starwoes,

you have just explained the need for a Con-di nozzle , and perhaps very well, obviously if you have NPR of over 3 then they become very efficeint,and obviously we must always have a jet exit velocity greater than the flight speed to provide forward thurst) but you don't make much mention of the need for a variable area nozzle.

As I understand it, its all to do with the ambient pressure and whether the jet is fully expanded or not. For maximum efficieincy you always want to fully expand your jet , so with a variable area geometry you can change the exit area of the divergent section such that the plume pressure matches the static pressure of the ambient air. And hence you do not end up with any shock system in the jet.

The control system controlling the final exit area then becomes a function of the ambient static pressure (altitude) and engine setting (nozzle pressure ratio) such that it dials in an area for each of these flight conditions.

The linear aerospike nozzle matches the exit plane pressure with the ambient pressure naturaly as part of its design , mainly as it was intended, (mounted on the X-33) to launch vertically and go up into space, i.e ambinet pressure is varying greatly. All anyone has to do is watch a rocket motor plume travel up into space and see the shape of it change enourmously from sea level to an almost vacumm. They choose not to mount variable area geometry on the nozzle for weight reasons and the fact that it is use once only (I believe, but sombody may correct me).


Nick