How Forward Movement of a Jet Engine is Achieved: Action and Reaction
One of the most fundamental principles behind jet propulsion is the action–reaction principle. Although it sounds simple, the way this principle actually produces forward motion in an aircraft engine involves several aerodynamic and fluid-dynamic processes.
To understand this properly, we must examine how the engine accelerates air and how the resulting reaction force produces thrust.
The Basic Physical Principle
The propulsion of a jet engine is governed by the principle described by
Isaac Newton
in Newton’s Third Law of Motion.
It states:
For every action, there is an equal and opposite reaction.
In the context of a jet engine:
Action: The engine accelerates air and combustion gases backward.
Reaction: The engine experiences an equal forward force.
This forward force is what we call thrust.
What Actually Happens Inside the Engine
In a gas turbine engine, the airflow passes through several stages:
Intake
Compressor
Combustion chamber
Turbine
Exhaust nozzle
By the time the air reaches the exhaust nozzle, it has been heated, expanded, and accelerated to a very high velocity.
The exhaust gases leave the nozzle at high speed and interact with the surrounding stationary ambient air.
The “Action” – Acceleration of Exhaust Gases
The action occurs when the engine pushes the exhaust gases backwards.
Inside the engine:
The compressor raises air pressure.
Fuel is burned in the combustor.
High-temperature gases expand through the turbine.
The nozzle converts pressure energy into high exit velocity.
The engine is essentially throwing a mass of air backwards at high speed.
This backward momentum change produces the action force.
In physics terms, thrust can be represented as:
Thrust = mass flow × change in velocity
When a large mass of air is accelerated backward, the engine must experience an equal and opposite force.
The “Reaction” – Force Acting on the Engine Surfaces
The reaction is not felt at a single point. It is distributed across many internal surfaces of the engine.
These include:
compressor blades
turbine blades
combustion chamber walls
nozzle surfaces
When gases expand and accelerate, they push against these surfaces.
This pressure force acts in different directions, but the overall result is a net forward force on the engine.
In simpler terms:
The gases push on the engine, and the engine pushes the gases backward.
Interaction with Stationary Ambient Air
A key part of the action–reaction process occurs at the exhaust jet leaving the nozzle.
The high-speed jet collides with the surrounding stationary ambient air.
This interaction causes:
mixing
turbulence
momentum transfer
The engine has effectively given momentum to the surrounding air mass.
Because the surrounding air initially had zero velocity relative to the aircraft, accelerating it backwards produces a reaction force pushing the engine forward.
Role of Engine Surfaces and Blade Profiles
Inside the engine, every aerodynamic surface contributes to the momentum transfer.
Compressor Rotor Blades
The rotating blades:
accelerate air rearward
Add kinetic energy to the flow
As the air is deflected and accelerated, it exerts reaction forces on the blade surfaces.
These forces are transmitted through:
the rotor
the shaft
the engine mounts
Eventually contributing to the overall thrust structure.
Turbine Blades
The turbine extracts energy from the gas stream.
When the expanding gases strike the turbine blade profiles, they exert pressure forces on the blade surfaces.
These forces rotate the turbine and also contribute to the overall force balance within the engine.
Nozzle Surfaces
The exhaust nozzle is particularly important.
In the nozzle:
gas pressure decreases
velocity increases dramatically
As the gases expand along the nozzle walls, they push on the nozzle surfaces.
These forces have forward components, which add to the engine thrust.
Momentum Change – The Real Source of Thrust
Although it is often explained using action and reaction, the most accurate way to describe jet propulsion is through momentum change.
The engine takes in air at a certain velocity and ejects it at a much higher velocity.
The difference in momentum between inlet and outlet produces thrust.
In simple terms:
Air enters slowly
Air leaves very fast
The increase in backward momentum produces a forward force.
Why Large Airflow Improves Efficiency
Modern turbofan engines improve efficiency by accelerating a larger mass of air at lower velocity.
Instead of throwing a small amount of air very fast, they push a large amount of air moderately fast.
This produces the same thrust with better fuel efficiency.
Visualizing the Process
A simple way to imagine this process is to think of standing on a skateboard and throwing a heavy object backward.
When the object is thrown backward:
it gains backward momentum
you move forward
The same principle applies to jet engines, except instead of throwing a solid object, the engine throws a continuous stream of high-speed gases.
Final Thoughts
The forward movement of a jet-powered aircraft is the result of a continuous momentum exchange between the engine and the surrounding air.
The engine accelerates air and combustion gases backward, and the reaction forces produced on the engine surfaces generate thrust.
Although the principle is rooted in Newton’s simple action–reaction law, the actual process involves complex aerodynamic interactions inside the compressor, turbine, and nozzle.
For engineers and inspectors who have spent time around gas turbine engines, it becomes clear that thrust is not generated at a single point but is the combined result of pressure forces acting across the entire flow path of the engine.
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