Design of Rocket Engine Exhaust

The physics used in rocket design is not very different to what we read in our schools but with a slight change that some laws of physics are changed a such high velocities. Earlier we posted about the blunt shape of rockets and missiles. This post is about the design of Rocket Engine Outlet or Throat. In this post you will also learn about the simulation of flow of exhaust from the rocket engine which will help you better understand the design of rocket engine throat.


An F-1 engine on display at NASA’s Marshall Space Flight Center.

To understand this, you will need some knowledge of mechanics, thermodynamics and fluid mechanics. Or basically about

So if you know about these basic things, you can infer that

  • Higher the speed of exhaust from rocket engines, higher will be the speed of rocket (as said by law of conservation of energy)
  • A Nozzle is used to increase the speed of fluid whereas a diffuser decreases the speed of fluid

But wait, did you see that the rocket engine had a diffuser as its throat. So basically why we use a diffuser at the throat at rocket engine instead of a nozzle? Let’s see.

First of all don’t judge a book by its cover. The throat you see is basically is not a diffuser, thought it is exactly like a diffuser but there is something hidden above it which you can’t see in the above image.

For Rocket propulsion, they use a different kind of Nozzle, known as a Convergent-Divergent or a C-d Nozzle. As the name suggests, this nozzle will first converge upto a defined area and then it will divert again. The cross section of a C-d Nozzle looks something like this:


The reason behind using a C-d nozzle is hidden in the laws of thermodynamics. The C-d nozzle is developed by a scientist named Gustaf De Laval. He developed this nozzle to accelerate hot, pressurised gas to supersonic speed by converting heat energy into kinetic energy.

How does it happen? We can understand the velocity transformation from subsonic flow to supersonic flow in a C-d Nozzle by simulating it in ANSYS.

Download files from:

Here is the video of “Simulation of C-d Nozzle for supersonic flows”



velocity contour

The Initial Velocity of flow from inlet was about 172 m/s but as the flow reaches the neck of throat, due to convergence, the velocity increases and crosses supersonic limit, in this case 689 m/s. But when the flow proceeds, instead of decrease in the velocity due to divergence, the velocity increases and goes upto 1370 m/s. In such cases the heat energy of gases is converted into Kinetic energy and thus we get a flow with extremely high velocity. At such velocities, we are able to get very high momentum and thus according to Newton’s 3rd Law, it gives a great thrust to Rockets.

So Now you know why Rocket engines have Wide open Exhaust.

Thanks for reading the post. I hope you liked it. Please like, Share and Comment for any suggestions.


2 Comments on “Design of Rocket Engine Exhaust

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