on video From turbojet construction to flight - just one step

General operating principle
The principle of jet aircraft propulsion is based on the third law stated by Issac Newton in 1687 in the first volume of his Philosophiae Naturalis Principia Mathematica: any body A exerting a force on a body B experiences a force of equal intensity , but in the opposite direction, exerted by the body B.
Action reaction
The reaction is the consequence of an action. Let's take an example of a balloon being inflated. The wall of the balloon undergoes a certain pressure and expands. If the balloon is properly closed, the resultant of the pressure forces inside the balloon is zero. (figure on the left)
Now let's open the mouth of the balloon, (right figure) the escaping gases create the propulsion energy (action) and cause the balloon to move (reaction as long as the internal pressure of the gases is sufficient).
Two categories of thrusters can be distinguished:
Propellants that carry their fuel (kerosene or other) but use ambient air as oxidizer:
- ramjets;
- pulsejets;
- turbojet engines;
- propeller engines (turboprops, helicopters).
And thrusters which carry their fuel (liquid hydrogen or kerosene etc.) and their own oxidizer (liquid oxygen or other)
- rocket engines.
Principle of operation
The operation of turbojet engines is based on the action-reaction principle: it involves accelerating a mass of air to create a thrust force which, by reaction, will propel the aircraft. All turbojet architectures are based on the same 3 stages, namely: Compression - Combustion - Expansion.
On start-up, compressed air supplied by the APU (auxiliary power unit) or by a pneumatic unit on the ground sets a compressor in rotation which draws in and compresses the ambient air to send it to one or more combustion chambers. Simultaneously a pump driven via a transmission box or AGB accessory gearbox (in English) injects fuel (kerosene).
This compressed air/fuel mixture is ignited in this or these chambers by a spark plug or several spark plugs (lighters), which greatly expands the gases.
These gases pass through one or more turbines which, using a rotating shaft, drive the compressor and the accessories essential to the operation of the reactor (fuel pump, alternator, etc.); which ensures the continuity of the movement. The gases then escape in a converging section nozzle in order to accelerate them at the exit.
After a few seconds, when the rotational speed is sufficient, the starter is uncoupled and the spark plug extinguished. The engine can then operate autonomously to reach its idle speed.
Below the Turbomeca Marboré VI visible at the Air Museum in Paris Le-Bourget


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