LIQUID PROPELLANT ROCKET ENGINE

 LIQUID PROPELLANT ROCKET ENGINE

In the Liquid propellant rocket engine liquid fuel (refined petrol, liquid hydrogen, hydra-zine, etc.) and liquid oxygen are used in this engine.Liquid fuel and liquid oxygen are stored separately in two different tanks. Preheater is used to heat the fuel and oxidizer. Nozzle is used to increase the velocity and decrease the pressure of the gases.

Working

* Liquid fuel and liquid oxygen are pumped separately into a combustion chamber through control valves.
* Since the liquid fuel and liquid oxygen are stored at very low temperature, they are preheated in the preheater to a suitable temperature.
* The preheated fuel-oxidizer mixture is injected into the combustion chamber through suitable injector and combustion takes place.
* When the combustion takes place in the combustion chamber, very high pressure and very high temperature gases are produced.
* The highly heated products of combustion gases are then allowed to expand in the nozzle section.
* In the nozzle, pressure energy of the gas is converted into kinetic energy. So the gases coming out from the unit with very high velocity. Due to high velocity of gases coming out from the unit, a force (or) thrust is produced in the opposite direction. This thrust propels the rocket.

Advantages of Liquid Propellant Rocket Engine

1. Liquid propellant engines can be reused after recovery. So it is economical.
2. Combustion process is controllable i.e., it is easy to stop the combustion by closing the fuel valve (or) oxidizer valve.
3. Speed regulation i.e., increase and decrease of speed is possible.
4. High specific impulse.
5. More economical for long range operation.
6. Malfunctions and accidents can be rectified at any stage.

Disadvantages of Liquid Propellant Rocket Engine

1. Liquid propellant rocket construction is more complicated compared to solid propellant rocket.
2. Manufacturing cost is high.
3. There are additional handling and safety problems if the propellants are poisonous and corrosive.
4. The size and weight of the engine is more compared to solid propellant rocket.
5. High vibration
6. Many liquid propellants can exist in liquid state at very low temperature. So proper insulation is needed.

Liquid Propellants

Liquid propellants are classified into the following two groups.
(a) Mono-propellants
(b) Bi-propellants

(a) Mono-Propellants -
A liquid propellant which contains both the fuel and oxidizer in a single chemical is known as a mono-propellant. It is stable at normal ambient conditions and liberates thermo-chemical energy on heating. Mono-propellants have been widely used in solid propellant rockets.
Examples: 1. Nitroglycerine
2. Nitromethane
3. Hydrogen peroxide
4. Hydrazine

(b) Bi-propellants-
If the fuel and oxidizer are different from each other in its chemical nature, then the propellant is called bi-propellant. Bi-propellants have been widely used in liquid propellant rocket system. The mostly used bi-propellant combinations are listed below.
Oxidizer- Fuel
1. Liquid oxygen - Gasolinc, Methane, Ethanol, Hydrazine Unsymmetrical Dimethyl Hydrazine (UDMH)
2. Red fuming nitric acid - Aniline, Ethanol
3. White funing nitric acid - Alcohol
4. Hydrogen peroxide - Hydrazine, Ethanol, Methanol

Properties of Liquid Propellants

1. Propellant should have high calorific value.
2. It density should be high.
3. It should have low values of vapour pressure and viscosity.
4. It should have higher specific heat and thermal conductivity.
5. Products of combustion should have low molecular weight to produce high jet velocity.
6. It should be non-corrosive and non-reactive with components of the engine.
7. It should not be poisonous and hazardous.
8. It should be cheap and easily available.
9. Energy released during combustion per unit mass of the propellant combination should be high.
10. It should be easily ignitable.
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