The hydronic systems could be really dangerous, but there are expansion tanks, temperature and pressure security valves, etc.
The problem with the rockets is that there is no shutter to automatically stop the burning process before the boiling temperature is reached.
What came to my mind is something like a well insulated "aprovecho institutional stove"
http://www.aprovecho.org/lab/institutional-stove-project/institutional-project with a copper coil inside the "boiling" barrel to transfer the heat to the hydronic system and a flue dumper to stop the air circulation after the burning has stopped.