"cap" on riser inside barrel

I've been reading a lot here and elsewhere for quite some time about rocket heaters but this is my first post.
I'm finally building my 1st RMH, and am using firebrick for the 26 sq" CSA J tube construction. As yet to be determined barrel size.
It's a basic design question. Would it work to have a 1/4" steel plate "cap" (with a diameter about the same as the O.D. of riser?) 1 1/2 -2" above the riser instead of having that gap between the top of barrel and the riser?
Why might I consider this? For 1 it would keep the high heat from deteriorating the top of the barrel, and perhaps make for easy fine tuning of flow relative to that gap.
Comments please

John Adamz : Honestly, you are making a problem were one does not exist, The gage of the ductile steel in a modern barrel is thicker than the gage of steel in a modern
Commercial Fossil Fuel Fired Forced-air Furnace's heat exchanger, while that furnace is producing more BTUs than you can get out of a sub 6" system, And the Heat
Exchanger is Guaranteed against failure for 20 years.

For the magic that turns heat radiated off of the barrel into 'Delta T' work being done, pushing the exhaust gases horizontally more than 20' we need the hot exhaust gases
leaving the Heat Riser to slam into the underside of the top of the barrel tumbling into a rolling toroid or doughnut shape with the increased turbulence creating some of the
Rockets load roar And additional heat to be radiated out of the barrel,cooling the gases and causing them to fall as they radiate more heat and contract becoming more
dense and being replaced by lighter hotter gases ! Your steel cap would likely disrupt and stop that process, several other people have come to this forum with similar plans
and if they actually tried them they did not come back here to record their success!

About barrel size selection, for the push-pull effect created by heat radiated off of the barrel, IF you had two Identical Rocket Mass Heaters RMHs, one with a 55 gal drum and
one with a 30 gal drum, both barrels must radiate off the same amount of heat to create the Delta T, counter-intuitively the smaller barrel must give off that heat energy at a
higher temperature.

This is why when your 3 year-old wants to get up in your lap they are such a good cuddle, with a smaller Surface Area to Mass or Skin to Weight ratio in order to maintain 98.6
they must radiate off the metabolic (mostly) heat energy at a higher Temp. I hope this clears this up for you ! For the Good of the Craft !

Think like Fire, Flow like a Gas, Don't be the Marshmallow ! As always your comments and questions are solicited and are Welcome ! PYRO - Logically BIG AL !

Thanks for the input. I actually would prefer less radiated heat, and more in to the mass (40ft of 6" duct under gravel floor) It's in a 22' geodesic dome greenhouse.
J tube specs: feed tube 9", burn tube 17", riser 35" all with a 26.6 sq" CSA.
Relative to the gap above the riser I've read that as long as it's not less than 1 1/2" your OK. I suppose there's a limit to that gap, but for ease of assembly for my particular setup I might end up with maybe 4". Would this be too much of a gap?

John Adamz :The gap between the top of the heat riser and the barrel will be fine between 1.5'' and 2'', above that we are running on anecdotal evidence, make sure
that your Heat Riser and Your barrel are both level and plumb, I know you will be careful setting the barrel down over the Heat Riser !

Because of the abundance of Crap out there in you tube Land, I mostly forgo the aggravation of visiting those videos. So for me little evidence and all 2nd hand reports
are discounted until I am provoked to check something out there, a high degree of suspicion is the proper attitude for these videos !

So now the very first thing I am going to do is to send you to you tube to the search field to type in web4deb channel NOTE there are no "S''s! This will take you to
a Geodome green house build, you are looking for the Rocket Mass Heater group of videos, don't blame me if you start browsing and get lost ! I THINK it's an 8''system

I am a little concerned ,40 ft of straight horizontal stove piping is doable with your system if well crafted, however the general rule that we follow pretty closely is that
for every (1) one 90* Elbow we subtract 5' of pipe for loss due to increased friction, (2) two elbows together to make a 180* turn will cost you 10 ' !

You may want to look at The Rocket Stove Forum/Thread - ' length of exhaust pipe in mass ' -posted by George Brabant, you will see from my comments, this is
not an exact science- a rough wall in the Transitional Area or a small difference in the open space needed through that area can greatly effect things downstream !

for the craft, think like Fire, Flow like Gas, Don't be the Marshmallow! As always, your comments and questions are welcome, and Solicited ! PYRO - Logically BIG AL !

Thanks again Allen. I have seen hundreds of RMH videos including the ones you referenced. I generally use them for inspiration and evaluation of concepts presented. A stepping stone to furthering my knowledge of RMH design and operation.

Since this is getting a bit off the original topic I'll finish up by giving details of the exhaust tube in the gravel floor. I added cardinal directions to better picture it without pictures
From bottom of barrel via a 10"x4" floor vent duct flange it goes directly in to a 90 (vertical to the horizontal), goes 5' east to a 90, 5' north to a 90, 10' west to a 90, 5' north to a 90, 10' east to a 45, then 5' NE to a 90 (horizontal to vertical) and a 5' stack which exits the ground about 15" from the outside edge of the dome. I saw no point in having it exit thru the wall from the inside.
Subtracting 5' for each 90(6 + 2.5' for a 45) does that mean I effectively only have 12.5' of vent pipe including 5' final vertical stack? What exactly does that mean in practical terms?
I will still need to figure some way to prevent ash from falling directly into the vent opening in the bottom of the barrel and directly into the first elbow.

John Adamz :The answers are easy, dealing with the answers is not, Your first elbow should be hand sculpted to the same general shape as the elbow but have
room there below the elbow, and your First clean out 'T' should be located just after to allow you to remove the cap and stick a vacuum in there !

In your situation you can generally adding an additional 10 ft horizontal for every 15 feet vertical that you go. so 6X5=30, 30 +2.5 =32.5, 40-32.5 =7.5 +10
(the 15' vertical) = 17.5 ' you can not do what you want with a 6'' system try 8'' how were you going to vacuum out the fly ash that gets by your ash pit !?

Have you been to rocketstoves.com the get Your PDF $15 Copy of 'rocket mass heaters' there id STILL no other book in any language with more Rocket Mass Heater information!

Not a lot of good news, unless you want to be dependent on an in duct 12 -24 volt booster fan and Face losing everything with a long power outage,
That is the best I can do for you ! the book will allow you to figure it out for yourself !Big AL !