Obviously in the simplest variant it's not complex at all. I've seen amateur rocket engine nozzles shaped through applying a nail to packed clay layer.

How complex is the science behind the shape of fixed nozzles used professionally - in space-faring rockets, in military missiles, solid rocket boosters etc? How much work - comparing to difficulty of other factors - is spent on designing the nozzle for a rocket?

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    $\begingroup$ Nozzle design is not something I have done, but I know that verifying the exhaust flow through the nozzle and looking at the resulting shockwave patterns is really important -- even though ideal nozzle shape is a solved problem. $\endgroup$ – Brian Lynch Nov 13 '15 at 17:56
  • $\begingroup$ What would it mean for the underlying science of some technology to be "complex"? I'm not sure I understand the question. $\endgroup$ – sintax Oct 8 '19 at 18:14
  • $\begingroup$ @sintax: Some problems of space operations require a lot of effort on the design and science front. Some - not so much. Some solutions are created by a think tank of geniuses working for months. Some - by an astronaut with a roll of duct tape, in a minute. Example problem: sustain high temperature of a dehydrated meal prepared with hot water from a dispenser, for long enough that the starch absorbs the water and gets soft and tasty. Solution: stuff it in a space suit boot, let it sit in there for five minutes. The procedure is in operation on ISS since Zvezda got its crew. $\endgroup$ – SF. Oct 9 '19 at 0:19
  • $\begingroup$ @SF. Ahh, okay. So, to me, I'd say you're asking rather about the effort of developing a particular engineering solution to a particular problem. I'm a research scientist in an engineering field, so I sometimes come across conceptions of what 'science' is or means that differ significantly from my own view on the subject and I like to try to understand what people outside my own head think when they think of science. To me, science is the process of discovery and, though not always a straight road, is never what I'd call "complex", though certainly some solutions enabled thereby can be. $\endgroup$ – sintax Oct 9 '19 at 15:47

The general shape of fixed nozzles is pretty close to a solved problem. Per Huzel, even a conical nozzle of sufficient length and 15º half-angle gives better than 95% efficiency. Bell nozzles can exceed 99% efficiency in much shorter form.

The actual engineering design of a nozzle for a large liquid-fueled rocket is more complex; cooling the nozzle becomes a large challenge, and in regeneratively cooled engines, it interacts with the propellant pump system.

In solid rockets, regenerative cooling isn't an option, so I believe ablative cooling is more common, helped by the fact that burn times are often shorter -- typically around 2 minutes instead of 4-6 minutes for liquid first stages. Substantial engineering issues involved in interfacing the casing to vectored nozzles, but again, shape-wise, it's generally a near-optimal bell nozzle.

Either the Huzel or Sutton book will take you through the next couple of levels of complexity on the topic.


"How complex" sounds like a judgement call, but if you wish to make up your own mind, take a look at Chapter 3 "Nozzle Theory and Thermodynamic Relations" in Sutton. Although this edition of the book is somewhat infamously "watered down", this particular chapter still seems good, and should give a handle on the basic theory.

Please note that you asked about the "science", which is covered in the work I cite. If you want to talk about the "engineering" of rocket nozzles, which involves turning this theory into a workable device, that's another question entirely.


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