What is the difference between thrust and thrust density?

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Discussion Overview

The discussion centers around the distinction between thrust and thrust density, exploring their definitions, measurements, and implications in the context of propulsion systems. Participants examine the relationship between thrust, nozzle area, and thrust density, considering both theoretical and practical aspects.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants clarify that thrust is measured in Newtons, while thrust density is measured in Newtons per square metre, indicating a relationship to the nozzle exit area.
  • One participant proposes that thrust density represents the thrust produced per square metre of nozzle area, using examples of different nozzles to illustrate this point.
  • Another participant questions whether thrust density can be classified as an efficiency metric for propulsion units, suggesting that it has units of force per unit area or energy per unit volume.
  • A later reply introduces the idea that thrust density alone does not provide information about exhaust velocity without additional data, such as the density of the exhaust stream.
  • One participant suggests that thrust density might be equated to pressure, raising further questions about the implications of this equivalence.

Areas of Agreement / Disagreement

Participants express varying interpretations of thrust density and its implications, with no consensus reached on whether it can be classified as an efficiency measure. The discussion remains unresolved regarding the relationship between thrust density and other performance metrics.

Contextual Notes

Limitations include the lack of information on exhaust velocity and density, which are necessary to fully understand the implications of thrust density in propulsion systems.

AdrianHudson
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0000126.001&db_key=AST&bits=4&res=100&filetype=.gif


The picture above has thrust density as a column and I am not too sure what that is. Can anyone explain to me what the difference between thrust and thrust density? Thank you for your answers!
 

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The clue is in the table headings.
  • "Thrust" is measured in Newtons.
  • "Nozzle exit area" is measured in square metres.
  • "Thrust density" is measured in Newtons per square metre.
It shouldn't be too difficult to work it out from that.
 
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DrGreg said:
The clue is in the table headings.
  • "Thrust" is measured in Newtons.
  • "Nozzle exit area" is measured in square metres.
  • "Thrust density" is measured in Newtons per square metre.
It shouldn't be too difficult to work it out from that.

Ahh! So its the thrust produced per m2 of the nozzle area. Therefore that means that if you had nozzle "A" that has a surface area of 0.5m2 with a thrust of 10 Newtons and Nozzle "B" that has a surface area of 1m2 with a thrust of 10Newtons. Nozzle "A" is producing more thrust density?

Am I wrong in classifying thrust density as an efficiency of the given propulsion unit?
 
AdrianHudson said:
Ahh! So its the thrust produced per m2 of the nozzle area. Therefore that means that if you had nozzle "A" that has a surface area of 0.5m2 with a thrust of 10 Newtons and Nozzle "B" that has a surface area of 1m2 with a thrust of 10Newtons. Nozzle "A" is producing more thrust density?

Am I wrong in classifying thrust density as an efficiency of the given propulsion unit?
Not really. It has units of force per unit area or, equivalently, of energy per unit volume. But unless you are also given something like the density (mass per unit volume) of the exhaust stream, it cannot tell you the exhaust velocity.

You could, for instance, have an ion drive with an ultra high exhaust velocity (very mass-efficient and very energy-inefficient). The exhaust stream would also be very sparse and would likely have a low thrust density.
 
Last edited:
DrGreg said:
  • "Thrust density" is measured in Newtons per square metre.

So it's pressure?

Cheers
 

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