Ideal area difference for rockets

  • Thread starter GiTS
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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.
 

FredGarvin

Science Advisor
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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.
 
957
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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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