A brief description of Propulsion - Rocket Propulsion

​A rocket engine is a type of jet engine that uses only stored rocket propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines, obtaining thrust in accordance with Newton's third law.  Rocket thrust results from the high speed ejection of material and does not require any medium to "push against", so they are well suited for uses at very high altitude and in space. There are two main categories of rocket engines; liquid rockets and solid rockets. Under normal temperature conditions, the propellants do not burn; but they will burn when exposed to a source of heat provided by an igniter.

A) Solid fuel rockets

Solid-fuel rocket engines were the first engines created by man. Solid rocket engines are used on air-to-air and air-to-ground missiles, on model rockets, and as boosters for satellite launchers. In a solid rocket, the fuel and oxidizer are mixed together and packed into a solid cylinder. Once the burning starts, it proceeds until all the propellant is exhausted. With a liquid rocket, you can stop the thrust by turning off the flow of propellants; but with a solid rocket, you have to destroy the casing to stop the engine. One of the biggest problems with solid fuel rocket engines is that once started, the reaction cannot be stopped or restarted. This makes them considered uncontrollable. Therefore, solid fuel rockets are more widely used for missiles, or as booster rockets. A solid rocket is much easier to handle and can sit for years before firing.
 
A hole through the cylinder serves as a combustion chamber. When the mixture is ignited, combustion takes place on the surface of the propellant. A flame front (the surface of the interface that faces reactants is often termed as the flame front. Similarly, the surface the faces the products can be termed as the flame back) is generated which burns into the mixture. The combustion produces great amounts of exhaust gas at high temperature and pressure. The amount of exhaust gas that is produced depends on the area of the flame front and engine designers use a variety of hole shapes to control the change in thrust for a particular engine. The hot exhaust gas is passed through a nozzle which accelerates 

B) Liquid fuel rockets

In a liquid rocket, the propellants, the fuel and the oxidizer, are stored separately as liquids and are pumped into the combustion chamber of the nozzle where burning occurs. Liquid rockets tend to be heavier and more complex because of the pumps and storage tanks. The propellants are loaded into the rocket just before launch. Liquid rocket engines are used on the Space Shuttle to place humans in orbit, on many un-manned missiles to place satellites in orbit, and on several high speed research aircraft following World War II
 
All liquid rocket engines have tankage and pipes to store and transfer propellant, an injector system, a combustion chamber which is very typically cylindrical, and one (sometimes two or more) rocket nozzles. Liquid systems enable higher specific impulse than solids and hybrid rocket engines and can provide very high tankage efficiency.

Liquid rockets can be mono-propellant rockets using a single type of propellant, bi-propellant rockets using two types of propellant, or more exotic tri-propellant rockets using three types of propellant.
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According to the type of propellants used in the rockets we can classify liquid fuel rocket engines into cryogenic, Semi cryogenic & hypergolic

B1) Cryogenic 

A cryogenic engine uses cryogenic fuels. A cryogenic technology is the process of involvement or including of usage of rocket propellants at a cryogenic temperature. It can be combination of liquid fuels such as: liquid Oxygen (LOX), and liquid Hydrogen (LH2) as an oxidizer and fuel in the different mixtures or proportions. The mixture of fuels offer the highest energy efficiency for the rocket engines that produces very high amount of thrust. Here, the Oxygen remains liquid only at the temperature below (-183 C) and Hydrogen below (-253 C). This is a type of rocket engine that is functionally designed to use the oxidizer which must be refrigerated in the liquid state. Sometimes, the liquid nitrogen (LN2) is used as a fuel because the exhaust is also nitrogen.

The engine components are also cooled so the fuel doesn’t boil to a gas in the lines that feed the engine. The thrust comes from the rapid expansion from liquid to gas with the gas emerging from the motor at very high speed. The energy needed to heat the fuels comes from burning them, once they are gases. Cryogenic engines are the highest performing rocket motors. One disadvantage is that the fuel tanks tend to be bulky and require heavy insulation to store the propellants. Their high fuel efficiency, however, outweighs this disadvantage.
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A Cryogenic rocket stage is more efficient and provides more thrust for every kilogram of propellant it burns compared to solid and earth-storable liquid propellant rocket stages. Specific impulse (a measure of the efficiency) achievable with cryogenic propellants (liquid Hydrogen and liquid Oxygen) is much higher compared to earth storable liquid and solid propellants, giving it a substantial payload advantage. 

Dont forget to see this awesome video by ISRO - https://www.youtube.com/watch?v=nfZBIHGAiuk

B2) Semi-cryogenic
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Semi cryogenic engines using liquid oxygen and kerosene as propellants. They are considered relatively environment friendly, non-toxic and non corrosive. In addition, the propellants for semi-cryogenic engine are safer to handle & store. It will also reduce the cost of launch operations. Apart from this combination of Liquid oxygen (LOX) and alcohol (ethanol, C2H5OH) Liquid oxygen (LOX) and gasoline , Liquid oxygen (LOX) and carbon monoxide.

The main difference between cryogenic and semi cryogenic engine is its fuel. Cryogenic engine has better specific impulse than semi cryogenic engine so it is ideal for upper stages of rockets. Semi-cryogenic engines offer the best thrust to weight ratios , higher density of the exhaust gases of the semi-cryogenic contribute to high mass flow rates making it easier to develop high thrust engines. So semi cryogenic engine is ideal for lower stages. 

B3) Hypergolic
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A hypergolic propellant combination used in a rocket engine is one whose components spontaneously ignite when they come into contact with each other. The two propellant components usually consist of a fuel and an oxidizer. Although commonly used hypergolic propellants are difficult to handle because of their extreme toxicity and corrosiveness, they can be stored as liquids at room temperature and hypergolic engines are easy to ignite reliably and repeatedly.

C) Hybrid

Rocket engine, where a solid fuel is combined with a liquid oxidizer or vice versa, is known as a hybrid rocket. Hybrid rockets avoid some of the disadvantages of solid rockets like the dangers of propellant handling, while also avoiding some disadvantages of liquid rockets like their mechanical complexity.

A brief description of Propulsion - Introduction