Originally Posted By: Momos
You wrote some interesting stuff I didn't know, especially about the back-pressure and maximum exhaust speed, thanks!

I only wish I had explained it better in the beginning...

It is an interesting problem when it comes to rocket design. I remember waaaay back in the mid-1990's when I built my first rocket engines, they constantly preformed well below my expectations. I spent the better part of a year thinking the problem was in my design/fabrication. Turns out I had used the ideal gas formulas, and missed out on the choked flow issue.

When I used the correct math, my engine performance was dead-on predicted.

Later designed however did have some design/fabrication flaws - those ones go "whoosh - BOOM", instead of under preforming. Basically expensive confetti = you screwed up...