Ideas about my design appreciated - before I start building :-)

So I organize cool group travels to remote places, usually a hut somewhere high in the mountains, and I built a portable sauna to enjoy during these trips. The sauna is 150x200cm and 140cm high. In the first days of the trip it fits four people; after some days it fits 6 to 8. I built a rocket stove for heating it. I am not a welder and, apart from a drill and an angle grinder, don't really have access to tools for processing metal, so for that rocket stove I used materials that could work for me without thinking all that much about the best result. I used two barrels; the outer some 60cm high and 40cm in diameter and the inner one 46cm high and 30 in diameter. The inner completely 'floating' with space also between the bottom of the two barrels. I had a 10x10cm J-tube welded out of 3mm strong steel and the space between the J-tube and the inner barrel filled with vermiculite. It kind of did what it had to do (and burned quite clean and efficient) but I wouldn't build it again like that. Especially the J-tube seems to be a way to big mass (both in kilos to carry up the mountain as in heat bank).

It's time for a better solution. My main prerequisites are:
- preferably no welding, bending etc
- as light/portable as possible
- should transfer most heat to the environment in the stove itself, before the chimney
- the thing gets fired up max. some 25 times per year for 3 to 4 hours.

Below a drawing of my new to-be-built rocket stove. I would highly appreciate any input/remarks/suggestions etc. about this design, before I start building it
The core of it is the feed/burn/riser tube built out of 3cm thick vermiculite board and a 'barrel' constructed out of steel plates that I would connect with L-profiles and simply bolt it together. The top plate would rest on T-profiles so that there is a bit of an 'edge' to contain the sauna rocks and a bit of water when I do an 'aufguss' (is there an English word for that?) The top plate would be of thicker steel than the rest (in my first rocket stove, the lid of the barrel did get red hot).

In my design the heat riser length is limited by two factors: I tried to make everything in a way that I would be able to make the whole thing using only one 100x60cm vermiculite board; and it has to fit in my car.
So I tried to respect the proportions between heat riser/burn tunnel/feeder length and the channel would measure 9x9 cm throughout the three. The square 'barrel' around it with 3cm distance on all sides; this makes that the CSA of the 'downwards' part is three times the CSA of the 'upwards' part. I am not so sure about the best distance between the top of the heat riser and the bottom of the steel 'lid'. I made it 3cm as well.

My initial doubts/questions are:
(1) does it make sense to build the system out of 3cm thick vermiculite board? Will it isolate enough? Will it need reinforcements (at the top for example) and if so, how?
(2) does it make any difference if the bottom of the burner sits on the bottom of the 'barrel', or should there be space between them?
(3) can I use the 12cm diameter chimney parts that I already have or should I attach a chimney with a smaller diameter?
(4) I read a lot about burn tubes that are to long - but can it be to short also? For portability, I would like to make it shorter. Would that be a bad idea?

For sure the 'veterans' on this subject will spot other flaws in my design and I am happy to get informed!



Thank you everybody!

For the non-metric people out there, here is the sketch in inches:

-

And 12cm equals 4,72"

If your steel J-tube model worked at all, this vermiculite board one built to the same dimensions should work much better. I don't think you mentioned how you are holding the parts of the J-tube together. Will the J-tube be a permanent assembly, or come apart for transport? I would suggest some steel angle pieces with strapping at the corners to hold the board sections in alignment and make the whole unit strong enough to stand moving. Bolting the steel sections at junctions would help a lot. If there is no significant insulation outside of the steel, it should last well - the heat will be contained by the board. Regular RMH's have used chicken wire around the heat riser to contain insulation, so I think a steel band clamp around the top and possibly other places on the heat riser should be okay.

1: For the size of your system, the 3cm vermiculite board sounds good. If you are going to move it while assembled, you need reinforcements for durability.
2: If you have a steel floor to the "barrel" which rests on the ground, putting the base of the burner directly on it may lose combustion heat and damage the steel. I would suggest a few scraps of vermiculite board to space it up.
3: A somewhat larger chimney will probably not hurt and certainly won't choke the system. You can always make a new smaller chimney if you are not happy with the performance of the original.
4: I am not an expert in this, but I don't think a shorter burn tunnel would hurt, at least not enough to matter.

The 3cm top spacing may be too small for good airflow. This dimension in the books is a minimum, not an absolute; a larger space will make the top surface less hot and the sides hotter.

Hi Glenn
That doesn't sound to bad then
My first idea was to make the J a permanent thing, indeed with some metal straps around it. The protruding part might be detachable also to make transport easier.

Hello Klaas,

I have had trouble building smaller scale rocket mass heaters. They did not pull as strongly and didn't get as hot. It was also harder to work (assemble) in the smaller dimensions.

Would it be easier to build/transport a "pocket rocket"? The pocket rocket could be sacrificial, using it only for a month or two before making another. Please see the below video for a pocket rocket example. It looks light, simple, and easy to transport.

Brett, I have doubts about using a pocket rocket like that in such a small space as my 5' x 6,5' x 4,5' sauna...

Klaas,
The chances are that the dimensions are a bit too cramped. It's not easy to build a well running small diameter rocket heater. Having said that, there are limitations with the vermiculite board as a material. It's brittle and heat resistant up to a certain level. It isn't suitable for a burn tunnel because of the high temperatures and the feed tube is altogether off-limits because of abrasion by the fuel. And the temperature as well, the same reason as for the tunnel.

The vermiculite plate is not strong enough to support its own weight, especially when cut in narrow strips. When assembled it's OK but still awfully vulnerable. Above that, it's possible to use screws to assemble the thing but that can't be done more than once using the same holes. I'd say you need a steel support frame of some sort and insert firebrick splits for the feed in that plus the vermiculite plate for the rest. The tunnel material has to replaced every year I'd reckon.

I am not the bringer of good tidings, sorry.

Pocket rocket using a 55 gallon barrel?

Maybe this is an idea: When a pocket rocket is too dangerous because of the red hot barrel you could dig a hole and partly sank it into the soil.

Thanks everybody for the feedback! It seems that I'll be tinkering on a pocket rocket this weekend.

a pocket seems like it would work good. maybe a can around the bottom full of the rocks to steam off. would be a fairly easy build too.