Compact RMH Design,Need Opinions.

I am considering building a RMH to install in a small stick built house.The plan is to build the heater inside a 55 gallon drum/barrel with a removable lid.The lid will be sealed with fire resistant rope seal like a conventional wood stove.Also,the RMH will have a standard stove door with adjustable draft and spark arrestor.The plan is to build the RMH inside the barrel out of fire brick and use fire brick joint compound to hold it all together.The exhaust side will have standard six inch black pipe.Then back fill the barrel with perlite to 3 inches from the top of the exhaust pipe for my mass.I'm not sure how close the RMH chimney needs to come to the top of the barrel.I am thinking 2 inches because the heat will oil can the top up,expanding the airspace above the chimney.I have also considered feeding the RMH with fresh air from outside the house.The RMH will be L shaped and not J shaped.The unit will be set on a concrete block stand on a floor pad for wood stoves and a heat shield will be placed around the rear of the RMH.Have I missed anything or forgetting anything.I think the only problem I foresee is the air gap between the RMH and the top of the barrel.

Here's my mock up drawing.

Ben Mosley :Go for it! This is a very bold departure from the standard/traditional build and while I can see some areas that trouble me, you may end up with a version say 1 1/2 barrels high that makes for a simpler build !

Have you read Ianto Evans' great book 'Rocket mass heaters' there is no other place to find as much Rocket specific material in one place - in any language !

I need a clearer picture in my mind as to what your plans are/how you would use the 'Standard stove door with adjustable draft and spark arrester'.
I want you to go to You-Tube and type in 'Build a rocket stove step-by-step' 90% of what i want you to see is in the 1st 25 seconds.
That little piece of tin cut down from a soup can and stomped flat is a very important part of your build!
This allows the pre-warming of your air, and is vital to your Rocket if you are going with a ' Standard stove door' and is scaled to the size if the #10 can, yours will have to be bigger, Longer and a little bit stronger .
I think that your 1st design as drawn does not allow you to use much of the depth of the 55gal drum, I want to send you to ' ERICA WISNERS ROCKET CANNING / FRYING STOVE ' so you can see how she modified the bottom of the barrel, please note the total length of the burn tunnel should never be more than 1/2 the length of your Heat Riser. Her top 1/2 barrel is not like a conventional rocket, or your design ether !

As drawn you will certainly have a well insulated Heat Riser/chimney However, the insulation will not act like the Thermal mass that allows for any heat storage ,it will only act to block the radiation of any heat from the barrel !

The oil canning of the 55 gal drum is common enough to be a commonly heard sound and is used to describe the sound of naval destroyers "working' in a heavy sea' and their nickname! Being aware of it, I understand your concern !
It is actually uncommon enough to be missed as a cause of the The rocket stove starting to smoke after running well for the 1st 15 minutes - or even longer !
It is also a 'sign' that the drum was filled under pressure , or filled with a cold liquid 'right up to the bung hole' and left to heat in the sun, This is not as common a problem with the open top 55gal drums as we usually divert them
before they can get refilled ! - get Two removable lids -more on that latter !

The L Shape will actually need more' frequent fiddling to feed fuel to the fire ' shoving your wood forward to keep it in the sweet spot where it burns best.
Also the J-Bend creates a thermal - syphon trap to keep all your heat from going up the chimney if your Rocket is left un-attended and goes out !

You could replace the insulation in the top 1/2 of the barrel with a denser material that could absorb some of the heat from your Design but the insulating nature of your Heat Riser/Chimney works against you here.
I think that this will therefor limit you to a more conventional build with at least the top 1/2 of the barrel allowed to radiate off the heat that MUST be radiated off to create the Heat Engine that drives the exhaust gases horizontally through your Thermal Mass ! Good luck with your build, try to get two 55 gal tops that look identical and keep us posted with your build as it proceeds ! PYRO -magicly yours Allen L.