I've got my hackles up right now, YouTube suggested a video to me of a guy with 55 thousand plus subscribers (GreenShortz) who is teaching people to make "Rocket" stoves when they really just miniature fireplaces.
To top it off he demonstrates building out of various types of concrete.
If I heard right, he's actually selling some on Amazon.
I'd like to leave a constructive comment to point his viewers in a better direction.
RMH technology has moved from J tubes towards batch box's.
This makes Peter's site leading edge.
In my opinion Permies has the most current info on J tubes.
Of course the RMH builders guide by Ernie & Erica Wisner is the leading book on the subject.
Another wonderful site, is our friend Donkey's. (http://donkey32.proboards.com/) They cover builds very technically. Most are batch box's, but some are J tubes.
A Rocket core is , A horizontal burning fire with a large bore air feed and a super insulated riser (chimney) made with materials capable of withstanding temps in excess of 2000F