What is the difference between thrust and thrust density?

[AdrianHudson]

Main Question or Discussion Point

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!

 

[DrGreg]

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.

 

[AdrianHudson]

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?

 

[jbriggs444]

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.

 

[cosmik debris]

• "Thrust density" is measured in newtons per square metre.

So it's pressure?

Cheers