Hmmm, the problem I was thinking of was in pure vacuum. Does the pintle (adjustable central obstruction in the nozzle)
Yes, you'll be pleased to know, I know what the pintle is, I have had to extract it from the motor several times
improve performance in vacuum?
Compared to a bell nozzle, yes, since it is operating at optimum efficiency throughout the thrust regime.
Seems to me that if you wanted almost perfect ISP, you still need a high expansion bell on your nozzle.
Absolutely, you're spot on
Thanks for replying... I think some people forget just how complicated (and time-consuming) it is to design a vehicle for space travel when you don't have billions of dollars in your budget.
My pleasure. I'm sat here working on calculations connected with the ED nozzle this afternoon (the rocket side of the engine is the part I am lucky enough to work on), and thought it made sense to reply to some of the comments where I could.
As you can imagine, there is a lot more going on behind the scenes than is indicated on the website. The problem is, we are rocket people, not PR people, so getting the word out doesn't come naturally, and we spend the vast majority of our time working on the technical aspects
We also have probably an inefficient nozzle design with a tradeoff between greater bell size (and efficiency in vacuum) and lower air drag.
That's why we are developing the ED nozzle
The ED nozzle is a very efficient nozzle design and provides altitude compensation across the thrust operation range. Part of the engine development at the moment is concerned with development of the ED nozzle for this purpose.
Two can Live as Cheaply as One for Half as Long. -- Howard Kandel