Ideal area difference for rockets

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Discussion Overview

The discussion revolves around the ideal area difference between the nozzle cross-sectional area and the inner engine cross-sectional area of a rocket engine, particularly in relation to maximizing thrust. Participants explore theoretical considerations, practical limitations, and the influence of various parameters on rocket performance.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that a greater area difference between the nozzle and engine cross-sectional areas may lead to higher exit gas velocity, potentially increasing thrust, but acknowledges that this also raises internal pressure, which could limit flow rate.
  • Another participant argues that there is no single optimal area difference, as rocket nozzles are designed for specific altitude ranges and not for optimal performance across all conditions.
  • A further reply questions the need for parameters, stating that the discussion is not focused on realistic scenarios but rather on the variable of area at constant pressure.
  • Another participant emphasizes the necessity of specifying propellant type, noting that different propellants introduce various interlocking variables that affect optimal solutions, and highlights the complexity of the mathematical models involved.

Areas of Agreement / Disagreement

Participants express differing views on the existence of an ideal area difference for maximizing thrust, with some asserting that it varies based on conditions and others suggesting that it is a more complex issue without a straightforward answer.

Contextual Notes

Limitations include the dependence on specific propellant types, operational pressures, and the complexity of interrelated variables affecting rocket performance. The discussion does not resolve these complexities.

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What is the ideal area difference between the nozzle cross-sectional area and inner engine cross-sectional area for a rocket engine for getting the most thrust? The greater the difference in area the higher the velocity of the exit gas but that also increases the pressure inside limiting the flow rate. So it seems that at some point you reach and area difference that would create the maximum thrust. I've tried to figure this out a million times myself but I could never figure it out.
 
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There's no such thing really. Real rocket nozzles are designed to be optimal at a certain altitude range. They are not optimal over their entire flight envelope.

For models, you have to selsect what is your best case performance. Conidering that you can produce more thrust by increasing the mass flow through the engine, you need some kind of parameters to be able to specify an optimum.
 
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Im not sure what you mean by parameters. I'm not trying to come up with anything realistic. Just using the variable of area at a constant pressure.
 
You need to at least specify a propellant type. If its water, operating near ambient pressures its less of an ordeal mathematically. Otherwise you have several interlocking variables, which make for a huge spectrum of optimal soln's. For instance, its a solid propellant whose burn characteristics are affected by both temperature and pressure, you can choke down a nozzle in an effort to produce greater ejection velocity and insodoing exceed the limits of the exterior case as pressure becomes runaway. Too loose a nozzle, and you may not even get it to reliably ignite, to say nothing of the loss of thrust. There are entire books on the subject, and no single answer.

Try here for a start:
http://www.aerospaceweb.org/design/aerospike/nozzles.shtml
 

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