Building a j tube with raw brick

I am planning on building a 6” j tube water heater along the lines of shown on various forums by Tim Barker and Geoff Lawton.I have what appears to be good soil for making bricks which has25 to 30% clay and 65% sand and 5+% silt.I  have made some green bricks in a mould and they air dry with minimal cracking.I want to make enough to Build my rocket with the air dried green bricks and fire up the rocket to cure them in situ.Anyone who maybe done this or foresees a problem with this please comment.Thank you in anticipation

Hi Jock;  Welcome to Permies and the wonderful world of rocket science!

Nothing wrong with making your own bricks.
I suggest drying them near a fire rather than trying to dry in place.
I have not done such a thing, but I would be concerned they might crack or otherwise shift during firing.

Tim Barkers water heater is very cool.  Please document your build with text and photo's.  
A lot of folks try to  heat hot water, most fail in one way or another.
The water heater you are building is tried and true. I would build one myself if I had a need of low pressure hot water.
Good luck on your build !