Rockets Explained-The Nozzle

Remember that the energy expelled from the rocket comes mostly in the form of kinetic energy.The rest of it stored as temperature and pressure.If we could turn the energy stored as temperature and pressure into kinetic energy, we’d make the engine more efficient.This is done using a ‘converging-diverging nozzle’, ‘CD nozzle’ for short or ‘de Laval nozzle’.

First the nozzle converges,

it gets narrower, then it diverges, it becomes wider.

As the gas moves through the converging section, it must accelerate, as there is less and less space.Kind of like pushing water through a needle.At the throat, in between the converging and diverging sections, the gas has a special property.

Here it is travelling at a velocity so high,it can no longer exchange information with itself.It is now travelling at the velocity of sound,or the acoustic velocity, also known as Mach 1.Now the opposite happens.The gas accelerates as it gains more and more space.

The converging-diverging nozzle essentially turned the high pressure and high temperature into more kinetic energy.The exhaust gas is now faster, cooler, and at a lower pressure.This makes the propulsion system much more efficient.

Let’s take a look at how the velocity, pressure and temperature change throughout the nozzle.As expected, the pressure and temperature drop as the gas accelerates.Why the increases and decreases follow these specific relations will be seen once we get to the thermodynamics of nozzles.Notice how the nozzle has the ability to control the exhaust pressure of a propulsion system.The engineer designing the nozzle can set the exhaust pressure.

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