I built one of those rocket waterheaters. It works exceptionally well I do say. I used the pass through technique using 50 feet of 1/2" copper tubing running through a 40 gallon stainless steel kettle. There is no pressure to it. It has an over flow hose going to the outside. The core is a 30 gallon barrel turned on its side with 6" pipe holding the form and I have a clay perlite mixture filling the space. I insulated the outside of the whole thing with rockwool. Anyway here's some pictures.