The term propulsion is derived from two Latin words: pro meaning before or forward and pellere meaning to drive. Propulsion is a means of means of creating force leading to forward movement.
The principal of rocket propulsion depends on–
Newton’s third law of motion
It states that for every action there is always an equal (in magnitude) and opposite (in direction) reaction.
Law of conversion of momentum
It states that the total linear or angular momentum in an isolated system is constant, provides that no external force is applied.
The propellant (fuel + oxidizer) on burning produces a large amount of gas, which is allowed to pass through a nozzle generating high pressure (as shown in fig.1.1.1.) this exerts a force (action) on the rocket, propelling it forward (reaction). The propelling provide thrust to the rocks thrust depends on the nozzle design, propellant etc.
Propulsion techniques are classified as-
The propellants used are chemicals that produce a large number of hot gases on burning. They are farther classified depending upon their physical state as solid-liquid propellants.
Solid propellants-fuel (e.g. gun cotton, nitrocellulose) and oxidizer (e.g.sodium nitrate) are in solid state. They are usually safe and easy to store and handle. They are contained in the combustion chamber itself. However, once ignited combustion cannot be stopped and thrust cannot be controlled.
Liquid propellants- the fuel (e.g. aniline liquid H2 ethanol) and oxidizer (e.g. liquid O2 ) are stored separately in containers and are sent to combustion as required. Combustion can be easily controlled but handling and storage of liquid propellants are comparatively difficult.
Nuclear propulsion includes various propulsion methods that use a form of nucleus reaction as a primary power source. They are further classified as fission or fusion propulsion.
In fission propulsion, fission reaction is used to produce energy that initiates combustion of the propellants. The gases are forced out through the nozzle producing thrust.
The main disadvantages are-
- There are chances of the explosion, as fission is an uncontrolled reaction. So proper designing is required.
Products formed may be radioactive and so is a threat to the environment.
- Infusion propulsion, fusion reaction (lighter nuclei fuse to form a heavier nucleus)is used to produce energy that initiates combustion of propellants.
- The gaseous products formed provide thrust to the rocket. The specific impulse is the main advantages of fusion propulsion.
Infusion propulsion system is an electric motor which spacecraft use to power propellant to high speed to produce required thrust.
The various types of electric propulsion are ion\plasma drives, electrostatic ion thruster ( ion bombardment), electrothermal, electromagnetic, electrodeless plasma thruster etc.