RMH Specifications

Hello!

I have been deeply curious about wood stoves that use gasification as the primary source of fuel and now Rocket Mass Heaters sound similar in some respects, but exciting different as well. Are there any specifications for the heater that maximize efficiency and minimize and possible dangers? The main j-tube and heat riser must have some specs (diameter/height/thickness/insulation requirements)? Is there a proper ratio of cob mix (clay, sand and straw) to form the thermal mass necessary for cooling down the exhaust of the heater? Does the mass age or crack? Is there a minimum mass to heat ratio? Are the designs based on successful pragmatic usage or engineering? (I'm a geek...) How long does the Cob have to cure before firing up the RMH? Soooooooo fascinating and so many questions...!

Thanks all!

Ian