Category: Jet propulsion

Rocket Propulsion Systems | Rocket Engine Work | Rocket Engine Thrust

Rocket propulsion Systems

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A rocket engine is defined as an engine that expands thrust by ejecting a stream of matter (i.e. exhaust gas) backwards. Since the reaction (i.e. thrust force) principle involved assumes a self enclosed supply of energy. A rocket engine can operate in any medium including space (i.e. Outside the earth’s atmosphere), where there is no oxygen to support combustion.

Rocket Engine work

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The working principles of rocket engines are primarily governed by Newton’s law of motion. Newton’s first law states that there is no change in the motion of body unless a resultant force acts on it. The governing action such as gravitational force, lift force, drag force and the thrust force of the rocket engine all proceed on the vehicle to cause the resultant motion. The net amount of the resultant force and its directions decide the acceleration on the vehicle and the path of the flight trajectory, in accordance with Newton’s second law.

Rocket Engine Thrust

A rocket engine develops its thrust by ejecting a mass backward. The mass is accelerated backward by combustion that accelerates its velocity from 0 to 1000 m/s. The Newton’s Second law states that, the force for this acceleration is proportional to the mass of the exhaust gases. The force acting on the accelerating mass and the resultant exhaust gases, produce a thrust in accordance with Newton’s third law, which states that for every action, there is an equal and opposite reaction. So the thrust force in the rocket engine is developed by internal fluids within the rocket which accelerates equal but opposite external force.

The force vector (i.e. thrust), can be determined by investigating the change of momentum in the design and the sum of the forces that act on a closed duct in a control volume. The internal flow to the rocket experiences a change of momentum that is equal to the mass flow rate and the change in velocity of the gases. When the inlet velocity is low the change in momentum considered as negligible.

Force of a Rocket:

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The sum of all pressures on the surfaces perpendicular to the flow axis of the device reduces to a resultant force. Due to the pressure differential between the pressure at the nozzle exit plane and the ambient pressure that acts on the exit area of the nozzle.

Net pressure force = (Pexit – P ambient) A exit

The sum of the forces that acts on a rocket is equal to the change of momentum in accordance with Newton’s second law.

Thrust = m Vexit + (Pexit – Pambient) A exit

  • When Pexit equals Pambient expansion is optimum and performance is good.
  • When the nozzle exit pressure is less than ambient, the nozzle is said to be over-expanded.
  • When exit pressure is greater than ambient, the nozzle is said to be under-expanded.

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Generally the rocket flies through the atmosphere; it experiences variations in the ambient pressure. So it operates at optimum expansion at only one altitude. Resultant design and flight conditions based on the rocket exit areas in different altitudes.

Rocket Engines | Solid Propellant Rockets | Liquid Propellant Rockets

Jet propulsion

Jet propulsion is based on newton’s second law and third law of motion. Newton’s second law states that the rate of change of momentum in any direction is proportional to the force acting in that direction. Newton’s third law of motion states that for every action there is an equal and opposite reaction. A type of jet engine is explained below.

Rocket engines

A rocket does not depend upon the oxygen in the atmosphere for its operation. It carries all its supply of oxygen in addition to fuel with it.


Principles of working: In rocket engine, a large amount of gases are produced by the combustion of oxidizer and fuel. High pressure gases are created by burning the oxidizer and fuel in the combustion chamber. The gases emerge out of the exit nozzle with a supersonic velocity producing a thrust to propel the rocket.

Solid propellant rockets

In this rocket, the total mass of fuel is stored and burned within the combustion chamber. No separate fuel supply system is provided. It consists of a seamless tube made of steel. The one end of the tube is closed and the open end holds the nozzle. These rockets are classified as the following.

Restricted burning type Rocket: In this type, the propellants are burnt on only end surface and the burning of propellant takes place progressively to the other end. The burning of propellant takes place similar to that of a cigarette.

Unrestricted burning type rocket: In this type, propellant is burnt freely on all surfaces at the same time. Washers provided at the ends to prevent the ends from burning.

Nitrocellulose, ammonium nitrate, ammonium perchlorate are some of the solid propellants used in rockets.


Liquid Propellant Rockets

Liquid propellant rockets use liquid propellants. The propellants are stored in separate containers outside the combustion chamber. It consists of

· A fuel tank,

· An oxidizer tank,

· A fuel injection system,

· A combustion chamber and

· An exit nozzle.

The oxidizer and fuel are pumped into the combustion chamber. In the combustion chamber, burning of fuel and oxidizer takes place at high pressure and at high pressure, high temperature gasses are produced. These gases expand through the exit nozzle. In the nozzle, pressure energy is transformed into kinetic energy and the gases are emerging from the nozzle to the atmosphere with a very high velocity. As a result of the increased velocity of gases coming out of the nozzle, a reaction or thrust is produced in the opposite direction. This thrust propels the rocket engine.

Application of rocket engines

· Signaling and fireworks – small solid fuel rockets are used for sending distress signal from ships and signals from the ground army. They are also used for fire work display,

· Long range artillery – rockets are used in long range artillery enemy countries may be attacked by rocket assisted shells.

· Lethal weapon – naval ships and ground forces are equipped with short and long range missiles. Rocket propelled missiles are used in modern fighter aircrafts.

· Satellites – satellites are sent by rockets into outer space.

· Space ships – these are driven out of the earth with the help of rockets.

· Future expectation-rockets may be used to carry passengers from one place to another.

· JATO – jet assisted take off enables interceptor, fighters, bombers, etc. to climb vertically rapidly