To put this in the proper starting place - I am, this very minute, sitting in a nicely heated place courtesy of the rocket heater I built for it - and it's below 20F (and dropping) outside with some wind.
The rocket heater is amazing both in concept and in practical usage - even one as far from perfect as the one we have here that makes so much heat from so very little (and very poor quality) wood.
Having said all that, the heading of this posting refers to a few things that go off in different but related directions...please bear with me here ??
The first is that I am very glad I was able to learn from others how important masonry is in a RH - and as a result I only used the stovepipe parts as things to cast masonry around in the most heat-prone areas of my RH.
The heavy gauge stovepipe elbow between the burn tube and the rocket tube has shown me that steel is not a worthy opponent for the wood fueled fire in a RH; I pulled out a chunk of it with the poker earlier that had melted small enough to come right out...
I can also see where (in the same immediate area) there is a (melted ?!?) hole in the side of the inner wall of the rocket tube which is sort of 'dripping' perlite out of it.
Still - it functions well and the outlet temp remains low & safe, and what finally goes outside is just dirty steam.
If and when I ever re-do this one or make another it is now obvious to me that making the burn & rocket tubes with 100% masonry walls is THE best way for them to work & last longest.
I also wish to mention that when using a solid fuel for heat I've always far preferred using hard, graded coal over wood.
(Back when I still had a coal stove of course...)
A proper coal burner can be banked for overnight heating safely, and contrary to all the nasty propaganda most folks have believed about coal as a fuel - hard (low sulfur) coal, when burned properly - is a very clean, hot fuel.
Why do I mention coal, you ask ?!?
I have been in discussions with someone about the notion of using coal as a fuel in a RH.
He seems to have loads of great ideas to improve upon the basic RH concept.
Here is where I sort of get stuck when it comes to using coal in a RH:
No matter which sort of coal may be used in such a project - I believe that what one will make when combining rocket-style burning with enough coal to sustain it will NOT really turn out to be a rocket heater at all...
I believe it will be a wonderful, red-hot forge.
As such it will likely excel at making distorted and/or molten steel if that is used as it's wall material.
I'd love to be wrong in my suspicions, but decades of burning coal as well as knowing men with coal-fired forges makes me lean the way I've expressed.
Aside of that - considering the notion of automation along with the need to buy & store solid fuel - and combining those with the RH concept would lead me straight to the idea of making a pellet burning adapter & feeder rather than using firewood or coal at all.
That idea just makes the most practical sense to me if starting out fresh into such a project (and facing the need to buy & store whatever fuel will then be in use.).
Firewood's true expenses are seldom carried all the way forward so it tends to superficially look a bit better as a fuel than it really is IMO.
Also, to be fair in this comparison=>
Hard coal has become quite pricey here and to get a good stove is very pricey as well; and then one must also have a bin to load and carry bucketfuls of the stuff from.
The worst of it is the pouring or shoveling of it - as one will always end up breathing coal fines/dust which is not healthy a'tall.
But pellets ??
Can be bought in tonnage reasonably, can stay bagged outdoors for a very long time, and require no chainsaw, splitter - nor are the chimney fire worries as bad.
Best of all - the dirty dust problems can be negligible compared with any other solid fuel.
No, I haven't made or used one yet, BUT:
I imagine that a carefully made pellet-burning RMH with a good auto-feeder might be the very peak of this technology combining efficiency, convenience, economy and safety to maximally good effect.
Then again - I may be totally mistaken, but these are my thoughts on the matter and should I live long enough to test them 1st hand I may learn better one way or another sometime.
The one opinion I will definitely stick with is that masonry seriously trumps steel for the hottest parts of a RH.
I was advised thusly, and t'was very wise advice which I appreciate the opportunity to learn from.
Best Wishes to All !!!