Why the barrel?

I'm thinking of not using a barrel around my jtube and just wondering what the reason is for the barrel before I ditch it. I was just going to leave an appropriate space (i forget if it's 1 inch or 2) and brick around my j-tube. I planed to refractory cement in a frying pan at the top.
I certainly could put in a barrel and brick around it and still cement my fring pan just over the barrel. But if it makes no difference, why? I designed it with the barrel simply because everyone else is using the barrel.
So, can anyone fill me in on why we are always using barrels. My understanding is that steel melts before brick, so we aren't using the barrel to protect the brick, right.
Thanks,
Anomika

The reason for the barrel is because of the inexpensive nature and ease of placing them. Most individuals building a rocket mass heater have little to no experience with masonry, so this is a great way to "cap" the stove with extreme ease. Barrels can be found for free basically anywhere, so that is another reason. You also get a lot of immediate radiant heat off of the barrel, so for those looking to get heat fast, barrels will shed heat quickly and effectively to the room.

You can brick around the riser to make your first bell of the stove. Do more than two inches. Two inches is the minimum, and is not worth the headache of having problems down the road with a restricted airflow. Those with lots of rocket experience have found that you can basically increase the gap between riser and bell to as high as you would like as long as the ISA (internal surface area) of the bell doesn't exceed the maximum for your stove dimensions.

From Batchrocket.eu

Riser diameter   /   Internal Surface area

12.5 cm  (5")   ISA 3.7 m²    (39.8 sq ft)
15.0 cm  (6")   ISA 5.3 m²    (57 sq ft)
17.5 cm  (7")   ISA 7.2 m²    (77.5 sq ft)
20.0 cm  (8")   ISA 9.4 m²    (101 sq ft)
22.5 cm  (9")   ISA 11.4 m²  (123 sq ft)
25.0 cm  (10") ISA 14.7 m²  (158 sq ft)

Also, a single skin of brick around the riser has the potential for much leaking. The temperatures of the stove may expand the brick and mortar to the point where you get fractures. This might result in smoking on startup out of the bell, as well as drafting issues if the draw of the stove is coming from many locations around the stove. To do a proper brick bell, you would need to do a double layer with an expansion joint between the two layers. A lot of work in my opinion and easily solved by using a barrel.

The other reason for using a barrel is that it gives an "instant heat" component to the system, so you don't need to wait for all of the heat to get through the mass, and if you don't have a well-drafting chimney, the barrel helps improve the draft by cooling the exhaust as it travels downward. Some people have reported that when a barrel was encased in cob or masonry the draft suffered and there were increased issues with smokeback.

If draft is not an issue, you can get some instant heating effect by installing a metal access panel in the side of the masonry bell, which of course has the added benefit of easing inspection and possible maintenance.

We use them because they are awesome! They do the job well, and they are free. (And there's nothing wrong with how they look.😂)

Great answers. Thanks. Because my stove is outside and the entire brick structure will be 3 walls of brick, i'm not concerned with a lot of the plus sides of the barrel. I see how it makes a lot of sense for indoors. I was planing on bricking around the barrel anyhow. And maybe i still will. I mean it doesn't hurt to have the barrel there and like you say it's free - 10 bucks per barrel. I find it interesting that cobbling around the barrel reduced draft. If that is true, i may run into this issue with my 3 layers of bricks. I"m building more of a traditional masonry heater then a rocket stove, but i'm using an insulated j-tube, so technically I think it's should be called a rocket stove. But then instead of the hot air going through a bench or a cob mass, it will zig-zag thru 3 walls of bricks before exiting. The last layer of brick might get insulation next year, but i want to test out my design this winter without it.
Anyhow, thanks for the info. It all made sense.

Jordan Holland wrote:We use them because they are awesome! They do the job well, and they are free. (And there's nothing wrong with how they look.😂)

Trust me, i wasn't questioning them because i care about looks. lol. I just didn't want to waste if it wasn't needed, why put it in.
But after reading about the expansion joints, i realised that it might be good, to protect the bricks from quick heat up/cold down and probably worth throwing one in. You won't see it in mine. lol So, i don't care what it looks like anyhow. not that I would care what my heater looked like. It's a heater, not wall photo. But my stove will be all bricked in so you only see bricks no mater if i put a barrel in or not.

Daniel,   In Stove Chat 3, Matt talks about Peters isa numbers and mentions that they suited Peters goals but with a bypass, these numbers are not relevant.  

Anomika Anderson wrote:Great answers. Thanks. Because my stove is outside and the entire brick structure will be 3 walls of brick, i'm not concerned with a lot of the plus sides of the barrel. I see how it makes a lot of sense for indoors. I was planing on bricking around the barrel anyhow. And maybe i still will. I mean it doesn't hurt to have the barrel there and like you say it's free - 10 bucks per barrel. I find it interesting that cobbling around the barrel reduced draft. If that is true, i may run into this issue with my 3 layers of bricks. I"m building more of a traditional masonry heater then a rocket stove, but i'm using an insulated j-tube, so technically I think it's should be called a rocket stove. But then instead of the hot air going through a bench or a cob mass, it will zig-zag thru 3 walls of bricks before exiting. The last layer of brick might get insulation next year, but i want to test out my design this winter without it.
Anyhow, thanks for the info. It all made sense.

http://batchrocket.eu/en/

Getting rid of barrels!

https://permies.com/t/44806/Cobbling-workshop-heater-cooktop-oven

The internal surface area of a barrel is not large, and if cobbed over it could get very hot quickly so that it no longer absorbs much heat to cool the airstream. A larger cavity of masonry would have much more internal surface and could absorb more heat without getting so hot it fails to assist the draft. Also, if you have a good draft already, that assist becomes less important.