I've made lots of rockets, both the simple Winiarski L-rocket and the Ianto Evans J-rocket. I cook and heat with my latest model, right now. This one of yours is really nice; not least because - clearly - you understand the principles which make rockets work so well, and they're faithfully reproduced in this handsome quickie knock-up. I would question, though, sand or clay as insulators; heat sinks, more like. And as you point out, good insulation of the riser tube is critical to make the stove work effectively. Very high combustion temperatures - near 1000 degrees Centigrade for larger-diameter rockets - are an essential design principle; only achievable with a thick layer of good insulation. The proportions which you built in, and the absolutely-vital insulation, are the outcome of long, careful, very much field-tested development periods, by Larry W., and by Ianto independently, and with help from many colleagues.

Beware the YouTube vids about 'rocket' making! Far too many of the posters clearly don't understand the basic principles of rockets, and make very bad, cargo-cult-style sort-of-rocket-like stoves that don't work right, because they're not built right. Lots of cases, for example, where sand or soil are recommended as high-grade insulators, when they're nothing of the sort. Quite a few cases where the insulation is deliberately left out altogether (kiss of death!). Lots of other examples where the proportions are quite wrong, producing vastly inferior results. Sure, they still cook stuff, after a fashion; but with low temperature flames licking out of the top of the riser, and sooting up your pans. Proper rockets produce super hot carbon-dioxide and water vapour at the top of the riser tube, just as they hit the pan, and virtually nothing else except a very small amount of fly-ash.

To achieve this final, very hot completion of the fuel burn just at the top of the riser, chimney height needs to be two-and-a-half to three times the diameter; and the more high-grade insulation round the chimney and the inner end of the feed tube the better. Make it three inches thick, at least. Play around with quick, dirt-cheap knock-up rockets like this one, whilst you get the hang of it all. That's what I did. Proper rockets are in a class of their own, better than any standard wood-stove. PROPER rockets, that is! DRY fuel is a great help too.

BTW, don't try glass-fibre for insulation. A properly built rocket gets so hot, it melts the fibre back from the riser-tube's outer surface. Tried it. Doesn't work! Good insulators are: DRY wood ash, perlite, or vermiculite. They can take the bright-red-to-white-heat.

Once you're confident that you're up to speed with all this, try making a rocket in stainless steel tube, with 3mm wall thickness. Stainless appears to resist burn-through indefinitely; well, for at least fifteen years in my experience. Four inch diameter is a good starting size. Bigger diameter tube gives a very hot, strong fire, which will need a little more fuel. Still amazingly fuel-economical, though. That's part of the original design-spec. for rockets, for Third World and refugee-camp cooking, where fuel is very scarce, and is often nothing more than dried plant stalks. Tried that with nettle stalks; works amazingly well! The other main design spec. was a very clean burn, to reduce serious lung illnesses to people using firewood long-term for cooking. Only possible at very high temperatures.

Weld up the parts of the stainless fire tube, rather than just pushing them together. You will need an angle-grinder to cut them to shape. Outer can, to contain the fire-tube and the insulation, can be any suitable mild-steel available. Doesn't have to stand up to the high temperatures and the super-heated oxygen, the way the fire-tube must.