I What is the difference between thrust and thrust density?

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.

 

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.

 

So it's pressure?

Cheers