving at very high velocity, we end up creating more efficient rockets

To make it to the exosphere (space)

The fuel and the oxidizer required for the rocket engine are stored in two large tanks. During lift off the thrust produced by the main engine may not be sufficient so usually a few solid propellant strap boosters are used to assist the lift off. The solid propellant strap boosters get burned off rapidly so to reduce the weight they are abandoned after the burn off. Now the next engine takes the charge. This way the rocket’s weight is greatly reduced. Hence great acceleration can be achieved. This process is called staging.

After the rocket has reached the desired height the rocket’s nozzle is slightly tilted by high precision devices. Hence the rocket will turn to the desired angle. After the angle is made the nozzle comes back to its original position. Now the second engine is ignited to give the rocket enough inertia to reach its desired destination.

  

Liquid Propellant Engines have high specific thrust where thrust can be controlled and can be restarted.