Hi yea I learnt a few things off this. But I still don't like those explanations :P Seems to me a rarified gas would approach being ideal. Also not sure why it really has to go at the speed of sound. I think that's only an upper limit when you have way too much force pushing it. For a bottle rocket I bet there's no speeds of sound happening.

I tend to dislike engineering equations (despite using them every day ;), because of exactly these problems. They're usually simplifications that only apply to special cases. No room for doing your own thing, and very bad for extreme thought experiments.

Perhaps you should tell Paul directly that Ve isn't zero anymore wink