Heating barrel(s)

From a quick search it appears that wrapping copper tubing around the rocket mass heater barrel is not a good idea per se. But has anyone just made a RMH with just a couple more barrels (one sitting on each side of the heater barrel) and filled them with water? I possibly want to use the water in these two adjacent barrels in my greenhouse (pumped out to the greenhouse at least a couple times a day).

Hi Stevie;  
I have not heard of anyone sitting barrels next to the rmh barrel .  
I highly doubt you would gain enough heat that way. Water is an amazing heat sink as long as it stays a liquid. Flashing to steam of course is the danger of copper pipes.
Radiant heating from the barrel would give some of its heat to adjacent barrels but most would go up.   If you try to block that heat from going up you stand a chance of overheating your rmh barrel and stalling the flow.

Tell us more about how you want to use a rmh with your greenhouse.

Check out Matt Walkers boiler. Really great for hot water if that is your main goal.

https://walkerstoves.com/walker-boiler.html

I think an unpressurized container on top of the barrel would work fine.
Maybe another barrel,  cut down to a third in height and plumbed to circulate heated water to a series of other barrels.
Cover the open-top with a sheet of metal or piece of cement backer board.
I have been planning around the use of water heater tanks, as they are easy to come by and fitted for standard plumbing .

Fwiw,  I don't think cooling the top and sides of the barrel should be an issue.
I believe the combustion is supposed to be completed by that point.
In the original lore,  the barrel was supposed to shed heat,  and push the cooled gasses downward via gravity.
This thermal gravity pump was supposedly one reason why the the RMH didn't need a chimney to draft properly.
Currently most RMH practitioners seem to rely on a conventional chimney for draft.

Hi William,   Starting at about 53 minutes in Stove Chat 12, a question is asked about whether the metal barrel is absolutely necessary to make a RMH function correctly and Matt give his take on it. May be of interest to you.

Given the existence of rockets using bells instead if barrels and Matt's own innovations, the barrel is definitely not required, but I would like to see what he says about it.