All About Rocket Propulsion

What is propellant?

THE word "propellant" is used to signify the chemicals combined in a rocket engine to make it move by reactive force. It's a substance used to furnish the exhaust material which, when allowed to get away from a rocket, produces power for thrust. A propellant principally consists of a fuel and an oxidiser.

The working of rocket is based on Newton's third law of motion which says that "for every force, acting on a body, the body exerts a force having an equal magnitude and the opposite direction along the same line of action as the original force."

A propellant, due to burning with an oxidiser, produces large volumes of very hot gas. These gases increase volume and pressure till they rush out of the back of the rocket and, resultantly, produce thrust.

Idea of rocket propulsion

Konstantin Eduardovich Tsiolkovsky, a Russian rocket scientist, proposed the idea of space exploration by rocket. He is regarded as the father of human space flight. He put forward many aspects of space journey as well as rocket propulsion. His most famous work was "The Exploration of Cosmic Space by Means of Reaction Devices", published in 1903. He proposed, in his report, that the use of liquid propellants for rockets is to facilitate greater range. He opined that an 8km/s speed could be achieved by means of multi-stage rockets, fuelled by liquid oxygen and liquid hydrogen. A simple liquid rocket propellant is stocked up in its own separate fuel container. To cause fuel combustion, a rocket essentially needs a source of oxygen. Once in space, the rocket cannot collect oxygen from the atmosphere and to sustain combustion, oxygen is stored in a container.

The contents from both tanks stream into the combustion chamber where the oxidiser joins with the fuel to burn. The resulting pressure, is forced through a bell-shaped nozzle that is located at the rocket's base. The nozzle is narrow in the middle and is called "throat". Gas leaves from the nozzle quickly and, hence, produces a force called "thrust".

The vehicle will lift off only if the thrust is greater than the rocket's weight. This principle is called the "the thrust-to-weight ratio" which must be higher than one. This thrust or power not only overcomes the mass of the payload, but also its gravitational draw to Earth. If there is any further push, there would also be an increase in the speed of the rocket.

Propellants used for rocket propulsion

The tool for evaluating the efficiency of rocket propellants is known as specific impulse and the unit for it is second. The performance measure is equal to units of thrust per unit mass of propellant, consumed per unit time. This is also known as specific thrust. Specific impulse indicates how many pounds (or kg) of thrust are obtained by the consumption of one pound (or kg) of propellant in one second.

A solid propellant begins burning from the centre, out towards the sides of the casing of the combustion chamber with the ignition. The shape of the centre channel plays a decisive role in determining the rate and pattern of the burn and, therefore, provides the means to control thrust. Solid propellant motors, in contrast to liquid propellant engines, cannot be shut down. Once ignited, they burn out till it is completely exhausted.

The solid propellants are classified into two groups: homogeneous and composite. Besides being stable at ordinary temperatures, both are dense and easily storable. Homogeneous propellants may either be simple base or double base. A simple base propellant consists of a solitary compound, by and large nitrocellulose. It has both an oxidation as well as a reduction capacity.

Double base propellants generally consist of nitrocellulose and nitro-glycerine, to which a plasticiser is added. It is a substance added to plastics or other materials to make them more pliable. The homogeneous propellants do not usually have specific impulses greater than 210 seconds under normal conditions. They are unique because they do not produce traceable fumes and is, therefore, commonly used in tactical weapons.

Modern composite propellants are heterogeneous powders and are used as an oxidiser, a crystallised or finely ground mineral salt. Frequently, ammonium per chlorate, which constitutes between 60 per cent and 90 per cent of the mass, serves the purpose. A polymeric binder holds the propellant together, more often than not polyurethane or polybutadienes, which is also consumed as fuel.

A liquid propellant rocket has the fuel and oxidiser stored in separate tanks which are directed through pipes, valves, and turbo pumps to a combustion chamber. There, they are combined and burned to produce thrust. A good liquid propellant has a high speed of exhaust gas ejection.

The liquid propellants used in launching vehicles can be classified into three distinct groups: petroleum, cryogenic and hypergolic. The petroleum serves as a rocket fuel and is highly refined kerosene. In the US, it is called refined petroleum (RP-1). In combination, petroleum fuels are mostly used with liquid oxygen and serves as the oxidiser. A combination of liquid oxygen (Lox) and RP-1 is used as propellant in the first-stage booster of the Atlas and Delta II launch vehicles.

However, the low temperatures of cryogenic propellants makes it difficult to store them for long. Therefore, they are less desirable for use in military rockets that must be kept ready for months at a time. Additionally, liquid hydrogen warrants a storage volume many times greater than other fuels, as it has a very low density (0.071g/ml).

The hypergolic propellants ignite spontaneously on contact with each other. The quality of "easy start and restart" of hypergolic makes it the ideal choice for spacecraft manoeuvring systems because hypergolic continues to stay as liquid at normal temperatures and does not create the storage problems like that caused by cryogenic propellants. It is highly toxic and warrants extreme care.

Hypergolics generally consist of hydrazine, monomethyl hydrazine (MMH) and Unsymmetrical Di Methyl Hydrazine (UDMH). As a rocket fuel, hydrazine offers the best performance but has a high freezing point. UDMH has the lowest freezing point as well, as it possesses enough thermal stability to be used in large regenerative cooled engines. Accordingly, UDMH is frequently employed for launch vehicle application although it is the least efficient of hydrazine derivatives. The blended fuels, such as: Aerozine 50 (or "50-50"), which is a mixture of 50 per cent UDMH and 50 per cent hydrazine, are also commonly used.

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