Rocket mass heater with 1 meter long fire box

I love the idea of rocket mass heater and want to fit one in my kitchen which will function also as a oven and hot water supply. I plan to have a second chamber behind the firebox with an oven above it and then have the flue pipe leading out.

However I wish to fit a one meter long fire box because its such a pain cutting wood into small chunks. I get my wood delivered and those branches cant be axed and are about one meter long so it make sense to cut down on work My question is, is it safe? I assume as I will be putting in big pieces of timber so temperatures can become pretty hot. I mean my wood burning stove now is 10 inches wide and hits 4kw.How much more with one four times the size.

My second question is about the smoke, will this increase compared to the videos I have seen of the rocket stoves? Will I need to modify the design to have no smoke being released? its important I have no smoke as its illegal to burn wood without a smokeless approved woodburner so it needs to be covert. My neighbors would be straight onto the council and I would have a big fine. My plan is to have the flu pipe coming out the kitchen into a lean to conservatory so those nosy neighbors cant see the smoke. Does this mean my flue liner will have to be x meters long?

I appreciate your feedback, has anyone designed one with a big firebox? I would love to read about it

I suspect no one can give a definitive answer because you are changing the standard design as well as having the normal variation in materials (construction materials plus fuel), vent configuration, etc. Your best bet is to build your version of chamber length somewhere it is legal and work the kinks out befor you get busted. Once you lose the trust of neighbors they'll always be checking on you.

Just because your wood comes in 1m lengths, doesn't mean your fire box needs to be that long, nor does the feed tube. The fact that the diameter is such that it can't be axed is more of a cause for concern as most rocket stoves are run on smaller diameter stuff. You'll probably need to split it, and for wood that long a froe is much nicer than an axe.

It's hard to be sure whether you are talking about a typical RMH with J-tube, or a batch box type. Also, you don't say precisely what diameter you plan to use your wood at, but a J-tube needs smallish sticks and not logs 5-6" or more. The J-tube is intended to burn only at the bottom end as the draft pulls heat and flame sideways into the burn tunnel, so the log length is irrelevant to speed of heat output, but you would be right that you would need a riser at least 3 times the height of your feed tube. If you had a long feed tube, I would not correspondingly lengthen the burn tunnel, as there is a limit to how long the flame path during combustion will be, and just adding horizontal length will not help if there is sufficient overall length. There have been numerous people proposing an open cage of some sort to keep long sticks feeding straight down without extending the feed tube, but I would be wary of using such a system indoors.

A batch box can be filled with larger diameter logs, which all combust and pyrolize at the same time, feeding the gases into the heat riser via a constricted port that induces desirable turbulence and causes complete combustion. There woud be a limit to the amount of gas that could be handled at any one time for a given system size (nominal cross-section), and making the batch box too much longer than standard ratios would risk overwhelming the riser and hobbling efficiency (thus generating significant smoke). This would as suggested require significant experimentation to find workable dimensions.