Best design for rocket stove water heater

Hi Everyone! I'm long time natural builder recently moved backed to cooler climes. This is also my first post!

I'd like to heat a hot tub with a rocket stove and have been scouring the internet for a good design.

Basically if I build a standard RS with an insulated inner burn chamber, where would the best place be for a copper heat exchanger?

I thought maybe around the inner stack encased in insulation, then continue spiralling around the outside of the inner stack in the combustion barrel and finally out to the water return.

I would love to hear other folks' experience and / or advice.

Thanks!

Patrick

There is an Immersion Heater M-67 used by the military that should work perfectly for your use.  Will run on almost any fuel that burns including old engine oil, diesel, kerosene, gasoline.  They are still available surplus new and generally sell for around $100. US.   A great idea over reinventing the wheel.   Of course it isn't a rocket stove but the payoff is using discarded burnables.

For me spiraling isn't the correct method. You can use straight pipes with u-turns or 2 x 90 degrees turns. You can also easily replace damaged pipes which is not the case for spiraling. You can place straight pipes vertically or horizontal, water always find a way.

The best way I can think of is to fill your tub with cold water, and have the tub up high, have the rocket stove barrel up against the bottom of the tub, heating the water by heating the metal tub from below.  A cast iron tub wood work the best.  Be sure to place a wooden rack in the tub so that you don't scorch your butt.    I know people who have done this with a fire under the tub.  Same principal.