How does the rocket fly?

Rocket Propulsion

Introduction. There is a saying, “had I the wings of a dove I could fly in air!” On this idea, great artist ‘Leonardo de Vinchi’ painted his imaginary dreams on a portrait. To win the air even onto the planetary traveling became the thrust of men. The conquest of the air, so far as propelled ac concerned, really started in 1903 with the historic flight of the Wright brothers at Kittyhawk in USA and was advanced by Bleriot and other early pioneers. Further strides in the development of the aeroplane made during 1st (1914-18) world war.

After invention of jet eng, scientist inspired to study on space. They could able to learn that planetary traveling is quite possible with spacecraft that must the air independent along with capable of operating beyond the force of gravitation. In chain of hard working they made the craft and operate a successful flight in the mid of 20th century. Men conquered the Moon & the Mars. Presently the IT world is dependent on satellite. That was possible by the invention of rocket. Without rocket it is totally impossible to install an artificial satellite above the force of gravitation around the sphere.


Narratives. A rocket eng produces propulsive force on the same principle as that of a Jet eng. I.e, reaction to the product of mass and acceleration of the working fluid. The basic dif between the Jet & Rocket propulsion is that the Jet eng is an air dependent eng whereas; the rocket is an air independent eng. That is, the operation of a rocket eng does not depend on atmospheric air, it carries it’s own oxygen. That is why it is the only power plant that can operate in space. Not only this, it can even operate under water. But as the wt of the oxygen is at least six times greater than the wt of the fuel used in the rocket power plant. So the wt of the propellant becomes greater than other type of power plant used in heat eng and the rate of propellant consumption is very higher than turbojet & ramjet, because oxygen itself acts as working fluid (incase of air dependent eng air acts as working fluid and this reduces the rate of fuel consumption). As a result, this rocket could not be used in place of turbojet eng to power the conventional ac. Above all; rocket eng will remain an effective and important means to power the spacecraft.

The use of rocket in war purpose (V-2 rocket of Germany) began in 1944. Now this is used not only for war purpose but also for inter planetary travel, as booster for take off, starting ram jet etc.

Rocket Performance. The working fluid of rocket eng is oxidizer. The oxidizer and the fuel together are known as the propellant. As the rocket power plant is an air independent eng, so initial velocity for calculating thrust is zero and thrust becomes the product of mass of the air and the jet velocity. F = MVJ

Classification. The rocket engines are classified into three main types.

 Chemical Rocket
 Nuclear Rocket
 Electrical Rocket

(1) Chemical Rocket. These are the most commonly used rockets; it develops thrust by burning propellant in the combustion chamber and expanding the burnt gases through the jet nozzle. To obtain high thrust from the chemical rocket the molecular weight of the propellant must be as low as possible, to reduce the overall wt of the rocket power plant. Most of the propellants use now a days produces CO, CO2, N2 & H2 and average molecular wt of these are about 25. Because of the higher wt of the combustion product the performance of the chemical rocket cannot be reduced below certain limit. When hydrogen & fluorine (wt 8.9) or hydrogen & oxygen (wt 9) or lithium (7) are used as the fuel even than the limit can’t be lowered indefinitely because the use of light molecular wt propellant does not necessarily gives lighter combustion products. The lighter combustion product is required for gaining better momentum; another factor that limits the thrust is the maximum temperature that can be produced by chemical reaction. At very high temperature dissociation occurs which limits the maximum obtainable temperature.

Types. On basis of propellant chemical rockets are classified into three types.

 Solid Propellant
 Liquid Propellant
 Free Radical Propellant

(a) Solid Propellant. The type of propellant used in the rocket is greatly affects the overall shape & size of the rocket. In the solid propellant rocket, propellant is stored in the combustion chamber itself. As there is no moving parts, so it is simple to manufacture and easy to use. It consists of a combustion chamber, an expansion nozzle & an igniter. Ignite the propellant during starting, the burning occurs with rapid decomposition and heat release. This heat is conducted to the propellant that enables the process to be continuous and self-sustaining. As the propellant are plastic in nature and have very high viscosity that enables it to give required shape by casting or extrusion.

The propellant of the solid propellant rocket is classified into two types.

 Composite
 Double Base

i. Composite Propellant. The composite propellant consists of an oxidizer mixed in the fuel in plastic nature which when combines, provides uniform composition and form into solid state. Example: Lithium/ammonium per chlorate or nitrate.

ii. Double Base. Double base propellants are the combinations of nitro-glycerin & hydrocellulose with small quantities of additives. The additives are provided for stability of combustion.

(b) Liquid Propellant. A liquid propellant rocket consists of fuel & oxidizer tanks, combustion chamber, expanding nozzle, pumps and injectors. The propellant is stored in the tanks and is fed to the combustion chamber. In the process of fuel flow, it cools the combustion chamber. The burnt gases from the combustion chamber expanded in the nozzle to produce thrust.


The liquid propellants rockets are classified into two basic types.

 Mono propellant (nitro methane, nitro glycerin & hydrazine)
 Bi-propellant (Hydrogen peroxide & alcohol, liquid oxygen & liquid hydrogen, liquid fluorine & liquid oxygen)

(2) Nuclear Rocket. The principle of the nuclear rocket motor is similar to the chemical rocket. I,e. in the chemical rocket heat accelerates the fluids to extremely high velocity in case of nuclear rocket the vast store of atomic energy is utilized. The nuclear reaction releases enormous amount of energy 106 to 108 times greater than that of chemical rocket and very high specific impulses is obtained from this rocket.

(3) Electrical Propulsion. There are three dif type of electrical propulsion; are heating propulsion, ion propulsion & magneto-plasma type. In the arc heating method electrical energy is changed to increase the temperature of the propellant when it passes through the arc. The gas thermodynamically expands through the nozzle and is ejected at high speed. In the ion propulsion system, working fluid used is cesium, is ionized and the ions are accelerated to very high velocity. The magneto-plasma process has a conductor carrying current and located in magnetic field. The magneto plasma is being exerted by the magnetic field at right angle to both the conductor & the field. In this process it is possible to accelerate it to very high velocity.