As I see it, if you're going to calculate a twin-spool turbojet engine, you

  • Determine expected max aircraft speed, and engine thrust at that speed and when stationary.
  • Calculate peak mass of air flow
  • Calculate energy needed to propel this mass of air
  • Calculate bypass ratio
  • Calculate compressor properties

And from here I have no idea where to move on. Some books I've read just go through the engine from compressor to exhaust nozzle, but none explains which parameter of the engine is primary and where do calculations start and how they're performed.

Would you give me a clue? Thank you!

  • $\begingroup$ The real world answer is you start with your experience based knowledge of existing engines, find one that is close to your design goals, and incrementally adjust parameters to find an optimal design for your application. $\endgroup$
    – agentp
    Dec 19, 2016 at 19:36
  • $\begingroup$ @agentp This won't probably help me to understand principle from ground up. Also, what if there is no suitable engine? $\endgroup$
    – Eugene
    Dec 21, 2016 at 16:13

1 Answer 1


Determine expected max aircraft speed, and engine thrust at that speed and when stationary.

Well, sort of. At the very beginning, you may know the desired thrust at a given condition, but you have to work all the way through the calculations to get the actual thrust at that speed. If the actual thrust and desired thrust don't match, then you would need to change the design assumptions (e.g. change fan diameter, or compressor pressure ratio or something) in order to meet that.

What I'm trying to say is, I think you may be combining the concept of how do I design a jet engine to meet a certain performance goal, with how do I evaluate the performance of a particular jet engine design.

To do the later, you start with upstream pressure, upstream temperature, and velocity, and the known design of your engine, and walk from front to back. At the end of that process you will know thrust, mass flow, fuel consumption, etc for that given design and operating conditions.

To do the former, you start with a set of assumption, calculate the performance, and iterate until you have the desired design. For a large commercial turbofan, this process would typically be done to optimize the performance at cruise (since that is where the plane spends the majority of them), and then you would verify that other conditions (e.g. sea level static takeoff) are acceptable.

  • $\begingroup$ Thanks for the answer. Let's consider all that I have at start is desired thrust at, say, 3-5 km, and basic concept of design: twin-spool turbofan, 1-stage fan + 2 stage IP/HP compressor, later stage is centrifugal type; 2 or 3 axial turbines. How do I proceed? $\endgroup$
    – Eugene
    Dec 18, 2016 at 21:32
  • $\begingroup$ Well, the wikipedia page for jet engine performance has a worked example for a turbojet (en.wikipedia.org/wiki/Jet_engine_performance) I would start there and get an understanding of turbojet before moving on to a turbofan. NASA also has a lot of good material. E.g. grc.nasa.gov/WWW/K-12//airplane/ngnsim.html $\endgroup$
    – Daniel K
    Dec 19, 2016 at 2:27

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