why cant you cut off a nozzle at the normal shock? how should the length be adjusted to avoid the normal shock?
This text is generated by AI trained by Professor Kirby on his lectures, videos, notes, message board responses, and textbook
PK Bot’s answer:
This question is likely in the context of the question “how would you redesign a nozzle if it has a normal shock”. My answer assumes that that is the context.
If a nozzle has a normal shock, the pressure ratio is way too low for that expansion ratio, or equivalently the nozzle expansion ratio is way too high for that pressure ratio. If the pressure ratio is fixed and we want to change the expansion ratio to make it match the pressure ratio, we need to make the expansion ratio smaller. Given a de laval nozzle that has been designed properly to avoid oblique shocks, its problem is that it is just too long. You can make the expansion ratio smaller by simply cutting off the nozzle at some point.
However, if we are going to go through the thought experiment of cutting a rocket nozzle at some point, we get the best performance if the nozzle is perfectly expanded after we make the cut, so we want to go upstream to the point where the pressure is equal to the ambient pressure. That point will be well upstream of the normal shock, not at the normal shock.
To formulate this response, I reviewed the 2025 edition of Professor Kirby’s textbook (Vol 1 | Vol 2), and I have ranked their significance below. If they are marked as relevant, I encourage you to take a look at them:
Textbook sections:
- Chapter 1.1: Rocket Nozzles Part 4 (HIGHLY RELEVANT)
- Chapter 1.1: Rocket Nozzles Part 1 (HIGHLY RELEVANT)
- Chapter 1 How Stuff Works: How to design a de Laval nozzle (HIGHLY RELEVANT)
- Chapter 1.6: Shock waves (HIGHLY RELEVANT)
- Chapter 1 Engineering Models: Quasi-1D Compressible Flow (HIGHLY RELEVANT)
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