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It is a fact (to the best of my knowledge) that for optimum thrust from a rocket engine, one of the most important factor is the design of its converging/diverging Nozzle - the diverging cone to be very specific. This design should be able to provide "ideal expansion" of the exhaust gases i.e. nozzle exit pressure being equal to atmospheric pressure.

Since the atmospheric pressure will keep reducing as the rocket ascends, will it not be advantageous to have many more stages (say one stage for every 25 km), so that nozzles for each stage can be designed to match the atmospheric pressure at a given altitude?

Further; perhaps an "adjustable" profile of nozzle (w.r.t. altitude) would be the best, if possible. Has such an attempt been made?

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  • $\begingroup$ astronautix.com/r/rl-10a-5ka.html $\endgroup$
    – Organic Marble
    Aug 8, 2020 at 13:14
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    $\begingroup$ en.wikipedia.org/wiki/Altitude_compensating_nozzle $\endgroup$
    – Organic Marble
    Aug 8, 2020 at 13:16
  • $\begingroup$ Thanks Organic Marble. Looks like it has not been used so far. The site talks about "throttling" and not changing the profile of the cone - which is what I meant. If such an effort is successful, May be we can have many more stages, do away with burnt stage, and reach the moon with much better efficiency than the Saturn V $\endgroup$
    – Niranjan
    Aug 8, 2020 at 13:20
  • $\begingroup$ @Organic Marble. After the last comment I read about "Altitude compensating" nozzles. That is good. thanks. $\endgroup$
    – Niranjan
    Aug 8, 2020 at 13:22
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    $\begingroup$ @uhoh, no problems. most welcome. Thanks. $\endgroup$
    – Niranjan
    Aug 11, 2020 at 13:35

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Staging adds dead weight — interstage hardware and engines and plumbing that spend much of the flight unused, for example. Increasing the number of stages therefore incurs a much larger penalty than the potential gain in nozzle optimization.

Variable nozzles have been experimented with, as Organic Marble notes, but still aren’t in wide use.

Rocket engineering is complicated and many factors contribute and interact. Optimizing for one specific factor at the cost of others doesn’t give best performance.

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    $\begingroup$ With regards to your last paragraph, you could just delete the word "rocket", and you have pretty much the definition of the term "engineering". I once read the definition "an engineer is someone who can do for a dollar what every idiot can do for ten", which I also like very much. Engineering, in general, not just rocket engineering, is about trade-offs, one of which is cost. $\endgroup$
    – Jörg W Mittag
    Aug 9, 2020 at 9:27
  • $\begingroup$ @Russell Borogove. With due respect, dont feel offended. JUST FYI - Most of my doubts are based on Saturn V and Man on moon mission (1961-1969). I agree that optimizing for one factor may not always give the best results / efficiency. You seem to suggest that all such possibilities have been considered by people in those days, and what has succeeded is the best and perfect, which cannot be improvised upon. I was just trying to mentally extrapolate the philosophy of 3 stages to more stages. May be 3 stages is the best techno-commercial solution. Its Ok. thanks. $\endgroup$
    – Niranjan
    Aug 9, 2020 at 9:46
  • $\begingroup$ @Jorg Mittag. Guten Tag. Your accepted definition of engineering is valid for re-engineering / reverse engineering. E.g. No genius other than wright brothers, did any engineering to create an airplane in "tens". All had to wait for the first dollar to be spent, so that the first plane could be made. It was easier to improvise next versions and cut corners. $\endgroup$
    – Niranjan
    Aug 9, 2020 at 9:54

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