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Engines

Three Kinds of Engines Power Most Flying Machines

Piston engines, jet engines, and rocket engines all depend on the same basic principles to produce thrust.

  1. The engine mixes fuel with oxygen or another oxidizer in a combustion chamber.
  2. The mixture is ignited.
  3. The burning mixture creates hot, expanding gases.
  4. The expanding gases either produce thrust directly (in jet and rocket engines), or are used to push a piston or drive a turbine.

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How Do Engines Produce Thrust?

Newton’s Third Law of Motion is often given as an explanation for the generation of thrust. But it explains the effect of thrust, not the cause of thrust. Pressure and shear stress are the only two ways nature exerts an aerodynamic force on an object. Pressure is the basic source of thrust produced by a propeller, jet engine, or rocket engine.

A piston engine cannot produce thrust on its own. It provides power to a spinning propeller, which produces thrust by creating a pressure difference between the front and back of the propeller, resulting in a forward force. Jets or rocket engines produce thrust by increasing the pressure inside the engine. This increased pressure in the jet or rocket engine exerts more force in the forward direction than the rear direction.

The exhaust gases produced by a propeller, jet or rocket, due to Newton’s Third Law, are feeling a force opposite and equal to the thrust, and therefore are moved in the direction opposite to the thrust of the engine. Hence, the exhaust is the effect of thrust.

Ask an Explainer

Q:

What’s the difference between a jet engine and a rocket engine?

A:

Jet engines and rockets work on the same principle. They produce thrust through an internal pressure difference and, as explained by Newton’s Third Law of Motion, eject exhaust gases in an equal and opposite direction. The main difference between them is that jets get the oxygen to burn fuel from the air and rockets carry their own oxygen, which allows them to operate in space. ... more

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Watch a Video

Thrust: Rocket Engines

Explore how rocket engines generate thrust.

Did You Know?

A glider is an aircraft without an engine.  Three forces of flight (lift, weight, and drag) act on a glider in flight. A powered aircraft uses an engine to generate thrust, while a glider must be launched from a hill or towed aloft by another airplane. Once released, it relies on altitude and gravity to generate the velocity for forward movement, like a vehicle coasting downhill. A glider, like a soaring bird, must find upward-moving air currents (thermals) to gain altitude.

Pop Quiz

Which engine is more fuel-efficient?

A) Piston/Propeller
B) Jet
C) Rocket
D) They're all the same

Piston/Propeller.  Propellers are most fuel-efficient and rockets are least fuel-efficient.  A propeller moves a large amount of air slowly; a jet or rocket moves a small amount of air quickly.  The energy you must expend, by burning fuel, to move a large amount of air slowly is less than the energy you need to move a small amount of air quickly.