Thanks folks!
Hardiboard is available in a bunch of different thicknesses and intended applications. Been quite a while since I looked at their product portfolio, but I'd try looking around their corporate website to see if they have complete product catalog with MSDS or similar engineering worksheets. Odds are you are making do with thinner and cheaper Hardiboard products that aren't optimal for this application.
Other companies have straight cement board available that should do the job. Obviously the stuff isn't cheap or light. Hardi is just a well known and distributed brand
Not an engineer or chemist, but perhaps a hardboard with metal grid/bracing built in is the correct solution. Very common to pour cement with a metal rebar or similar mesh sandwich for integrity.
Was fussing with steel strips in my brick pile to support spans without brick obstructions in the burn chamber (that is why that all came to mind).
As for the 55 gallon drum, burnout and or cracking is inevitable. Unsure if the inner black pipe or the barrel will fail first. I'll post elsewhere to see how others are fairing with their older rocket stoves and metal failures. Expect if the chemical reaction, condensation / oxidization doesn't create the problem the temperatures will warp the barrel in no time.
The solution (if there is one) in general with these parts is to stick with stainless steel 55 gallon drums (hard to find here and expensive new) and ditto for the inner duct black pipe, stainless instead. Ceramic would be even better, but I never see it around here and consider it exotic (can't salvage it from teardowns, scrap, etc.)
As for the 2000 degree temperatures, it is possible but only in tight spots in very optimal fire with a rather big pile of hardwood coals. Get 6 inches away and the temps drop massively. What we are doing with these stoves typically is two notable things:
1. Reducing the burn chamber down to a typically very small area.
2. Reflecting and concentrating heat in the burn chamber
I have yet to see a rocket loaded with massive logs or what most people use to bank a fire in a box stove overnight. Since the burn chamber accepts and burns less we are heating the same very common area. As where in a traditional stove, you have fire wherever it has fuel and is ignited. Meaning, the cold spots in the traditional stove are many and always different.
In many ways this concentrated burn spot is similar to the designs we see for pelletized biomass. That is why those stoves are also far more efficient per se than boxwood stoves.
The reflection and concentration are common in these stoves and good stoves of all types. Problem with other stoves is we are reflecting fire temps in a much larger burn area. It is like burning a match in a 10 sq ft room compared to burning a match in a can. You would notice the heat in a can, but in the room, only the smell/smoke.
Wondering how suitable most rockets would be for prepping batches of biochar/charcoal. Seem to be very close to environment one would want to create such.