Simple rocket

 How a simple rocket works?

The problem has arisen in the 19th century when experimental balloon flights showed how the Earth's atmosphere thins out with the increasing altitude. It was clear that such devices that create forward and upward forces, such as wings and propellers, would not work in space. Also engines that generate power by burning fuel in the oxygen from the Earth's atmosphere wouldn't work in airless environment. 

So what is the solution? Rocket!

Rockets generate thrust from the engine. Rockets burn fuel and turn it into hot gas, which is pushed through a nozzle. The force of the gas moving backward pushes the rocket upward or forward. The process is called jet propulsion. Unlike jet engines, rockets don't need air to work and carry everything they need. 

A simple rocket consists of a tall cylindrical body constructed from relatively thin metal. The fuel is fitted inside the body of the rocket, while the engines that provide thrust to propel the rocket are placed at the bottom of the body of the rocket. Each engine contains a device that injects rocket fuel into an are located at the top of the nozzle  called the combustion chamber. The fuel burns in the chamber and turns into a hot gas that expands in all direction. The nozzle directs the expansion in a direction opposite of the desired direction of the rocket motion. 

A rocket's ability to lift its payload into space depends on how it is able to overcome gravitational force. The rocket must provide enough thrust to overcome weight of the rocket's body. 

Newton's Third Law states that "every action has an equal and opposite reaction". In case of a rocket, burning fuel creates a push on the front of the rocket pushing it forward. This creates an equal and opposite push on the exhaust gas backwards. The combined acceleration of all the molecules of oxidized fuel as they emerge from the rocket's nozzle create the thrust that accelerates and propels the rocket.

source: Britannica for kids

Rocket engines typically use two different types of chemicals at the same time to support the burning reaction. Together, these chemicals are called propellants. The first chemical, called fuel, provides the substance that the engine burns, while the second chemical, called oxidizer, supplies the oxygen for the burning reaction. Compared to cars and airplanes, where their engine gets the oxygen to support the combustion of fuel from the Earth's atmosphere. 

Many different types of propellants exist. Specific impulse provides a method to measure how much thrust a propellant produces when in burns. the rating number, measured in seconds, depends on different factors, such as the velocity with which the exhaust from the combustion leave the rocket nozzle, and how much internal energy is stored in the fuel and oxidizer. Propellants with higher specific impulse burn more effectively than propellants with lower specific impulse. 

There are two main categories of rocket engines: liquid rockets and solid rockets. 

In case of a liquid rocket, the propellant, the fuel and the oxidizer, are stored in different tanks as liquids and they are pumped into the combustion chamber where the burning occurs. One of the most used combinations involves using combination of liquid hydrogen (LH2) as a fuel and liquid oxygen (LOX) as the oxidizer. This combination results in specific impulse of 400 seconds at sea level, and 453 seconds in vacuum. 

In case of a solid rocket, the propellants are mixed together and packed into a solid cylinder. This type of propellant consists of a flammable putty or rubber like material. 

Propellants do not burn under normal temperature. When they are exposed to a source of hear provided by an igniter, they burn. Under normal temperature conditions, the propellants do not burn; but they will burn when exposed to a source of heat provided by an igniter. When the burning starts, it continues until all the propellant is exhausted which is different in case of liquid rocket. You can stop the burning by switching off the flow of the propellant. 

Liquid rockets tend to be heavier and more complicated because of the pumps and storage tanks. The propellants are loaded into the rocket just before launch. A solid rocket is much easier to manage and can sit for years before firing.

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