Hight of the feeding batch box and other questions...

So now that I have seen the batch box, I can do what I thought impossible: have more than 12m flue!
For my special configuration below a cliff, I need 16m and the chimney away from the house and cliff. I would have only 2 or 3 direction changes, the 1st half being underground.
My 1st question is to know what system size I need for this lenght, still 8”, or would less be possible? I have somehow understood that the power of the batch box doubles the power, so 24m for the 8” size?

My system is underground, so I cannot open the front. I had planned the top of the feeding tube (or batch box) exactly at floor level.
2nd question: I can still dig the floor, which is rocky. So I need to know the depth. What is the advised hight of the feed box, and what range for the insulation thickness?

Please, we are digging the floor right now, and I hope that some one can answer this of the hight of the feeding part of the batch rocket!
I insulate the ground with a local light stone (does not float, but real light) that will be mixed in cement, and then I build with fire bricks and clay the normal way.

I have another important question to check out: is it possible to charge the batch from above? Why is it usually a front load?

I am no expert, but I don't think it's a good idea to have your feed tube much higher than half the barrel hieght. And what I understand, the better insulative the j, the harder the burn.

Yes, I will see for the hight of the barrel afterward (I will have it welded, i cannot find correct oil drums). If you saw some pics of the batch load rocket, this is a big beast, and there needs to be a certain height for this batch box, as you can see in the innovator event of last september.

I am also conscious that the size will vary for a 6 or 8" system.
That is why I mention the length I need for the flue pipe.

I originaly planned a 8" so that I could have a large enough feed tube, and a long enough flue pipe.
The batch load changes a lot of things...
Then if I can have a good result with a smaller section, then I would go for it.