LIQUID PROPELLANT ROCKET ENGINE

   A liquid-propellant rocket or liquid rocket is a rocket engine that uses liquid propellants. Liquids are desirable because

their reasonably high density allows the volume of the propellant tanks to be relatively low, and it is possible to use lightweight centrifugal turbopumps to pump the propellant from the tanks into the combustion chamber, which means that the propellants can be kept under low pressure. This permits the use of low-mass propellant tanks, resulting in a high mass ratio for the rocket.

There are two types of liquid propellents :-

a)      Monopropellants

b)      Bipropellants

a)      Monopropellant

A liquid propellant which contains both fuel and oxidizer in a single chemical is known as monopropellant. It stable at ambient conditions and liberates thermo-chemical energy on heating. Monopropellants have been widely used in solid propellant rockets.

Examples : Nitro glycerin, nitro methane, Hydrogen peroxide, Hydrazine

b)     Bipropellants

If the fuel and oxidizer are different from each other in its chemical nature then the propellant is called bipropellant. Bipropellants have been widely used in liquid propellant rocket system.

PROPERTIES OF LIQUID PROPELLANTS

• Propellant should have high calorific value
• Its density should be high
• It should have low values of vapor pressure and viscosity
• It should have higher specific heat and thermal conductivity
• Products of combustion should have low molecular weight to produce high jet velocity
• It should be non corrosive and non reactive with components of the engine
• It should not be poisonous and hazardous
• It should be cheap and easily available
• Energy released during combustion per unit mass of the propellant combination should be high
• It should be easily ignitable

LIQUID PROPELLANT ROCKET ENGINE

Construction

1. Liquid fuel(refined petrol, liquid hydrogen, hydrazine etc) and liquid oxygen are used in this engine

1. Liquid fuel and liquid oxygen are stored separately in two different tanks
2. Preheater is used to heat the fuel and oxidizer
3. Nozzle is used to increase the velocity and decrease the  pressure of the gases
4. The construction of liquid propellant rocket engine is shown in figure.

Working

1. Liquid fuel and liquid oxygen are pumped separately into a combustion chamber through control valves.

1. Since the liquid fuel and liquid oxygen are stored at  very low temperature , they are preheated in the preheated to a  suitable temperature
2. The preheater fuel-oxidizer mixture is injected into the combustion chamber through suitable injector and combustion takes place.
3. When the combustion takes place in the combustion chamber , very high pressure and temperature gases are produced
4. he highly heated products of combustion gases are then allowed to expand in nozzle section.
5. In the nozzle, pressure energy of the gas is converted into kinetic energy. So the gases coming out from the unit with the very high velocity.
6. 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

1. Liquid propellant engines can be reused after recovery. So it is economical .
2. Combustion process is controllable.
3. Speed regulation is possible
4. High specific impulse
5. More economical for long range operation
6. Malfunctions and accidents can be rectified at any stage

Disadvantages

1. It’s construction is more complicated compared to solid propellant rock
2. There are additional handling and safety problems if the propellants are poisonous and corrosive
3. Manufacturing cost is high
4. High vibration
5. Size and weight of the engine is more compared to solid propellant rockets

Any liquid propellants can exist in liquid state at very low temperature. So proper insulation is needed