Thickness of perlite layer around the fire box inside manifold

Hello, we are in germany and have nearly completed our rocket stove. The heat raiser stack is set and the fire box at the bottom is set.
Now we need to put the perlite layer around the fire box.
However, one thing is still not clear to us from the pictures we got out of the manual:
How thick should be the perlite layer around the fire box at the bottom part inside the manifold?
Connected to that is the other question: how big should be the gap be between the perlite surface inside the manifold and the outer brick wall of the manifold, necessary for the air to reach the bench outlet?
Those dimensions are not clear to us...

Thank you in advance for any tips!

There are lots of designs for rocket stoves, different barrel sizes, riser sizes, etc. In general I would try and center the riser as much as possible in the barrel, although some slight eccentricity might be necessary to accommodate the manifold --especially when you're using lots of insulation on a large riser.

In general I try and get as much insulation and distance as I can between manifold and back of firebox. I also try and get a smooth transition with no constrictions/obstructions between the space around the riser and the manifold. Picture a  smooth downward flow into  the ductwork.

Also,  I don't just use perlite, but stabilize it with some clay. It can always be broken apart and reformed with some fresh clay later if you need to rebuild or change anything

Thank you Bob for your reply.
Our riser seems to be quite central. Underneath is a square fire box (bricks), also pretty central. Around it is the wall of bricks again.
Now according to the plan we have we should put some perlite/clay layer around it and we do not know how thick it should be and also how wide the gap..

Rocket Mass Heaters are very much an individualistic sort of thing, so without knowing  the specific plans / dimensions it is quite difficult to know how to advise you.

Let's just say that the back of the firebox is a pretty hot place, and the exhaust leaving the manifold should be cooling to facilitate the push of exhaust out through the mass (bench). So with that thought you can manipulate your distances/insulation to maximize that temperature differential. It would probably be a good rule of thumb to have at least as much insulation between the back of the firebox and the manifold as you have surrounding your heat riser, more if you can fit it in

Correct me if i am wrong.The book says to put 1 to 2 inches of perlite insulation around the firebox.
The gap is different if you have a 6 inch or an 8 inch system.Does anyone know if the book make it clear that the gap should be measured from the perlite layer to the brick plenum that defines the manifold?Or from the brick of the firebox to the plenum?

From a plan of a 6inch system that i just saw,i can see that there is an 8 cm(bit more than 3 inches) gap between the brick plenum and the perlite insulation.
For a 8 inch system there isa gap of 4 inch/10 cm.

dear Panagiotis,
could you please specify the 6 inch oder 8 inch system - what is "system" in this context"?
Thanks!

RMH system sizes are based on the duct diameter, which should remain constant through most of the system length except in barrel and manifold.

Rikka, the "manifold" is usually called the plenum or flue transition.

Here's a thread about it i tried to start.  

https://permies.com/t/61657/Flue-exhaust-transition-plenum-pictures

Regarding system size, as Glenn explained, it's usually stated as the diameter of the pipe used. Which gives you a "CSA" or cross sectional area. Which should remain constant, except in the plenum. Usually the  minimum accepted number there, is 1.5 times CSA, best practice is to reach, if possible, for 3 times CSA, ass ashes love to pile up in there.

System size, also gives you the maximum length of pipe you can useOr the biggest ISA for bells.

HTH.

Max.