The Converging-Diverging Nozzle: Unveiling How Rocket Engines Soar to the Stars

At the heart of every space mission lies a marvel of engineering – the rocket engine. A complex yet fascinating technology, it propels spacecraft beyond Earth’s atmosphere and into the cosmos.

Rocket engines utilize the principles of momentum, energy, and force; they burn rocket propellants to produce high-pressure and high-temperature gasses.

These gasses are then expelled through a nozzle, causing the rocket to move in the opposite direction with tremendous velocity.

A typical rocket engine comprises several crucial components:

Each of these components plays a pivotal role in ensuring successful propulsion. In this article, we will be diving into the science behind the converging-diverging nozzle and its importance to the entire rocket.

Converging-Diverging Nozzle Design

Diagram of a converging-diverging nozzle
Credit: Spirax Sarco

A converging-diverging nozzle, also known as a De Laval nozzle, is a specially designed cylindrical duct where exhaust exits to generate thrust in a rocket.

The nozzle is designed to accelerate the flow of a fluid, typically a gas, to a supersonic speed while efficiently converting the internal energy of the fluid into kinetic energy.

The nozzle has three main sections:

  • Converging section: This part of the nozzle is tapered, gradually decreasing the cross-sectional area as the fluid flows through it.
  • Throat: The throat of a nozzle is the narrowest section where the flow velocity reaches exactly mach speed (speed of sound), which means the flow is sonic. The throat size is chosen to “choke” the flow and set the mass flow rate through the system. Choked flow is when the flow velocity will no longer accelerate from the higher pressure in the converging section.
  • Diverging section: After reaching a certain point in the converging section, the nozzle expands or diverges, increasing the cross-sectional area until the nozzle exits.

The Science Behind the Converging-Diverging Nozzle

The genius behind this nozzle design lies in its simplicity yet effectiveness. De Laval’s design uses principles of fluid dynamics where gasses are compressed (converged), sped up to a choke point (throat), and then allowed to expand (diverge) while accelerating to supersonic speeds.

nozzle design equations
Credit: NASA

Here’s how that works:

By conservation of mass, the mass flow rate of the gas throughout the nozzle is the same regardless of the cross-sectional area.

Therefore, the velocity changes as the cross-sectional area changes and the results depend on the Mach number. If the flow is subsonic, an increase in the area produces a decrease in the velocity.

Hence, the decrease in area in the converging section creates an increase in velocity. As the flow enters the diverging section, the flow becomes supersonic. For a supersonic flow, an increase in the area produces an increase in the velocity.

Hence, under the conditions of supersonic flow, the increase in area in the diverging section creates an increase in velocity.

Why the a big difference between subsonic and supersonic?

Well, to conserve mass in a supersonic (compressible) flow, both the density and the velocity change as we change the area.

For subsonic (incompressible) flows, the density remains fairly constant, so the increase in area produces only a change in velocity. But in supersonic flows, there are two changes; the velocity and the density.

For supersonic flows, the change in density is much greater than the change in velocity.

To conserve both mass and momentum in a supersonic flow, the velocity increases, and the density decreases as the area is increased.

Importance of the Throat Size

The throat size in a Converging-Diverging (CD) nozzle is of critical importance in determining the nozzle’s overall performance and efficiency. The throat acts as a crucial flow constriction point in the nozzle, where the exhaust gasses become a “choked” flow.

Temperature, pressure, and velocity diagramed in a CD nozzle
Credit: Wikipedia

The importance of the throat size lies in achieving and maintaining choked flow conditions.

If the throat is too small, the gasses may not achieve choked flow, leading to under-expanded exhaust gasses in the diverging section.

This condition results in reduced thrust and overall engine efficiency.

On the other hand, if the throat is too large, the gasses may not reach sonic or supersonic speeds at the throat, leading to over-expanded exhaust gasses.

This condition also leads to reduced efficiency and can even cause instability and performance issues.

Accurately designing and controlling the throat size is essential in CD nozzles to achieve proper choked flow conditions, enabling the engine to produce the maximum thrust and propulsion efficiency, which is crucial for successful rocket launches and space exploration missions.


In conclusion, the converging-diverging nozzles importance to the rocket engine cannot be overstated. Just like the Aerospike Nozzle, it efficiently accelerates the exhaust to supersonic speeds, increasing thrust and ultimately the power of the rocket.

Its ability to efficiently convert thermal energy into kinetic energy has paved the way for the development of powerful rockets, supersonic aircraft, and cutting-edge space exploration vehicles.

By allowing exhaust gasses to reach supersonic speeds, it has opened up new frontiers in high-speed rocketry and advanced space missions.

As we continue to explore the cosmos with ever-changing rocket designs, the converging-diverging nozzle remains an essential component in the quest to explore the cosmos.

Check out the other articles in this series!

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